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Applied Surface Science 383 (2016) 261–267

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

onhomogeneous surface properties of parylene-C film etched by antmospheric pressure He/O2 micro-plasma jet in ambient air

ao Wang a,b,c,d, Bin Yang a,b,c,d, Xiang Chen a,b,c,d, Xiaolin Wang a,b,c,d,hunsheng Yang a,b,c,d, Jingquan Liu a,b,c,d,∗

National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic ofhinaKey Laboratory for Thin Film and Micro fabrication of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of ChinaCollaborative Innovation Center of IFSA, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of ChinaDepartment of Micro/Nano Electronics, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 15 March 2016eceived in revised form 20 April 2016ccepted 30 April 2016vailable online 3 May 2016

eywords:onhomogeneousurface properties

a b s t r a c t

Surface properties of parylene-C film etched by an atmospheric pressure He/O2 micro-plasma jet inambient air were investigated. The morphologies and chemical compositions of the etched surface wereanalyzed by optical microscopy, SEM, EDS, XPS and ATR-FTIR. The microscopy and SEM images showedthe etched surface was nonhomogeneous with six discernable ring patterns from the center to the outsidedomain, which were composed of (I) a central region; (II) an effective etching region, where almost allof the parylene-C film was removed by the plasma jet with only a little residual parylene-C being func-tionalized with carboxyl groups (C O, O C O−); (III) an inner etching boundary; (IV) a middle etchingregion, where the film surface was smooth and partially removed; (V) an outer etching boundary, where

arylene-C filmtmospheric pressure plasma jetilm etching

the surface was decorated with clusters of debris, and (VI) a pristine parylene-C film region. The analysisof the different morphologies and chemical compositions illustrated the different localized etching pro-cess in the distinct regions. Besides, the influence of O2 flow rate on the surface properties of the etchedparylene-C film was also investigated. Higher volume of O2 tended to weaken the nonhomogeneouscharacteristics of the etched surface and improve the etched surface quality.

© 2016 Published by Elsevier B.V.

. Introduction

Parylene-C (poly(monochloro-p-xylylene)), is one of the mostell-known thin film polymer because of its superior mechani-

al properties and remarkable bio-compatibility [1–4]. It is a USPUnited States Pharmacopeia) Class VI polymer, which is the highestevel of biocompatibility [5]. It is used in the application of flexiblemplant microdevices and bioMEMS with more and more attention1–5]. Therefore, the patterning of Parylene-C film is of great signif-cance in parylene microfabrication technology. Dry plasma-basedemoval techniques like reactive ion and deep reactive ion etch-ng are widely used in Parylene-C patterning [6–8]. However, the

acuum environment and pattern mask based on photolithographechnique limit the fabrication of flexible and three dimensionaltructure of Parylene based devices.

∗ Corresponding author at: Shanghai Jiao Tong University, Shanghai 200240, Peo-le’s Republic of China.

E-mail addresses: taowang89@163.com (T. Wang), jqliu@sjtu.edu.cn (J. Liu).

ttp://dx.doi.org/10.1016/j.apsusc.2016.04.191169-4332/© 2016 Published by Elsevier B.V.

To overcome those drawbacks, more attention has been focuson the application of atmospheric pressure plasma jets whichcan generate plasma plumes in ambient air condition with lowtemperature [10–12]. Besides, with the benefits of maskless andcost effective characteristics, they are widely used in etching andmodification of polymers like PI (polyimide), PMMA (polymethyl-methacrylate) and photoresist [13–18]. Therefore, atmosphericpressure plasma jets have great advantages in maskless pattern-ing of three dimensional and flexible Parylene-C devices in theatmospheric pressure environment.

