Improvement of chemical, physical, and electrical properties of parylene-D deposited by chemical vapor deposition by controlling the parameters process

Materials Chemistry and Physics xxx (2016) 1e14

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Improvement of chemical, physical, and electrical properties of parylene-D deposited by chemical vapor deposition by controlling the parameters process
c, A. Sylvestre a M. Mokni a, b, A. Kahouli a, b, *, F. Jomni b, J.-L. Garden c, E. Andre a b c

Univ. Grenoble Alpes, G2Elab, F-38000 Grenoble, France Laboratoire Mat
eriaux Organisation et Propri
et
es (LMOP), Universit
e de Tunis El Manar, 2092 Tunis, Tunisia CNRS, I. Neel, F-38000 Grenoble, France

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Surface and bulk properties of ne D are sensitive to the Tpyr paryle and Tsub.
The sublimation temperature strongly affects the layer growth rate of parylene D.
Nearly amorphous parylene-D is obtained when sublimation temperatures are height.
Pyrolysis temperatures (Tpyr) impact more the surface quality compared to the Tsub.
Dielectric properties are very sensitive to the parameter processes change.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 September 2016 Received in revised form 17 November 2016 Accepted 21 November 2016 Available online xxx

The effect of chemical vapor deposition (CVD) parameters on the surface morphology and the molecular structure of parylene-D (also called poly-dicloro-xylylene or PPX D) films were investigated. Relevant process parameters are defined by the sublimation temperature, pyrolysis temperatures, growth rate, and time of the deposition process. The sublimation temperatures strongly affects the layer growth rate which increases by a factor of about 4 as the temperature increases from 120 to 160  C. The sublimation temperature at which the parylene-D cannot be deposit is estimated to be 105  C. The present CVD experimental parameters have shown that appropriate sublimation temperatures can yield a controlled growth of the layer thickness ranging from tens of nanometers up to at least 8 mm with a good control of the amorphous and the crystalline amounts. Nearly amorphous parylene-D is obtained when sublimation temperatures are height. However, the crystallinity is increased considerably when the dimer Tsub is decreased. A decrease in the surface roughness is achieved by reducing the Tsub and the deposition rate. These experimental conditions giving rise to dense and transparent parylene-D films. The pyrolysis temperatures favorite growing of globules at the surface compared to the sublimation temperatures. The size of the globules increases from 1.3 mm for Tpyr. ¼ 650  C to 13.7 mm for Tpyr. ¼ 690  C. It inferred also that the crystallinity content and the crystalline size decrease by increasing the pyrolysis temperatures. Electrical properties are very influenced by the CVD-processing parameters especially by modifying the pyrolysis temperature. Using these optimized conditions, state-of-the-art parylene-D films with

Keywords: Parylene CVD process Crystallinity Sublimation temperature Pyrolysis temperatures Morphology

* Corresponding author. Univ. Grenoble Alpes, G2Elab, F-38000 Grenoble, France. E-mail address: [email protected] (A. Kahouli). http://dx.doi.org/10.1016/j.matchemphys.2016.11.042 0254-0584/© 2016 Published by Elsevier B.V.

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promising properties are reproducibly processed. These results significantly expand the range of processing variables compatible with large applications domains. © 2016 Published by Elsevier B.V.

1. Introduction The research works and the applications incorporating the various families of parylene (parylene-N, -C, -D, VT4, and AF4) are rapidly expanding in the last ten years. This success is largely due to the protective properties and the excellent conformal capabilities of these materials and more specifically they are used as a coating layer [1e3] and as an insulation materials [4,5] for microelectronic applications and electronic devices. Thus, almost every industrial sectors use parylene: medical [6,7], integrated optoelectronic devices [8], organic devices like protective coatings insulation layers in organic light emitting diode (OLED) [9,10], passivation layer in organic field effect transistors (OFET) [11], coating film for flexible electronics devices [12], and dielectric insulation layers [13,14] for nanotechnology devices [15,16]. This huge success in the use of these thermoplastic family is also due to the industrial process which is made at room temperature by chemical vapor deposition (CVD) basing on the Gorham method [17] allowing to introduce into the deposit chamber diverse devices on which the parylene can easily settle. The CVD technique of parylenes provides the room temperature coating process and produces parylene films with various physical and electrical interesting properties, uniform and excellent control of the thickness with good conformability even for complex geometry of devices or still extraordinary penetration abilities and purity [18,19]. Large varieties of molecular topologies are also available if a specific sample-holder is installed in the CVD chamber [20,21] which is useful for specific medical fields [22]. These properties allow to expands the spectrum of applications of these thermoplastic parylenes family. The vast majority of studies have focused on the optimization of CVD deposition parameters for parylene-N (PPX N) and parylene-C (PPX C) materials. PPX N is the reference material of the thermoplastic parylenes family obtained from an unsubstituted cyclic [2.2] paracyclophane. Aromatic chlorination of the PPX N gives rise to PPX C and parylene-D (PPX D) with one and two chlorine atoms on average per repeat unit (monomer) [18], respectively (Fig. 1). Recently, we realized a study demonstrating the potentialities of parylene D relating to its electric and dielectric properties [23]. Particularly, it possess a high crystalline melting temperature (Tm) of 360  C more than parylene-C (Tm ¼ 290  C) [19], a good thermochemical stability at high temperature ant it shows two various dielectric responses according to the frequency and the temperature range: it works like a non-polar material at very low temperature (similar to PPX N) and like a polar material at high

