HfO2 moisture barrier films grown by low-temperature plasma-enhanced atomic layer deposition

Accepted Manuscript Highly-impermeable Al2O3/HfO2 moisture barrier films grown by low-temperature plasma-enhanced atomic layer deposition Lae Ho Kim, Jin Hyuk Jang, Yong Jin Jeong, Kyunghun Kim, Yonghwa Baek, Hyeokjin Kwon, Tae Kyu An, Sooji Nam, Se Hyun Kim, Jaeyoung Jang, Chan Eon Park PII:

S1566-1199(17)30383-X

DOI:

10.1016/j.orgel.2017.07.051

Reference:

ORGELE 4243

To appear in:

Organic Electronics

Received Date: 10 May 2017 Revised Date:

23 July 2017

Accepted Date: 31 July 2017

Please cite this article as: L.H. Kim, J.H. Jang, Y.J. Jeong, K. Kim, Y. Baek, H.-j. Kwon, T.K. An, S. Nam, S.H. Kim, J. Jang, C.E. Park, Highly-impermeable Al2O3/HfO2 moisture barrier films grown by low-temperature plasma-enhanced atomic layer deposition, Organic Electronics (2017), doi: 10.1016/ j.orgel.2017.07.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Highly-impermeable Al2O3/HfO2 moisture barrier films grown by low-

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temperature plasma-enhanced atomic layer deposition

Lae Ho Kima, Jin Hyuk Janga, Yong Jin Jeonga, Kyunghun Kima, Yonghwa Baeka, Hyeok-jin

Polymer Research Institute, Department of Chemical Engineering, Pohang University of

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Kwona, Tae Kyu Anb, Sooji Namc*, Se Hyun Kimd*, Jaeyoung Jange*, and Chan Eon Parka*

Science and Technology, Pohang, Republic of Korea 37673. E-mail: [email protected] b

Department of Polymer Science & Engineering and Department of IT Convergence, Korea

National University of Transportation, Chungju, Republic of Korea 27469. Smart I/O Control Device Research Section, Electronics and Telecommunications Research

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c

Institute, Daejeon, Republic of Korea 34129. E-mail: [email protected] School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Republic

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d

of Korea 38541. E-mail: [email protected] Department of Energy Engineering, Hanyang University, Seoul, Republic of Korea 15588.

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e

E-mail: [email protected]

Corresponding Author *E-mail: [email protected], Fax: +82-54-279-8298, Tel: +82-54-279-2269 (C.E. Park) 1

ACCEPTED MANUSCRIPT *E-mail: [email protected], Fax: +82-54-810-4686, Tel: +82-53-810-2779 (S.H. Kim) *E-mail: [email protected], Fax: +82-2-2220-2334, Tel: +82-2-2291-5982 (J. Jang)

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*E-mail: [email protected], Fax: +82-42-860-5202, Tel: +82-42-860-1479 (S. Nam)

Abstract

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Graphical Abstract

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Polymer substrates are essential components of flexible electronic applications such as OTFTs, OPVs, and OLEDs. However, high water vapor permeability of polymer films can significantly reduce the lifetime of flexible electronic devices. In this study, we examined the water vapor permeation barrier properties of Al2O3/HfO2 mixed oxide films on polymer substrates. Al2O3/HfO2 films deposited by plasma-enhanced atomic layer deposition were transparent, chemically stable in water and densely amorphous. At 60 °C and 90% relative 2

ACCEPTED MANUSCRIPT humidity (RH) accelerated condition, 50-nm-thick Al2O3/HfO2 had water vapor transmission rate (WVTR) = 1.44 x 10-4 g m-2 d-1, whereas single layers of Al2O3 had WVTR = 3.26 x 10-4 g m-2 d-1 and of HfO2 had WVTR = 6.75 x 10-2 g m-2 d-1. At 25 °C and 40% RH, 50-nm-thick

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Al2O3/HfO2 film had WVTR = 2.63 x 10-6 g m-2 d-1, which is comparable to WVTR of conventional glass encapsulation.

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Keywords

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Al2O3, HfO2, mixed oxide film, plasma-enhanced atomic layer deposition (PEALD), thin-

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film encapsulation (TFE).

