Growth of thin barrier films on flexible polymer substrates by atomic layer deposition

    Growth of thin barrier films on flexible polymer substrates by atomic layer deposition Karyn L. Jarvis, Peter J Evans PII: DOI: Reference:

S0040-6090(16)30878-1 doi:10.1016/j.tsf.2016.12.055 TSF 35712

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Thin Solid Films

Please cite this article as: Karyn L. Jarvis, Peter J Evans, Growth of thin barrier films on flexible polymer substrates by atomic layer deposition, Thin Solid Films (2017), doi:10.1016/j.tsf.2016.12.055

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ACCEPTED MANUSCRIPT Growth of thin barrier films on flexible polymer substrates by atomic layer

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Karyn L. Jarvis1,* and Peter J Evans2

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deposition

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1. ANFF-Vic Biointerface Engineering Hub, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, VIC 3122 2. Institute of Materials Engineering, Australian Nuclear Science and Technology

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Organisation, Lucas Heights, NSW 2232

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Abstract

Organic electronics research has received significant attention in recent years. The majority of this research has focused on the development of organic photovoltaic cells (OPVs) and

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organic light emitting diodes (OLEDs). Polymer substrates are used for organic electronics as

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they are lightweight, cheap, transparent, printable and flexible. One significant disadvantage of polymers, however, is their high gas/vapour permeability. For both OPVs and OLEDs to

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have sufficient lifetimes for commercial applications, barrier films are required to reduce the degradation resulting from exposure to water vapour and oxygen. Atomic layer deposition (ALD) is a thin film deposition technique ideally suited to the deposition of inorganic films

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for barrier applications as it produces conformal pinhole-free coatings. Inorganic films produced by ALD have achieved lower water permeation rates with thinner layers than other techniques due to film integrity. ALD alumina has been the most frequently studied barrier coating on polymer substrates in work to date. While single ALD films layers have been successfully used to reduce the water vapour transmission rate (WVTR) of polymer substrates, the lowest WVTRs have been achieved using multilayer films. Inorganic multilayer barrier films have been produced with alternate layers of two ALD metal oxides such as alumina and titania. In addition, alternating layers of ALD metal oxide and a polymer have also been combined to form inorganic/organic multilayer barrier films. The use of a multilayer structure reduces diffusion pathways through the entire film thickness, thus producing lower WVTR values. This review highlights the effectiveness of ALD barrier films in reducing water permeation through polymer substrates, which is an essential requirement for the longevity of organic electronic devices. 1

ACCEPTED MANUSCRIPT Contents

Introduction ........................................................................................................................ 4

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Atomic layer deposition...................................................................................................... 5

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2.2

Spatial ALD................................................................................................................. 8

2.3

Roll-to-Roll ALD ...................................................................................................... 10

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ALD background ......................................................................................................... 5

Determining the water vapour transmission rate .............................................................. 12 MOCON .................................................................................................................... 13

3.2

Ca Test....................................................................................................................... 13

3.3

Tritiated Water Permeation ....................................................................................... 15

3.4

Comparison of WVTR techniques ............................................................................ 17

3.5

Effect of film defects on WVTR ............................................................................... 18

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Single Al2O3 barrier films................................................................................................. 19 Influence of film thickness and deposition temperature ........................................... 20

4.2

Influence of purge times ............................................................................................ 23

4.3

Influence of precursors .............................................................................................. 24

4.4

Influence of the substrate .......................................................................................... 25

4.5

Influence of PEALD plasma parameters ................................................................... 25

4.6

Comparison of thermal and plasma ALD ................................................................. 26

4.7

Film flexibility........................................................................................................... 27

4.8

Double-sided films .................................................................................................... 28

4.9

Summary ................................................................................................................... 28

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Single-layer non-Al2O3 barrier films ................................................................................ 31

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Inorganic/inorganic multilayer barrier films .................................................................... 34 6.1

Al2O3/TiO2 films ....................................................................................................... 34

6.2

Al2O3/ZrO2 films ....................................................................................................... 36

6.3

Al2O3/SiO2 films ....................................................................................................... 39

6.4

Al2O3/SiN films ......................................................................................................... 40

6.5

Al2O3/ZnO films........................................................................................................ 40

6.6

Non-Al2O3 films ........................................................................................................ 40

6.7

Summary ................................................................................................................... 41

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ACCEPTED MANUSCRIPT Inorganic/organic multilayer barrier films ....................................................................... 43 Organic films deposited by molecular layer deposition ............................................ 43

7.2

Organic films deposited by vapour phase deposition................................................ 46

7.3

Organic films deposited by spin coating ................................................................... 46

7.4

Organic films deposited by plasma polymerisation .................................................. 48

7.5

Summary ................................................................................................................... 50

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Spatial and roll-to-roll deposited barrier films ................................................................. 52

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Barrier films in OLEDs .................................................................................................... 54

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10 Factors to consider ............................................................................................................ 58

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10.1 WVTR test configuration .......................................................................................... 58 10.2 Deposition directly onto Ca test pads ........................................................................ 60

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10.3 Effect of temperature and relative humidity on WVTR ............................................ 60 11 Conclusions ...................................................................................................................... 62 12 Acknowledgements .......................................................................................................... 63

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13 References ........................................................................................................................ 64

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ACCEPTED MANUSCRIPT 1

Introduction

Organic electronics have been the subject of significant research in recent years, which has

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investigated the development of organic photovoltaic cells (OPVs) and organic light emitting

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diodes (OLEDs). Polymer substrates are typically used for organic electronics due to their flexibility, price and printability but unfortunately they also have highly permeability. Barrier

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films are therefore required to retard the degradation resulting from water vapour and oxygen exposure to enable OPVs and OLEDs to have adequate lifetimes for commercial applications [1]. It has been widely accepted that water vapour transmission rates (WVTRs) of 10-6 g.m/day are required to produce OLEDs with sufficient lifetimes [2]. Atomic layer deposition

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(ALD) can be used to deposit a number of inorganic films, some of which possess barrier

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properties. ALD is a self-limiting technique in which gas-phase deposition is used to produce conformal pinhole-free inorganic coatings. It is based on the alternate pulsing of precursor gases and inert gases in which pulses of the latter are used to purge the reactor chamber of

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excess precursor and by-products [3]. Inorganic films produced by ALD have achieved lower

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WVTRs than those of similar thickness obtained with other techniques; a finding attributed to the integrity of ALD films [4]. In much of this work, alumina (Al2O3), either singly or in

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combination with other metal oxides, has been the most extensively investigated film material [5]. Thermal ALD frequently requires temperatures greater than 200 °C to produce high quality films. However, many polymeric materials have glass transition temperatures less than 100 °C and cannot withstand such high temperatures. Therefore, lower temperatures

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are typically used to deposit barrier films onto polymer substrates [6]. For example, the commonly used polyethylene terephthalate (PET) is generally limited to processing temperatures up to 120 °C [7]. This limitation can be overcome through the use of plasmaenhanced ALD (PEALD), which has been utilised as an alternative to thermal ALD in a number of studies. PEALD exploits plasma excitation of precursors to promote reactions at lower temperatures, thus enhancing film quality without subjecting the coated material to elevated temperatures.

ALD was initially referred to as atomic layer evaporation [8], then atomic layer epitaxy [9, 10] before finally being known as ALD. The term “epitaxy” was used initially instead of deposition to describe the growth of single-crystal layers [9]. Early ALD work was undertaken to deposit zinc sulfide thin films for electroluminescent thin film displays [11] and zinc telluride films [8]. A number of reviews have appeared in recent years documenting 4

ACCEPTED MANUSCRIPT the advances and breadth of ALD research [9, 12-24]. Other reviews have also been published which summarise the use of ALD for specific applications such as photovoltaics [25], OLEDs [4], semiconductors [26] and nanotechnology [10, 27, 28]. While ALD is used

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in a wide variety of applications, the use of ALD to deposit barrier films is a more recent

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application, with the earliest papers appearing in early to mid-2000s [29-34]. Although a number of ALD review articles have been published, only a few discuss the use of ALD for

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barrier films. Several reviews include ALD amongst other techniques for depositing barrier films [4, 35-37]. Other reviews discuss the use of ALD as different components, including as a barrier, in a photovoltaic cell [25] or the mechanisms of ALD deposition onto polymers and

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how they can applied as a barrier [23]. The intent of this paper is to provide a comprehensive and critical review of the ALD barrier film literature and to discuss the deficiencies and

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possible future directions of the field. The first part will outline the types of ALD that have been used for barrier film research followed by a discussion of the techniques used to measure the WVTR. The various film structures (i.e. single and multilayers) used to produce

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Atomic layer deposition

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barrier films and their WVTRs will then be summarised and assessed.

ALD background

ALD is based on a reaction sequence in which a surface is exposed sequentially to two or

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more precursors as shown in Figure 1. A typical ALD growth cycle consists of 4 steps: 1) first precursor pulse, 2) reactor purge, 3) second precursor pulse and 4) reactor purge. This cycle is repeated as many times as necessary to deposit a film of desired thickness [15]. Separation of the precursors pulses by purging with an inert gas, usually nitrogen or argon, removes unreacted species and products from the deposition chamber, thereby avoiding gasphase reactions as used for film growth in conventional chemical vapour deposition (CVD). The use of purge pulses also plays an important function in low-temperature depositions for which physisorbed precursor molecules can contribute to the film growth process. The presence of such species on the surface may lead to a higher film growth rate, which is accompanied by a decrease in density and an increase in impurity levels.

One of the main advantages of ALD is the precise control over film thickness by the number of reaction cycles. In addition, ALD can reproducibly deposit films with large area 5

ACCEPTED MANUSCRIPT uniformity and excellent conformality. The self-limiting nature of ALD also produces smooth, continuous and pinhole-free films as no active sites are retained on the surface during film growth [4], and is therefore ideal for producing barrier films. A disadvantage of

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ALD is its slow deposition rates due to the long cycle times resulting from cycling the

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precursor and purge pulses [19]. The deposition rates for ALD are typically 100 – 300 nm/hr [22], but the specific rate is dependent on a number of factors including deposition

Figure 1

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temperature, precursors, reactor setup and sample geometry.

Schematic diagram of ALD using self-limiting surface chemistry [17].

ALD precursors must be sufficiently volatile and thermally stable for efficient transportation within the reactor so that uniform conditions prevail during film deposition. The precursors also must have high enough vapour pressure and reactivity to provide the concentration and energy to react with all the available chemisorption sites on the surface in the shortest time possible. In addition, the precursors should not decompose at the deposition temperature nor etch the deposited film [38]. Finally, precursor cost is also an important issue, particularly for ALD commercial applications, since their synthesis can be both time consuming and expensive. A wide variety of films can be deposited by ALD, with the majority being oxides, nitrides, sulfides and pure elements [22]. Many of the requirements for ALD mentioned above have been discussed in more detail in several review articles [16, 17, 22]. 6

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ALD reactions carried out at temperatures above ~200 °C usually produce good quality films. Film growth at lower temperatures can be affected by two competing processes. First,

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thermally-activated chemisorption is reduced resulting in a decrease in deposition rate [38].

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In contrast, physisorption of precursors may increase leading to a higher growth rate. A number of studies have investigated the ALD of Al2O3 films at temperatures between 33 –

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177 °C as this range is generally required for coating flexible polymeric materials [39-41]. It has been shown that such temperatures cause a decrease in both the density and refractive index of the films and may lead to a higher growth rate due to physisorption. In addition, the

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level of impurities in the films can increase significantly with decreasing temperature. For example, an Al2O3 film deposited at 177 °C had a hydrogen concentration of ~5% whereas

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the hydrogen content increased to >20% at 33 °C. It was suggested that this large increase contributed to the lower film density obtained at the lower temperature [39]. The precursor pulse and reactor purge times are also dependent on the deposition temperature and are

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therefore factors which influence ALD film quality. In particular, ALD precursor pulse times

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should be long enough to saturate the deposition surface, while sufficiently long purge times are required to completely remove completely all unreacted precursor and reaction products.

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It has been shown for example that precursor pulses of 1 s and purge times of 5 s were sufficient for the ALD of Al2O3 films at 177 °C using trimethylaluminium (Al2(CH3)3) and water (H2O) [42]. In contrast, Al2O3 deposition at 58 °C required an H2O pulse time of 2 s and the purge times following the trimethylaluminium (TMA) and H2O pulses had to be

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increased to 10 and 30 s respectively. These substantial increases in pulse times at the lower temperature were necessary to remove physisorbed species and avoid CVD growth, both of which lead to a greater mass gain per cycle [39].

Both thermal ALD and PEALD have been used to deposit barrier films. Systems have been developed with different substrate configurations, but the deposition mechanisms remain the same. In thermal ALD, the substrate is heated to initiate surface reactions for which the deposition temperature has been shown to have a significant impact on deposition rate and film chemistry [39]. The precursors used in thermal ALD have negative heats of reaction and therefore undergo spontaneous surface reactions within specific temperature ranges. The latter requirement may necessitate the use of temperatures above those compatible with certain substrate materials such as polymers. PEALD provides a means of depositing films at lower temperatures than those often needed to produce films of comparable quality by 7

ACCEPTED MANUSCRIPT thermal ALD. PEALD utilises plasma excitation to expose the deposition surface to energised precursor species to promote reactions at lower temperatures than those achievable with only thermal energy [18]. Plasma activation is usually applied at the second precursor

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step in which O2, N2 or H2 gases are typical replacements for the ligand-exchange precursors,

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such as H2O or NH3, frequently used in thermal ALD [19]. Single element films, most notably metals, are also more readily deposited by PEALD as these are difficult to produce

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using thermal ALD [18]. In addition, PEALD circumvents problems associated with purging species like H2O from the deposition chamber at temperatures ≤ 120 °C, which may require purge times greater than 20 s [39, 43]. Even the latter does not ensure complete removal of

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H2O as evidenced by the increased growth per cycle often observed below 120 °C. Using an O2 plasma in place of H2O, shortens the second purge time which, in turn, reduces the single

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cycle time and the total deposition time. Thus, PEALD is a versatile technique for barrier film applications due to its lower operating temperatures and shorter deposition times [44].

Spatial ALD

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Spatial ALD facilitates rapid film deposition by moving the substrate through separate

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precursor exposure zones which are isolated from each other by inert gas shields. This approach results in faster deposition times by effectively removing the purging steps required in a conventional ALD cycle. Thus, the total deposition time is only dependent upon the individual precursor exposure times required to saturate the surface active sites and the

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substrate transport time [45]. Conventional ALD is typically not feasible for applications which require cost effective large-area barrier films due to the small substrates sizes which can be accommodated in an off-the-shelf laboratory-scale ALD reactor. Fast ALD deposition rates are required to deposit large-scale barrier films on large sheets of printed OLEDs and OPVs. Spatial ALD reactors have mostly been based on thermal ALD as the plasma processing step is not readily accommodated in the various reactor configurations developed thus far. Recently, silver films have been deposited using atmospheric pressure plasma enhanced spatial ALD [46]. However, the deposition of relevant barrier films such as Al2O3 by plasma enhanced spatial ALD has not yet been reported in the literature. Such a development would be advantageous for the application of spatial ALD systems for the largescale production of barrier films.

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ACCEPTED MANUSCRIPT Several different configurations of spatial ALD reactors have been developed and these are illustrated in Figures 2 and 3. A previous paper has summarised the companies and universities which have developed spatial ALD reactors [47]. Drum spatial ALD reactors

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attach a length of flexible substrate to a drum which rotates within a heated cylindrical

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reaction chamber. Precursor and purge gas inlets and outlets are situated around the drum to create precursor and purge zones, as shown in Figure 2. In a recent study, a flexible substrate

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was fitted to the drum that was rotated at speeds in the range 10 - 250 revolutions per minute (RPM) for 1000 rotations which resulted in coating times of 100 to 4 minutes respectively. For rotation speeds below 100 RPM, the growth rate increased linearly with decreasing

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rotation speeds. In addition, a linear relationship between the number of rotations and film thickness was observed for films deposited at 20 and 50 RPM. The thicknesses measured at

Figure 2

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points across the substrate were also found to be relatively consistent at 20 – 40 RPM [48].