temperature (similar to PPX C). For our previous study [23], conventional parameters for the temperature of sublimation (Tsub.) and pyrolysis (Tpyr.) were used for the manufacture of these PPX D for microelectronic applications. Tsub. of the precursor determine the vapor pressure of the gaseous molecules, which is postulated to have the greatest effect on the deposition rate, whereas Tpyr control the deposition rate at which the reactive molecules species are generated from the precursor. Therefore it affects the concentration of the reactant arriving on the substrates surface. The role of the sublimation and pyrolysis temperatures is not well explored and discussed yet for parylenes family. With regard to PPX D, this role is absent in the scientific literature. However, these two factors can contribute in-depth for the deposition rate, the structural and physical properties of this family of parylenes. Since our first study shows interesting properties of parylene-D for microelectronic applications and in the other hand this material bring new advantageous properties in comparison to parylene-N and parylene-C, it appears interesting to deeply investigate its potential properties for some specific requirement applications necessity specific deposition parameters. In addition, this material can be employed in many fields of applications when the other parylenes family could not respond to the applications demands or when they reach their use limits. Further work is required on this polymer to reach an optimization in the CVD process in order to guarantee the film quality and the desired properties. In this context, our work is focused on the understanding of the effect of the CVD process parameters on PPX D properties, i.e., relationships process/structure/properties. Chemical analyses and structural properties (crystallinity, morphology) were explored. In order to well control the properties, to give more information about this material and to offer new routes in several areas of applications, this study is carried out for different sublimation and pyrolysis temperatures, various deposition rate and time of the deposition process.

2. Experiment part 2.1. Material The main attractive reason to deposit the parylenes family lies in its large temperature deposition [18] without catalyst elements making conformal and pure films (pinhole free). The growth rate follows an exponential decreasing law with the substrate temperature [18].

Fig. 1. Chemical structure of parylene N, C and D.

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M. Mokni et al. / Materials Chemistry and Physics xxx (2016) 1e14

LogðdÞ ¼

K1  k2 TS

using a Digital Instrumental Nanoscope V-AFM (Veeco Nanoscope V Controller) in order to analyze the surface topography and to quantify the associated surface roughness as a function of the sublimation and the pyrolysis temperatures.

(1)

where d is the film thickness, Ts is the substrate temperature and k1; k2 are two constants. parylene-D films are deposited on various substrates including, Si, Si/Au, Si/Al, Si/Ag, and Si/Cu by chemical vapor deposition (CVD) at room temperature using the Gorham
el Laboratory (Grenoble-France). In this method in the institut Ne process, the cyclic dimer (tetrachlorinated-di-para-xylylene) is sublimated at a temperature between 120 and 160  C with a pressure around 1 torr. The vapor of the dimer is cleaved into reactive vapor monomer (dicloro-chlorinated-p-xylylene) in a pyrolysis chamber at high temperature between 650 and 690  C and lower pressure around 0.5 torr (Fig. 2). Under these experimental conditions a transparent, uniformity, and conformity coating film of parylene-D is obtained on all substrates.

3. Results and discussion 3.1. Effect of sublimation temperature Thermal process can have important effects on the properties of the parylene-D, therefore FT-IR analysis of the parylene-D films was carried out by varying the sublimation temperatures from 120 to 160  C while the other experiment parameters were kept constant: at pyrolysis temperatures of 650  C, a substrate temperature at 30  C ± 1, a dimmer mass of 5.2 g, and pressure ~0.1 torr. It is clear seen from FTIR analysis (Fig. 3) that the chemical structure of parylene-D is just slightly affected by the variation of the sublimation temperature: at increasing Tsub. all the typical peaks of parylene-D continue appearing in the spectra. Moreover, nor shift of parylene peaks neither appearance of new peaks are found thus indicating that the formation of new adverse species like carbonyl (eC]Oe) at 1750 cm1 or hydroxyl (eOHe) at 3500 cm1 groups does not appear to occur. On the other hand, a decrease of intensity and a broadening of some specific peaks is found as the sublimation temperature decreases, especially the peaks at 2929 and 2866 cm1 (CeH aliphatic stretching of methylene groups dCH2), 1454 cm1 (CeH bending), 887 cm1 (one adjacent CeH bending on benzene ring), and 823 cm1 (two adjacent CeH bending on benzene ring). All these peaks correspond to vibration modes of the CeH groups, which are particularly sensitive to the surrounding environment of the polymer chains, and indicate an increase of the disorder in the parylene-D films as the Tsub increases. A net increase of absorption peak appears at 1076 cm1 (CeCl bending) with decreasing the sublimation temperature is clearly observed. FTIR analysis show that the chemical bonds of parylene-D films

2.2. Chemical and structural characterizations: FTIR and XRD X-ray diffraction (XRD) analyses were performed with a CubiXXRD Philips diffractometer using a Cu-Ka radiation with a wavelength l of 1.54 Å. Fourier transform infrared spectroscopy (Nicolet 380 model FTIR Spectrometer in the wavenumber region 400 to 4000 cm1) was used to characterize the chemical composition of the polymer. 2.3. Morphological analyses: FESEM and AFM The surface morphology of the material was investigated by examining the sample using an Ultra-Zeiss Field Emission Scanning Electron Microscope (FESEM). Representative SEM micrographs of the film morphology were studied as a function of the sublimation and the pyrolysis temperatures. Atomic force microscopy (AFM) measurements were performed

Tetrachloro-di-para-xylylene

3

Parylene-D

dichloro-p-xylylene

Cl CH2

CH2

Pyrolysis

Cl

Cl

CH2

Cl CH2

CH2 Cl

CH2 Cl

Cl

CH2

CH2 Cl

Monomer

Dimer

Deposition

n

Polymer

Fig. 2. Process of chemical vapor deposition of parylene-D using Gorham method.