However, because of intense electrostatic and hydrodynamicinteractions between the plasma jet and the surrounding air, thechemical and physical processes in the downstream of a plasmajet are complicated [19–21]. Sakiyama et al. [19] have found thatthere exist two different patterns of ground state atomic oxygennear a surface in the helium plasma needle discharge. And the

patterns are greatly influenced by gas flow rate. Yonemori et al.[20] also investigated the density distribution, temporal behaviorand flux of OH and O radicals on a surface using laser-inducedfluorescence. They found that the concentration distribution of

262 T. Wang et al. / Applied Surface Science 383 (2016) 261–267

F jet sysw aract

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ig. 1. (a) The schematic diagram of the atmospheric pressure He/O2 micro-plasma

hite box in (b) indicates the inner wall of the capillary tube. (c) voltage-current ch

H and O radicals was not identical and nonhomogeneous, andhe distribution varied with the surface condition and surround-ng air conditions. Similar experiment was also carried out byirer in Ref. [21]. He investigated the reactivity zones around andtmospheric pressure plasma jet though XPS mapping of chemicalpecies on a polyethylene film and found that the spatial distribu-ion of chemical species showed a ring pattern from the center tohe outside area [21]. Generally, plasma etching of polymer filmsonsists of three different mechanisms, namely chemical etch-ng, physical bombardment and UV radiation [22]. Therefore, theonuniform chemical and physical processes in a plasma plumeill greatly influence the etching results of polymer films. As for

arylene-C, the USP Class VI polymer, it might be sensitive to com-licated etching process of atmospheric pressure plasma jet in thembient air. And the surface properties, such as uniformity, sur-ace chemistries and morphologies of parylene-C film selectivelyemoved by a plasma jet significantly influence its performance inarious applications, such as in biomedical application the topog-aphy and surface chemistry of the treated parylene-C film directlyffect cell viability [8]. Besides, in the fields of flexible implanticrodevices and lab on chip applications, the patterned surface

roperties of parylene-C film also play a key role in their perfor-ance. Thus it is necessary to investigated the surface properties

f plasma jet treated parylene-C film [1–9]. However, as far as wenow, there are no detailed investigations on the surface propertiesf parylene-C film etched by an atmospheric pressure plasma jet.

tem; (b) photograph of the AP�PJ operating for parylene-C film etching. The dashederistics of AP�PJ; (d) optical emission spectrums of AP�PJ.

In this paper, we report the detailed investigation on the non-homogeneous surface properties of parylene-C film etched byan atmospheric pressure He/O2 micro-plasma jet (AP�PJ) in theopen air. Properties of etched surface from the center of theetched region to the outer domain were characterized by opticalmicroscopy, scanning electron microscopy (SEM), energy disper-sive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS)and attenuated total reflectance fourier transform infrared spec-troscopy (ATR-FTIR). Besides, the influence of oxygen flow rate onthe nonhomogeneous characteristics of the etched surface proper-ties was also investigated.

2. Experimental details

2.1. Parylene-C film preparation

A 2 cm × 2 cm Si wafer (p-type with resistivity of 10 � cm,0.5 mm thick) was prepared as the deposition substrate. A 5 �mthick parylene layer was chemical vapor deposited (CVD) on the Siwafer in a PDS2010 system (Specialty Coating Systems, USA).

2.2. Atmospheric pressure He/O2 micro-plasma jet

The parylene-C film was etched with a homemade atmosphericpressure He/O2 micro-plasma jet (AP�PJ) in the ambient air, asshown in Fig. 1(a) and (b). AP�PJ was composed of a high-voltageelectrode, a ground electrode and a capillary quartz glass tube. The

T. Wang et al. / Applied Surface Science 383 (2016) 261–267 263

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ig. 2. (a) Microscopic image of parylene-C film etched by AP�PJ of 90 s; (b) local enf region V; (e) local enlarged image of region II and III. The solid yellow boxes repre

nterpretation of the references to colour in this figure legend, the reader is referred

nner and outer diameter of the capillary tube was 0.2 mm and.7 mm, respectively. Helium (He) and oxygen (O2) gases were fed

nto the tube with He flow rate of 150 sccm and O2 flow rate of sccm in this experiment. Two copper tapes (5 mm in length) wererapped around the capillary tube as high-voltage electrode and

round electrode with the distance of 2 cm and 1 cm to the nozzlef the tube, respectively. A sinusoidal high voltage source with fre-uency of 20 kHz was applied to the two electrodes. And the peako peak applied voltage was fixed at 12 kV in this experiment. Theistance between the nozzle exit and the parylene-C film surfaceas fixed at 1 mm, and the sample etching time was fixed of 90 s.