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ACCEPTED MANUSCRIPT 1. Introduction Organic electronic and optoelectronic devices based on polymer substrates are flexible, portable, and can be produced at low cost; therefore, they have applications in flexible

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displays, wearable sensors and medical devices.1 An important impediment to commercialization of flexible electronics is that they have a short lifetime because they are susceptible to degradation by moisture and oxygen. Electrodes based on low-work function

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metals and organic-based materials can oxidize, delaminate, or crystallize upon exposure to water or oxygen.2-5 These reactions create trap states and interrupt charge carrier injection or

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transport in active materials, and thereby degrade the electrical characteristics of the devices. Also, polymers are highly permeable to water vapor due to their properties such as free volume and low cohesive energy between carbon chains.6 Therefore, polymer substrates and devices must be encapsulated in a thin water-resistant film. Furthermore, to be applicable to

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flexible polymer substrate or devices, such thin-film encapsulation (TFE) methods (1) require low processing temperature (typically ≤ 100 °C) to minimize thermal degradation of the polymer; (2) must have low numbers of defects such as pinholes, cracks, grain boundaries,

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and impurities, which provide pathways for gas permeation; and (3) must be highly

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transparent to visible light, to ensure high efficiency of optoelectronic or optical devices. The water vapor transmission rate (WVTR) is an important figure-of-merit for moisture permeation barriers. Typically, stable operation of flexible electronic devices for > 10,000 h requires 10-3 ≤ WVTR ≤ 10-6 g m-2 d-1.7-8 Various inorganic compounds (e.g., SiOx, SiNx, SiCx, SiOxNy, SiOxCy, Al2O3, AlOxNy, ZrO2, TiO2, MgO, ZnO, SnO2) have been evaluated as TFE layers. Various thin-film deposition methods such as solution processing,9-10 evaporation,11-14 sputtering,15-20 and plasma-enhanced chemical vapor deposition (PECVD)214

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techniques have been used to form TFE layers. However, single-layered barrier films have

relatively high 10-1 ≤ WVTR ≤ 10-3 g m-2 d-1, mostly because of the presence of defects, loosely packed oxide structure, or both demerits.6,13-15,24,26 Organic/inorganic multilayer films

and may incorporate contaminants during vacuum breaks.

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can reduce WVTR to 10-6 g m-2 d-1,31 but this approach can be time-consuming and costly

Atomic layer deposition (ALD) can produce thin-film TFE layers that are pinhole-free,

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dense, and highly uniform.32 Sequential exposure of reactants separated by an inert gas purge results in a self-limiting process with precise control over thickness and homogeneity.

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Therefore, ALD can deposit very dense films even at low processing temperatures with superior WVTR ≈ 10-4 g m-2 d-1.33-35 Plasma-enhanced ALD (PEALD) uses O2 plasma as an oxidizing reactant for oxide coating, and is particularly useful because it has short deposition time.36-37 Furthermore, additional energies provided by radicals, electrons, ions and photons

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in the plasma produce sufficient reactivity to grow a high quality film without increasing the deposition temperature. Therefore, PEALD is a promising technique to deposit TFE on polymer substrates and flexible electronic devices.

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PEALD-based Al2O3 films have been widely studied as barrier layer due to their low gas

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permeability, high optical transparency, and good thermal and mechanical stability. However, Al2O3 thin films deposited at low temperature readily corrode in moderately acidic or basic solutions, and even in water.38-40 The gas impermeability of an Al2O3 single layer deteriorates under high humidity due to corrosion of the film by water that condenses at the oxide surface. Al2O3/TiO2 mixed oxide TFE films formed by alternating PEALD processes can improve the water resistance of Al2O3,40 but the composite film is not sufficiently transparent.