160°C 150°C

Absorbance (a.u)

Absorbance (a.u)

160°C

140°C

130°C

150°C

140°C

130°C

120°C

120°C

(a) 4000

3500

3000

2500

Wavenumbers (cm-1)

2000

(b) 1800 1600 1400 1200 1000

800

Wavenumbers (cm-1)

600

400

Fig. 3. FTIR spectra of parylene-D versus sublimation (Tsub) temperature in the wavenumber ranges (a) from 4000 to 1800 cm1 and (b) from 1800 to 400 cm1.

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XC ð%Þ ¼ ðA=A0 Þ  100

Fig. 4. XRD patterns of as-deposited parylene-D for various sublimation temperatures (Tsub).

are independent of the sublimation temperature and the material kept the same chemical composition. However, the decreasing of the peaks intensity and their broadening are associated to the increasing of the disorder materials and changes of the crystallinity. On the other hand, EDX analysis (not shown here) was used: i) to detect the main chemical elements which constitute the material in relation to the sublimation temperatures and ii) to look deeply if the deposit films contain some impurities or adverse chemical species. The EDX spectra reveal the same chemical elements and the absence of any oxygen or adverse species in the material for tye different sublimation temperatures. This result is consistent with its chemical structure (Fig. 2) and is in good agreement with results obtained by FTIR analysis (Fig. 3). The obtained parylene-D coating film is pure and presents a good chemical quality. In order to characterize the structure and to investigate the difference in the extents of chain stacking in the present films with various deposited sublimation temperatures, the-angle X-ray diffraction measurements were performed at room temperature. Fig. 4 shows the XRD patterns of parylene-D for various sublimation temperatures [18]. The diffraction peaks at about 2q ¼ 12 and an exceedingly broad peak at 2q ¼ 22 correspond to the (020) and (110) diffraction planes of the crystal monoclinic unit cell which corresponds to the a-phase are observed [18]. They are treated as the main signature of the crystalline phase's structure of paryleneD deposited in that CVD conditions. All these results mean that parylene-D exhibits a semi-crystalline structure independently to the sublimation temperature. It is also shown that there is no effect of sublimation temperature on the physical structure of parylene-D. XRD results show that increasing the sublimation temperature of the deposit parylene-D gives rise to its amorphization (Fig. 4). The physical structure remains unchanged, except that peak intensities varied showing a difference in terms of the crystallization percentage (Xc) and size of crystalline lamellae (L). The enhancement of sharpness of the diffraction peaks intensity with decreasing sublimation temperature can be ascribed to the increase of polymer chains ordering leading to the higher crystallinity. The average crystallite size (L) and degree of crystallinity Xc for various sublimation temperatures are estimated through the XRD patterns using the Scherrer equations [24]:

L ¼ K l= ðb cosqÞ

(2)

(3)

where b is the full width at half maximum (FWHM) of the peak (in radian). l is the X-ray wavelength (l ¼ 1.5406 Å in our case for CuKa radiation), q is Bragg angle in degree, K is the shape factor (Known as Scherrer constant), A is the area of the diffraction peaks 0 (area corresponds only to the crystalline peaks), and A is the total area under the diffractogram (area of crystalline peaks and amorphous broad peaks). The shape factor k depends on the miller indices of the reflection plane and normally its value is 0.89. Thermoplastic polymer is a disordered system and the diffraction peak in such materials arises due to the arrangement of polymer chains. We have approximately calculated the crystallite size L, average inter-chain distance (dspacing) and degree of crystallinity Xc from the diffraction peak. The principal diffraction peak at 2q ¼ 12 is considered for calculating the average crystallite size and interchain distance. The calculated values of crystallite size, interchain separation, and degree of crystallinity are presented in Table 1. The calculated values of percentage of crystallinity show the increase of crystallinity with decreasing sublimation temperature, which could be attributed to the increase of systematic alignment of the polymer chains. On the other hand, the crystallinity percent shows a transition temperature in the vicinity of 140  C suggesting two distinct growing behaviors. The material decreases from 54% at TSub ¼ 120  C to 18% at TSub ¼ 160  C (Table 1). Below TSub ¼ 140  C the larger crystallites size gives more ordered molecular orientation probably due to the predominant of the nucleation and growing mechanisms, whereas above TSub ¼ 140  C the smaller crystallites size favorites a disordered molecular structure and an increase in the amorphous phase. The crystallization of the polymeric chain becomes more difficult if the polymer is formed rapidly. It is known that the Parylene process manifested in 3 steps, initialization, addition (or propagation) and finally polymerization (or polycondensation) [19,25]. At high sublimation temperature, the monomer spaces do not find the time to be added and to be rearranged together in order to form a certain order (crystalline phase). We think that in this stage, the addition step is inhibited in comparison to the initialization during the pyrolysis process. Fortin et al. [25] showed that the chemisorption model of Parylene process under such conditions fails to predict the correct deposition rate since the initiation reaction becomes important. They show also that the chemisorption model does not explicitly include any parameter for diffusion of the monomer into the bulk of the film. It means that the monomers are deposit on the surface and they don't have time to diffuse on the bulk in order to form a dense film. In this case the disordered structure is more favorited and an amorphous structure is obtained for the film. This scenario can explain the structure results obtained when the sublimation temperature goes to increases. Surendran et al. [26] shows that the d-spacings in Parylene-C are independent of sublimation rate of dimer. In the case of parylene-D, d-spacing increases slightly as a function of sublimation temperatures (Table 1). This is due to the decreasing of the crystallinity percent and the crystallinity size with increasing the sublimation temperature. We show that the sublimation temperatures has a very pronounced effect on the physical properties (crystallinity, dspacing, …) of this polymer. Nearly amorphous parylene-D result is obtained when the sublimation temperatures of the dimer is height. However, more crystalline material is obtained when the Tsub of the dimer is decreased. Further bulk properties of the parylene-D such as its glass transition temperature (Tg) may be significantly affected by sublimation temperatures since it is very sensitive to the crystallinity

