Plasma Polymerization of Hexamethyldisiloxane

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Plasma polymerization of hexamethyldisiloxane: Investigation of the effect of carriergas related to the lm properties

C. Chaiwong a,b,⁎, P. Rachtanapun c, S. Sarapirom a, D. Boonyawan a,b

a Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai, Thailandb ThEP Centre, Commission of Higher Education, 328 Sri Ayuddhaya Road, Bangkok, Thailandc Department of Packaging Technology, Faculty of Agro-Industry, Chiang Mai University, Chiang Mai, Thailand

a b s t r a c ta r t i c l e i n f o

Available online 31 August 2012

Keywords:

Plasma polymerization

Hexamethyldisiloxane

RF discharge

Compressive stress

In this work we present the inuence of carrier gases in the deposition of low-pressure discharge plasma of hexamethyldisiloxane (HMDSO). Plasma polymerized HMDSO lms were deposited with an inductively-coupled

discharge reactor using Ar and O2 as carrier gases. The lms deposited in Ar contained polymeric structure in the

form ofSiOxCyHz andcould signicantly improve thebarrier to water vapor of poly(lactic acid) (PLA).The SiOx-like

structure of HMDSO lmswas obtained when using O2 as the carrier gas. However, thelmssupported some state

of residual stress leading tolmfailuresand a signicant loss of barrier performanceof PLA. Theformationof organ-

ic and inorganic contents in the lms was conrmed by X-ray photoelectron spectroscopy (XPS). The discharge

power hadan effect on the topography of thelms. Rough surface with coarse texture was obtained when thepro-

cess was done in Ar at high discharge powers. On the other hand, the deposition process in O2 induced smoother

surface of plasma‐polymerized lms.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

An increasing interest in organo-silicon thin lms has been ob-

served owing to their potential applications in, for example, protec-

tive coatings [1], gas barrier coatings [2], and chemical sensors [3].

These lms can be prepared via wet chemical processes such as hy-

drolysis and ring-opening polymerization of dimethyltrichlorosilane

and hexamethylcyclotrisiloxane [4]. Wet chemical processes need

solvents or catalysts and are multiple steps. The lms obtained from

these processes are unfortunately thick and non-uniform; therefore

their applications are restricted. Plasma polymerization is a single-

step process effective to deposit thin lms with high degree of

cross-linking and pinhole-free on various substrates. Chemical solvents

and catalysts are not necessary. In the process of plasma polymerization

induced by a plasma-enhanced chemical vapor deposition (PECVD),

polymerlms are deposited through reactions between reactive species

produced by the plasma discharge and the substrates. A number of

plasma sources and monomers have been employed to fabricate

organo-siliconlms. Thecharacteristics of thelm arehighly dependent

on the process conditions [5–7]. Hexamethyldisiloxane (HMDSO;

(CH3)3\Si\O\Si\(CH3)3) is one of the monomers being employed

to produce polymer thin lms via PECVD because it is non-toxic, com-

mercially available, and has a high vapor pressure at room temperature.

Usually, the deposition of HMDSO is carried out in oxygen. Other gases,

such as argon and helium, may be used to stabilize the discharge. Theproperties of the lm could vary from polymeric to SiOx-like structures

depending on the process conditions[8–10]. In this work, we investigat-

ed theeffect of differentkindsof carrier gas, i.e. argon andoxygen, on the

properties of HMDSO lms. The properties of thelms were analyzed in

terms of water vapor barrier performance, surface topography, and

chemical compositions.

2. Experimental details

2.1. Experimental arrangement

Plasma polymerized HMDSO lms were prepared in a low-pressure

radio frequency (RF) plasma reactor at the frequency of 13.56 MHz. The

detail of the reactor was described in our previous work [11]. Commer-

cially available HMDSO solution was obtained from Sigma Aldrich Inc.

(St. Louis,USA) andused as received. Aask containing HMDSO solution

was connected to the inlet port of the plasma reactor through a mass

ow controller. Ar and O2 were used to carry the vapor of HMDSO into

the plasma reactor. The ow rate of the carrier gas was kept at 1 l/min

whereas that of HMDSO was adjusted to obtain the pressure of

100 mTorr. The lm depositions were done with the RF powers of

10 W, 30 W, 40 W, 50 W, and 60 W. The deposition time was 10 min.

Si wafers and poly(lactic acid) (PLA) lm with a nominal thickness of

30 μ m were used as substrates. The Si substrate was used for measuring

the lm thickness. Before the lm deposition, the substrates had been

sputtered with Ar plasma for 10 s. The Ar plasma was obtained at a

Surface & Coatings Technology 229 (2013) 12–17

⁎ Corresponding author at: Department of Physics and Materials Science, Faculty

of Science, Chiang Mai University, Chiang Mai, Thailand. Tel.: +66 53 943 379;

fax: +66 53 222776.