Schematic diagram of a rotating drum spatial ALD reactor [48].

A ring spatial ALD reactor incorporating a rotating, circular substrate table is shown in Figure 3a. This system was used to rotate a substrate at 600 RPM under the dedicated precursor inlets, located in the reactor head, to deposit a 30-mm-wide annular track of Al2O3. The thickness of the Al2O3 film increased linearly with the number of rotations and resulted in a deposition rate of 1.2 nm/s [45]. A ring spatial ALD reactor has also been used to investigate the low-temperature deposition of Al2O3 on silicon wafers. Deposition temperatures of 75 – 200 °C were used to deposit films for 1000 cycles at 120 RPM. The highest growth per cycle was achieved at 75 °C, but relative humidity (RH) values above 10% were required for homogeneous deposition. In the range 100 – 200 °C, the growth per 9

ACCEPTED MANUSCRIPT cycle was significantly lower with RH less than 10% [49]. A “back and forth” spatial ALD reactor has been developed in which a 370 mm x 470 mm piece of glass was moved back and forth at a speed of 0.8 m/s under bar-type gas injectors, shown in Figure 3b. Two cycles of

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ALD occurred during each scan. The film thickness increased linearly with the number of

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ALD cycles, resulting in a thickness of approximately 90 nm after 1000 cycles and a growth

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rate of 0.7 nm/min [50].

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Figure 3

Schematic diagrams of a) ring (Adapted from [45]) and b) back and forth (Adapted from [50]) spatial ALD reactors.

2.3

Roll-to-Roll ALD

Roll-to-roll (R2R) processing is another technique that can be used to rapidly coat flexible substrates. In R2R ALD, the flexible substrate travels from the supply reel to the take-up reel 10

ACCEPTED MANUSCRIPT and passes through the precursor gas zones in-between. As the substrate continually moves relative to the precursor sources, the deposition rate is dictated by the substrate speed rather than the cycle time as in conventional ALD [51]. An important consideration in R2R ALD is

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the fragility of the freshly-deposited ALD films. These films can be damaged when

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contacting surfaces such as guide rollers or during rewinding. In addition, surface particles and imperfections, coated during ALD, rub against the adjacent surface on the roll resulting

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in the formation of defects [52].

A number of different R2R systems have been developed, as shown in Figure 4. A drum R2R

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ALD reactor has been used to deposit Al2O3 onto 500-mm-wide polyethylene naphthalate (PEN) at coating head speeds of 0.02 – 0.4 m/s. The configuration of the reactor is shown in

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Figure 4a. The coating head is comprised of 29 sections with each containing a nozzle for precursor delivery and a surrounding exhaust slit. The nozzles are also separated by inert gas curtains to prevent precursor mixing. The coating head moves in a pendulum motion which

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results in two ALD cycles per swing. It was shown that increasing the speed of the head

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reduced the growth rate per cycle from 1.1 – 0.494 Å/cycle [53].

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An atmospheric R2R ALD has been used to deposit Al2O3 films onto PET at a substrate speed of 7 mm/s. [51]. The gas delivery head was positioned above the moving substrate and consisted of precursor heads separated by inert-gas streams, as shown in Figure 4b. At a substrate speed of 7 mm/s, a growth rate of 0.98 Å/cycle was achieved. For substrate speeds

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greater than 7 mm/s, deposition rates of greater than 3 Å/cycle resulted from CVD growth.

An alternative to the above systems is provided by a R2R ALD system based on a serpentine configuration, as shown in Figure 4c. The chamber is separated into 3 zones with narrow slits that allow the substrate to travel between the zones. The precursors are continuously pumped into the top and bottom zones while the central purge zone, which is not directly pumped, operates at a positive pressure to prevent ingress of precursors. The substrate in this serpentine configuration travels between the top and bottom precursor zones such that one complete pass of the substrate is equivalent to 8 ALD cycles. For the deposition of thicker coatings, the direction of the substrate is repeatedly reversed at the end of each pass to achieve the desired thickness. This system was used to deposit Al2O3 onto a 100-mm-wide substrate using TMA and water as precursors. Due to the serpentine setup of the system, film is deposited on both sides of the substrate. The speed of the substrate was varied from 0.33 – 11

ACCEPTED MANUSCRIPT 1.5 m/s. The growth rate increased with increasing substrate speed and reached a maximum of approximately 0.22 nm/cycle at a substrate speed of 1.5 m/s. The high growth rates observed at the faster substrate speeds were attributed to insufficient time between alternating

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precursor exposures for complete desorption of excess water from the substrate surface

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leading to non-ALD film growth [52].

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b)

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Figure 4

Schematic diagrams of a) drum [53], b) atmospheric (Adapted from [51]) and c) serpentine roll to roll ALD reactors [52].

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Determining the water vapour transmission rate

The WVTRs of polymers and barrier films are most frequently measured using the calcium (Ca) test or the MOCON instrument. Less commonly used is the permeation of tritiated water 12

ACCEPTED MANUSCRIPT (HTO). In addition, a number of other techniques have been used to determine WVTR including mass spectroscopy [54], laser adsorption spectroscopy [55], gravimetric [56] and cavity ringdown infrared spectroscopy [57]. A number of studies have modelled water

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permeation through barrier films [58-62]. As MOCON, Ca test or HTO permeation are the

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techniques most often used to determine the WVTRs of high barrier films, these will be

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described in greater detail below.

MOCON

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The commercially available MOCON instrument, with three different models available, is frequently used to evaluate the WVTR of barrier films. The MOCON Permatran uses a

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modulated infrared sensor and has a detection limit of 5x10-3 g.m-2/day [63]. The detection limit is extended in the MOCON Aquatran, which uses a coulombmetric sensor with a detection limit of 5x10-4 g.m-2/day [40]. The MOCON Aquatran Model 2 instrument has an

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even lower detection limit of 5x 0-5 g.m-2/day [64]. In all instruments, the substrate is placed

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between two chambers in a test cell, as shown in Figure 5. The inner chamber is filled with nitrogen carrier gas and the outer chamber is filled with water vapour. Water molecules

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diffuse through the substrate to the inner chamber and are transported to the sensor by the nitrogen carrier gas. The sensor monitors the increase in water vapour concentration and calculates the WVTR. Due to the aforementioned detection limits, some barrier films currently being produced have WVTRs that cannot be measured with MOCON instruments

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[50, 52, 53]. Thus, more sensitive measurement techniques such as the calcium test and tritiated water permeation have been developed and utilised for barrier films with WVTRs in the 10-5 - 10-6 g.m-2/day range.

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Schematic diagram of the MOCON instrument [65].

Ca Test

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ACCEPTED MANUSCRIPT The Ca test is widely used to determine the WVTR of barrier films, where permeation rates as low as 3x10-7 g.m-2/day have been detected [4]. The Ca test is based on the change in properties of a calcium film due to the oxidation caused by exposure to water/oxygen. Upon

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exposure to water vapour and/or oxygen, the reflective and conductive metallic calcium

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oxidises to the less conductive and more transparent calcium hydroxide [66]. By in-situ monitoring of either the change in conductivity or transparency, the Ca test can be used to

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determine the WVTR. One perceived disadvantage of the Ca test is its inability to distinguish between oxygen and water permeation as both cause oxidation [4]. However, it has been shown that the oxidation attributed to water is dominant at room temperature [66, 67] and that

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oxygen accounts for less than 5% of Ca oxidation [7]. Therefore, the Ca test provides a

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reliable technique for assessing the water permeation rates of barrier films [68].

Although different configurations of the Ca test have been developed [68], there are two distinctly different types: the electrical Ca (e-Ca) test [55, 68-73] and the optical Ca test (o-

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Ca) [33, 74, 75]. In the e-Ca test, metallic calcium is deposited onto gold or silver electrodes

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and the barrier film under investigation is then coated on top. A typical set-up is shown in Figure 6. The sensor depicted is comprised of silver electrodes deposited 500 µm apart on a

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glass substrate. Ca pads, 150-µm-thick and 500-µm-wide. are then deposited onto the electrodes followed by a coating of 130 nm of ALD Al2O3 film directly on the Ca and electrodes [76]. The diffusion of moisture and oxygen through the film coverts to metallic Ca to calcium oxide (CaO) and calcium hydroxide (Ca(OH)2) [70]. The rate of conversion is

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determined by measuring the change in conductivity of the Ca film under controlled temperature and RH conditions. An example is plotted in Figure 6 where a linear decrease in conductance was observed following a short induction period.

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ACCEPTED MANUSCRIPT Figure 6

Conductance versus time for a calcium test and schematic of permeation sensor [76].

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The setup of the o-Ca test differs from that used for the e-Ca test as the Ca pads are deposited

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directly onto the glass substrate rather than on pre-deposited electrodes. The metallic calcium is initially a highly reflective mirror which becomes increasingly transparent as it is

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converted to CaO and Ca(OH)2. Images of the Ca pads are taken at regular intervals to monitor the extent of oxide formation with the oxidized calcium appearing white due to its transparency. Automated image analysis is then used to determine the optical transmission of

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the Ca layer. Subsequent optical modelling of this transmission data provides a measure of the distribution of oxidized Ca in the pads as a function of time, which can then be used to

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determine the WVTR of the barrier film [74]. The e-Ca and o-Ca techniques have been compared by determining the WVTR at 70 °C/28% RH of an 18.7-nm-thick Al2O3 barrier film [75]. With the o-Ca test, it was found that Ca oxidation occurred at pinhole defects in the

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film around which time dependent radial growth was observed. In addition, new sites of

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oxidation appeared suddenly at later times and these also grew radially. These were attributed to water corrosion of the Al2O3 film. For the e-Ca test, the Ca conductance initially changed

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little over time. A sudden drop in the conductance was then observed which corresponded to when a significant portion of the Ca was visually oxidized.

The accuracy of the Ca test relies on the assumption that Ca oxidation is linear with water

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exposure. However, previous studies have shown this is not the case [77, 78]. Initially, the Ca conductance remains constant, which is known as the lag region. The conductance then rapidly decreases at longer water exposure times [72]. This effect has also been observed in a study of Ca oxidation kinetics using a quartz crystal microbalance. The mass gain as a function of time was once again shown to be non-linear. Three distinct regions were identified; lag region, oxidation region and sensor lifetime. Different WVTR values were calculated, depending on which region the data was taken from. The non-linearity of Ca oxidation raised doubts about the accuracy of the WVTRs determined by the Ca test though it was proposed that reliable values could be obtained at short lag times [79].

3.3

Tritiated Water Permeation

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ACCEPTED MANUSCRIPT HTO permeation has been shown to offer a viable method for determining the WVTR of films [80, 81]. Tritium (T) is the radioactive isotope of hydrogen which decays through the emission of low-energy beta particles. HTO is radioactive water with a specific radioactivity

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and is composed of a mix of HTO, H2O and T2O molecules. Two different methods for

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detecting the beta particles have been utilised in water permeation studies. One system uses a combination of lithium chloride (LiCl) and liquid scintillation counting while the other

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measures the emitted radiation with an inbuilt beta ray detector. In the former, the barrier film separates the top and bottom chambers of a vessel, as shown in Figure 7a. The vapour from a droplet of HTO placed in the bottom chamber permeates through the film where it is

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absorbed by the hygroscopic LiCl in the top chamber. At a predetermined time, the LiCl is removed, dissolved in water and scintillation cocktail added. The activity of this solution is

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then measured by liquid scintillation counting from which the WVTR can be calculated. This method assumes that all the tritium diffuses through the film as molecular HTO [32, 82]. Independent of temperature, the gap between the HTO reservoir and film becomes saturated

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with HTO vapour and thus the measurement is undertaken at ~100% RH [83]. A system with

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a beta-ray detector is shown in Figure 7b. HTO is loaded into a reservoir at the bottom of the vessel and temperature controlled heaters are used to regulate the vapour pressure. Once

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again, the substrate is mounted between the two halves of the vessel. HTO permeates through the membrane and is transported by a carrier gas to the detector [84]. The detection limit of

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WVTR using tritium permeation is reported to be below 1x10-6 g.m-2/day [4].

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b)

Figure 7

Schematic diagrams of systems used to determine water permeation with HTO

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with a) liquid scintillation counting [82] and b) a beta-ray detector [84].

The permeation of tritiated species has been investigated by comparing the transmission of tritated water, tritiated propanol and tritiated hexanol [82]. It was shown that for a 26-nm-

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thick Al2O3 film, the permeation rate from all three tritiated molecules was similar. However, for films with thicknesses of 2.5, 5 and 10 nm, the transmission rate increased as the molecule size decreased. For these thinner films, it was proposed that both molecular and atomic tritium diffusion occurs whereas atomic diffusion was predominant with the 26-nm film due to tritium atoms exchanging with hydrogen atoms on the hydroxyl groups of the Al2O3 film. 3.4

Comparison of WVTR techniques

The WVTRs of barrier films are typically determined with MOCON, Ca test or HTO permeation alone, but a few studies have used a combination of these techniques. In particular, the MOCON test has been used to determine the WVTR of a PET substrate while the barrier performance of multilayer films was measured with the Ca test [85]. The same 17

ACCEPTED MANUSCRIPT combination of techniques has also been used in several studies for which the Ca test was applied when the WVTR was found to be below the MOCON detection limit [40, 53, 86-89]. Although both techniques were used in these studies, direct comparisons between the two

T

were restricted by the lack of matching data points. A direct comparison has been made

IP

between HTO permeation and the o-Ca test, which were used to measure the WVTR of a 10nm ALD Al2O3 film [32]. Under ambient conditions, WVTR values of 2x10-3 g.m-2/day and

SC R

1.5x10-3 g.m-2/day were measured using the HTO permeation and o-Ca methods, respectively. Paetzold et al [71] reported the development of an e-Ca test device for studying the WVTR of PET. At 38 °C/90% RH, they found good agreement between their results and

NU

those obtained using a MOCON instrument. In a multi-laboratory study, the WVTR of a multilayer barrier film was investigated using a number of techniques: e-Ca, o-Ca, MOCON,

MA

isotope-marking mass spectrometry, tuneable-diode laser absorption spectroscopy and cavityringdown spectroscopy [90]. The WVTR was determined at 20 °C/50% RH and 38 °C/90% RH, although it was not possible to measure both sets of conditions with every technique. In

D

particular, the MOCON technique did not yield useful results at 20 °C/50% RH due to its

TE

detection limit. Reasonable agreement in the WVTRs was obtained across the range of

3.5

CE P

techniques.

Effect of film defects on WVTR

Water permeation increases due to the presence of pinholes or defects in barrier films [91].