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ACCEPTED MANUSCRIPT In this study, we used PEALD-based Al2O3/HfO2 mixed oxide films on polymer substrate at processing temperature of 100 °C to improve impermeability to moisture without loss of transparency. The Al2O3 and HfO2 layers can each compensate for the other’s weakness, so

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Al2O3/HfO2 films were densely amorphous, stable against water, and transparent. In detail, thermally stable and dense Al2O3 suppresses the formation of grain boundaries and voids that originate from crystalline morphology and higher impurity levels of HfO2 layers. These are

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weaknesses of HfO2 layers since grain boundaries and voids can provide moisture permeation pathways. On the other hand, HfO2 improves the chemical stability in the Al2O3/HfO2 mixed

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oxide films, compared with Al2O3 single layer films. Because the materials have relatively low refractive index, the Al2O3/HfO2 films have high optical transmittance OT ≈ 95 % (light wavelengths 400 to 800 nm). We also studied optimization of PEALD process, surface morphology, actual thicknesses of oxide layers formed on polymer substrate, and the effects

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of oxide thickness on the moisture permeability for Al2O3, HfO2, and Al2O3/HfO2 films.

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ACCEPTED MANUSCRIPT 2. Experimental Section Al2O3, HfO2, and Al2O3/HfO2 films were deposited using a 6-inch PEALD reactor (LTSR150, Leintech). PEALD films were grown on p-type Si(100) wafer, soda lime glass, or

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polyethylene naphthalate (PEN) to investigate the films’ moisture permeability, and their chemical and physical properties. The Si wafer and glass were cleaned sequentially with acetone and isopropyl alcohol in an ultrasonic bath for 20 min each, dried after each step

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under an N2 stream, then exposed to UV radiation for 20 min to remove carbon contaminants. The PEN substrate was only washed with isopropyl alcohol in an ultrasonic bath for 1 min,

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then dried in a vacuum oven at 100 °C after N2 blowing. Before being coated with the oxide, the PEN substrate was exposed to UV radiation for only 5 min to prevent UV degradation. The substrates were fixed using imide tape onto 6-inch Si wafers before the deposition step. The PEALD substrate heater was maintained at 100 °C, and the warm wall of the reactor was

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maintained at 80 °C. The substrates were placed onto the PEALD heater for 30 min to permit thermal equilibration. The base pressure was 0.01 Torr, and processing pressure inside the reactor

was

0.5

Torr.

Trimethylaluminium

(TMA,

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and

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tetrakis(ethylmethylamino)hafnium (TEMAH, EG Chem) were used as metal-organic

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precursors for Al2O3 and HfO2 deposition, respectively. The canister containing TMA was maintained at room temperature (24 °C); the canister containing TEMAH was heated to 75 °C to increase the vapor pressure. O2 plasma was used as oxygen source for oxide coating with radio frequency (RF) of 13.56 MHz and RF power of 100 W in all PEALD processes. One PEALD cycle for Al2O3 deposition consisted of 0.1-s precursor injection, 10-s Ar purge, 1.5-s O2 injection, 1-s O2 injection with RF plasma, and 5-s Ar purge. The sequence of one cycle for HfO2 deposition consisted of 0.6-s precursor injection, 10-s Ar purge, 1.5-s O2 7

ACCEPTED MANUSCRIPT injection, 2-s O2 injection with RF plasma, and 5-s Ar purge. O2 injection step prior to RF plasma activation was required to increase the stability and reactivity of the O2 plasma. Al2O3/HfO2 mixed oxide films were prepared by the alternate deposition of an Al2O3 and an

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HfO2 layer until the desired thickness is obtained. The growth per cycle (GPC) was 0.175 nm cycle-1 for Al2O3 and 0.132 nm cycle-1 for HfO2.

We prepared a Ca test cell to determine the WVTR of PEN substrate (thickness: 125 µm;

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Tg 121 °C) coated with PEALD oxide film. A thermal evaporator under high vacuum (5 x 10-6 Torr) was used to deposit a 250-nm-thick Ca layer (20 x 20 mm) on 120-nm-thick Al

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patterned glass (50 x 50 mm). A patterned Al electrode was connected to the Ca layer for the conductance measurements. UV-curable sealant (XNR 5570-B1, Nagase ChemteX) was used to encapsulate the Ca layer with a PEN substrate coated with PEALD oxide film. A 4-point conductance measurement (GDM-8255A, GW INSTEK) was performed to monitor temporal

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change in the conductance of the Ca layer at 60 °C and 90% RH or at 25 °C and 40% RH. The film thickness and refractive index of the PEALD films on Si wafer were measured using a spectroscopic ellipsometer (M-2000, J.A.Woollam). OT between 400 nm and 800 nm

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of the PEALD films on glass was measured using a UV-Vis spectrophotometer (V-670, Jasco).