E-mail address: [email protected] (C. Chaiwong).

0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.surfcoat.2012.08.058

Contents lists available at SciVerse ScienceDirect

Surface & Coatings Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s u r f c o a t

http://dx.doi.org/10.1016/j.surfcoat.2012.08.058http://dx.doi.org/10.1016/j.surfcoat.2012.08.058http://dx.doi.org/10.1016/j.surfcoat.2012.08.058mailto:[email protected]://dx.doi.org/10.1016/j.surfcoat.2012.08.058http://www.sciencedirect.com/science/journal/02578972http://www.sciencedirect.com/science/journal/02578972http://dx.doi.org/10.1016/j.surfcoat.2012.08.058mailto:[email protected]://dx.doi.org/10.1016/j.surfcoat.2012.08.058


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pressure of 30 mTorr with the RF power of 30 W. Thesubstrates were atoating potential during the process.

Species in the plasmas were analyzed using optical emission spec-

troscopy (OES). The spectrometer employed in this work was a S2000

ber optics spectrometer (Ocean Optics Inc., USA) with a wavelength

range from 200 nm to 800 nm. The resolution was 0.3 nm.

2.2. Characterization techniques

Water vapor transmission rates (WVTR) of the HMDSO lms on PLA

substrates were measured with a Permatran™ C3/31 (Modern Controls

Inc., MN, USA) according to ASTM E96-93 [12]. The testing temperature

was 25 °C with 100% relative humidity. Three samples were measured

in each deposition condition.

Scanning electron microscopy (SEM) was used to examine the

surface of the samples. The SEM employed in this work was a Jeol

JSM-5910LV (Jeol, Japan).

Atomic force microscopy (AFM) was carried out to observe surface

topography of the lms on PLA substrates. The AFM used was a

NanoScope III (Digital Instrument, USA), operated in tapping mode.

The images were collected at a xed scan rate of 1 Hz. The sampling

rate was 256 samples/line.

X-ray photoelectron spectroscopy (XPS) was used to obtain chemi-cally specic information of thelms on PLA. XPS spectra were obtained

with a Kratos Axis DLD Ultra (Kratos Analytical Ltd., UK). Allpeaks in the

spectra were referenced to the signature C1s peak for adventitious

carbon at 284.6 eV.

The thickness of the lms on Si wafers was measured using an

alpha-SE spectroscopic ellipsometer (J.A. Woollam Co. Inc., NE, USA)

in a spectral range of 300 nm–900 nm. The spectrum was tted

using a Si with absorbing lm model.

3. Results and discussion

3.1. Optical emission spectroscopy analyses

Fig. 1 shows the emission spectra of HMDSO plasma discharged at40 W in Ar (Ar/HMDSO) and O2 (O2/HMDSO). The emission line as-

signments are listed in Table 1 [7,13,14]. Methyl group abstraction

is a key step in the fragmentation of HMDSO upon electron impact

in RF discharge due to the low bond energy of Si\C. The dissociation

of methyl groups could be observed through the emission of H, H2,

and C2. The presence of SiO emission lines in the spectral range

216–293 nm indicated the fragmentation of Si\O\Si. We expected

that species like SiO, C, and H played a role in the lm growth. The

emission lines of Ar were dominant in the spectrum of Ar/HMDSO

(Fig. 1(a)). The presence of weak OH and CO lines can be attributed

to the interaction between oxygen atoms in the residual water

vapor in the system and carbon or hydrogen containing species

issued from the dissociation of HMDSO. For O2/HMDSO plasma

(Fig. 1(b)), the emission lines of hydrogen were obvious. It is notablethat the emission of O could be observed at 777 nm in O2/HMDSO

whereas it could not be detected in Ar/HMDSO. The atomic hydrogen

can react with oxygen and produce OH. In O2/HMDSO, a reaction

between R 1–Si–O and R 2–Si–O–Si can be the initiation of the long

chains Si\O\Si and produce SiOx-like structure in the lm [15].

3.2. Water vapor transmission rate measurements

The results of water vapor transmission rate (WVTR) measurements

of HMDSOlms on PLAsubstrates are shown in Fig. 2. HMDSOlms de-

posited in Ar (Ar/HMDSO) at the discharge powers of 10 W, 30 W, and

40 W could substantially decrease the WVTR of PLA. When the dis-

charge power was further increased to 50 W and 60 W, the transmis-

sion rates of the composite lms became higher. The lms deposited

in O2 (O2/HMDSO) deteriorated the barrier to water vapor of PLA. The

transmission rates were higher than that of uncoated PLA.