AC

These defects may occur from rough handling subsequent to deposition. Contaminants present on the substrate surface prior to film deposition, such as dust particles, will be enveloped due to the high conformality of ALD. The coating of these surface contaminants may then result in defect formation as the high points are easily abraded during handling [92]. Water permeation also occurs through nanoporosity in the inorganic barrier film [91]. Pores larger than 1 nm with a relative content greater than 1% are responsible for WVTRs of 10-210-3 g.m-2/day, while intrinsic WVTRs of 10-4-10-6 g.m-2/day have been attributed to pores of 0.3 to 1 nm [93]. The intrinsic WVTR is the water permeation though the bulk barrier whereas the extrinsic WVTR is the total water permeation through both the film and pinholes/defects [91]. Typically, the extrinsic or effective WVTR is higher than the intrinsic WVTR due to the increase in water permeation through pinholes and defects [94]. In the Ca test, defect assisted degradation of the Ca, resulting from water permeation through pinholes or defects, appears as white spots in the Ca pads, as shown in Figure 8, as the Ca(OH)2 is 18

ACCEPTED MANUSCRIPT transparent, thus allowing light reflection from the white background placed under the test piece. In the absence of defects, the calcium layer degrades uniformly [94]. The intrinsic WVTR is calculated by excluding the Ca pads that have undergone degradation due to

T

defects, which appear as white spots. Thus, the WVTR calculated is only due to water

IP

permeation through a “perfect” barrier film devoid of defects. A few studies have reported only intrinsic WVTRs [91, 95], while others have reported both the intrinsic and extrinsic

SC R

values [94, 96]. Kim et. al. [94] have shown that a silicon nitride (SiNx)/Al2O3/hafnia (HfO2) multilayer barrier film is relatively free of defects due to the similar intrinsic and extrinsic WVTR of 1.41x10-4 and 1.89x10-4 g.m-2/day, respectively. Starostin et. al. [96] have however

NU

shown that for a silica (SiO2)/Al2O3 bilayer, there was a significant difference between the intrinsic and extrinsic WVTR. The extrinsic WVTR was in the 10-3 g.m-2/day region while

MA

the intrinsic WVTR was 10-5 - 10-6 g.m-2/day, suggesting the presence of film defects. Although calculating the intrinsic WVTR can indicate whether the WVTR of a barrier film is dictated by film properties or by pinholes/defects [94], its value is an indication of how the

D

film would ideally behave rather than its actual behaviour due to the inherent presence of

TE

defects. The extrinsic or effective WVTR gives the most accurate estimation of how a barrier film will perform in real-world applications and is therefore the most relevant in the

AC

CE P

development of barrier films.

Figure 8

o-Ca test images of a) fresh calcium pads and b) after 506 hours of degradation at 50 ºC/85% RH for a SiNx/Al2O3/HfO2 multilayer barrier film (Adapted from [94]).

4

Single Al2O3 barrier films

Physical vapour deposition (PVD), CVD and ALD are established techniques for the deposition of metal oxides. Thus, all three have been used to investigate their potential usefulness for producing barrier layers with WVTRs below 10-3 g.m-2/day. However, incomplete layer coverage is an issue with PVD, while Al2O3 films deposited by CVD tend to 19

ACCEPTED MANUSCRIPT have high surface roughness, a property that may degrade barrier performance. In contrast, ALD produces uniform and conformal films, which in the case of Al2O3 are amorphous over the temperature range of interest for coating polymeric materials. This latter feature leads to

T

reduced surface roughness and eliminates grain boundary diffusion, which can contribute to

IP

the WVTR in polycrystalline films. These properties of ALD Al2O3 films have made it the material of choice for barrier layers deposited by means of this technique. This has resulted in

SC R

the large number of published articles investigating its barrier performance on different polymer substrates.

NU

The precursors most commonly used for the ALD of Al2O3 films are TMA and either H2O for thermal ALD or O2 plasma for PEALD. An advantage of these reactants is the relatively high

MA

film deposition rate (>1 Å/cycle) at temperatures lower than 150 °C [39, 97], which is an upper limit for deposition on many polymer substrates. In addition, the high vapour pressure and reactivity of TMA make it an ideal choice for the deposition of ALD films at low

D

temperatures. Several other ALD precursors can be used for Al2O3 films including aluminium

TE

trichloride, aluminium isopropoxide and dimethylaluminium propoxide. However, these reactants are generally more suitable for higher temperature depositions due to their lower

4.1

CE P

reactivity and vapour pressure at room temperature.

Influence of film thickness and deposition temperature

AC

Several studies have shown that increasing either the film thickness [64, 98], deposition temperature [76, 99] or both [29-32, 40, 100, 101] decreases the WVTR. Groner et al. [32] investigated the effect of film thickness on WVTR and their results are shown in Figure 9. In this study, the WVTR was measured using HTO permeation under ambient conditions. While the thinnest Al2O3 film (2.5 nm) did not reduce the WVTR in comparison to the uncoated substrate, a decrease of several orders of magnitude in WVTR was observed with a 10 nm film. Surprisingly, increasing the film thickness to 26 nm produced only a small improvement in barrier performance. Thicker Al2O3 films of 100 and 130 nm have been studied by Meyer et al. [76]. In their work, the WVTR of Al2O3 films were measured by means of the e-Ca test using films deposited directly on Ca pads. At 70 °C/70% RH, WVTR values of 3.5x10-4 and 9.9x10-5 g.m-2/day were reported for the 100 and 130 nm films, respectively. The effect of film thickness on Al2O3 films deposited by PEALD has been investigated Kim et al. [100]. A WVTR of 4x10-2 g.m-2/day was reported for a 48.5 nm film on polyethersulfone (PES), 20

ACCEPTED MANUSCRIPT which represented a decrease of ~3 orders of magnitude relative to the uncoated substrate. Increasing the film thickness from 15.1 to 48.5 nm resulted in a small decrease in WVTR,

MA

NU

SC R

IP

T

consistent with the other studies discussed above.

Figure 9

WVTR versus Al2O3 ALD film thickness on PEN [32].

D

A more comprehensive study of the effects of temperature and thickness on barrier

TE

performance was that undertaken by Carcia et al. [40]. Thermal ALD Al2O3 films were deposited on PET for 75, 125 or 250 cycles at 50, 75 or 100 °C with respective growth rates

CE P

of 0.85, 0.86 and 0.926 Å/cycle at the three temperatures. The WVTR was determined by means of the o-Ca test at 38 °C/85% RH after aging for ~1200 hours. Additional measurements were also performed on a MOCON Aquatran instrument, which had a

AC

sensitivity of 5x10-4 g.m-2/day. The results of the latter analysis are shown in Figure 10. In accordance with the studies discussed above, the most significant reduction in water permeation was observed for film thicknesses in the range 0-11 nm, with a more gradual change between 12-20 nm. A temperature effect is also evident in Figure 10 which shows that at 50 and 75 °C, thicker films were necessary to achieve the same performance as the thinnest film deposited at 100 °C. Several other studies have shown behaviour similar to that discussed above. In particular, the barrier properties of ALD AlOx with thicknesses ranging from 20-50 nm deposited on PES at temperatures of 80, 90 and 100 °C have been reported by Park and co-workers [29-31]. Once again, increased film thickness and higher temperature both resulted in lower WVTRs which were reduced by 2-3 orders of magnitude relative to the uncoated PES.

21

Figure 10

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

WVTR values at 38 °C and 85% RH as a function of film Al2O3 thickness

TE

D

deposited on PET [40]. (Note x-axis scale is mg.m-2/day not g.m-2/day).

For Al2O3 films deposited by PEALD, an opposite relationship of increasing WVTR with increasing deposition temperature has been observed [102]. Deposition temperatures were

CE P

varied from room temperature to 100 °C, while film thicknesses ranged from 10 to 40 nm. For 20-nm films deposited on PEN, the WVTR at 100 °C was approximately four times higher than the room temperature value, which is the opposite to the trend observed with

AC

thermal ALD. While such behaviour was not fully understood, it was attributed to increased hydroxyl content at lower temperatures. These results may also suggest non-optimised deposition conditions. The effect of thickness on the WVTR of films deposited at room temperature was also investigated. A significant reduction in WVTR was observed for thicknesses of 10 and 20 nm. However, films of 20 and 40 nm had similar WVTRs of 5x10-3 g.m-2/day as determined by the o-Ca test at 21 °C/60% RH.

In contrast to the above report, several more-recent studies using PEALD have observed decreases in WVTRs with increasing temperature [99, 100]. Kim and Kim [100] measured a WVTR of ~3.5x10-1 g.m-2/day for a 15 nm Al2O3 film deposited on PES at 50 °C. Increasing the deposition temperature to 120 or 150 °C lowered the WVTR to below the MOCON Permatran detection limit of 4x10-3 g.m-2/day. They attributed the decrease to densification of the Al2O3 film and more efficient removal of side products at higher deposition temperatures 22

ACCEPTED MANUSCRIPT [100]. A similar strong dependence on temperature has been found by Choi et al. [99] who used PEALD to deposit 100 nm Al2O3 films on PES at selected temperatures between 50 and 200 °C. The lowest WVTR of 5x10-4 g.m-2/day was measured at 200 °C by means of the e-Ca

T

test at 50 °C/50% RH [99]. The improvement in barrier performance was also attributed to an

IP

increase in Al2O3 film density with temperature, which was supported by X-ray photoelectron spectroscopy (XPS) and Fourier Transform infrared spectroscopy. The levelling off in

SC R

WVTR with Al2O3 film thickness has also been reported by Lee et al. [64] who studied PEALD films deposited at 120 °C. While a significant decrease in WVTR was observed for film thicknesses of 5-20 nm, the WVTR remained relatively constant at ~5x10-3 g.m-2/day

Influence of purge times

MA

4.2

NU

between 20-60 nm.

Early ALD research identified a temperature window in which a nearly constant film growth

D

rate was observed [17, 18]. At temperatures above and below this window, reactions other

TE

than those between chemisorbed species on the substrate surface and the incoming precursor pulse were found to occur. In the case of lower temperatures, both precursor reactivity and

CE P

adsorption have the potential to affect the growth rate and properties of the deposited films. Of particular importance is the contribution from physisorbed precursor molecules which will increase the film growth rate relative to that observed in the temperature window. Since much of the work on ALD barrier films has been conducted at temperatures from room temperature

AC

to 150 °C, the process parameters have to be optimised to achieve the best quality films deposited at the selected temperature. Furthermore, the particular set of parameters will be dependent on ALD system design and must be optimised accordingly. Thus, a number of studies have reported the effect of various process parameters on the barrier performance of Al2O3 films. The effect of purge time following both the TMA and plasma O2 pulses on WVTR has been investigated for Al2O3 films deposited at 120 °C by PEALD [100]. The purge time was varied from 3 to 15 s and maintained following both precursor pulses. The selected purge times produced films with thicknesses of 11.8 to 17.8 nm, though the trend was not linear, with the thinnest film deposited using a 10 s purge. Increasing the purge time reduced the WVTR, as determined by MOCON, shown in Figure 11. For purge times of 10 and 15 s, the WVTR was below the MOCON Permatran detection limit of 4x10-3 g.m-2/day. The lower WVTRs at 23

ACCEPTED MANUSCRIPT longer purge times were attributed to the removal of reaction products and unreacted precursors, thus preventing CVD growth. The combined effect of temperature and purge time has also been investigated by Li and co-workers [103]. They used thermal ALD to deposit 80

T

nm Al2O3 films at 80 and 200 °C with purge times of 30 and 10 s respectively. Similar

IP

surface roughness was observed for both films indicating that uniform deposition could be achieved by increasing the purge time at the lower deposition temperature. The 80 and 200

SC R

°C films had WVTRs of 1.5x10-4 and 8.6x10-4 g.m-2/day, respectively, as measured using the e-Ca test at 25 °C and 60% RH. The significantly higher WVTR at 200 °C is somewhat surprising as it is inconsistent with other studies that have reported improved barrier

[100]).

4.3

WVTR of Al2O3 ALD films on PES for purge times of 3 – 15 s (Adapted from

AC

Figure 11

CE P

TE

D

MA

NU

performance with increasing temperature [29-31, 40].

Influence of precursors

The use of TMA+H2O for thermal ALD and TMA+O2 for PEALD have been the preferred reactant combinations for Al2O3 barrier films, but TMA+Ozone (O3) has also been investigated [1, 104-111]. In particular, the barrier properties of films deposited by thermal ALD at 80 °C using either H2O or O3 as the oxidant have been studied by Yong-Qiang et al [111]. WVTRs of 8.7x10-6 g.m-2/day with O3 and 2.1x10-4 g.m-2/day with H2O for film thicknesses of 81 and 73 nm, respectively, were determined using the e-Ca test. Several other studies from the same research group have also shown that O3 produces better barrier films than H2O [104, 106]. Two other research groups have, however, shown that Al2O3 films deposited with H2O are better barriers than those deposited with O3 [108, 110]. These varied

24

ACCEPTED MANUSCRIPT results make it difficult to definitively conclude which precursor produces superior barrier films as the results appear to be system dependent. Influence of the substrate

T

4.4

IP

A few studies have investigated the effect of the underlying polymer substrate on the WVTR after Al2O3 film deposition. 25-nm Al2O3 films were deposited onto cellulose, polylactic acid

SC R

and polyimide (PI) by R2R ALD. At 23 °C/50% RH, the WVTR of the uncoated substrates were 144, 39 and 3 g.m-2/day respectively. The corresponding WVTRs decreased to 15, 10 and 2 g.m-2/day after Al2O3 film deposition [56]. Al2O3 films approximately 12-nm thick

NU

were deposited onto PES, polycarbonate and PEN. The WVTR measurement conditions were not reported, but resulted in uncoated WVTRs of 60, 50 and 2 g.m-2/day respectively. After

MA

Al2O3 deposition, the WVTRs were reduced to 4.1x10-3, 4x10-3 and < 4x\10-3 g.m-2/day respectively [100]. In both studies, the highest WVTR after Al2O3 barrier film deposition resulted from the substrate with the highest uncoated WVTR, therefore indicating that the

Influence of PEALD plasma parameters

CE P

4.5

TE

D

substrate influences the WVTR of a barrier film deposited onto a polymeric substrate.

The effect of plasma power on the barrier performance of a 24-nm Al2O3 film deposited on PEN by PEALD has been investigated at plasma powers of 100 – 700 W [86]. The WVTR was determined using a combination of MOCON and the e-Ca test at 38 °C/100% RH and 38

AC

°C/90% RH, respectively. Increasing the plasma power from 100 to 300 W decreased the WVTR by a factor of ~20 to the lowest reported value of 3.12x10-3 g.m-2/day, shown in Figure 12. However, plasma powers of 500 and 700 W resulted in successive small increases in WVTR. This behaviour was attributed to incorporation of additional carbon in the Al2O3 film at the higher powers such that the lowest carbon concentration at 300 W yielded the lowest WVTR. Lee et al. also studied, at a plasma power of 300 W, the effect of the working pressure on carbon concentration and WVTR. Increasing the working pressure from 200 to 1000 mTorr led to a gradual increase in carbon concentration and the WVTRs exhibited a similar trend. The lowest WVTR of 1.11x10-3 g.m-2/day was measured at the lowest pressure of 200 mTorr. The final variable investigated by these authors was the electrode-substrate distance, which influenced both the O/Al ratio and the WVTR. At an electrode-substrate distance of 50 mm, they measured their lowest WVTR of 8.85x10-4 g.m-2/day which coincided with the highest O/Al ratio as determined from XPS analysis. 25

SC R

IP

T

ACCEPTED MANUSCRIPT

WVTR of Al2O3 PEALD films on PEN for plasma powers of 100 – 700 W

NU

Figure 12

MA

[86].

The effect of film density on WVTR has been investigated by Jung et al. [112]. By varying the O2 plasma pulse time in a PEALD system, films of different densities, including bilayer

D

structures, were deposited. For O2 plasma pulse times of 6, 9 or 12 s, the density of 100-nm

TE

Al2O3 films increased with pulse time. Thus, the lowest WVTR for a single density film was 1x10-4 g.m-2/day produced at a pulse time of 12 s. Multi-density Al2O3 films were also

CE P

produced by depositing two 50-nm films with different O2 plasma pulse times. As with the single density films, bilayer films deposited with combinations of longer reactant times exhibited increased densities and lower WVTRs. The lowest WVTR, 4.7x10-5 g.m-2/day,

4.6

AC

resulted from a film with pulse times of 9 and 12 s.

