Plasma Removal Of Parylene C - University Of Southern .
IOP PUBLISHINGJOURNAL OF MICROMECHANICS AND . Micromech. Microeng. 18 (2008) 045004 (13pp)Plasma removal of Parylene CEllis Meng1, Po-Ying Li2 and Yu-Chong Tai31Department of Biomedical Engineering, University of Southern California, 1042 Downey Way,DRB-140 Los Angeles, CA 90089-1111, USA2Department of Electrical Engineering, University of Southern California, 3737 Watt Way,PHE-604 Los Angeles, CA 90089-0271, USA3Department of Electrical Engineering, California Institute of Technology, 1200 E California Blvd.,Pasadena, CA 91125, USAE-mail: ellis.meng@usc.eduReceived 20 November 2007, in final form 13 January 2008Published 22 February 2008Online at stacks.iop.org/JMM/18/045004AbstractParylene C, an emerging material in microelectromechanical systems, is of particular interestin biomedical and lab-on-a-chip applications where stable, chemically inert surfaces aredesired. Practical implementation of Parylene C as a structural material requires thedevelopment of micropatterning techniques for its selective removal. Dry etching methods arecurrently the most suitable for batch processing of Parylene structures. A performancecomparison of three different modes of Parylene C plasma etching was conducted usingoxygen as the primary reactive species. Plasma, reactive ion and deep reactive ion etchingtechniques were explored. In addition, a new switched chemistry process with alternatingcycles of fluoropolymer deposition and oxygen plasma etching was examined to producestructures with vertical sidewalls. Vertical etch rates, lateral etch rates, anisotropy and sidewallangles were characterized for each of the methods. This detailed characterization was enabledby the application of replica casting to obtain cross sections of etched structures in anon-destructive manner. Application of the developed etch recipes to the fabrication ofcomplex Parylene C microstructures is also discussed.(Some figures in this article are in colour only in the electronic version)Parylene C (poly(monochloro-p-xylylene)) has a longhistory of use in the medical industry as a coating forstents, cardiac assist devices, surgical tools, electronics andcatheters [5]. The use of Parylene C as a structural materialin microelectromechanical systems (MEMS) devices and inparticular bioMEMS and microfluidics has gained traction[6–10]. Among the many polymers used as structuralmaterials, Parylene C is an increasingly popular choice due toits deposition method, low process temperature, transparencyand compatibility with standard microfabrication processes[11]. It is possible to perform multilayer processing ofParylene films to produce complex structures and devices.Parylene C is a USP (United States Pharmacopeia)Class VI polymer. This designation is the highest level ofbiocompatibility possible for polymers permitting its use inapplications where long-term implantation is required. Itsbiocompatibility, biostability, low cytotoxicity and resistanceagainst hydrolytic degradation [1, 12, 13] have resultedin increasing popularity of Parylene C in micro- and1. IntroductionParylene, or poly (p-xylylene), is one of the most well-knownchemical vapor deposited (CVD) thin film polymers. It isused in a wide range of applications, particularly as a coatingfor biomedical implants and microelectronics. Its desirableproperties include chemical inertness, conformal coating andexcellent barrier properties [1]. Originally discovered in1947 by Swarc, Parylenes were not commercialized by UnionCarbide until 1965 following the development of a CVDpolymerization process by Gorham [2–4]. Although overtwenty types of Parylene have been developed, only threeare commonly available: Parylenes N, C and D (figure 1).With increasing interest in Parylenes, newer commercialproducts have recently been introduced including, Parylene HT(Specialty Coating Systems, Indianapolis, IN), a fluorinatedversion of the polymer, and diX A and AM (Kishimoto SangyoCo., Ltd, Japan), having amino groups attached to the benzenerings.0960-1317/08/045004 13 30.001 2008 IOP Publishing LtdPrinted in the UK