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The surface roughness of the PEALD films was measured using atomic force microscopy (AFM, VEECO Dimension 3100). The film coverage, effective barrier thickness, and interface morphology of PEALD films deposited on PEN were measured using highresolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F). The atomic concentration of carbon and nitrogen impurity were quantified using magnetic sector secondary ion mass spectroscopy (SIMS, CAMECA IMS-6f).

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ACCEPTED MANUSCRIPT 3. Results and Discussion Al2O3 and HfO2 films deposited by PEALD process were optimized by confirming saturation behavior of GPC at deposition temperature of 100 °C. We investigated the effects

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of precursor injection duration, purge duration after precursor injection, O2 plasma exposure duration, and purge duration after O2 plasma exposure on GPC (Fig. 1). The GPC of the films saturated at 0.1-s TMA injection (for Al2O3) and at 0.6-s TEMAH injection (for HfO2) (Fig.

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1a) due to their vapor pressure difference (at 25 °C, TMA: ~11.6 Torr; TEMAH ~0.25 Torr). Increasing the purge duration caused decrease in GPC in both cases by suppressing the CVD-

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like gaseous reaction (Figs. 1b, d). Increasing in the O2 plasma exposure duration increased the GPC of Al2O3, but decreased the GPC of HfO2 (Fig. 1c); we speculate that the increase in the GPC of Al2O3 may be a result of the presence of sufficient energy to generate reactive surface –OH groups, whereas the decrease in GPC of HfO2 may be due to the relatively low

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reactivity of TEMAH compared to TMA, so that O2 plasma is more involved in the reaction of ligand-exchange from –NEtMt (ethylmethylamino group) to –OH group than in creation of new surface active sites. Based on the GPC analysis, optimized pulse sequence (precursor

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injection / purge / O2 injection / O2 plasma exposure / purge) for Al2O3 was 0.1 s / 10 s / 1.5 s / 1 s / 5 s and for HfO2 was 0.6 s / 10 s / 1.5 s / 2 s / 5 s. Al2O3/HfO2 mixed oxide film was

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prepared by alternating deposition of one layer of Al2O3 and one layer of HfO2. The thickness of oxide films on the Si wafer increased linearly as the number of PEALD deposition cycles increased (Fig. 2). The GPC were 0.175, 0.132, and 0.295 nm cycle-1 for Al2O3, HfO2, and Al2O3/HfO2, respectively. These GPC values agree well with previous reports.41-42 The Al2O3/HfO2 mixed oxide film is thinner than the sum (0.307 nm cycle-1) of GPC of Al2O3 and HfO2; the difference occurs because HfO2 is polycrystalline and 10

ACCEPTED MANUSCRIPT Al2O3/HfO2 is amorphous. AFM was used to measure the surface roughness of PEALD oxide films with thickness of 50 nm on Si wafer (Fig. 3). Al2O3 and Al2O3/HfO2 film were very smooth with root mean

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square roughness RRMS = 0.260 nm and 0.284 nm, respectively, whereas the HfO2 film had RRMS = 1.550 nm because of the presence of HfO2 crystallites. This result is consistent with HR-TEM results, which we discuss later.

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High OT of barrier film is important to maintain the efficiency of optical devices such as

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OLEDs or OPVs. The refractive indices n of 50-nm-thick Al2O3, HfO2, and Al2O3/HfO2 films were 1.6446, 1.9553, and 1.7962, respectively at 550-nm wavelength (Fig. 4a). The average OT was 97.63 %, 88.93 %, and 94.78 % for Al2O3, HfO2, and Al2O3/HfO2 film, respectively. The Fresnel equation

 − 1  + 1

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=

can predict the reflectance of oxide film from its refractive index. As n increases, the

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reflectance of the oxide film increases, so its transparency decreases. The moisture permeation barrier properties of PEALD oxide films were investigated using