Comparison of thermal and plasma ALD

Only one paper has been found that compares the effect of thermal and PEALD on WVTR. 20 nm Al2O3 films were deposited at 80 ºC with thermal ALD and 100 ºC with PEALD. The WVTR was determined using the e-Ca test at 38 ºC/90% RH. The film deposited by thermal ALD had a WVTR of 3x10-3 g.m-2/day which decreased to 3x10-5 g.m-2/day for PEALD. To enable the direct comparisons of WVTRs produced by thermal and PEALD films, the deposition temperature should have been kept the same. Although the deposition temperature would have had the effect of lowering the thermal ALD WVTR, these results still suggest that lower WVTRs can be produced by PEALD in comparison to thermal ALD. No possible explanation for the observed difference was suggested by the authors [109]. 26

ACCEPTED MANUSCRIPT

4.7

Film flexibility

T

The relationship between ALD film thickness and flexibility has been the subject of several

IP

publications [113, 114]. Jen et al. [113] studied Al2O3 films with thicknesses of 5–80 nm deposited on heat-stabilised PEN by thermal ALD at 155 ºC. The films were then placed

SC R

under tensile strain to determine the cracking density. The critical tensile strain was subsequently calculated from a plot of crack density versus tensile strain. It was found that the critical tensile strain decreased with increasing film thickness, shown in Figure 13. The

NU

critical tensile strain was also used to calculate the critical bending radii. Values of the latter were smaller for the thinner films which demonstrated increased flexibility [113]. Similar

MA

behaviour has also been observed for titania (TiO2) and Al2O3 films deposited by thermal ALD at 90 °C on as-received and functionalised PET [114]. For a 10 nm Al2O3 film, cracks did not appear until 2% strain was reached, whereas cracks were observed at 1% strain in a

D

30 nm Al2O3 film [114]. A different approach was adopted by Zhang et al. [115] who used

TE

laser scanning confocal microscopy to investigate surface and sub-surface cracking in ultrathin barrier films. ALD Al2O3 films with thicknesses of 5, 12.5, 20 and 40 nm were deposited

CE P

on PEN at 155 °C using TMA and H2O. Cracks were produced in the films by deflection bending in which the film was clamped between two parallel plates. The distance between the two plates was then reduced, thus axially displacing the sample and inducing cracking. Increasing the bending strain increased the cracking density such that the thinner films (5 and

AC

12.5 nm) had approximately twice the number of cracks as the thicker films (20 and 40 nm) at the maximum bending strain of 2.5%. The thinner films appeared however to be more flexible; cracking was not observed until the bending strain reached 1% for the 12.5 nm film and 1.5% for the 5 nm film.

27

Figure 13

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Critical tensile strength of Al2O3 ALD films on PEN for film thicknesses of 5

Double-sided films

TE

D

4.8

MA

– 80 nm [113].

A few studies have produced ALD barrier layers with films on both sides of the substrate. For 30-nm Al2O3 films deposited at 80 °C, adding a second 30 nm onto the other side of the PES

CE P

substrate decreased the WVTR from 4.09x10-1 to 6.15x10-2 g.m-2/day [29-31]. Al2O3 doublesided films of 25 and 100 nm have also been deposited onto polyetheretherketone (PEEK). For the 25-nm film, the WVTR decreased from 9.59x10-1 to 4.64x10-1 g.m-2/day by

AC

depositing a second 25 nm Al2O3 on the other side of the substrate. For the 100-nm film, the WVTR decreased from 8.86x10-2 to 5.04x10-2 g.m-2/day following the addition of a second layer on the reverse side [98].

4.9

Summary

The WVTRs reported for single-layer Al2O3 barrier films are listed in Table 1. Clearly, there is considerable variation in the values which range from 5x10-1 to 8.7x10-6 g.m2/day. This is due in part to differences in film properties, deposition conditions, substrate materials and WVTR test conditions. For example, most of the lowest reported values were measured with the Al2O3 barrier layers deposited directly onto the Ca test pads, thus avoiding any substrate related contributions. Such deposition produces lower WVTRs, but is less relevant for realworld applications of barrier films due to the absence of a polymeric substrate. The results in 28

ACCEPTED MANUSCRIPT Table 1 also illustrate some of the apparent inconsistencies present in this area of research. An example can be seen in the WVTRs of 3x10-1 g.m2/day [44] and 1.9x10-4 g.m2/day [116] measured for 100-nm Al2O3 barrier films. In this case, the large variation is most likely due to

T

the substrate materials used rather than small differences in the deposition and measurement

IP

parameters. Nevertheless, it does raise questions as to why this relatively thick ALD film on the PP/paper substrate does not result in better barrier performance. Even when conditions are

SC R

similar, the type of ALD utilised may influence the results as illustrated by the results of two studies [105, 117]. The use of PEALD resulted in a WVTR of 9.5x10-3 g.m2/day [117] whereas a value of 7.05x10-4 g.m2/day [105] was obtained using thermal ALD with O3

NU

precursor. These and other examples from the results of Table 1 highlight the difficulty in identifying a combination of deposition conditions and techniques that could serve as the

MA

basis for producing ALD films with the best possible barrier properties for a range of real-

AC

CE P

TE

D

world applications.

29

ACCEPTED MANUSCRIPT

Table 1

PES PEEK PEN PEN PES PES

WVTR

WVTR

WVTR 2

Temp. (°C)

(nm)

Technique

Temp. (°C) / RH (%)

(g.m- /day )

Thermal

60

60

e-Ca

85 / 85

5x10-1

50 / 23

-1

Thermal spatial

100

Thermal R2R

100

100

Plasma

25

100

Thermal

40

110

Thermal

30

85

N.D

30

120

Plasma

18.7

80

100

23 / 50

MOCON

38 / 100

MOCON

37 / 100

o-Ca

25

100

Thermal

Gravimetric

50

N.D

Thermal

MOCON

T

Type

IP

PI

Thickness

SC R

PP/paper

ALD

e-Ca

NU

PES

ALD

MOCON

o-Ca & e-Ca

<1x10 8x10

[44]

-1

-2

5.04x10

-2

70 / 80

-2

38 / 100

3x10

2.8x10

2x10 * -2

PES

Plasma

100

100

e-Ca

50 / 50

9.5x10

Thermal R2R

50

40

MOCON

37.8 / 100

6x10-3

NS PEN PI PES PEN PES PEN PEN PEN -

Plasma

120

Thermal spatial

100

Thermal Thermal Thermal Thermal Thermal Plasma Thermal Plasma Plasma

Thermal (O3)

12

D

120

85 120

o-Ca

50

100 80

200 80

26

HTO

26

HTO

100

25 / 85 Ambient / Ambient / -

e-Ca

30

e-Ca

100

e-Ca e-Ca e-Ca

50

80

37.7 / 100

e-Ca

50

100

38 / 90

MOCON

20

100

NS

e-Ca

30

175

21 / 60

MOCON

24

TE

PEN

Plasma

20

CE P

PEN

25

AC

PEN

Plasma

e-Ca

100

e-Ca

5x10

[100]

-3

[86]

< 4x 10

3.12x10

-3

2.38x10 1x10

-3

1x10

-3

-3

50 / 50

7.02x10

85 / 85

7x10

-4

[122]

5x10

-4

[118]

5x10

-4

50 / 50 85 / 85 38 / 90 60 / 90

[123]

3.75x10

[124]

3.69x10

-4

[125]

-4

38 / 20

3x10 * -4

100

o-Ca

50 / 60

1.9x10

90

48

e-Ca

85 / 85

1.83x10-4

PEN PEN -

Thermal Thermal Plasma Thermal R2R Thermal Thermal (O3)

80

130

80

e-Ca

38

100

e-Ca

20

105

e-Ca

20

120

o-Ca

25

80

o-Ca

81.4

e-Ca

#

(*-Deposited directly onto calcium) ( -Double sided film) (NS-Not specified)

30

25 / 60 70 / 70 40 / 100 38 / 90 38 / 90 38 / 85 NS

[105]

-4

80

e-Ca

[121] [82]

-4

Thermal

80

[50] [32]

Atm. Plasma

80

[117] [102]

PET

Thermal

[75]

-3

PEN -

[29-31]

[51]

-3

< 3x10

[120]

[97]

-3

PEN PEN

[98] [72]

-2#

-2

2x10

[56] [119]

-2#

3.7x10

28 / 70

MOCON

3x10

Ref. [118]

85 / 85

38 / 100

MA

Substrate

Summary of deposition conditions and WVTRs for ALD Al2O3 barrier films.

[1] [116] [126]

-4

[103]

-5

[76]

1.5x10 * 9.9x10 * -5

[110]

3x10 *

[109]

9.64x10 * -5

3x10

-5

1.7x10

-5

-6

8.7x10 *

[53] [33] [111]

ACCEPTED MANUSCRIPT 5

Single-layer non-Al2O3 barrier films

While Al2O3 has predominated in studies of ALD barrier films, the WVTRs of several other

T

single-layer metal oxide ALD films have also been reported. These studies typically report

IP

the WVTR of the non-Al2O3 films in conjunction with Al2O3 to demonstrate the improvement in the WVTR by inorganic/inorganic multilayers in comparison to the single

SC R

inorganic films alone. The WVTRs of SiO2 [82, 93], TiO2 [52, 97, 124, 127], zinc oxide (ZnO) [128], magnesium oxide (MgO) [129], tin oxide (SnOx) [130] and zirconium oxide (ZrO2) [6, 104, 105, 121, 131] deposited by ALD are shown in Table 2. For example, Duan et

NU

al. [104] deposited ZrO2 barrier films at 80 and 140 °C using tetrakis(dimethylamido) zirconium (Zr[N(CH3)2]4) and either H2O or O3. The WVTR was then determined using the

MA

e-Ca test at 20 °C/60% RH. It was found that films deposited at 80 °C exhibited better barrier performance, relative to those grown at 140 °C, due to increased crystallinity at the higher deposition temperatures. In addition, the use of O3 yielded films that performed better than

D

those deposited with H2O. For films deposited directly onto the Ca pad using O3 at 80 °C, a /day for an H2O based film at the same temperature.

CE P

2

TE

WVTR of 6.09x10-4 g.m-2/day was achieved. This compared with a WVTR of 3.74x10-3 g.m-

SnOx films appear to have the lowest WVTRs of any non-Al2O3 ALD single-layer barrier films. 200-nm-thick SnOx films were deposited at 100, 150 and 200 ºC by thermal ALD using either H2O or O3 [130]. For both H2O and O3 precursors, the WVTR decreased as the

AC

deposition temperature increased due to the increased film density at higher deposition temperatures thus retarding water permeation. For the majority of deposition temperatures, lower WVTRs were observed for H2O in comparison to O3, which has also been previously observed for Al2O3 films [108, 110]. The SnOx films deposited by O3 typically had more oxygen than those deposited by H2O; this was predicted to facilitate water permeation due to the increased concentration of hydroxyl groups.

The influence of MgO thickness on WVTR was investigated with 20 to100-nm-thick films deposited by thermal ALD at 70 °C. The WVTR, determined by the e-Ca test at 30 °C/90% RH, decreased with increasing film thickness up to 60 nm at which a value of 5.83x10-2 g.m2

/day was reported, as shown in Figure 14 [129]. The performance of 100 nm thick Al2O3 and

TiO2 films have been compared at 38 ºC/100% RH using MOCON [97]. WVTRs of

31

ACCEPTED MANUSCRIPT approximately 2x10-1 and 2x10-2 g.m-2/day were reported for TiO2 and Al2O3 respectively,

NU

SC R

IP

T

thus indicating that Al2O3 is a better barrier than TiO2.

MA

WVTR of MgO ALD films on PET with thicknesses of 20 – 100 nm (Adapted

Figure 14

D

from [129]).

TE

The effect of plasma power and film thickness on WVTR has been investigated for TiO2 films deposited at 90 °C [127]. As determined by MOCON at 37.8 °C/100% RH, the WVTR

CE P

significantly decreased as the plasma power increased from 30 to 50 W and the thickness increased from 10 to 20 nm. Similar WVTRs resulted from varying the power from 100 to 200 W and the thickness from 20 to 80 nm. The lowest WVTR of 2.4x10-2 g.m-2/day was

AC

found for an 80 nm thick TiO2 film deposited at 100 W. Apart from the SnOx film, non-Al2O3 ALD metal oxide films typically have higher WVTRs than Al2O3 films of similar thicknesses, as shown in Table 2. Non-Al2O3 ALD barriers have their lowest reported WVTRs in the range of 10-1 g.m-2/day for SiO2, 10-2 g.m-2/day for ZnO and 10-4 g.m-2/day for TiO2 and ZrO2. The low WVTR of the SnOx film was attributed to its increased density at high deposition temperatures [130], which is unsuitable for deposition onto many polymeric substrates. SnOx films however also exhibited WVTRs in the 10-4 – 105

g.m-2/day range for deposition temperatures of 100 and 150 ºC, similar to those that can be

achieved with Al2O3. The presence of grain boundaries in crystalline films, such as ZrO2, provide potential permeation pathways that can lead to increased H2O diffusion [117]. ALD Al2O3 films are, however, amorphous over a wide temperature range, which avoids grain boundary diffusion that can occur with polycrystalline films. This may help to explain the

32

ACCEPTED MANUSCRIPT generally lower WVTRs achieved with ALD alumina films in comparison to those of the other materials listed in Table 2. However, it cannot be the only reason for the difference as the low temperature ALD of other metal oxides often results in amorphous films. ALD Al 2O3

T

films have also been shown to be to be more flexible than those of other metal oxides. For

IP

example, TiO2 films were found to have a lower crack-onset strain and less flexibility than their Al2O3 counterparts [114]. An advantage of non-Al2O3 barrier films is that they are more

SC R

resistant to corrosion when exposed directly to high temperatures and humidities [124, 132, 133]. Consequently, these non-Al2O3 metal oxide films still exhibit barrier properties and they can be combined with Al2O3 layers to form bilayer and multilayer structures, as

Summary of deposition conditions and WVTRs of SiO2, SnOx, TiO2, ZrO2 and ZnO ALD barrier films.

MA

Table 2

NU

discussed in the next section.

ALD

WVTR

WVTR

WVTR

Substrate

Type

Temp. (°C)

(nm)

Technique

Temp. (°C) / RH (%)

(g.m-2/day )

SiO2

PI

Thermal

175

60

HTO

Ambient / -

1x10-1

Thermal

PES

Plasma

PEN

Thermal R2R

PEN PEN

[82]

-6

200

e-Ca

70 / 70

3.1x10

80

100

MOCON

38 / 100

2x10-1

75

18

MOCON

38 / 90

9x10-4

[52]

Plasma

100

50

e-Ca

60 / 90

6.52x10-4

[125]

Plasma

100

50

e-Ca

38 / 90

6.32x10-4

[124]

[130] [97]

-4

PES

Plasma

90

80

MOCON

37.8 / 100

2.4x10

PES

Plasma

100

100

e-Ca

50 / 50

1.6x10-2

[117]

Thermal (O3)

100

100

e-Ca

50 / 50

8.87x10-3

[105]

Thermal

85

30

e-Ca

25 / 85

4.5x10-3

[121]

PES NS ZnO

Ref.