.3. Diagnostic methods

Optical discharge image was recorded by a CCD camera (IMG5S-, Image Technology Company). A Tektronix P6015A voltage probeonnected to an Agilent DSO-X-2024A oscilloscope to record volt-ge and current was calculated by voltage drop across a 50 �oninductive resistor. Optical emission spectroscopy (OES) wasetected using a Wyoptics spectrometer with variable wavelength

rom 185 nm to 1100 nm. The microscopy morphologies of etchedarylene-C film were observed by optical microscopy (BA310Met-, Motic) and high vacuum scanning electron microscopy (ULTRA5, Zeiss, Germany). XPS analysis was performed by X-ray photo-lectron spectrometer (AXIS ULTRA DLD, Kratos) with an excitationource of Al K� radiation (� = 1486.6 eV). ATR-FTIR spectroscopy ofifferent regions of parylene-C film etched by AP�PJ was obtainedy fourier transform infrared spectrometer (Nicolet iN 10 MX, Ther-oFisher).

. Results and discussion

.1. Electrical and OES characteristics of AP�PJ

Fig. 1(c) shows the voltage-current characteristics of AP�PJ.ulti-spikes appeared in both positive and negative half cyclesith the peak value of about 50 mA. With so high discharge cur-

ent, various excited and reactive species were generated. Fig. 1(d)resents the OES of AP�PJ. As can be seen, excited N2, excited He,

eactive O atom at 777.1 nm and 844.4 nm, as well as OH radical at09.1 nm are successfully generated. And the presence of excited N2nd OH radicals were generated by the interaction between plasmaet and ambient humid air

d image of region II–V; (c) local enlarged image of region I; (d) local enlarged imagehe six different regions from the center of the etched area to the outer domain. (Fore web version of this article.)

3.2. Surface morphology of parylene-C film etched by AP�PJ

Fig. 2 shows the microscopic image of parylene-C film treatedby AP�PJ of 90 s. It is obvious that the etching surface is nonho-mogeneous. Different ring patterns are observed from the centerto the outside domain. According to the discernable patterns, wedefine six distinct regions, which are marked by solid yellow boxesin Fig. 2. Region I–V were the etching regions, while region VI wasthe pristine parylene-C film that was taken as control. Here, the sixregions can be denoted as (I) a central region, (II) an effective etch-ing region, (III) an inner etching boundary, (IV) a middle etchingregion, (V) an outer etching boundary and (VI) a pristine parylene-C film region. Closer inspection of the surface morphologies of thesix distinct regions was carried out by SEM, as shown in Fig. 3. Itshows that the nanostructures of the six etched regions are alsodifferent.

The film surface of the central region I was seriously carbonizedand damaged, which indicated the high energy deposition on thisregion. The high temperature and large amount of high energeticreactive species in the center of the plasma jet accelerated thecarbonization and erosion of the parylene-C film. The peripheralcontour was the irregular circle and the equivalent diameter ofthis region was about 100 �m. From the SEM image, this regionwas full of condense particles with the size from tens to hundredsnanometers.

Region II was the effective etching region. Almost all of theparylene-C film was removed by the plasma jet in this region andthe edge was a typical circle with the diameter of about 400 �m.The surface morphology of this region was uniform and smoothfrom both optical microscopy and SEM images.

Outside the effective etching region, there was an obvious etch-ing boundary with 200 �m to 250 �m away from the center. Fromthe local enlarged image shown in Fig. 2(e), it is clearly visiblethat the surface was partially eroded, though not as serve as thecenter region. And the surface characteristics of this region werealso nonhomogeneous. Close to the effective etching region, therewere many micro-holes along the outer circle of region II. Thesemicro-holes might be physical sputtered by energetic He atoms orHe+ ions. Continue to look outward, the number of erosion micro-holes decreased. But many circular Newton’s rings were observed,

as shown in Fig. 2(b) and (e). Two reasons might lead to this phe-nomenon. The first one was that the nonhomogeneous distributionof chemical reactive species caused the non-uniform etching rate,

264 T. Wang et al. / Applied Surface Science 383 (2016) 261–267

Fig. 3. SEM of the regions of the six solid yellow boxes shown in Fig. 2. And (a)–(f) represent the regions of (I)–(VI), respectively.