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a Calcium (Ca) test. The critical strain at which cracks begin to form in the film by tensile or compressive strain, increases as film thickness decreases, because its internal stress also decreases (Fig. S1 and S2, which present bending test results for Al2O3/HfO2 films on PEN).43 Therefore, we studied the thickness effects on the barrier properties of PEALD films with thickness ≤ 50 nm on PEN substrate. A 250-nm-thick Ca film was deposited onto patterned Al electrodes and encapsulated with Al2O3, HfO2, or Al2O3/HfO2 films formed on 11

ACCEPTED MANUSCRIPT PEN by using epoxy glue. The conductance of Ca decreased over time as the chemical reaction of Ca with water vapor [Ca + 2H2O → Ca(OH)2 + H2], oxygen [ 2Ca + O2 → 2CaO] or both proceeded. The reaction of Ca to Ca(OH)2 dominates the reaction rate in a thermos-

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hygrostat chamber under accelerated aging conditions of 60 °C and 90% RH.30 WVTRs of the films on PEN were calculated as44

  1⁄
      ",    !

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= −

where m is the molar equivalent of the degradation reaction [m = 2, Ca + 2H2O → Ca(OH)2 +

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H2], $ % = 18 g mol-1 is the molecular weight of H2O, &' = 40.1 g mol-1 is the molecular weight of Ca, ρCa = 1.55 g cm-3 is the density of Ca, σCa = 3.4 x 10-8 Ω m is the resistivity of Ca, SCa = 20 x 20 mm is the area of Ca, and Swindow = 26 x 26 mm is the transmission area of water vapor. The basic set up procedure has been described previously.35

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The conductance change of the Ca d(1/R)/dt was calculated based on the slope at the normalized conductance of 0.5 for HfO2. For Al2O3 and Al2O3/HfO2, WVTR were extracted from the slop of the normalized conductance change between 12 and 36 h to avoid initial

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fluctuations in Ca resistance and epoxy degradation after 50 h at 60 °C and 90% RH.35 The conductance of Ca encapsulated with HfO2, Al2O3, or Al2O3/HfO2 films formed on

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PEN as well as with bare PEN all decreased over time due to Ca corrosion (Figs. 5a-c). WVTR values decreased as oxide film thickness increased (Table 1; Fig. 5d). The conductance of Ca encapsulated with bare PEN rapidly decreased within approximately 1 h (WVTR = 6.75 x 10-1 g m-2 d-1) because water molecules can easily permeate through the free volume and the gaps between carbon chains in the polymer (Fig. 5a). The single Al2O3 layer showed low permeability than the single HfO2 layer. With 50-nm-thick layers, WVTR was 12

ACCEPTED MANUSCRIPT 3.26 x 10-4 g m-2 d-1 for Al2O3 and 6.57 x 10-2 g m-2 d-1 for HfO2. The Al2O3/HfO2 mixed oxide films had much lower WVTR than did single Al2O3 and HfO2 films. The lowest measured WVTR of Al2O3/HfO2 film was 1.44 x 10-4 g m-2 d-1, which is ~4700 times better

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than that of the bare PEN. This value is similar to those of previously reported ALDprocessed Al2O3/ZrO2 and Al2O3/TiO2 films based on both TMA-based Al2O3 and other oxide with a high metal-oxygen bond dissociation energy.33,40 Initial conductance change of Ca

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encapsulated with Al2O3 film/PEN was similar to that of Al2O3/HfO2 film/PEN, but over time, the conductance of Ca encapsulated with Al2O3 film/PEN decreased faster than that of

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Al2O3/HfO2 film/PEN, because of the low chemical stability of Al2O3.40 Under mild condition (25 °C, 40% RH), 50-nm-thick Al2O3/HfO2 film had remarkable WVTR = 2.63 x 10-6 g m-2 d1

, which surpasses that of conventional glass encapsulation (Fig. 6). Cross-sectional TEM images of Al2O3, HfO2 and Al2O3/HfO2 films on PEN substrate show

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that each PEALD oxide films had thickness close to the target of 50 nm (Fig. 7). The differences in minimum oxide thickness, which affects the effectiveness of the gaspermeation barrier, were not significant. Only HfO2 film showed channels along the grain