200

AC

ZrO2

TE

PEN

TiO2

CE P

SnOx

Thickness

D

ALD Material

-3

[127]

Thermal

80

28

o-Ca

20 / 60

3.03x10

-

Thermal (O3)

80

80

e-Ca

20 / 60

6.09x10-4*

[104]

PET

Thermal

70

60

e-Ca

30 / 90

5.83x10-2

[129]

PET

Thermal

70

30

e-Ca

30 / 90

1.49x10-2

[128]

(*-Deposited directly onto calcium) (NS-Not specified)

33

[131]

ACCEPTED MANUSCRIPT 6

Inorganic/inorganic multilayer barrier films

Al2O3 has been combined with a number of other metal oxides to form bilayer and multilayer

T

barrier films. This approach provides a means of reducing moisture permeation through two

IP

main mechanisms. Firstly, the addition of a different inorganic oxide layer on top of Al2O3 can protect it from the corrosion that has been observed in high humidity environments [124].

SC R

Secondly, the additional interfaces in multilayer structures may reduce the WVTR by blocking defects present in one layer, thereby reducing continuous pathways for moisture

Al2O3/TiO2 films

MA

6.1

NU

diffusion through the entire film [76, 105].

A number of studies have investigated Al2O3/TiO2 multilayers as barrier films [97, 98, 108, 109, 119, 124, 125]. A 50-nm Al2O3/TiO2 nanolaminate film comprising alternate layers of

D

0.18-nm Al2O3 and 0.075-nm TiO2 was deposited by PEALD at 100 °C. A WVTR of

TE

1.81x10-4 g.m-2/day was determined for this film using the e-Ca test at 60 °C/90% RH. This result was lower than those of single component Al2O3 or TiO2 films of the same total

CE P

thickness, which yielded WVTRs of 3.75x10-4 and 6.32x10-4 g.m-2/day respectively [124].

Kim and co-authors [125] have investigated the effect of Al2O3/TiO2 ratio on the WVTR of nanolaminates by varying the ratio of deposition cycles from 4:1 to 1:7. Films with a total

AC

thickness of 50 nm were deposited by PEALD and their permeability was measured at 60 °C/90% RH by the e-Ca test. The WVTRs of the nanolaminates decreased in comparison to a 50-nm Al2O3 film up to an Al2O3/TiO2 ratio of 1:3, after which it increased. The lowest WVTR reported by these authors was 9.16x10-5 g.m-2/day for a film with an Al2O3/TiO2 ratio of 1:3.

The beneficial effect of adding a TiO2 layer to Al2O3 barrier films has also been reported by Nehm and co-workers [109]. These authors deposited 10 nm of TiO2 on top of 20 nm of Al2O3 at 80 ºC with thermal ALD and determined the WVTR using the e-Ca test at 38 ºC/90% RH. The 20-nm Al2O3 film had a WVTR of 3x10-3 g.m-2/day which decreased to3 x10-4 g.m-2/day with the addition of the TiO2 protective layer. Similarly, the barrier properties of 40-nm Al2O3/TiO2 bilayer and multilayer films have been shown to be superior to Al2O3 alone in a study focussed on the passivation of flexible organic devices [119]. Films were 34

ACCEPTED MANUSCRIPT deposited at 80 °C and the WVTRs were measured by MOCON at 38 °C/100% RH. The bilayer and multilayer films had WVTRs below the MOCON Permatran detection limit of

T

5x10-3 g.m-2/day, whereas the 40-nm Al2O3 film alone had a WVTR of 9.5x10-2 g.m-2/day.

IP

The barrier properties of Al2O3/TiO2 bilayer films, deposited by thermal ALD at 110 °C, have also been studied. Films of Al2O3 with thicknesses of 25 or 100 nm were deposited on

SC R

one or both sides of PEEK substrates followed by 10 nm of TiO2. Both increasing the Al2O3 thickness and depositing a second bilayer on the other side of the substrate decreased the WVTR with the lowest value of 2.42x10-2 g.m-2/day obtained with 100-nm Al2O3 and 10-nm

NU

TiO2 bilayers on each side [98]. This WVTR is significantly higher than the previously discussed Al2O3/TiO2 bilayer [109], despite the Al2O3 layer being significantly thicker. A

MA

lower WVTR may be due to the PEEK substrate, but cannot be definitively determined without further investigation.

D

Atmospheric PEALD has been used to deposit 30 – 100 nm thick Al2O3 films onto indium tin

TE

oxide (ITO) coated PET. The WVTR was measured with the o-Ca test using the set-up shown in Figure 15 [116]. In this study, water permeation was stated to occur through the PET, ITO

CE P

and Al2O3 film. It was not specified if the 93-nm thick Al2O3/TiO2 nanolaminate layer was exposed to the humid atmosphere or contributed to the barrier measurements. At 50 ºC/60% RH, the WVTRs were in the range of 1.9 to 7x10-4 g.m-2/day with the lowest value observed

AC

for the 100-nm thick Al2O3 film.

Figure 15

Diagram of o-Ca test setup for atmospheric PEALD deposited Al2O3 films [116].

The flexibility of Al2O3/TiO2 structures has been studied by performing compressive bending testing of 40-nm thick bilayer and multilayer structures using a bending radius of 12.5 mm. The bilayer was composed of 10 nm of TiO2 and 30 nm of Al2O3, while the multilayer 35

ACCEPTED MANUSCRIPT consisted of four 10-nm alternating layers of TiO2 and Al2O3. The multilayer structure was more flexible as it required approximately 1600 bending cycles before the WVTR significantly increased, while that of the bilayer film significantly increased after only 800

Al2O3/ZrO2 films

SC R

6.2

IP

T

cycles [97].

Al2O3/ZrO2 films have proved effective in reducing water vapour transmission relative to Al2O3 films alone. The ZrAlxOy phase that forms at the interface between the two layers is

NU

more thermodynamically stable than either Al2O3 or ZrO2 separately and has a higher packing density, thus increasing the density at the interfaces, which inhibits water permeation [122].

MA

Evidence for the latter has come from the investigation of a number of different Al2O3/ZrO2 multilayer configurations that were deposited at 100 °C by thermal ALD using O3 as a precursor. The WVTRs of Al2O3/ZrO2 multilayers with a total thickness of 100 nm,

D

comprising either 4x25 nm or 10x10 nm layers, were compared with those of 100 nm of

TE

Al2O3 or ZrO2 films. In addition, the WVTR of a 100-nm ZrAlxOy film, deposited by alternating single cycles of Al2O3 and ZrO2, was also measured. The two multilayer films had

CE P

WVTRs, at 50 °C/50% RH, of 4.1x10-4 and 3.97x10-4 g.m-2/day for the 4x25 nm and 10x10 nm layer structures, respectively, both of which were lower than the values for the single Al2O3 and ZrO2 films. The WVTR of the ZrAlxOy mixed structure was slightly lower at 3.26x10-4 g.m-2/day. In the Al2O3/ZrO2 multilayer structures, crystallization is supressed by

AC

the amorphous Al2O3 layers, thus reducing water permeation [105]. Similar 100-nm Al2O3/ZrO2 multilayer structures have also been investigated using PEALD at 100 °C to deposit 4x25, 10x10, 20x5 and 50x2 nm multilayer films [117, 122]. A ZrAlxOy mixed film, once again produced by alternating single cycles of Al2O3 and ZrO2, was also included in this study. The ZrAlxOy film yielded the lowest WVTR of 9.9x10-4 g.m-2/day, in agreement with the aforementioned study. The influence of deposition technique on WVTR has been demonstrated by comparing the performance of Al2O3/ZrO2 multilayer films deposited by thermal ALD with O3 [105] and PEALD [117]. In these studies, the thermal ALD with O3 method yielded lower WVTRs than PEALD. Various 30-nm Al2O3/ZrO2 multilayer structures have also been investigated by Seo and co-workers [122]. The films were deposited by thermal ALD at 80 °C and the WVTRs were determined with the e-Ca test at 85 °C/80% RH. The WVTRs decreased as the number of multilayers increased with a lowest value of 2x10-4 g.m-2/day. 36

ACCEPTED MANUSCRIPT

The effect of the total thickness of Al2O3/ZrO2 multilayer films on the WVTR has been investigated by depositing 100 and 130-nm-thick films directly onto arrays of Ca pads [76].

T

The Al2O3 and ZrO2 layers had thicknesses of 2.6 and 3.6 nm, respectively, and were

IP

deposited by thermal ALD. The WVTR, which was measured with the e-Ca test at 70 °C/70% RH, was reduced from 6.4x10-5 to 4.7x10-5 g.m-2/day by increasing the total film

SC R

thickness from 100 to 130 nm. In addition, the presence of film defects was investigated by comparing the performance of a 100-nm Al2O3/ZrO2 multilayer film with that of a 100-nm of Al2O3 film. The films were deposited onto arrays of 64 Ca pads that were then exposed to 70

NU

°C/70% RH. The results of this experiment are displayed in Figure 16 in which a greater number of the pads remain reflective for the nanolaminate films. Such behaviour indicates

MA

that there were fewer defects in the nanolaminate films thus resulting in less corrosion. Meyer et al. [6] expanded the previous study by depositing Al2O3/ZrO2 nanolaminate films with total thicknesses of 37 to 145 nm by thermal ALD at 80 °C. It was found that total thicknesses >48

D

nm had similar WVTRs. For the 48-nm-thick multilayer, a WVTR of 3.2x10-4 g.m-2/day was

TE

measured at 80 °C/80% RH with the e-Ca test. Single and multiple layers of Al2O3 and ZrO2 were also deposited onto arrays of 100 Ca pads to characterize pinhole density. The arrays

CE P

were coated with 100-nm Al2O3, 100-nm ZrO2, 95-nm Al2O3 followed by 5-nm ZrO2 or a 100 nm Al2O3(2.1 nm)/ZrO2(3.1 nm) nanolaminate. After 90 hours, the Ca pads covered with 100-nm Al2O3 were fully corroded while only ~14% of the pads coated with 100-nm ZrO2 or the 95-nm Al2O3/5-nm ZrO2 bilayer were corroded. In contrast, only 2% of the pads coated

AC

with the 100-nm Al2O3/ZrO2 multilayer were fully corroded after 90 hours.

37

Images of Ca pads encapsulated by 100-nm single Al2O3 and nanolaminate

MA

Figure 16

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Al2O3/ZrO2 films at 70 ºC/70% RH [76]. The fraction of ZrO2 in Al2O3/ZrO2 multilayer structures has been investigated by varying the

D

ZrO2:Al2O3 ratio from 5:1 to 1:5. The results of this variation are shown in Figure 17, where

TE

a rapid increase in WVTR was observed for ZrO2 volume fractions >0.5. Below 0.5, the

AC

CE P

WVTR was near the MOCON Aquatran detection limit of 5x10-4 g.m-2/day [134].

Figure 17

WVTR of 10 nm Al2O3/ZrO2 multilayer films as a function of ZrO2 volume fraction [134].

The barrier performance of Al2O3/ZrO2 multilayers with varying numbers of dyads has been investigated by Lee et al [64]. PEALD at 120 °C was used to deposit films with a total thickness of 50 nm containing 1, 2, 3 or 5 dyads. The ZrO2 layer thicknesses were fixed at 2 nm, while that of the Al2O3 was varied to maintain a constant total thickness. The ZrO2 38

ACCEPTED MANUSCRIPT thickness was maintained at 2 nm as layers thicker than 4 nm tend to be polycrystalline, which decreases the flexibility of the film. As the number of dyads increased, the WVTR decreased, as shown in Figure 18. For 5 dyads, the WVTR was less than the MOCON

T

Aquatran Model 2 detection limit of 5x10-5 g.m-2/day. The flexibility of the 5 dyad multilayer

IP

film was also investigated by subjecting it to 5000 bending cycles with a bending radius of 2 cm. The WVTR of the film did not increase, thus demonstrating the high flexibility of this

WVTR of ALD Al2O3/ZrO2 multilayers with 1, 2, 3 and 5 dyads (Adapted

CE P

Figure 18

TE

D

MA

NU

SC R

multilayer structure [64].

from [64]).

Al2O3/SiO2 films

AC

6.3

Dameron et al. [82] have investigated the barrier properties of Al2O3/SiO2 bilayers and multilayers and demonstrated their effectiveness in decreasing the WVTR. A 26-nm Al2O3, deposited at 175 °C onto PI, had a WVTR of 1x10-3 g.m-2/day as measured by HTO permeation under ambient conditions. The addition of a 60-nm ALD SiO2 film decreased the WVTR to 7.8x10-5 g.m-2/day. Subsequent tests on films comprising of 2, 3 and 4 dyads yielded mixed results. The WVTR of the 2 dyad film decreased further to 4.2x10-5 g.m-2/day. However, the 3 and 4 dyad films yielded WVTRs that were similar to those found for the Al2O3 film. This result was attributed to film stress and cracking due to the increased film thickness. Starostin and co-workers [96] have also studied the permeability of Al2O3/SiO2 bilayers that were produced by depositing 80 nm of SiO2 by atmospheric PECVD followed by various thicknesses of Al2O3 by PEALD. The addition of 80 nm of SiO2 to 5 nm of Al2O3 39

ACCEPTED MANUSCRIPT reduced the WVTR from 4x10-2 to 1x10-2 g.m-2/day, as determined by a Technolox Deltaperm instrument at 40 °C/90% RH.

Al2O3/SiN films

IP

T

6.4

Barrier films based on Al2O3/SiN bilayers have been the subject of a number of studies [29-

SC R

31, 87, 95]. Several of these have shown that the WVTR of a 30-nm Al2O3 film grown at 80 °C can be reduced from 4.09x10-1 to 5.8x10-2 g.m-2/day by the addition of 200 nm of SiN deposited by PECVD, as measured by MOCON at 38 °C/100% RH [29-31]. The influence of

NU

Al2O3 thickness on an Al2O3/SiN bilayer has also been investigated by combining various thicknesses of Al2O3 with 100 nm of SiN. The latter had a WVTR of 6.5x10-3 g.m-2/day, as

MA

determined by the o-Ca test at 38 °C/85% RH. The addition of 2.5 nm of Al2O3 on top of the SiN reduced the WVTR to 3x10-4 g.m-2/day. Increasing the thickness of the Al2O3 to 5 or 10 nm reduced the WVTR further to less than 2x10-5 g.m-2/day, the detection limit of the o-Ca

Al2O3/ZnO films

CE P

6.5

TE

D

setup [87].

Bilayer and multilayer films of Al2O3/ZnO with a total thickness of 60 nm were produced by depositing either 30 nm of Al2O3 and 30 nm of ZnO onto PES or alternating 10-nm layers of the two. 60-nm films of Al2O3 and ZnO films alone had WVTRs of 5x10-1 and 1.28 g.m-2/day

AC

respectively. The WVTRs of the bilayer and multilayer structures were 2.39 and 2.8x10-1 g.m-2/day, respectively, as measured with the e-Ca test at 85 °C/85% RH [118]. Significantly higher WVTRs were observed for these multilayer structures in comparison to other inorganic/inorganic multilayers, which is likely due to the higher temperature and RH used for the e-Ca tests.

6.6

Non-Al2O3 films

A mixed metal oxide barrier film has been deposited by alternating 4 cycles of ZnO and 1 cycle of mixed ZnO/HfO2. The mixed ZnO/HfO2 layer was deposited by simultaneously pulsing the ZnO and HfO2 precusors followed by a single H2O pulse. The film was produced by a total of 1000 cycles resulting in a film thickness of approximately 180 nm. The WVTR was initially tested using the MOCON Aquatran Model 1, but the WVTR was lower than its 40

ACCEPTED MANUSCRIPT detection limit of 5x10-4 g.m-2/day. The WVTR was then measured using the e-Ca test at 85 °C/85% RH resulting in a WVTR of 6.3x10-6 g.m-2/day [89].