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Fig. 4. (a)–(f) ATR-FTIR spectr

hich eventually induced the different film thickness along theadical direction. The other one was that the parylene-C film slightlyetached from the silicon substrate and it was caused by ther-al stress, which came from the non-uniform heat distribution on

he film surface. The SEM image of this region showed that thereere many elongated particles with the diameter and the length of

undreds of nanometers and several micrometers, respectively.Along the expansion of Newton’s rings, there was another annu-

ar discernable region: the middle etching region. The appearancesf the optical microscopic image and SEM image showed that

he film surface was smooth without carbonization and sputtered

icro-holes. But, as mentioned before, the film in this region wasartially etched by AP�PJ with different film thickness. Besides, the

f the region I–VI, respectively.

diameter of the outer circle of region IV was about 800 �m, whichwas four times larger than the inner diameter of the capillary tube.This was due to the expansion effect of the plasma jet when con-tacting with the sample surface and intense interactions betweenthe plasma jet and the surrounding air [23].

The enlarged image of the outermost region is shown in Fig. 2(d).It could be seen that the surface was decorated with clusters ofdebris. Combined with the SEM image shown in Fig. 3(e), the sizeof the debris was varied from nanometers to micrometers. Thesefragments might be carried outwards by gas flow from the inner

region [21].

The diameter of the outermost region V was about 1020 �m,which was approximately 5 times lager than the inner diameter of

T. Wang et al. / Applied Surface Science 383 (2016) 261–267 265

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Fig. 5. EDS of the regions of (a) region I and (b) region II show

ozzle exit of the capillary tube. As could be seen from Fig. 1(b), thelasma jet was also expanded on the sample surface. And the diam-ter of the expansion area was about 900 �m. Besides, the color ofhe expansion plasma was different from the center to the outside,hich indicated that the chemical species in the expansion plasmaas different from the radical direction. Hence, the nonhomoge-

eous circular patterns of the etching surface was attributed to theomplex process of the expansion effect of the plasma jet whenontacting with the sample surface, as well as the hydrodynamicsf working gases with the ambient air.

.3. Composition analysis of parylene-C film etched by AP�PJ

.3.1. ATR-FTIR spectrumsIn order to investigate the chemical compositions of the six dis-

inct etched regions, ATR-FTIR was carried out, as shown in Fig. 4.ig. 4(f) shows the characteristic bands of the pure parylene-C filmnd the inset is the molecular formula of parylene-C [24]. The peaksrom 1449.4 to 1607.6 cm−1 were assigned to C C and C C in-planeibration of benzene ring and the peak of 1048.9 cm−1 denoted

Cl bond. The peaks from 2800 to 3100 cm−1 were attributed to H bond on aromatic ring and the peak 823.9 cm−1 representedhe two adjacent hydrogen atoms (C H). There were no obvioushanges of ATR-FTIR spectrums of Fig. 4(e) and (f), which indicatedhat the chemical structure was unchanged of region V.

As for Fig. 4(c) and (d), there was also no significant changef the characteristic bands of the film surface. It showed that theain chemical composition was still unchanged of region III and

V. However, as the red arrows marked in Fig. 4(c) and (d), twoew weak bands appeared at the wavenumbers of 1840.3 cm−1

nd 1849.4 cm−1, which implied that carboxyl groups (C O) wereormed in region III and IV [25,26]. Therefore, the parylene-C filmurface of these two regions had been slightly functionalized byP�PJ. Besides, the transmission percentage of the characteristicands of the film surface increased from region V to region III, whichemonstrated that the film thickness decreased. It showed that thetching rate of inner regions was faster than the outer area. Sinceoncentration distribution of the reactive species, such as OH and Oadicals was not identical and nonhomogeneous in the downstreamf the plasma jet [19–21], the different etching rate of the treatedrea was attributed to the different reactive species distributionsnd etching mechanisms.

Fig. 4(b) shows the ATR-FTIR spectrum of region II. A signifi-

antly reduction of characteristic bands magnitude was observed,ith unchanged spectra shape and peak location. It indicated that

he majority of the parylene-C film had been etched by AP�PJ.oreover, a new band at 1749.6 cm−1 was also observed as the

ig. 2. Insets are the atomic percentage of different elements.

red arrow marked in Fig. 4(b), which also implied the appearanceof C O or C O O− carboxyl groups [26–28].