J. Micromech. Microeng. 18 (2008) 045004E Meng et alFigure 1. Commercially available poly (p-xylylene) types.Parylene is readily removed in oxygen-based plasmas andthe possible etching reactions that govern Parylene removalhave been suggested [23, 24]. The etching mechanism forParylene C is thought to be similar to that of Parylene Nwhich is different only by the absence of a chlorine atom.Plasma removal of Parylene N is attributed to the opening ofthe benzene ring which is necessary in etching of aromaticpolymers. This process is thought to occur as follows. First,hydrogen is abstracted by an oxygen radical from the ethylcarbons between the benzene rings in the polymer chain and ahydroxyl radical is formed. Then the exposed reactive site issubjected to molecular or atomic oxygen absorption to forman unstable peroxy radical. Rearrangement of the unstablespecies can then result in the formation of volatile carbonmonoxide (for atomic oxygen adsorption) or carbon dioxide(for molecular oxygen adsorption). Further oxygen attack onthe radical site results in ring opening.While the mechanisms governing Parylene C removal arelikely similar to those for Parylene N, the presence of a chlorineatom on the ring reduces the available reactive carbon sites byone. If ring opening is the rate limiting step in Paryleneetching, then the reduction in reactive sites may explain theobserved reduction in Parylene C etch rate compared to thatof Parylene N by 17% [23].Parylene etching has been demonstrated in multiplemodes including plasma etching [19, 25, 26], reactive ionbeam etching (RIBE) [27], reactive ion etching (RIE) [28, 29]and high-density plasma etching [30]. However, no attempthas been made to optimize anisotropy or employ sidewallpassivation to produce high aspect ratio structures. Yehdemonstrated patterning fine features ( 2.5 µm) in thin filmsbut observed an etch selectivity near unity between ParyleneC and photoresist by RIE [29]. Vertical profiles were achievedwhen using metal or oxide masks and under conditions of lowpressure, low power and a biased substrate. However, etchedfeatures exhibited significant roughness and the presence ofdense micrograss structures due to redeposition of the hardmask [28, 29]. Parylene removal by RIBE achieved smoothersurfaces but at the expense of greatly reduced etch rates ( 10’sof Å min 1 compared to 102–103 Å min 1) [27].Recently sidewall passivation and inductively coupledplasma sources have been explored to achieve anisotropicnano-fabricated devices for microfluidic and bioMEMSapplications. For example, Parylene C-based devices havebeen demonstrated as platforms for neuronal growth [14–16]and in implantable neuronal probes [17]. Furthermore, newdevelopments on the functionalization of Parylene surfacesmay expand its use in biomedical applications [18–20].Pattern transfer of masks into Parylene C films is acritical enabling step in Parylene microfabrication technology.Dry plasma-based etching techniques are likely the mostsuitable means for achieving fine features in Parylene Cfilms. The characterization of Parylene C removal by oxygenbased plasmas was investigated for three plasma etchingmodes: plasma, reactive ion and deep reactive ion etching.In particular, the focus of this study was on identifyingprocess parameters that will enable anisotropic etching towardachieving high aspect ratio (HAR) structures desirable forMEMS applications. Removal rates for the photoresistmasking layer were also monitored.2. Patterning Parylene2.1. Chemical removalA key feature of Parylene is chemical inertness whichcomplicates its chemical removal.Below the meltingpoint, Parylene is resistant to dissolution by solvents. Attemperatures above 150 C, it is possible to remove Parylene ineither chloronaphthelene or benzolyl benzoate [21]. However,this method is not compatible with most commonly usedlithographic processes. The highly conformal natural ofParylene films prevents patterning via lift-off processes.2.2. Plasma removalOxygen (O2) plasmas are used to etch many polymers but thespecific mechanisms for this removal are not well understood.Polymer etching in pure oxygen plasmas is linked to thepresence of atomic O; etching enhancement is associated withan increase in atomic O by increase of dissociation, reductionof losses due to recombination, and increase of O atom fluxfrom the plasma to the sample [22]. Increases in electrondensity or electron energy can increase O2 dissociation.2