In plasma polymer deposition process, lm deposition is achieved

by the interactions between the generated active species on the sur-

face of substrates. Process parameters, monomers, and methods of

discharge have a great inuence on the lm properties. In the case

of inductively-coupled discharge, the degree of ionization and disso-

ciation of the plasma becomes higher and leads to higher frequency

I n t e n s i t y ( a . u . )

Wavelength (nm)

SiO H2

Hα

O

C O

H

OH

C O

200 300 400 500 600 700 8000

50

100

150

200

250

300

I n t e n s i t y ( a . u . )

Wavelength (nm)

OH

Ar

Hα

H

Ar

S i O

C O

C 2

H2

Ar

SiO

200 300 400 500 600 700 8000

50

100

150

200

250

300

350

400

450

500a

b

Fig. 1. Optical emission spectra of (a) Ar/HMDSO, and (b) O2/HMDSO obtained at RF

power of 40 W.

Table 1

Emission lines of HMDSO plasma.

Species Wavelength (nm)

SiO 216–293 (main system)

424 (weak system)

SiH 414.0

H2 586–670

427.1

Hα, Hβ, Hγ 656.3, 486.1, 434.0

C2 512.9–516.5

CO 575–800

OH 281–309

O 777

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of electron-neutral collisions when the RF power increased. The plas-

ma density is generally proportional to the power absorbed by the

plasma. For a high supplied RF power, a high plasma density is re-

sponsible for a strong skin effect i.e. most of the RF energy is deposit-

ed withina certain skin depth of the plasmaand reduces the absorbed

power. Some electrons undergo thermal motion and traverse the skin

layer quickly thus the electric eld has short time to interact with

them. The electrons gain less energy from the electric eld. However,

they acquire some energy which can be transferred to the heavy par-

ticles outside the skin layer producing more excited species and rad-

icals due to their shortened mean free path inside the discharge. As

the plasma density is raised, the plasma potential decreases to main-

tain the quasineutrality. This induces an increasing DC-sheath voltage

on the substrate resulting in a greater ion bombarding energy. There-

fore, a competition between sputtering anddeposition in theprocess oc-

curs. Sputtering of the surface may cause a coarse texture with voids in

between which in turn contributes to the higher WVTR in the Ar/

HMDSO lms prepared with the RF of 50 W and 60 W. Addition of oxy-

gen in O2/HMDSO is expected to promote lms with dense SiOx-like

structure that can decrease the gas transmission of the substrates. How-ever, our results are inconsistent with the expectations. In the next sec-

tion, we report the observation of the lm topography to clarify the

factors that affect the WVTR of the lms.

3.3. SEM results

In order to elucidate the lm properties that have an inuence on

the WVTR, SEM was carried out. The results are shown in Fig. 3. All

the samples coated with Ar/HMDSO lms were relatively smooth.

On the contrary, the O2/HMDSO lms were covered with cracks.

Film delamination was also observed. It is obvious that cracking of

HMDSO lm had a profound impact on the WVTR as previously ob-

served. Deformation of the substrate was also found. The fractureand deformation of the lm and the substrate lead to signicant loss

in the barrier of the composite lms [16].

It is known that cracking and delamination of thinlms are the con-

sequences of residual stress. In general, the residual stress results from

the contribution of intrinsic and thermal stresses. In our work, the con-

tribution of thermal stresscan be ruled out since the deposition temper-

ature was relatively at room temperature. Intrinsic stress is caused by

the deposition conditions during the growth and typically involves the

microstructure of a lm. Moreover, the mismatch between mechanical

properties of the lm and the substrate can contribute to the intrinsic

stress [17]. The cracking of the O2/HMDSO lms on PLA indicates that

high level of intrinsic stress was generated. According to Körner et al.

[18] a high compressive stress of −487 MPa could be generated in the

HMDSO lm deposited in oxygen onto polyethylene terephthalate

(PET). It can be implied that SiOx-like structure with high bond energy

wasformed andinduced an increase in theelastic modulus of thedepos-

ited lm. Therefore, a mismatch between mechanical properties was in-

duced leading to the fracture of the lms.