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boundary, which may be moisture permeation pathways. The higher permeability of HfO2 films might result from the presence of these channels. The large interface roughness at oxide

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film/polymer may occur because nonpolar TMA and TEMAH precursors can be infiltrated and react with oxygen reactants at the PEN surface.45 The oxide clusters might first formed at the carbonyl groups in the PEN main backbone,46 then coalesce to form continuous oxide films. Therefore, we expect that oxide layer thickness above 10 nm is needed to obtain homogeneous oxide thin-film when PEN substrates are used at growth temperature of 100 °C. The high moisture permeability of Al2O3 and Al2O3/HfO2 films with thickness of 10 nm (Figs. 13

ACCEPTED MANUSCRIPT 5b, c) support this hypothesis. To confirm the morphology of Al2O3, HfO2, and Al2O3/HfO2 films on PEN substrate, cross-sectional HR-TEM analysis was performed (Fig. 8). Al2O3 and Al2O3/HfO2 films had a

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homogeneous amorphous structure, but HfO2 film showed crystalline morphology. Because the grain boundaries in crystalline structure can provide moisture-permeation pathways in the film, the WVTR of HfO2 film was two orders of magnitude higher than WVTRs of

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stable Al2O3 suppressed the crystallization of HfO2.47-48

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amorphous Al2O3 or Al2O3/HfO2 films (Fig. 5d). In Al2O3/HfO2 mixed oxide films, thermally

The excellent moisture impermeability of Al2O3/HfO2 films could be explained by the following effects. Firstly, the addition of HfO2 to the Al2O3 matrix resulted in the formation of strong Al-O-Hf bonds that improved the chemical resistance of the mixed oxide films.40 HfO2 has the highest metal-oxygen bond dissociation energy of 801 ± 13 kJ mol-1 (501.9 ±

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10.6 kJ mol-1 for Al-O bond) except oxides containing rare earth metals, so Al2O3/HfO2 films have high chemical stability.49 To evaluate the water tolerance, we measured film thickness and refractive index change over time in the PEALD oxide films upon immersion in water

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(pH 7.2) at room temperature (Fig. 9). The film thickness of the Al2O3 film increased and the

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refractive index of the Al2O3 film decreased rapidly after 400 h, but those of the HfO2 and Al2O3/HfO2 films did not change over 800 h; this result shows the good chemical stability of the films that contain HfO2. The second possible cause of the excellent water impermeability of the Al2O3/HfO2 films is that a low level of impurities in Al2O3 can form a very dense structure in Al2O3/HfO2 films. We used SIMS analysis to investigate the impurity levels (carbon and nitrogen contents) of 14

ACCEPTED MANUSCRIPT the Al2O3, HfO2, and Al2O3/HfO2 films (Table 2). All films had < 1.2% C or N; this result means that the metal-oxide bonds are formed effectively by PEALD, even at low temperature. The impurity levels were lower in Al2O3 film than in HfO2; the difference can be attributed to

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the higher reactivity of TMA than TEMAH at the deposition temperature of 100 °C. TEMAH is a relatively low-reactivity precursor with large-sized ligands; it may degrade the impermeability of the oxide film. Formation of a densely-packed metal-oxide network in the

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film may be interrupted by impurities due to insufficient energy for ligand exchange, loosely packed oxide structure due to steric hindrance of large size of TEMAH molecule, or to both

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phenomena. We expect that highly-reactive and small TMA precursor molecule could react with OH groups on oxide surface more effectively than that of TEMAH precursor. From these results, we conclude that Al2O3/HfO2 mixed oxide films have both advantages of a

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densely-packed amorphous structure due to Al2O3, and high chemical stability due to HfO2.

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ACCEPTED MANUSCRIPT 4. Conclusion We demonstrated that ultralow WVTR values of 1.44 x 10-4 g m-2 d-1 at 60 °C/90% RH and 2.63 x 10-6 g m-2 d-1 at 25 °C/40% RH can be obtained using 50-nm-thick Al2O3/HfO2 mixed

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oxide films on PEN substrate. Low temperature PEALD-processed Al2O3/HfO2 mixed oxide films showed high water stability, high optical transmittance of ~95%, and dense amorphous structure. Al2O3 formed by highly reactive and small TMA molecules can acts as densifier,

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and HfO2 can improve the chemical stability of Al2O3/HfO2 mixed oxide film. This exploitation of the advantages of dense Al2O3 and chemically-stable HfO2 can be a good

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strategy for TFE of polymeric substrates.