Summary

IP

T

6.7

The barrier performance of various inorganic/inorganic bilayers and multilayers are listed in

SC R

Table 3. The table shows that the addition of one or more non-Al2O3 layers typically reduces the WVTR compared to a single-layer film. Furthermore, the non-Al2O3 layers serve to block underlying defects and, if used as the outermost layer, will protect the Al2O3 from corrosion

NU

caused by exposure to high temperature and humidity. In addition, the presence of additional interfaces in bilayer and multilayer films helps to retard moisture permeation. The WVTRs in

MA

Table 3 vary widely from 2.8x10-1 to 6.3x10-6 g.m-2/day. It appears that the material of the non-Al2O3 layer does not significantly influence the WVTR as TiO2, ZrO2, SiO2 and ZnO bilayer and multilayer films have all yielded WVTRs in the range of 10-5-10-6 g.m-2/day. In

D

Table 2, TiO2 appeared to be the best performing non-Al2O3 barrier film at temperatures

TE

below 150 °C. However, when used as a component of multilayer films with Al2O3, other metal oxide layers perform equally well, or even better. Such behaviour suggests, therefore,

CE P

that the protection of the Al2O3 film is a critical factor in reducing the WVTR. The majority of the films with WVTRs lower than 3x10-4 g.m-2/day are multilayer films, thus demonstrating that adding additional interfaces lower the WVTR. The three barrier films with WVTRs below 4.7x10-5 g.m-2/day were all deposited at temperatures greater than 100 °C.

AC

These results are consistent with previous studies which have shown that higher deposition temperatures produce lower WVTRs [76, 99]. The only inorganic/inorganic film with a WVTR in the 10-6 g.m-2/day range, was a nanolaminate composed of discreet layers which were up to only 4 cycles thick [89]. While this result shows considerable promise for the application of inorganic/inorganic multilayer barrier films, it is worth noting that it was achieved with a 180-nm-thick film deposited at 150°C. In addition, the flexibility of such multilayer structures, a subject considered in the next section, has yet to be significantly investigated with only a few studies undertaking such analyses.

41

ACCEPTED MANUSCRIPT

Summary of deposition conditions and WVTRs ALD for inorganic/inorganic bilayer and multilayer barrier films.

Total Structure

Thickness (nm)

Technique

Temp. (°C) / RH (%)

(g.m- /day )

Thermal

60

10 nm Al2O3

10 nm ZnO

Multilayer

60

e-Ca

85 / 85

2.8x10-1

38 / 38

-2

Thermal

80

20 nm Al2O3

200 nm SiNx

100 nm Al2O3

10 nm TiO2

PEN

Plasma

80

5 nm Al2O3

80 nm SiO2

PEN PEN PEN

Thermal Thermal (O3) Thermal Thermal Thermal Plasma Plasma

100

1 cycle Al2O3

70 100 80

3 nm Al2O3 1 cycle Al2O3 2.1 nm Al2O3

80 80 100 100

20 nm Al2O3 1 cycle Al2O3 0.18 nm Al2O3 1 cycle Al2O3

Thermal

80

2.6 nm Al2O3

Thermal

175

26 nm Al2O3

PEN PEN PEN

Plasma Thermal Thermal

120

8 nm Al2O3

125 150

(*-Deposited directly onto calcium)

3 nm ZnO 1 cycle ZrO2 3.1 nm ZrO2 10 nm TiO2

1 cycle ZrO2

0.075 nm TiO2 3 cycles TiO2

5 nm Al2O3

Bilayer Bilayer

Multilayer Multilayer Multilayer Multilayer Bilayer

Multilayer Multilayer Multilayer

220 110 85 100 30 100 40 30 30 50 50

MOCON MOCON Technolox Deltaperm e-Ca e-Ca e-Ca e-Ca e-Ca e-Ca e-Ca e-Ca

40 / 40

1x10-2

50 / 50

[105]

80 / 80

-4

38 / 38 85 / 85 60 / 60 60 / 60 70 / 70 Ambient / -

1 cycle Al2O3 4 cycles ZnO ( -Double sided film)

Multilayer

#

42

180

o-Ca e-Ca

[117]

3.26x10

e-Ca

105

[96]

-4

50 / 50

HTO

Bilayer

[98]

[128]

85

100 nm SiNx

[29-31]

-4

130

MOCON

[118]

4.92x10

90 / 90

Bilayer

50

9.9x10

Ref.

-4

Multilayer Multilayer

-2#

2.42x10

59 nm SiO2 2 nm ZrO2

5.8x10

37 / 37

3.6 nm ZrO2

AC

PI

1 cycle ZrO2

Bilayer

TE D

-

Plasma

CE P

-

2

Layer 2

110

PES

WVTR

Layer 1

Thermal

PEN

WVTR

Temp. (°C)

PEEK

PES

WVTR

Type

CR

PES

ALD

US

PES

ALD

MA N

Substrate

IP

T

Table 3

38 / 38 38 / 38 85 / 85

3.2x10 * -4

3x10 * 2x10

-4

[6] [109] [122]

1.81x10

-4

[124]

9.16x10

-5

[125]

-5*

[76]

4.2x10-5

[82]

< 2x10

-5

[64]

< 2x10

-5

[87]

-6

[89]

4.7x10

6.3x10

ACCEPTED MANUSCRIPT 7

Inorganic/organic multilayer barrier films

Increasing the thickness of barrier films has been shown to improve barrier performance, but

T

these films are more prone to cracking due to intrinsic or externally applied stress [34].

IP

Cracking is a major issue in barrier films as it significantly increases water permeation and is therefore of particular concern when barrier films are applied to flexible substrates as the

SC R

films are required to bend, flex or roll during production or end use [115]. The flexibility of barrier films can be improved by combining organic and inorganic layers in multilayer structures. The organic layers increase the flexibility while the inorganic layers inhibit

NU

permeation [5]. In addition, the combination of organic and inorganic layers decreases water permeation by blocking defects that may occur in single inorganic barrier films [135], thus

MA

resulting in longer moisture diffusion paths [136]. It has been suggested on the basis of X-ray and neutron reflectivity experiments that the effectiveness of inorganic/organic multilayer barrier films is due to the Al2O3/organic interface acting as a desiccant by accumulating

D

water. Further permeation through the barrier is therefore retarded by its strong adsorption to

Organic films deposited by molecular layer deposition

CE P

7.1

TE

Al2O3 [137].

One alternative approach for producing inorganic/organic multilayer barrier films utilises a combination of molecular layer deposition (MLD), O3 treatment and ALD. Seo et al. [85] an

organic

AC

formed

self-assembled

monolayer

(SAM)

of

7-octenyltricholorsilane

(CH2(CH(CH2)6SiCl3) using MLD. The terminal C=C groups of the SAM were then converted to carboxylic acid groups using O3 treatment. This conversion was used to produce bilayer and multilayer films with alternating layers of 9.6 nm of TiO2 and 80 nm of SAM. The multilayers consisted of 1, 3 or 5 dyads. Increasing the number of dyads extended the lifetime of the barrier film, determined by the e-Ca test at 60 °C/85% RH, as shown in Figure 19. The best result was achieved with the 5 dyad barrier film, which had a WVTR of 7.0x10-4 g.m-2/day.

43

Calcium tests for SAM/TiO2 multilayers with 1, 3 and 5 dyads [85].

NU

Figure 19

SC R

IP

T

ACCEPTED MANUSCRIPT

MA

MLD has also been used to produce Al2O3/aluminium alkoxide (alucone) bilayers comprised of 50 nm of Al2O3 deposited by thermal ALD at 85 ºC and 50 nm of alucone by MLD at 85 ºC. Using the o-Ca test at 85 ºC/85% RH, the WVTR was reduced from 3.73x10-2 g.m-2/day

D

for a single 50 nm Al2O3 layer to 2.08x10-2 g.m-2/day for the bilayer [120]. The effect of an

TE

additional inorganic layer has been investigated by Zhang et al. [121] who deposited 15 nm each of Al2O3 and ZrO2 by thermal ALD at 85 ºC followed by 30 nm of alucone by MLD

CE P

using TMA and ethylene glycol at 85 °C. They also used the same combination of layers to form 3 dyad multilayer structures. The addition of the alucone layer decreased the water permeation rate and increased device lifetime. The 3 dyad multilayer structure yielded the

AC

lowest WVTR of 8.5x10-5 g.m-2/day as determined by the e-Ca test at 25 ºC/85% RH.

The influence of different ALD oxidants on barrier performance has been investigated in a study of Al2O3/alucone films in which the Al2O3 was deposited using either H2O or O3. In this work, bilayers and 2 and 3 dyad multilayers were formed by depositing 20 nm of Al 2O3 by thermal ALD at 80 °C and 4 nm of alucone by MLD at 80 ºC. Both the H2O and O3 films showed a decrease in the WVTR as the number of dyads increased. However, for the same number of dyads, the films deposited with O3 had lower WVTRs with the lowest value of 2.37x 10-5g.m-2/day resulting from a 5 dyad O3 multilayer film, as measured with the e-Ca test at 20 °C/60% RH [106].

The effect of coating both sides of the alucone with Al2O3 has been investigated by Choi et al. [126] who studied an Al2O3/alucone/Al2O3 trilayer structure. The film is comprised of 18

44

ACCEPTED MANUSCRIPT nm Al2O3 layers deposited by thermal ALD at 90 °C on both sides of a 280 nm alucone layer prepared by MLD. A WVTR of 1.61x10-4 g.m-2/day at 85 °C/85% RH was obtained for the sandwich structure by means of the e-Ca test. In comparison, a single sided Al2O3/alucone

T

sample yielded a WVTR of 2.48x10-4 g.m-2/day under the same conditions. The relatively

IP

small difference between these two values suggests that the additional Al2O3 layer does not

SC R

greatly enhance barrier performance.

Other factors that influence the barrier properties of films containing alucone are the Al2O3:alucone cycle ratio. The relationship between the Al2O3:alucone ratio and film

NU

flexibility has been investigated by Jen et al. [138]. 25-nm nanolaminate films were deposited at 135 °C using thermal ALD and MLD with Al2O3:alucone ratios that varied from 1:1 to 6:1.

MA

A film with an Al2O3/alucone ratio of 3:1 was found to be the most flexible with the highest critical tensile strain of 0.99%. However, increasing the Al2O3:alucone ratio, lowered the WVTR such that films with ratios of 5:1 and 6:1 yielded values below the MOCON Aquatran

D

detection limit of 1x10-4 g.m-2/day, as shown in Figure 20. The effect of varying the

TE

Al2O3:alucone ratio has also been studied by Sun et al. [107]. 75nm-thick nanolaminate films with ratios of 5:1, 6:1 and 7:1 were deposited using thermal ALD at 80 °C with O3 as the

CE P

oxidant and alucone by MLD. The WVTR, measured with the o-Ca test at 20 ºC/60% RH, decreased as Al2O3 proportion increased with the lowest value of 8.68x10-5 g.m-2/day for the

AC

Al2O3:alucone ratio of 7:1.

Figure 20

WVTR of 25 nm Al2O3/alucone nanolaminate films grown with varying ALD:MLD cycle ratios [138]. 45

ACCEPTED MANUSCRIPT 7.2

Organic films deposited by vapour phase deposition

Vapour phase deposition has been used as an alternative to MLD for producing an organic

T

layer in multilayer barrier films. Kim and Graham combined SiOx(100 nm)/Al2O3(50 nm)

IP

bilayers with 1-µm-thick protective layers of parylene ((CH2C6H3ClCH2)n)deposited from e-Ca test at 20 °C/50% RH [7].

Organic films deposited by spin coating

NU

7.3

SC R

vapour.. This multilayer barrier film had WVTR of 2.0x10-5 g.m-2/day as measured with the

Al2O3/ZnO nanolaminates have been combined with spin coated silica sol-gel/epoxy films to

MA

create inorganic/organic barrier films. Alternating 3-nm layers of Al2O3 and ZnO were deposited by thermal ALD at 70 °C to form 30-nm thick inorganic layers after which 120-nm of silica sol-gel epoxy was spin coated on top. A comparison of the WVTRs of 1.5 and 2.5

D

dyad structures with that of the Al2O3/ZnO nanolaminate alone found that the addition of the

TE

silica/epoxy film improved barrier performance. The best result was obtained with the 2.5 dyad film which had a WVTR of 1.91x10-5 g.m-2/day, as determined by the e-Ca test at 30

CE P

°C/90% RH. The flexibility of these films and its effect on WVTR were also investigated in this study by subjecting the films to 1000 bending cycles with a 3 cm bending radius. For the Al2O3/ZnO nanolaminate alone, the WVTR increased from 4.92x10-4 to 1.73x10-3 g.m-2/day after bending. In contrast, the addition of the spin coated organic layer increased film

AC

flexibility such that bending of the 2.5 dyad film only increased the WVTR from 1.91x10-5 to 4.05x10-5 g.m-2/day [128]. The WVTR and flexibility of Al2O3/silica sol-gel epoxy inorganic/organic multilayer barrier film has also been demonstrated by depositing a 3.5 dyad structure with four 30 nm Al2O3 layers and three silica sol-gel epoxy layers [139]. An 111-µm cycloaliphatic epoxy hybrimer was then spin coated on top to act as a buffer. The structure was subjected to 1000 bending cycles with a bending radius of 1 cm and the WVTR measured by the e-Ca test at 30 ºC/90% RH. The multilayer barrier film had an initial WVTR of 4.4x10-5 g.m-2/day prior to bending. Without the hybrimer the WVTR increased to 2.2x10-2 g.m-2/day after bending, shown in Figure 21. The addition of the hybrimer resulted in the WVTR increasing to only 8.2x10-5 g.m-2/day after bending, thus significantly increasing flexibility.

46

Before bending

Figure 21

SC R

IP

T

ACCEPTED MANUSCRIPT

After bending After bending W ith hybrimer W ithout hybrimer

Glass

WVTR of Al2O3/silica sol-gel epoxy inorganic/organic multilayer barrier films

NU

before and after bending with and without hybrimer addition (Adapted from

MA

[139]).

Spin coated silica sol-gel/epoxy films have also been combined with Al2O3 to produce inorganic/organic multilayers. Multilayers comprising 1.5, 2.5 and 3.5 dyads were prepared

D

from 330 cycles of Al2O3, deposited by thermal ALD at 70 °C, and a 190-nm-thick spin

TE

coated organic layer. The WVTRs of these films were found to decrease as the number of dyads increased with the 3.5 dyad multilayer film having the lowest WVTR of 1.26x10-5 g.m/day. The flexibility of these films was also investigated by applying 100 bending cycles at a

CE P

2

bending radius of 3 cm. The WVTR did not significantly increase after bending, thus demonstrating high film flexibility [140]. A similar approach has been investigated by Nehm

AC

et al. [109]. In their work, acrylate EGC-1700 was spin coated at 1500 or 3000 RPM, or nLOF photoresist at 3000 RPM onto 20-nm ALD Al2O3 films. The WVTR was reduced from 3x10-3 g.m-2/day for 20 nm of Al2O3 on PET to 2-4x10-5 g.m-2/day following deposition of the organic layer. The spin coating speed or the type of organic layer did not have a significant effect on the WVTR [109]. Finally, 1, 1.5 and 2 dyad barrier films were produced by depositing a 28 nm ZrO2 film by thermal ALD and a 16 µm layer of spin coated UVcurable epoxy. The addition of the spin coated layer decreased the WVTR from 3.03x10-3 to 8.75x10-4 g.m-2/day, as determined by o-Ca at 20 ºC/60% RH. Adding a second 28 nm ZrO2 layer further decreased the WVTR to 2.22x10-4 g.m-2/day while that for the 2 dyad structure was 1.27x10-4 g.m-2/day [131].