The ATR-FTIR spectrum of region I is shown in Fig. 4(a). It wasclearly that the chemical structure of the film surface was signif-icantly changed. No characteristic bands of parylene-C film couldbe observed, which indicated that the parylene-C was completelyremoved in this region. The existence of the absorbance feature of1039.2 cm−1 suggested the formation of SiO2 [29]. Detailed resultsand discussion are presented in the following EDS analysis.

3.3.2. EDS analysisIn order to further investigate the etching result of the region

I and region II, energy dispersive spectrums of these two regionswere detected, as shown in Fig. 5(a) and (b). It revealed that themain element compositions of region II were C, O, Si and Cl withthe atomic percentage of 65.26%, 15.95%, 16.46% and 2.34%, respec-tively. And region I was composed of C, O and Si elements withatomic percentage of 6.47%, 57.51% and 36.03%, respectively. Com-bined with the surface morphology and ATR-FTIR spectrum ofregion I, it could deduce that the C peak was from the carboniza-tion of parylene-C film and Si peak came from the Si substrate andthe oxidization of Si. Thus it further verified the high energy depo-sition of the plasma jet in region I that accelerated the removaland carbonization of the parylene-C film and the oxidization of theSi substrate. In region II, C and Cl peaks were from the residualparylene-C and Si peak was from the Si substrate. This indicatedthat most of the parylene-C film had been etched with only a smallamount of residuals, which was consistent with the ATR-FTIR spec-trum of region II. Besides, the detection of O element in region II alsoindicated that the injection of functional groups, which agreed wellwith the ATR-FTIR spectrum. The detailed analysis of the type of thefunctional groups is discussed in XPS analysis.

3.3.3. XPS analysisTo examine the functional groups which were generated by

AP�PJ, XPS analysis was carried out. Fig. 6(a) shows the XPS ofthe pristine parylene-C film (region VI) and the etched parylene-C film in region II. It was apparent that O 1s, Si 2p, Si 2s spectrumsappeared in region II after exposure to AP�PJ. The signals from the Si2p core level and Si 2s core level came from the Si substrate, whichindicated that the parylene-C film had been greatly removed bythe plasma jet. The signals from the Cl 2p core level and C 1s corelevel demonstrated that there was still some residual parylene-C in

this region. But the presence of O 1s core level in region II showedsignificant surface oxidization. Meanwhile, we also observed anadditional weak peak of N 1s in region II, which implied the nitro-gen doped in the parylene film with the formation of N H bond

266 T. Wang et al. / Applied Surface Science 383 (2016) 261–267

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Fig. 6. (a) XPS of the region II and region VI shown in

r C N bond [30]. And the nitrogen was mainly from the ambientir, which further indicated the complex hydrodynamic interactionetween the plasma jet and the surrounding air.

Fig. 6(b) represents the deconvolution of the C 1s spectrum ofhe parylene-C film in region II. As shown in this figure, the carboneak can be fit with a principal core level at 284.8 eV and threeubsidiary core levels at 286.1 eV, 288.1 eV and 291.3 eV. The peakt the binding energy of 284.8 eV was attributed to aromatic (C C)nd saturated (C H, C Cl) carbons [10]. The peak at the bindingnergy of 291.3 eV corresponded to the �-�* shake-up transitionatellite of the aromatic ring [31]. Moreover, the peaks at 286.1 eVnd 288.1 eV were owning to the generation of carboxyl groupsC O, O C O−) [32].

According to the XPS results, the residual parylene-C film inegion II had been widely functionalized and showed significanturface oxidation, which was in accordance with the ATR-FTIR spec-rum and EDS analysis.

.4. Influence of O2 flow rate

According to previous studies [5,10], the chemical etchingechanism plays the most important role in plasma etching of

arylene film and the reactive oxygen species (ROS) dominate thehemical etching process. Thus the O2 flow rate will have a greatmpact on the etching results of parylene-C film. Fig. 7 shows theptical microscopy images of parylene-C film etched by AP�PJ withifferent O2 flow rate.