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Table 1 The main structure properties of semicrystalline parylene-D at various sublimation temperatures (Tsub). 2qmax [ ]

dspacing [Å]

FWHM [ ]

Crystallite size [Å]

Degree of crystallinity [%]

120 130 140 150 160

12.52 12.49 12.45 12.41 12.34

7.06 7.08 7.10 7.12 7.16

1.82 1.94 2.10 2.30 2.66

43.91 41.19 38.05 34.74 30.04

54 52 43 26 18

14

9.0

Linear fits

12

8.8 8.6

10 8.4 8

8.2 8.0

6

7.8

4

Film thickness (μm)

Time of deposition process (hours)

TSub [ C]

7.6 2

120

130

140

150

160

Sublimation temperature (°C) Fig. 5. DSC spectra of parylene-D versus sublimation temperature (Tsub) in the temperature range from 60 to 160  C.

Fig. 6. Film thickness and time of the deposition process of parylene-D versus sublimation temperatures.

change. Fig. 5 shows the DSC analysis for different sublimation temperature. The experimental Tg values are given in Table 2. One can see that the Tg value is sensitive to the TSub variation. After increasing the sublimation temperature the glass transition temperature decreases. This means that the ordered polymer chains decrease and consequently the structure state of PPXD favorites its amorphous character at higher Tsub. This result is very consistent with the obtained ones of the XRD analyses (Fig. 4). Fig. 6 depicts the variation of film thickness and time deposition of parylene-D as a function of sublimation temperature. It is observed that the overall thickness uniformity obtained after deposition are good and films are transparent in the exception of Tsub ¼ 160  C which presents a partially opaque film. In relation to the sublimation temperature, the deposit parylene-D presents a gradual increase in thickness towards the lower sublimation temperature. For all sublimation temperatures, we find a linear film growth with thickness. In the case of parylene-D, a lesser dimer weight results in a thicker film compared to Parylene-C and N [18,19]. The parylene-D monomer is a heavier molecule, therefore, condenses more readily on the substrate chamber than the monomer of Parylene-C and N [18,19] and a less gaseous monomer can be condensed on the cold trap maintained at the liquid nitrogen temperature. As can be seen also from Fig. 6, the dependence of the deposition time process versus the sublimation temperatures exhibits a threshold temperature at 140  C. As observed by the XRD analysis this temperature reflects the change of the deposition

regime from the lower sublimation temperatures to the higher ones. The deposition rates reported in this work were calculated by dividing the actual final film thickness by the deposition time. It shows linear sublimation temperatures dependence. From this dependence, the sublimation temperature at which the parylene-D cannot be deposit is estimated. This temperature is about 105  C. The deposition rate of parylene-D increases with an increase in the sublimation rate of the dimer as observed for parylene-N and parylene-C [28]. However, the decrease in crystallinity is similar to parylene-N and in contrast to parylene-C [26,27]. This behavior can be in relation to the polar character of the monomer space since parylene-D and parylene-N are nonpolar and parylene-C is a polar material. The observed decreasing of crystallinity when the Tsub increases by using the same dimer mass can be explained as follow: the higher sublimation temperatures increase the sublimation rates and may increase the kinetic energy of the deposition molecules at the substrate surface. These molecules with a higher thermal energy of sublimation give a higher deposition rate. Therefore, it may be concluded from literature that the higher deposition rates are due to the increased condensation of the monomer on the surface during growth [28] and an increase in the sublimation temperatures results in an increase in the partial pressure of the monomers, which should cause an increase in the deposition rate [29]. V. Simkovic [30] assigned that the sublimation temperatures in parylene-C contributed 25% with increasing sublimation rates leading to higher deposition rates. This scenario does not provide the time necessary for the molecules to be arranged in an ordered crystal structure, consequently, a more amorphous state is favorited by the molecular orientations. In the same time, a more monomer molecules are collected to the cold trap due to the higher densities of monomer coming from the pyrolysis compartment to the

Table 2 Glass transition temperature versus sublimation temperatures of parylene-D deduced by DSC analyses. Tsub ( C) Tg ( C)

120 C 109

140 C 105.5

160 C 103

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ln( Growth Rate, Å/sec)

2.2

Table 3 RMS roughness of parylene-D for various sublimation temperatures.