47

ACCEPTED MANUSCRIPT 7.4

Organic films deposited by plasma polymerisation

Plasma polymerization is another technique that has been used in conjunction with ALD

T

films to produce inorganic/organic multilayers [123, 136, 141, 142]. The influence of

IP

inorganic/organic multilayers on film flexibility, cracking and subsequent water permeation has been investigated by testing a number of different film structures. The barrier

SC R

performance of three multilayer structures, comprising 4, 8 or 20 dyads, were compared to that of a single 20-nm Al2O3 film [136]. The multilayers were produced by alternating inorganic and organic layers with combined thicknesses of approximately 20 nm of Al2O3

NU

deposited by PEALD at 80 ºC and 200 nm of plasma polymerised hexane. The influence of bending on water permeation was determined using the e-Ca test at 85 ºC/85% RH. Under

MA

these conditions, the single 20-nm Al2O3 layer resulted in complete Ca oxidation in 18 hours. Following 10,000 bending cycles, the moisture permeation rate accelerated and was found to be strongly dependent on the bending radius. In the case of the film subjected to a bending

D

radius of 0.5 cm, the calcium was completely oxidised in less than one hour. In contrast,

TE

complete oxidation of the Ca with the 8 dyad structure required 25 hours and was found to

AC

CE P

reduce slightly to 23-24 hours for all bending radii tested, as shown in Figure 22.

Figure 22

Calcium tests for ALD Al2O3 single and multilayer barrier films before and after 10,000 bending cycles with radii of 0.5 – 3 cm (85 ºC/85% RH) [136].

The effect of the thickness of plasma polymerised organic layers has been investigated by the fabrication of 2 dyad multilayers with 10 nm of Al2O3 deposited by thermal ALD at 80 °C and 100 nm, 500 nm or 1 µm of plasma polymerised hexane/hexamethyldisiloxane [141]. At 85 °C/85% RH, increasing the thickness of the organic layer reduced the water permeation. 48

ACCEPTED MANUSCRIPT In particular, after 10,000 bending cycles at a radius of 2 cm, the time taken for complete Ca oxidation of the multilayer with a 1-µm organic layer decreased from 21 to 16 hours. In comparison, the same test on a single 20-nm Al2O3 layer decreased the corresponding time

T

from 18 to 10 hours, demonstrating the effectiveness of organic layers in increasing the

IP

flexibility of barrier films. It is worth noting that the relatively short permeation times reported in the above studies were a consequence of the high temperature and humidity

SC R

conditions used in the e-Ca tests. The WVTRs of the three multilayers were determined using the e-Ca test at 85 °C/85% RH. The films had an average WVTR of 7.2x10-4 g.m-2/day with

NU

the thickness of the organic layer not significantly affecting the WVTRs.

Plasma polymerised hexane (C6H14) (PPhex) combined with ALD Al2O3 films has been

MA

studied by Lim et al. [123]. These authors reported a WVTR of 5x10-4 g.m-2/day for a 20-nm Al2O3 film deposited on PEN by thermal ALD at 80 °C. Subsequent bending tests at radii of 20 and 10 mm increased the WVTR to 9x10-4 and 1x10-3 g.m-2/day respectively, as shown in

D

Figure 23. Films of 20 and 200 dyads formed from either 1-nm Al2O3/20-nm PPhex or 1

TE

cycle of Al2O3/20-nm PPhex, respectively, were also investigated. All the multilayers were then subjected to bending tests for 10,000 cycles at bending radii of 3, 5, 10 and 20 mm. In

CE P

contrast to the single Al2O3 layer, both multilayer structures did not exhibit much change in WVTRs at bending radii of 5, 10 and 20 mm for which values in the range 2-4x10-4 g.m-2/day were reported. However, a bending radius of 3 mm resulted in a significant increase in the

AC

WVTRs of both multilayer configurations to ~ 1x10-1 g.m-2/day.

Figure 23

WVTR of Al2O3, 20 and 200 dyad Al2O3/PPhex multilayers [123].

49

ACCEPTED MANUSCRIPT 7.5

Summary

A number of inorganic/organic barrier films have been developed and their WVTRs are

T

summarised in Table 4. The WVTRs for these films vary significantly from 1.99x10-1 to

IP

4.33x10-6 g.m-2/day. Different techniques were used to deposit the organic layer such as MLD, plasma polymerisation and spin coating. Spin coating with silica sol-gel/epoxy resin

SC R

produced the lowest reported WVTR of 4.33x10-6 g.m-2/day. The last two structures listed in Table 4 suggest that both the thickness of the organic layer and the total film thickness influence barrier performance with the lowest WVTRs reported for both the thickest organic

NU

layer (200 nm) and total film (1 µm) [129]. Interestingly, the lowest WVTR was achieved with an MgO inorganic layer rather than Al2O3. However, as this appears to be the only study

MA

that has used MgO, it is not possible to assess the performance of this material relative to that of Al2O3, which has been extensively investigated.

D

The studies discussed in this section show the benefits of inorganic/organic multilayer barrier

TE

films for applications in which flexibility is an issue. While barrier films with WVTRs <1x10-6 g.m-2/day have been reported for many of the systems discussed in this review, the

CE P

increased flexibility of inorganic/organic multilayer films reduces the likelihood of cracking and ensures a longer operational life. Furthermore, the goal of maintaining barrier performance in the presence of extensive bending has been shown to be achievable with several different multilayer systems. However, based on work to date, there is no consensus

AC

as to the most suitable system for the wide variety of potential applications that require flexible moisture barriers. Nevertheless, some progress has been made in applying inorganic/organic moisture barriers to OLED devices and this work is discussed in the next section.

50

ACCEPTED MANUSCRIPT

Organic

Type

Temp. (°C)

Layer

Layer

technique

Thermal

80

20 nm Al2O3

3 µm Parylene

N.D

Thermal

85

50 nm Al2O3

50 nm Alucone

PET

Thermal

150

9.6 nm TiO2

80 nm 7-OTS SAM

PEN

Thermal

80

1 cycle Al2O3

20 nm Hexane

PEN

Thermal

90

18 nm Al2O3

280 nm Alucone

PI NS

Thermal Thermal Thermal (O3) Thermal

80 135 80 85

28 nm ZrO2 5 cycles Al2O3 7 cycles Al2O3 15 nm Al2O3

1.6 µm Epoxy resin 1 cycle Alucone 1 cycle Alucone 30 nm Alucone

15 nm ZrO2 70

30 nm Al2O3

PET

PECVD

110

100 nm SiOx

NS PET

ALD

50 nm Al2O3

PECVD

400 nm SiOx

Thermal (O3) Thermal Thermal

80 80 70

20 nm Al2O3 20 nm Al2O3 15 nm Al2O3

120 nm Silca solgel/epoxy resin 1 µm Parylene

CE P

Thermal

AC

PET

MLD

4 nm Alucone

Photoresist 120 nm Silca solgel/epoxy resin

Structure Bilayer

MLD Plasma polymerisation MLD

TE D

-

Deposition

T

Inorganic

CR

PEN

ALD

US

PES

ALD

MA N

Substrate

Summary of deposition conditions and WVTRs for ALD inorganic/organic barrier films.

IP

Table 4

Spin coating MLD MLD

Bilayer

No.

WVTR

Temp. (°C) / RH (%)

(g.m- /day )

1

MOCON

38 / 100

1.99x10-1

1

o-Ca e-Ca

Multilayer

20

e-Ca

Multilayer

1.5

e-Ca

Multilayer

2

Technique

5

Multilayer

WVTR

Dyads

Multilayer

Multilayer

WVTR

2 NS NS

o-Ca MOCON e-Ca

85 / 85 60 / 85 85 / 85 85 / 85 20 / 60 38 / 85 20 / 60

2.08x10 7.0x10

-2

-4

Ref. [29-31] [120] [85]

2x10-4

[123]

1.61x10-4

[126]

-4

[131]

-4

[138]

-5

[107]

-5

[121]

4.4x10-5

[139]

1.27x10 * < 1x10

8.68x10 *

MLD

Multilayer

3

e-Ca

Spin coating

Multilayer

3.5

e-Ca

Vapor phase deposition

Multilayer

2

e-Ca

20 / 50

2.4x10-5#

[7]

MLD

Multilayer

3

e-Ca

20 / 60

2.37x10-5

[106]

Spin coating

Bilayer

1

e-Ca

Spin coating

Multilayer

2.5

e-Ca

Spin coating

Multilayer

3.5

e-Ca

Spin coating

Multilayer

4.5

e-Ca

25 / 85

30 / 90

38 / 90 30 / 90

8.5x10

-5

2x10 *

[109]

-5

[128]

1.26x10-5

[140]

4.33x10-6

[129]

1.91x10

15 nm ZnO PET

Thermal

70

330 cycles Al2O3

PET

Thermal

70

40 nm MgO

190 nm Silca solgel/epoxy resin 200 nm Silca solgel/epoxy resin

#

(*-Deposited directly onto calcium) ( -Double sided film) (NS-Not specified)

51

30 / 90 30 / 90

ACCEPTED MANUSCRIPT 8

Spatial and roll-to-roll deposited barrier films

Spatial and R2R ALD have been investigated to assess their viability for applying barrier

T

layers to large areas in shorter times than those required with conventional (i.e. static) ALD.

IP

A “back and forth” spatial ALD system was used by Choi et al. [50] to deposit 50-nm Al2O3 films on a 100x100 mm2 PEN substrate. The WVTR of this film at 37.7 °C/100% RH was

SC R

found to be less than 3x10-3 g.m-2/day, the detection limit of the MOCON system. Lahtinen et al. [44] also used spatial ALD to compare its performance with that of conventional ALD. 100-nm Al2O3 films were deposited on different polymer coated papers and the WVTR

NU

measured using a MOCON instrument at 23 ºC/50% RH and 38 ºC/90% RH. The WVTRs of all samples were reduced following Al2O3 deposition although for 7 out of 8 test conditions,

MA

conventional ALD resulted in lower WVTRs [44].

The use of ALD to deposit barrier films in R2R processes has been investigated in several

D

studies. Ali et al. [51] deposited 15-40 nm Al2O3 films on PET in an atmospheric pressure

TE

R2R ALD system. At a substrate speed of 7 mm/s, the WVTR at 37.8 ºC/100% RH was found to be ~6x10-3 g.m-2/day. In another study, the effect of winding and unwinding during

CE P

operation was investigated by comparing TiO2 films deposited using either R2R or band-toband (B2B) modes in the same ALD reactor. In B2B mode, the majority of top and bottom guide rollers are removed and the substrate travelled in a continuous loop such that one revolution was equivalent to one ALD cycle. TiO2 films with thicknesses in the range 5-30

AC

nm were deposited although only films with thicknesses of 5-8 nm had similar WVTRs for both modes. For film thicknesses greater than 8 nm, lower WVTRs were observed with the B2B mode when measured at 38 ºC/90% RH with a MOCON instrument. Thus, a WVTR of 9x10-4 g.m-2/day was measured for an 18-nm TiO2 film deposited in R2R mode whereas a similar film using B2B mode yielded a value below the MOCON detection limit of 5x10-4 g.m-2/day. This difference in performance was is attributed to abrasion of the film surface during rewind in R2R mode [52].

A drum R2R ALD reactor was used to deposit 20 nm of Al2O3 onto PET at coating head speeds of 0.03 – 0.4 m/s. The WVTR was determined using MOCON at 38 ºC/90% RH. To avoid damage to the films from rewinding, the samples for permeation measurements were cut from the roll before reaching the rewinding roll. Decreasing the coating head speed decreased the WVTR, which was attributed to the lower TMA doses at higher coating head 52

ACCEPTED MANUSCRIPT speeds resulting in incomplete surface coverage. WVTRs below the MOCON limit of 5x10-4 g.m-2/day were measured for coating head speeds of ≤0.1 m/s. The o-Ca test was then used to determine the WVTR of 20 nm of Al2O3 deposited at a coating head speed of 0.1 m/s, which

IP

T

yielded a WVTR of ~3x10-5 g.m-2/day at 38 ºC/90% RH [53].

The surface properties of the polymer substrate materials used in R2R systems is another

SC R

variable that may affect the performance of barrier films. Rolls of these materials often used in R2R processes typically have the underside coated with a low friction, slip layer to assist in winding and unwinding. In R2R ALD, both sides of the substrate can be coated

NU

simultaneously so that the influence of the slip layer on barrier film growth and associated WVTR needs to be considered. Carcia et al. [101] at DuPont compared the WVTRs of films

MA

deposited on the bare and slip coated sides of a PET web as a function of the number of ALD cycles. For up to 80 cycles (< 8 nm), the WVTR was greater for the slip slide than the bare polymer side, which was attributed to a slower film nucleation rate for the former. The two

D

sides coated with thicker films (80-100 cycles) both had WVTRs below the MOCON

TE

Aquatran detection limit of 5x10-4 g.m-2/day. In a subsequent study, Carcia et al. [88] used the o-Ca test to measure the WVTRs of ALD Al2O3 films deposited for 75, 125 or 250 cycles.

CE P

All three films on the slip coated side failed in less than 2000 hours, as shown in Figure 24. For the same number of cycles, the slip side coated PET failed earlier than those on the bare polymer side. It was suggested that this behaviour was due to different modes of nucleation

AC

during the early stages of film growth which still had an effect on the WVTR of thicker films.

53

o-Ca-test transmission of Al2O3 ALD films deposited for 75, 125, and 250

MA

Figure 24

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

cycles on the slip-coated side of PET [88].

D

In summary, the above studies of spatial and R2R systems have investigated some of the

TE

issues associated with the incorporation ALD barrier film technology. These include contact with rollers, precursor exposure times with moving substrates and abrasion of surfaces due to

CE P

contact during winding. In addition, the surfaces properties of a commercial PET polymer, suitable for R2R processing, have been shown to affect the performance of deposited barrier films. Defects are also an issue in R2R ALD due to the abrasion of contaminant particles on

9

AC

substrate surfaces when they are in close contact.

Barrier films in OLEDs

A number of studies have investigated the use of ALD films as barrier layers for OLED devices, although in some cases only ex-situ WVTRs have been reported [67, 103, 106, 111, 128, 131, 140]. This is partly due to the requirements of the various measurement techniques, which make it difficult to determine the barrier properties of a single component in a fully assembled OLED. With techniques such as HTO permeation and MOCON, other components of the OLED would contribute to the resulting WVTR as the water permeation would occur across the entire device. The Ca based methods are also not readily adapted to the measurement of barrier layer WVTRs in OLEDs. For such measurements, the Ca pad would have to be embedded within the OLED under the barrier layer or the entire device

54

ACCEPTED MANUSCRIPT would have to be sealed onto the pad. Neither of these approaches is readily achievable, so alternative evaluation methods have been adopted. OLEDs have commonly been protected from moisture and oxygen by encapsulating them with UV cured epoxy loaded with a strong

T

desiccant. However, this method is unsuitable for top-emitter OLEDs as light cannot pass

IP

through the opaque desiccant and placing the desiccant around the edge of the device is not sufficient to eliminate moisture. Therefore, the barrier films applied to top-emitter OLEDs

SC R

need to be fully transparent. In addition, they need to fulfil a number of other criteria including low stress, low deposition temperature, densely packed, continuous, conformal and pinhole free; all of which are properties that can be achieved with ALD films [34]. The

NU

lifetime of an OLED is usually defined as the time it takes for the luminance to decay to half its initial value at constant current. This definition has been used as a useful means of

MA

evaluating the effectiveness of ALD barrier layers deposited on top emitter OLEDs [143].