As the O2 flow rate increased from 6 sccm to 20 sccm, the surfaceroperties tended to be more homogeneous and less ring patternsere observed. As can be seen from Fig. 7(a) and (b), region V dis-

ppeared and the diameter of region III and region IV enlarged with2 flow rate increased to 9 sccm. With the increase of O2 flow rate,

he spatial distribution of ROS and gas temperature changed. When2 flow rate increased to 9 sccm, the addition of oxygen gave highermount of ROS and enlarged the expansion area of the ROS, whichventually enlarged the diameter of region IV. However, as an elec-ronegative gas, the addition oxygen might absorb the electrons anduench the metastable He atoms [33]. The decrease of energeticlectrons and metastable He atoms made the physical sputteringffect weakened, thus eventually decreased the amounts of theputtering fragments from the film surface. And this might explain

he disappearance of region V. Besides, the attachment process oflectrons by oxygen released heat, which rose the gas temperaturef the plasma jet. As a result, more parylene-C film was carbonized,hich clarified the increased diameter of region III.

; (b) C 1s spectrum of the parylene-C film in region II.

By increasing the O2 flow rate to 12 sccm, region IV disap-peared and the diameter of region III reduced. Continue to increasethe O2 flow rate to 15 sccm and 20 sccm, the shrink of region IIIwas more apparent, and the diameter of region I and region IIexpanded. When O2 flow rate increased to 12 sccm or higher vol-ume, the power supply was saturated. The amount of ROS no longerincreased with the increase of O2 volume. In contrast, the increaseof O2 volume might quench the plasma by electron attachment[34]. Therefore, there was no enough energy to dissociate all theaddition oxygen and ionize the ambient air. Thus the spatial distri-bution of ROS and gas temperature changed. Consequently, regionIV vanished and the diameter of region III reduced with O2 flow ratehigher than 12 sccm. But in the center of the plasma jet, the increaseof O2 flow rate accelerated the etching and oxidation processes. Asa result, the region I and region II expanded. Further increased theO2 flow rate more than 20 sccm, the plasma was quenched withpeak to peak applied voltage fixed at 12 kV. Anyway, with the O2flow rate increased from 6 sccm to 20 sccm, the surface propertiestended to be more homogeneous and surface quality became bet-ter. Therefore, in some extent, the addition of O2 volume is helpfulto improve the uniformity and quality of surface etched by AP�PJ.

4. Conclusions and future work

In this study, surface properties of parylene-C film etchedby an atmospheric He/O2 micro-plasma jet in ambient air wereinvestigated. The complex etching process associated with theinteractions between the plasma jet and the surrounding aircontributed to the nonhomogeneous surface properties of theparylene-C film etched by AP�PJ. When the O2 flow rate was 6 sccm,we found that the etched surface showed six distinct etchingregions from the center to the outer domain, which were presentedas follows: (I) a central region; (II) an effective etching region;(III) an inner etching boundary; (IV) a middle etching region; (V)an outer etching boundary; (VI) a pristine parylene-C film region.Although the morphologies of region III to V were distinct, it wasinteresting that the main chemical composition was unchanged,which was slightly functionalized with carboxyl groups. Besides,we also found that the etching rate of inner regions was faster thanthe outer area and the diameter of the outermost region V wasabout 1020 �m. These phenomena may reflect a complex local-ized etching process of AP�PJ. And the nonhomogeneous surface

properties of the etched parylene-C film were attributed to threedifferent reasons: (1) the different spatial distribution of ROS, (2)different etching mechanisms, (3) interaction between the plasmajet and the ambient air.

T. Wang et al. / Applied Surface Science 383 (2016) 261–267 267

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ig. 7. Optical microscopy images of parylene-C film etched by AP�PJ with differeate was fixed at 150 sccm.

The increase of O2 flow rate showed that it had greatly influ-nce on surface properties of the etched parylene-C film. Higher O2ow rate tended to weaken the nonhomogeneous characteristicsf the etched surface. But with the O2 flow rate more than 20 sccm,he plasma was quenched significantly in this experiment throughlectrons attachment.

Based on this preliminary study, it is helpful to better under-tand the localized etching mechanism and surface properties ofarylene-C film etched by an atmospheric pressure plasma jet inhe ambient air. In future work, comprehensive investigation ofhe influence of applied voltage, gas species, gas flow rate, workistance and the etching time on the etching surface properties iseeded.

cknowledgements

The authors thank to partly financial support from the Nationalatural Science Foundation of China (No. 51475307), 973 Program

2013CB329401), SRFDP (20130073110087). The authors are alsorateful to the colleagues for their essential contribution to thisork.

eferences

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