Data Linear fit

2.0

TSub RMS [nm ]

120  C 4.295

130  C 4.442

140  C 4.686

150  C 5.086

160  C 5.439

1.8 1.6 1.4 1.2 1.0 0.8 0.6

2.30

2.35

2.40

2.45

2.50

2.55

1000 /TSub (1/K) Fig. 7. The Growth rate versus sublimation temperatures.

deposition chamber. Thus some amounts of monomers are lost implying a film thicknesses less than those obtained where the sublimation temperatures is low (case of lower growth rate). Fig. 7 shows the dependence of the growth rate r with the inverse of the sublimation temperature. The growth rate of paryleneD is rather low and can be described by the empirical linear relation:

Lnr ¼ 14:016  5:19  1000=TSub :

(4)

Where r is the growth rate of parylene-D and Tsub. is the sublimation temperature of the dimer. This relation shows that the

natural logarithm of the growth rate is linearly related to the reciprocal of the sublimation temperatures for a given dimer and a constant substrate temperature. The same behavior's shown by Kramer et al. in parylene-N and C [28]. The present CVD experimental parameters have shown that appropriate sublimation temperatures can yield a controlled growth of the layer thickness ranging from tens of nanometers up to a few tens of micrometers with a good control of the amorphous and the crystalline amounts. Fig. 8 shows the topographic surface images with 5 mm X 5 mm scan area of parylene-D at various sublimation temperatures. AFM images of all samples were captured to more understand the surface morphology of parylene-D. All the active layers of PPXD showed good film formation with smooth and uniform surfaces and absence of surface impurities like aggregations and clusters. The calculated surface roughness values in function of the sublimation temperature are given in Table 3. It shows the increase of the surface roughness with the Tsub. The roughness is larger at high sublimation temperatures (Tsub 160  C) with a value remains good suggesting a promising surface property in the case of polymer materials. Moreover, AFM images reveal the existence of a globular structure of various particle distributions meaning that the growing mechanism is sensible to the thermal energy of the sublimation. As deduced from the XRD analysis, when the sublimation temperatures is above 140  C, the materials present a lower crystallinity ratio with lower crystalline sizes. This phenomenon reflects a higher roughness in comparison to the phenomenon obtained for sublimation temperatures below 140  C. As observed on the AFM images, parylene-D at lower sublimation temperatures

Fig. 8. AFM topographic images of parylene-D for various Tsub (a ¼ 120  C, b ¼ 140  C, and c ¼ 160  C).

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Fig. 9. Field emission scanning electron microscopy (FESEM) morphology of parylene-D. for various sublimation temperatures (a ¼ 120  C, b ¼ 140  C, and c ¼ 160  C).

defined layer formation, sublimation temperatures between 120 and 140  C are preferred for a fixed pyrolysis temperature at 650  C, as shown in Fig. 10. It means that an increase of sublimation temperatures provide an increase of the deposition pressure which leads to a partially non-transparent films giving rise to inferior mechanical and optical proprieties [32]. We showed that a decrease in the surface roughness can be achieved by reducing the Tsub and the deposition rate. These experimental conditions giving rise to dense and transparent parylene-D films. 3.2. Effect of pyrolysis temperatures In order to more understanding the effect of the deposition parameters on the molecular structure and properties of paryleneD, the impact of the pyrolysis temperature has been also studied. For such study, sublimation and deposition temperatures were

650°C

Absorbance (a.u)

is denser than that obtained at higher sublimation temperatures, showing a less surface roughness as well as the peak-to-valley. In order to more refine the surface structure properties of parylene-D FESEM analysis is required for this study. Fig. 9 shows FESEM images of parylene-D films with various sublimation temperatures of 120  C (a), 140  C (b), and 160  C (c), respectively. As the sublimation temperature increases from 120  C to 160  C, the morphology of materials changes witch results from an increase of globules density on the surface. This result is in good adequacy with the roughness obtained by AFM analysis. The reason given for the existence of these globules can be explain as follows: a set of molecules of the dimer does not polymerize and are not dissipated via the cold trap because of their relatively high molecular weight in comparison to the monomer gas and/or due to the monomers which do not polymerize due to the diffusion rate which is lower than the speed of the initiation monomers to the surface of the substrate. This leaves time for the monomers to aggregate and condense on the surface without diffusing to the bulk. Beach [31] showed that these globules are derived from the condensation of thousands of molecules that strike the growth surface of the parylene film and stay on the surface without reacting with the chains bulk. The same observations are obtained recently by kahouli et al. [18]. The increasing of the globules density at the surface of the film with the sublimation temperature can be related also to the residence time of the gaseous dimer cycle at the pyrolysis chamber. As the sublimation temperature increases the flow of the dimer gaseous molecules increases and they reach rapidly the deposition temperature. In this case some dimer gaseous molecules passed rapidly or swept with the monomer molecules flow because they don't have the necessary residence time in the pyrolysis chamber to be converted into monomer molecules. Thus, globules dimer molecules are formed at the surface of the parylene-D films. It means that the globules density increases as the residence time of the dimer gaseous molecules decreases in the pyrolysis chamber due to the increasing of molecules flow from the sublimation chamber. The topology of sample at TSub ¼ 120  C is transparent but an increase in sublimation temperatures results in polymers that becomes slightly opaque (TSub ¼ 160  C). In order to achieve a well-

670°C

690°C 4000 3500 3000 2500 2000 1500 1000

Wavenumbers (cm-1)

500

Fig. 10. FTIR spectra of parylene-D versus pyrolysis temperature in the wavenumber ranges from 4000 to 400 cm1.