Acknowledgments

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This research was supported by a New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean Government through the Ministry of Knowledge Economy (20123010010140), the Samsung

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Display Corporation, and the Center for Advanced Soft Electronics under the Global Frontier

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Research Program (Grant No. 2013M3A6A5073175).

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ACCEPTED MANUSCRIPT Figure and Table captions Figure 1. Dependence of GPC on (a) precursor injection duration, (b) purge duration after precursor injection, (c) O2 plasma exposure duration, and (d) purge duration after O2 plasma

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exposure. All PEALD films were grown at 100 °C. The PEALD pulse sequence (precursor injection / purge / O2 injection / O2 plasma / purge) is represented in the legend of each graphics; the circles represents the adopted pulse duration in this study.

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Figure 2. Increase in film thickness with the number of PEALD cycles. One cycle of

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Al2O3/HfO2 consists of one cycle of Al2O3 and one cycle of HfO2.

Figure 3. AFM surface images of (a) Al2O3, (b) HfO2, and (c) Al2O3/HfO2 film on Si substrate.

Figure 4. (a) The refractive index and (b) optical transmittance of PEALD films prepared

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with a thickness of 50 nm.

Figure 5. Normalized conductance change of Ca encapsulated with (a) HfO2, (b) Al2O3, and

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(c) Al2O3/HfO2 films formed on PEN at 60 °C and 90% RH. (d) WVTR values for PEALD films as function of oxide thicknesses at 60 °C and 90% RH.

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Figure 6. Normalized conductance change of Ca encapsulated with 50-nm-thick Al2O3/HfO2 film on PEN substrate at 25 °C and 40% RH. Figure 7. Cross-sectional TEM micrographs of (a) Al2O3, (b) HfO2, and (c) Al2O3/HfO2 film formed on PEN substrate. Figure 8. Cross-sectional HR-TEM images (left) and electron diffraction patterns (right) of (a) Al2O3, (b) HfO2, and (c) Al2O3/HfO2 film. 24

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Table 1. WVTR of HfO2, Al2O3, and Al2O3/HfO2 mixed oxide films vs. film thickness at 60 °C and 90% RH.

Table 2. Content of carbon and nitrogen impurities in PEALD films, as determined by SIMS

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analysis.

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Figure 4.

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Figure 8.

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Figure 9.

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WVTR (g m-2 d-1)

(nm)

Al2O3

10

6.72 x 10-1

3.69 x 10-2

20

4.29 x 10-1

2.92 x 10-3

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1.81 x 10-1

1.77 x 10-3

40

8.70 x 10-2

50

6.57 x 10-2

Al2O3/HfO2

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HfO2

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Layer Thickness

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5.40 x 10-3 1.46 x 10-3 7.19 x 10-4

8.33 x 10-4

6.05 x 10-4

3.26 x 10-4

1.44 x 10-4

ACCEPTED MANUSCRIPT Table 2.

Al2O3

HfO2

Al2O3/HfO2

C

0.234

1.108

0.495

N

0.009

0.990

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Atomic %

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0.467

ACCEPTED MANUSCRIPT Highlights - Al2O3, HfO2, and Al2O3/HfO2 mixed oxide films were deposited on Si wafer, glass, or polyethylene naphthalate (PEN) by plasma-enhanced atomic layer deposition (PEALD) at

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low temperature of 100 °C, and the chemical, physical, and moisture permeation barrier properties of the films were investigated.

- Al2O3/HfO2 mixed oxide films were prepared by alternating deposition of Al2O3 and HfO2

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(one layer of each per cycle).

high chemical stability due to HfO2.

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- Al2O3/HfO2 mixed oxide films had densely-packed amorphous structure due to Al2O3 and

- All Al2O3/HfO2 mixed oxide films were less permeable to moisture than were single Al2O3

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and HfO2 films in the thickness range 10 nm to 50 nm.

1