8-hydroxyquinoline aluminium (C27H13AlN3O3) (AlQ3) is an electron transporting and

D

emitting molecule in OLEDs. Upon exposure to water and/or oxygen, degradation by-

TE

products form and result in a decrease in the OLED electroluminescence [144]. Protection of the luminescent AlQ3 layer has been investigated with 2.5–20 nm thick Al2O3 films. For 2.5,

CE P

5 and 10-nm-thick Al2O3 films, the AlQ3 layer shows no fluorescence after a few hours at 85 °C/85% RH. For the 20 nm thick Al2O3 film, the fluorescence spectrum did not significantly change after 168 hours at 85 °C/85% RH, thus indicating that a 20-nm-thick film is required

AC

to prevent the loss of AlQ3 luminescence [145]. Typical of the aforementioned work is a study by Ghosh et al. [34] who assembled an OLED on a glass substrate which was then capped with 180 nm of Al2O3 deposited at 130 ºC using thermal ALD with O3 followed by 2 µm of paralyene. The paralyene layer was added to prevent corrosion of the Al2O3 film resulting from direct contact with the moist test environment. The optical transmission of the device was in excess of 92% and negligible degradation was observed after exposure to 85 ºC/85% RH conditions for 1300 hours [34]. In a similar study [135], an OLED was prepared by directly depositing a 100-nm Al2O3/ZrO2 nanolaminate onto the device using thermal ALD at 80 ºC. Under ambient condition, this OLED was reported to have an extrapolated lifetime of 22,000 hours, which demonstrated the feasibility of applying ALD barrier films directly onto organic devices without damaging the underlying organic functional layers.

55

ACCEPTED MANUSCRIPT The effect of Al2O3 barrier layer thickness on OLED degradation has been investigated by Klumbies and co-workers [1]. Barrier films of 15–100 nm thick Al2O3 were deposited on OLEDs by thermal ALD at 80 °C using TMA and O3. The degradation rate at 38 °C/15-

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20%RH decreased exponentially for film thicknesses in the range 15–25 nm after which the

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rate exhibited a sub-exponential decrease up to 50 nm. For the 100-nm Al2O3 film, rapid degradation was observed due to the appearance of large cracks in the Al2O3 film, which may

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occur with thicker films due to stress.

The doping of barrier films such as Al2O3 provides a means of altering their properties and

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performance in specific applications. This approach has been adopted in an assessment of OLED lifetime resulting from the deposition of 300-nm Al2O3:N films by PEALD at 80 ºC. hours after Al2O3:N deposition [143].

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Without the Al2O3:N film, the OLED had a lifetime of 105 hours, which increased to 650

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Inorganic/organic multilayers have also been investigated as suitable barriers for OLED

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devices. For example, the lifetimes of OLEDs with Al2O3/alucone multilayer barrier films have been investigated by Xiao et al. [106]. The multilayers were produced by depositing 20-

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nm Al2O3 films using thermal ALD and 4 nm of alucone by MLD, both at 80 ºC. In addition, the Al2O3 was deposited using either H2O or O3 as the oxidant. The multilayers deposited with O3 had a lifetime of 50 hours, which was nearly double that of the film deposited with H2O. The effect of barrier films with varying Al2O3/alucone ratios on the lifetimes of OLEDs

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has also been investigated for cycle ratios of 5:1, 6:1 and 7:1 [107]. Figure 25 shows the luminance as a function of time for the three ratios and an uncoated sample when subjected to 25 ºC/80% RH. Clearly, increasing the Al2O3 proportion extended device lifetime.

56

Figure 25

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Luminance of OLEDs with no ALD barrier film and Al2O3/alucone

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nanolaminate barrier films as a function of time at 25 ºC/80% RH [107].

Another study on OLED lifetimes used 1, 1.5 and 2 dyad barrier layers consisting of 28-nm

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ZrO2 films deposited by thermal ALD and 16-µm-thick spin coated UV-curable epoxy layers.

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At 20 ºC/60% RH, the lifetime increased as the number of dyads increased, yielding lifetimes of 79.2, 120 and 186 hours for 1, 1.5 and 2 dyads respectively [131]. The improvement in

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OLED lifetimes offered by 40-nm-thick, single Al2O3 and, Al2O3/TiO2 bilayers and multilayers has also been investigated. An unprotected OLED had a lifetime of ~10 hours which was increased to ~45 hours following the addition of a 40-nm Al2O3 film. The use of Al2O3/TiO2 bilayer and multilayer films further increased the device lifetime to 112 and 267

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hours, respectively [97].

The performance of a 3.5 dyad SiO2 nanoparticle embedded/Al2O3 nanocomposite multilayer barrier film on OLED luminance has been compared to that of a glass lid by Han et al. [140]. After 720 hours, the luminance of the OLED with the barrier had decreased to 50.5% of its initial value while that of the OLED with the glass lid was only slightly better at 55.2%.

Finally, two studies that have used single layer Al2O3 barrier films are worth mentioning. Li et al. [103] have compared the lifetime of an unprotected OLED with that of one coated with an 80-nm Al2O3 barrier film. A lifetime of ~35 hours obtained for an unprotected device was increased to ~500 hours for a device with the Al2O3 barrier film. In a similar study by the same group [111], the luminance of OLEDs protected with 80-nm-thick Al2O3 films deposited via thermal ALD using either H2O or O3 as the oxidant precursor were compared to 57

ACCEPTED MANUSCRIPT an unprotected OLED. As shown in Figure 26, the device lifetimes were approximately 10, 20 and 150 hours for the unprotected and, H2O oxidant and O3 oxidant barrier films,

Figure 26

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respectively.

Luminance of unprotected OLED and OLED’s protected by ALD Al2O3 films

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10 Factors to consider

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as a function of time at 25 ºC/60% RH [111].

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10.1 WVTR test configuration

It has been mentioned several times in this review that Al2O3 exhibits corrosion when directly

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exposed to high temperature and humidity [124, 132, 133]. Such behaviour has been demonstrated by depositing 100 nm of SiOx then 20 nm of Al2O3 directly onto a Ca test pad. The bilayer structure was then exposed to 20 °C/50% RH, but only remained an effective barrier layer for 20 days. Upon deposition of a 1-µm-thick parylene layer onto of the bilayer, the effectiveness of the barrier increased to 4 months as the parylene layer protected the Al2O3 layer from corrosion [7]. Many Ca tests which report low WVTR values, orient the Al2O3 ALD coated substrate with the Al2O3 side down [33, 53, 101, 116, 118]. The Al2O3 is therefore not exposed to the high humidity measurement conditions. Only occasionally have studies reported WVTRs determined with the barrier film exposed to the high humidity [32, 82]. Often the literature does not specify as to which way the ALD film is oriented when measuring the WVTR [40, 102]. Although when WVTRs such as 1.8x10-5 g.m-2/day are reported for a 21.5 nm thick 58

ACCEPTED MANUSCRIPT single Al2O3 layer deposited at 75 °C, it can be reasonably assumed that the test was undertaken with the Al2O3 film sealed within the Ca test [40]. Only one study could be found that investigated the effect of film orientation on WVTR [82]. For a single Al2O3 layer 26 nm

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thick, an average WVTR of 1.2x10-3 g.m-2/day was measured with HTO permeation when the

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substrate was oriented with the Al2O3 film facing away from the high humidity environment. When the substrate was oriented with the Al2O3 film facing the high humidity environment,

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the WVTR was initially 1x10-3 g.m-2/day but began to increase after 130 hours and failed after 160 hours of exposure, as shown in Figure 27. It was proposed that film failure resulted from exposure to the 100% RH environment which gave rise to film degradation. To protect

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the Al2O3 film, a SiO2 layer approximately 60 nm thick was deposited on top. The WVTR of the Al2O3/SiO2 films were reduced to 3.5x10-4 and 7.8x10-5 g.m-2/day for films oriented

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towards and away from the high humidity environment respectively. The reduction in the WVTR by the addition of a SiO2 layer to Al2O3 was attributed to the inability of SiO2 to form

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a hydrate with water.

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Al2 O3 away from RH

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Al2 O3 towards RH

Figure 27

WVTR of Al2O3 ALD films oriented away from and towards RH(Adapted from [82]).

Atomic force microscopy (AFM) and X-ray reflectivity (XRR) measurements have been undertaken to investigate the effect of exposing a 20-nm Al2O3 to 38 ºC/90% RH conditions. The initial AFM roughness of the Al2O3 film was 1.1 nm, but after only 5 minutes of exposure to 38 ºC/90% RH conditions, increased to 3.7 nm. XRR measurements were undertaken after the film was exposed to 38 ºC/90% RH conditions for 5, 15 and 60 minutes. XRR determined that the film had an initial density of 2.9 g/cm3 which then decreased to 2.7 after only 5 minutes while the film thickness did not increase. 15 and 60 minute exposure 59

ACCEPTED MANUSCRIPT times did not further decrease the density or film thickness. The changes in surface roughness and density were attributed to restructuring of the film, which is likely to increase water

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permeation due to the formation of low density regions, cracks and pinholes [109].

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10.2 Deposition directly onto Ca test pads

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WVTRs of for several barrier films have been determined by deposition directly onto the Ca pads used for the Ca test [1, 6, 75, 76, 103, 104, 107, 109-111, 131, 146], as denoted in Tables 1-4. Direct Ca deposition is likely to result in lower WVTRs than if the barrier film

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was deposited on a flexible polymer substrate as it removes any influences from the substrates such as surface contaminants or the introduction of cracking and defects by manual

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handling, which is relevant for the real-world application of barrier films.

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10.3 Effect of temperature and relative humidity on WVTR

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The temperature and RH at which the WVTR is measured, significantly influences the calculated values. The WVTR increases with increasing temperature due to the kinetic theory

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of gases. At higher temperatures, the velocity of the water vapour molecules increases, thus resulting in greater diffusion through polymers and barrier films [147]. The WVTR also increases at higher RH for a constant temperature, as the mass of water per unit volume of air increases. For a greater mass of water vapour, the water vapour on one side of the film has a

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greater diffusional driving force, thus accelerating water permeation. It has previously been shown that increasing the temperature and RH increased the WVTR of polypropylene and polyvinyl alcohol films [148]. It has also been shown that increasing the RH increases the WVTR of PEN [72].

Only a few studies have measured the WVTR of barrier films at different temperatures or RH [33, 44, 87, 146]. An example of the effect of humidity on WVTR is provided by the work of Carcia et al [33]. These authors used the o-Ca test to determine the WVTR of 25-nm Al2O3 deposited onto PEN at 120 °C. At 38 °C/85% RH, the measured WVTR was 1.7x10-5 g.m2

/day which increased to 6.5x10-5 g.m-2/day at 60 °C/85% RH, as shown in Figure 28. In

comparison, the WVTR at 23 °C was estimated to be 6x10-6 g.m-2/day. The WVTRs of Al2O3 films deposited onto polymer coated paper were determined at 23 °C/50% RH and 38 °C/90% RH. For all samples, higher WVTR values occurred at 38 °C/90% RH [44]. For 2060

ACCEPTED MANUSCRIPT nm Al2O3 deposited directly onto the e-Ca test at 38 ºC, the WVTR increased almost linearly

Figure 28

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with increasing RH [146].

o-Ca-test transmission of a 25-nm Al2O3 ALD film at 38 °C/85% RH and 60

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°C/85 %RH (Adapted from [33]).

A few studies have used the measured WVTR to extrapolate the WVTR at different

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conditions [33, 53, 87, 122]. The WVTR of a 20-nm Al2O3 film deposited by R2R ALD was experimentally determined at 85 °C/85% RH, but the value not specified. This value was then

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stated to be equivalent to a WVTR of 5x10-6 g.m-2/day at 20 °C/50% RH or 3x10-5 g.m-2/day

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at 38 °C/90% RH, but the equations used to calculate this extrapolation were also not specified [53]. The WVTR of a 30-nm Al2O3/ZrO2 multilayer structure at ambient conditions was determined using the time taken for complete Ca oxidation at 85°C/85% RH. This calculation used an activation energy of 52 kJ/mol (0.54 eV) and a humidity factor of 6 to

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relate ambient condition to 85% RH. The acceleration factors determined that 1 hour at 85 °C/85% RH corresponded to 240 hours at ambient conditions. Experimental results showed that at 85 °C/85% RH, complete Ca oxidation occurred in 70 hours and it was therefore stated that the WVTR of the barrier film at ambient conditions was 2x10-4 g.m-2/day. The equations used to calculate this WVTR were again not specified [122].

There is very little consistency in the temperature and RH conditions used to measure the WVTR of barrier films. As shown in Tables 1-4, the WVTR measurement conditions range from ambient temperature and humidity [32, 82] to 100% RH [29-31, 50, 51, 64, 97, 98, 119] and 85 ºC [118, 120, 122, 123, 126]. This range of values indicates that a set of standard conditions for measuring the WVTR have not been adopted in the majority of studies. This is particularly relevant when making comparisons between the results obtained using different techniques, precursors and deposition conditions. It is apparent from the range of values for 61

ACCEPTED MANUSCRIPT temperature and RH collated in Tables 1 - 4 that direct comparison of the results is difficult at best. As the WVTR is significantly affected by temperature and RH, these conditions need to be closely matched to accurately compare the WVTRs of different samples. To enable direct

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comparison between films, the standardisation of WVTR measurements conditions would be

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advantageous to enable benchmarking of experimental barrier films.

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11 Conclusions

This critical review has focussed on research into the use of ALD films as barriers against

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moisture permeation. This research has achieved considerable success in terms of identifying film structures with WVTRs below1 x10-4 g.m-2/day, while the best performing systems have

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produced moisture permeation rates of less than 5x10-5 g.m-2/day. In addition, some of the cited studies have also considered the limitations of specific film materials and problems associated with adapting ALD technology to large-scale production. The latter is particularly

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relevant in the case of R2R processing systems due to relatively slow ALD film deposition

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rates. Nevertheless, a number of research groups have addressed this issue through the design and testing of ALD R2R systems. While these systems show promise, their viability as part

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of a complete R2R production process has yet to be established.

Al2O3 films have been featured in the majority of studies on ALD barrier layers. This has been partly due to the ease of depositing conformal, amorphous films at temperatures

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≤120°C; a limit compatible with many common polymers. However, ALD Al2O3 films have been found to corrode under the high humidity conditions typically used to assess the performance of barrier films. This renders them unsuitable for long-term use in applications where exposure to such conditions is likely. As a consequence, ALD alumina films have been combined with those of other metal oxide or organic films in bilayer or multilayer structures. These mixed structures have yielded many WVTRs in the 10-4–10-6 g.m-2/day range for which Al2O3/organic multilayer films have been found to predominate. While some inorganic/inorganic bilayer and multilayer structures have resulted in WVTRs comparable to those obtained with inorganic/organic films, the flexibility afforded by the organic layer offers significant advantages in applications that utilise flexible web materials such as R2R processing.

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ACCEPTED MANUSCRIPT Most of the reports considered in this review have studied the performance of moisture barriers as stand-alone films or in combination with a variety of substrate materials. In a few cases, films have been deposited directly onto working OLED devices and the effectiveness

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of the barrier layer determined by monitoring the change in operational parameters with time.

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These studies have reported some significant increases in device lifetimes even with the deposition of a single Al2O3 barrier layer. Such findings suggest that ALD barrier films have

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considerable potential for specific commercial applications.

Finally, the studies considered in this review encompass a broad range of film materials,

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deposition techniques and conditions, and WVTR measurement methods and conditions. This complicates direct comparisons between the various published results. Thus, it is not possible

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at the present time to select a combination of materials, methods and conditions that could serve as the basis for the routine, large-scale production of ALD-based barrier layers. In this review, we have highlighted some of the issues that need to be resolved for this goal to be

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12 Acknowledgements

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achieved.

The Cooperative Research Centre for Polymers (CRC-P) is gratefully acknowledged for their funding of the ALD barrier film project which enabled the writing of the literature review on which this paper was based. The authors would also like to acknowledge Gerry Triani for his

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contributing expertise to the CRC-P ALD barrier film project. K. Jarvis thanks Prof. Sally McArthur for allowing time to work on this paper.

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