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M. Mokni et al. / Materials Chemistry and Physics xxx (2016) 1e14 Table 5 RMS roughness of parylene-D for various pyrolysis temperatures. Tpyr. ( C) RMS [nm]

Fig. 11. XRD patterns of as-deposited parylene-D for various pyrolysis temperatures.

maintained constant (Tsub ¼ 140  C, Tdep ¼ 23  C, and p ¼ 0.1 torr), while the pyrolysis temperature has been changed in the range of (650e690  C). Fig. 10 shows the FTIR analysis of parylene-D for different pyrolysis temperature from 650 to 690  C. As observed in the case of sublimation temperature effect, the variation of pyrolysis temperature in some temperature range slightly affects the chemical structure of the material. It is inferred that nor shift of parylene peaks and neither appearance of new peaks are found in the chemical structure of parylene-D. However, as the pyrolysis temperature increases the peak intensity decreases and the broadening peaks increases. This result shows the deterioration of the molecular order and the system favorites the amorphous structure. Thus, parylene-D becomes more amorphous as the pyrolysis temperature is increased. Fig. 11 shows the XRD diffraction peaks of parylene-D films for different pyrolysis temperatures. It is clearly seen from the XRD pattern that the intensity of the diffraction peak increases as the temperature of pyrolysis decreases. As the pyrolysis temperatures increases from Tpyr ¼ 650  C to Tpyr ¼ 690  C the thermal and kinetic energies of monomers increase that means more gaseous molecules flux passing through the pyrolysis chamber towards the deposition chamber. Thus these molecules preserved less time to arrange and to polymerize giving lower crystallinity to the material (Table 4). Table 4 illustrates the main structure properties of semicrystalline parylene-D as a function of successive pyrolysis temperatures. It inferred that the crystallinity content and the crystalline size decrease by increasing the pyrolysis temperatures without changing in the crystallographic structure. The same behavior is observed in the case of the sublimation temperatures effect as explained in the previous part. The roughness of the parylene-D is also fulfilled in this part with varying the pyrolysis temperatures. As given in Table 5, the roughness value of paryleneD increases of about 14% with increasing the pyrolysis temperatures from 650 to 690  C. The increase of the amorphous phase of

650  C 4.686

670  C 4.840

690  C 5.329

parylene-D is accompanied by an increase in the surface roughness. Morphology of parylene-D in various pyrolysis temperatures was also studied by FESEM as shown in Fig. 12. It is clearly seen that the film is relatively smooth and homogenous except for the presence of some 'globules' on the surface of the deposited parylene-D. These globules are found in all the FESEM figures but with more elongated in the case of high pyrolysis temperature of 690  C (Fig. 12-c). The size of the globules increases from 1.3 mm for Tpyr. ¼ 650  C to 13.7 mm for Tpyr. ¼ 690  C. Temperature of pyrolysis favorites growing of globules compared to the sublimation temperature. The change of deposition parameters of parylene-D have shown a significant effect in the surface morphology and the bulk crystallinity properties but did not affect noticeably the chemical structure. In addition, the deposition rate of parylene-D is found to increase as the pyrolysis temperature rises, which is the same behavior observed in the case of parylene-C [30].

3.3. Dielectric analysis The relationship process/structure/properties proposed in this work is completed by studying the dielectric measurement for different parameter processes. Fig. 13(aec) show the frequency dependence of dielectric properties of parylene-D for various sublimation temperatures from 120 to 160  C. In the glassy state (Tdielectric measurement ¼ 20  C ¼ Tg e [80e90  C]) of the material, the dielectric constant ε' (Fig. 13a) and the dissipation factor tan d (Fig. 13b) of parylene-D decrease as the sublimation temperature decreases, while the electrical conductivity is very slightly affected. As deduced from the XRD analysis (Fig. 4), the parylene-D becomes more crystalline as the sublimation temperature decreases. The crystalline phase may causes a confinement effect on the amorphous phase of parylene-D and blocks the charges transfers between molecules chains which are more mobile in the amorphous phase. Since the electrical conductivity is due to mobile charges, it should obtain a decrease in the electrical conductivity with decreasing the sublimation temperature. This behavior is in good agreement with our experimental results but it is not very significant because the dielectric measurements were made at the glassy state of the materials where the mobility chains are frozen and the charges are not activated by the chains mobility in the glassy state. On the other hand, since the dielectric constant is proportional to the polarisation content (dipoles orientation) present in the material, the experimental results show that the dipoles (CeCl dipoles in parylene-D) are more easily oriented with the applied electric field in the amorphous phase. Consequently, the dielectric constant increases as the amorphous phase of parylene-D increases (i.e. as the sublimation temperature increases). In addition since the dissipation factor is more sensitive to the relaxation phenomena and very slightly to the dc-conductivity especially in the case of good insulator materials, tan d increases as the polarisation content

Table 4 The main structure properties of semicrystalline parylene-D at various pyrolysis temperatures. Tpyr [ C]

2qmax [ ]

dspacing [Å]

FWHM [ ]

Crystallite size [A ]

Degree of crystallinity [%]

650 670 690

12.45 12.22 12.15

7.06 7.23 7.27

2.1 2.25 2.5

38.05 35.51 31.96

43 27 19

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9

Fig. 12. Field emission scanning electron microscopy (FESEM) of parylene-D for various pyrolysis temperatures (a ¼ 650  C, b ¼ 670  C, and c ¼ 690  C).

Fig. 13. Frequency dependence of (a) dielectric constant, (b) dissipation factor, and (c) the electrical conductivity of parylene-D versus different sublimation temperature. Dielectric measurements were made at 20  C: Glass state (ordered system).