3.4. XPS analyses

The elemental composition of the HMDSO lms on PLA substrates

were obtained using XPS. The samples prepared at 10 W and 30 W

were chosen to present the effect of the carrier gases. The atomic

W

V T R ( g / m 2 . d a y )

RF powers (W)

argon+HMDSO

0

5

10

15

20

25

30

10W 30W 40W 50W 60W uncoated

oxygen+HMDSO

Fig. 2. Water vapor transmission rates (WVTR) of Ar/HMDSO and O 2/HMDSO lms on

PLA. The lms were deposited at varied RF powers with a xed deposition time of

10 min. WVTR of uncoated PLA is also shown in the gure.

a

b

c

Fig. 3. SEM micrographs of (a) smooth Ar/HMDSO lm on PLA,(b) cracking and delam-

ination of O2/HMDSO lm on PLA, and (c) delamination of O2/HMDSO lm and defor-

mation of PLA substrate. The lms were deposited at 40 W.

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groups [15]. These ndings support that the lms with organic struc-

ture SiOxCyHz was formed when Ar was used as a carrier gas mean-

while inorganic structure was induced in the lms deposited in O2.

3.5. AFM measurements

AFM measurements were carried out for uncoated PLA and select-

ed samples of HMDSO lms on PLA. The results are shown in Fig. 5.

The uncoated PLA has a smooth surface with the root mean square

(rms) roughness of 2.99 nm. The Ar/HMDSO of 10 W shows a rather

smooth surface with some protrusion annular structures. A consider-

able change in the surface topography occurred in the lm deposited

at 50 W. The surface was rather rough with a coarse texture. The rms

roughness increased to 37.70 nm. This feature can be attributed

to the impinging of high energy ions caused by the increased

DC-sheath voltage on the substrate at high plasma density. It is

likely that the coarse surface deteriorates the barrier to water

vapor of the lms. The bombardment of plasma ions during the

lm growth could contribute to the high surface roughness resulting

in more defect sites; such as pinholes, voids and microcracks. Moreover

the deformation of the PLA substrate caused by the failure of the lms

increased the surface of the composite lm exposing to the water

vapor. These defects enhance diffusion of the water vapor through the

lms. For the O2/HMDSO lm of 10 W, the surface was covered with

random protrusions. When the RF power was increased to 50 W, a

smoother surface was observed. This can be explained by the etching

of the surface of the lm due to the presence of oxygen, as seen in the

emission spectrum.

3.6. Film thickness measurements

The thickness of the lms on Si substrates was shown in Table 3.

The thickness of O2/HMDSO lms was less than that of Ar/HMDSO

lms. This can be described by the etching of the lm during the de-

position process caused by oxygen in the plasma. The thickness of the

lms was not proportion to the RF powers. Cracking occurred in the

O2/HMDSO lms although the thickness was half of the Ar/HMDSO

lms indicating that the lms were under stress.

Fig. 5. Surfacetopographyobtained fromAFM measurementsof (a)uncoated PLA,(b) Ar/HMDSOlmat 10 W, (c) Ar/HMDSOlmat50 W,(d) O2/HMDSO lmat10 W,and(e) O2/HMDSO at

50 W. Films were deposited on PLA for 10 min. Note the difference in z-axis scales.

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4. Conclusions

We report the effect of carrier gases; Ar and O2; on the properties of

plasma polymerized HMDSO lms. In the process of plasma polymeriza-

tion, HMDSOmonomer wasbroken into smaller fragments as seen in the

emission spectra of the plasmas. The complexity of chemical structure

and the dissociation process of HMDSO in the plasma state as well as

the interactions between the plasma and the substrate result in a variety

of structures. The structure of thelm can be either organic or inorganic.

The organic structure is similar to polymethylsiloxane whereas the inor-

ganic one is similar to silicon oxide.

The lms deposited in Ar contain organic structure in the form of

SiOxCyHz. The lms could signicantly enhance the barrier to water

vapor of PLA. The inorganic structure was obtained when O2 was used

as the carrier gas. However, the mechanical mismatch between the

lm and the PLA substrate led to lm cracking and delamination. The

failures of the lm resulted in a complete destruction of the barrier to

water of PLA. The discharge power contributed to the plasma species

and the energy of the ions traversing through the plasma sheath leading

to etching and diverse surface topography of the lms.

Acknowledgments

C. Chaiwong would like to acknowledge the nancial support

from the Thailand Research Fund (TRF) under the contract num-

ber MRG5380224 and the Thailand Center of Excellence in Physics

(ThEP).

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Table 3

Thickness of the lms on Si substrates.

RF powers (W) Thickness (nm)

Ar/HMDSO O2/HMDSO

10 95.6 ±0.5 51.6 ±0.2

20 164.7 ±0.7 43.5 ±0.8

30 188.7 ±0.9 54.5 ±0.2

40 131.3 ±0.3 51.9 ±0.3

50 172.1 ±0.9 88.6 ±0.9

60 167.4 ±0.4 66.0 ±0.3

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Plasma Polymerization of Hexamethyldisiloxane

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