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increases. Thus tan d, increases as the amorphous phase increases and consequently as the sublimation temperature increases. Let us introduce now the dielectric behavior of parylene-D versus sublimation temperature in the liquid phase (disordered system). Fig. 14(aec) show frequency dependence of (a) dielectric constant, (b) dissipation factor, and (c) the electrical conductivity of parylene-D versus different sublimation temperatures. Dielectric measurements were made at 300  C: liquid state. It is clearly seen that all the dielectric parameters (ε', tan d and s') are significant affected in the liquid phase than that in the glassy state, especially at low frequencies range, where the dielectric response is governed by the cooperative molecules chains mobility (cooperative motion of dipoles). However, all dielectric parameters are slightly affected at high frequencies range since this frequency region is more sensitive to the local scale motion of dipoles. As obtained in the glassy state, the dielectric parameters are more affected when the amorphous phase is increases. The same results are obtained when the system is in the liquid phase but the effect of the sublimation temperature in this case is more pronounced since the system is nearly entirely amorphous. For more clear understanding the effect process/structure/properties relationship, a summary curves are given in Fig. 15(a) and (b) for both glassy and liquid state. As shown, the structure (crystallinity percentage) presents a negative exponential behavior and the other dielectric parameters show a positive exponential dependence with the sublimation temperatures. In the liquid phase of parylene-D, the dielectric constant is enhanced more than 100% at low frequency (@0.13 Hz) by varying the sublimation temperature from 120 to 160  C. On the same time, the dissipation factor increases sharply at low frequency but remain

less than one decade by varying the sublimation temperature from 120 to 160  C. However, all dielectric properties are more sensitive with pyrolysis temperature (Fig. 16(a) and (b)), especially when the parylene-D is treated in the liquid phase. It indicates that the dielectric constant is enhanced more than for times by passing from the sublimation temperature of 650e690  C. In addition the dielectric losses and the electrical conductivity are also present a noticeably enhancement by increasing the sublimation temperature. This behavior can be related to the increase of the polarisation character of the materials and the mobility of charges in the liquid phase. The high electrical conductivity for the high sublimation temperature at liquid phase can be related to the enhancement of the contribution of the surface globules (non-evaporated dimer). Since the dimer molecules of parylene-D started the sublimation for temperature more than 105  C as estimated in the previous part of this study (Fig. 6), the dielectric measurement at liquid phase (300  C) can be influenced by the presence of the globules particles. These globules can be sublimated to gaseous molecules or present as impurities and after can move to accumulate at the interface electrode/parylene-D gives rise to an interfacial polarisation effect. In dielectric spectroscopy of polymers [33,34], the interfacial effect is observed generally at low frequency and high temperature. For this reason, the observed feature dielectric of parylene-D at low frequency in the liquid state (300  C) is in part due to the contribution of the globules sublimation. The presence of measured electrical conductivity in a dielectric polymer material (non-perfect insulator material) is due to the presence of some defects. The energy levels of these defects can be deduced from curves of the dc conductivity in Arrhenius plots. Fig. 17(a) and (b) show the

Fig. 14. Frequency dependence of (a) dielectric constant, (b) dissipation factor, and (c) the electrical conductivity of parylene-D versus different sublimation temperature. Dielectric measurements were made at 300  C: liquid state (disordered system).

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11

Fig. 15. Process/structure/properties relationship of parylene-D in (a) glassy and (b) liquid state for different sublimation temperatures. All the dielectric parameters of this figure are given for the frequency of 0.13 Hz.

Fig. 16. Process/structure/properties relationship of parylene-D in (a) glassy and (b) liquid state for different pyrolysis temperatures. All the dielectric parameters of this figure are given for the frequency of 0.13 Hz.

Fig. 17. Arrhenius plots of dc-conductivity for different (a) sublimation and (b) pyrolysis temperatures.

Arrhenius plots of the dc-conductivity versus the sublimation and the pyrolysis temperature, respectively. The energy values are in the same order for both the sublimation and the pyrolysis temperature with some slightly higher values in the case of the

pyrolysis temperature. This means that the conduction mechanism is took place in the bulk of the materials and the slightly difference between both energies levels my due to the additional effect of the species coming from the sublimation of the surface globules. For

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the same charge density, a higher electrical conductivity is related to a lower energy level. In our case the electrical conductivity is higher for the lower energy level in the case of the Tpyr. ¼ 690  C. The energy level of dc-conductivity is decreases by decreasing the sublimation and the pyrolysis temperature. As observed by XRD (Fig. 4) for sublimation temperature effect and Fig. 11 for pyrolysis temperature), the crystalline phase increased as the sublimation and the pyrolysis temperature decreased. This means that when the crystalline phase increases, the charge cannot move easily and becomes more confined in the amorphous phase. Consequently, this structure can present a higher energy level which is the case of the activation energies of Tsub. ¼ 120  C and Tpyr. ¼ 650  C. In semicrystalline polymer materials, the interfacial amorphous/ crystal presents an effect when the polymer is subjected at a temperature more than its glass transition temperature (Tg) [33,34]. As deduced by DSC analysis (Fig. 5), the Tg of parylene-D is in the range 100e110  C. On the other hand, the dielectric measurements were made at 300  C (~Tg þ 200  C). This suggests that the dielectric response at high temperature and low frequency can be generated by the interfacial polarisation called also the Maxwell-WagnerSillars polarisation. This kind of polarisation is associated to the interfacial polarisation mechanism occurred at the crystal/amorphous interfaces. The most suitable electrical formalism used to overcome the interfacial polarisation in polymer materials is the electrical modulus formalism [33,34] which given by the following equation.


00 M * ¼ 1 ε* ¼ M0 þ iM

(5)

Where

.  00 2 00 00 2 2 2 M 0 ¼ ε0 =ðε0 Þ þ ε and M ¼ ε ðε0 Þ þ ðε0 Þ

(6)

M' and M00 are the real and the imaginary part of the complex electric modulus, respectively. Fig. 18 (a) and (b) show the frequency dependence of the imaginary part, M00 , of the complex electric modulus of parylene-D for different sublimation and pyrolysis temperatures, respectively. Well-defined asymmetric peaks are observed and are well fitted at low frequency by the Bergmann equation [35]. 00

00

M ðuÞ ¼

ð1  bKWW Þ þ



Mmax 

bKWW 1þbKWW

bKWW fmax þ f



f

bKWW 

(10)

fmax

where M00 max is the maximum peak of the imaginary part of the

Fig. 19. Interfacial polarisation of two parylene-D materials with different structure morphology.

electric modulus and fmax is the corresponding peak frequency.

bKWW is the Kohlrausch exponent with the condition 0 < bKWW  1. The calculated theoretical bKWW is found to be 0.82 independently to the sublimation and the pyrolysis temperature values. However, the maximum relaxation frequency fmax is sublimation and pyrolysis temperature dependence as observed in the inset of Fig. 18(a) and (b). It inferred that the maximum relaxation frequency is less dependence on the sublimation temperature (Fig. 18(a)) than the maximum relaxation frequency in the case of the pyrolysis temperature (Fig. 18(b)). The relaxation frequency moves towards the lower frequencies range as the sublimation and the pyrolysis temperatures decreases. This implies that the inhomogeneity of the material is increased. Since the degree of crystallinity and size of crystallites of parylene-D increases when the temperature of sublimation and pyrolysis decreases, the number of interface increases, as well as the inhomogeneity of the material. We deduce that the inhomogeneity of the parylene-D is more dependent on the variation of the pyrolysis temperature than the variation of the sublimation temperature. Fig. 19 shows a qualitative comparative example between two different systems with different inhomogeneity percentage basing on the imaginary part of the electric modulus (M00 ). In this figure we present the comparative between M00 responses observed for the pyrolysis temperatures of 650 and 670  C. From this figure, it is possible to have the same

Fig. 18. Interfacial properties of parylene-D versus different (a) sublimation and (b) pyrolysis temperatures.

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contribution of the interfacial polarisation for two systems with different morphology. It should make the parylene-D deposited at pyrolysis temperature of 670  C at a measurement temperature less than the measurement temperature of parylene-D deposited at pyrolysis temperature of 650  C as clearly illustrated in Fig. 19. In this case the electrical measurements were made at 270  C for Tpyr. of 670 to be identical to the electrical measurement made at 300  C for Tpyr. of 650. This procedure can decrease or slackened the melting of some crystallite and consequently increases the crystallinity portion of the material. However, increasing the temperature helps the destruction of the ordered region and the migration of the system from a crystalline phase to an amorphous phase.

[4] [5]

[6] [7]

[8]

[9]

4. Conclusion Using a CVD tool adapted to achieve a parylene-D with controlled properties, we explore the influence of large parameter process over parylene-D growth. Sublimation temperatures from 120 to 160  C and pyrolysis temperatures from 650 to 690  C at 0.1 torr and 30  C ± 1 are evaluated. This study gives the keys to control the surface morphology and the bulk structure properties of parylene-D by modifying the molecular kinetics and the thermal process of the gaseous molecules (monomers). Nearly amorphous parylene-D is obtained. Experimentally this can be achieved by increasing the deposition rate, increasing the sublimation temperatures of organic molecules, and increasing the pyrolysis temperatures. Crystallinity form is favorited when both the sublimation and the pyrolysis temperatures decreased individually. Owning to these variations, XRD analysis show that no polymorphism is observed by increasing the sublimation and pyrolysis temperatures and the structure remained unchanged. However, the degree of crystallinity decreases and the topography changes with increasing the sublimation and pyrolysis temperatures. FESEM studies reveal the presence of globules on the surface of films varying from 1.3 mm for Tpyr. ¼ 650  C to 13.7 mm for Tpyr. ¼ 690  C. This leads to an increase of 14% in the surface roughness of the parylene-D with a partial opaque film obtained at (Tsub ¼ 140  C, Tpyr. ¼ 690  C, Tdep ¼ 23  C, and p ¼ 0.1 torr). In this study, we clearly show that these two strategies are successful and can be used to define a process window for the growth of high-quality parylene-D films. Using these conditions we reproducibly fabricate state-of-the-art of a new dielectric material which adopting both dielectric properties of the non-polar parylene-N and the polar parylene-C due to its specific chemical structure. This allows to expand the applications domains of this material as a coating dielectric film. The dielectric analyses show that the process parameters of parylene-D have a clear influence on the dielectric constant, the dielectric losses and the electrical conductivity. This influence is more pronounced in the case of modifying the pyrolysis temperature and in the liquid state of the material. The inhomogeneity of the materials was study basing on the electrical modulus.

[10]

[11] [12]

[13]

[14]

[15]

[16]

[17] [18]

[19]

[20]

[21]

[22]

[23]

[24] [25] [26]

Acknowledgments

[27]


gion Rho ^ ne-Alpes and Universite
Grenoble Alpes” in the The “Re framework of the International Doctoral thesis is acknowledged.

[28]

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