J. Micromech. Microeng. 18 (2008) 045004E Meng et al(a)(f)(b)(e)(c)(d )Figure 2. Illustrations (a)–(d) of the process used to fabricate silicone rubber replicas of etched Parylene C features and (e) the replicamounting method in preparation for SEM viewing. (f ) SEM image of a sectioned silicone rubber replica that has been Au sputter-coated.(a)(b)(c)Figure 3. Au-coated cross sections of PDMS replicas of 10 µm line features etched using (a) plasma etching, (b) RIE and (c) DRIE modes.The scale bar in each image measures 5 µm. Cracking of the Au conductive coating is evident.etching of polymers. Zahn achieved aspect ratios up to 20:1 fordeep reactive ion etching (DRIE) of polymethylmethacrylate(PMMA) by using a switched chemistry etching techniqueinspired by the Bosch process for silicon removal [31].SiO2 masks and an inductively coupled plasma source wereused to create vertical Parylene N sidewalls in high-densityoxygen-based plasmas (Ar/O2) [30]. The anisotropy was inpart attributed to the sidewall passivation by redeposition ofoxygen-deficient etch products which prevent lateral erosiondue to reflection of atomic and molecular oxygen. Anisotropywas further improved by increasing the substrate bias which isconsistent with the results of earlier studies.Figure 4. Illustration showing parameters measured to obtain thevertical etch rate, lateral etch rate and sidewall angle.patterned mold and then thermocompression bonding with asecond Parylene film was developed as an alternative to surfacemicromachining of microchannels [39, 40].3. Experimental methods2.3. Alternative methods3.1. Preparation of etched Parylene C couponsAlternative methods for the selective deposition and removal ofParylene insulation layers on implantable electrodes have beenreported. The thickness of deposited Parylene is a function ofsubstrate temperature [32, 33] so biased resistors were usedto generate a localized heat gradient that prevented depositionin regions held above 70 C [34]. Parylene has been usedas an insulation coating for microelectrodes used in neuralrecordings. Selective removal by ultraviolet laser ablation[35, 36] and manual mechanical removal on wiremicroelectrodes [37] has also been reported.More recently, thermal imprint patterning in Ni moldsand micromolding techniques have also been investigated.Thermal imprinting achieved 25 µm high 10 µm wideline features in 30 µm thick parylene, but required hightemperature processing at 250 C to enable accurate patternreplication [38]. Micromolding of Parylene films to aFour inch silicon wafers were treated with A-174 silaneadhesion promoter (Specialty Coating Systems, Indianapolis,IN). Next, the back side of each wafer was covered with adicing saw tape (Nitto Denko Corporation, Osaka, Japan) torestrict Parylene coating to only the front side. 10 µm ofParylene C (Specialty Coating Systems, Indianapolis, IN) wasdeposited (PDS 2010 Labcoter, Specialty Coating Systems,Indianapolis, IN). The Parylene coating was gently scored witha sharp razor and the dicing saw tape was carefully removedfrom the back side. After priming with hexamethyldisilazane(HMDS), 14 µm of photoresist (AZ 4620, AZ ElectronicMaterials, Branchburg, NJ) was applied by spin coating(1 krpm for 40 s) and then patterned with an etching calibrationpattern consisting of lines, trenches and other geometricalfeatures.3
J. Micromech. Microeng. 18 (2008) 045004E Meng et al(a)(b)(c)(d)Figure 5. Etch data for Parylene C samples (n 4) processed by plasma etching using oxygen plasma: (a) vertical etch rate, (b) lateral etchrate, (c) anisotropy and (d) the measured sidewall angle. Process pressure (200 and 400 mTorr) and power (100, 200, 300 and 400 W) werevaried.parameters examined in each etching mode. Table 2 shows indetail the process recipes used in DRIE of Parylene C. Onetest coupon was processed for each condition examined. Forplasma etching, 8 different process conditions were studied.RIE and DRIE studies involved 18 and 21 different processconditions. In total, 47 test coupons were processed (94 dies,2 dies each process condition).Table 1. Process parameters examined in each of the etching modes.PlasmaetchingReactive ionetchingDeep reactiveion etchingPowerPressurePowerPressureO2 flow ratePowerPressureO2 flow rateAddition of ArSidewall passivation with C4F8Etch step time3.2. Photoresist mask etch rate measurementThe step heights of the etched structures (photoresist Parylene) were measured using a surface profilometer (AlphaStep 200, KLA-Tencor, San Jose, CA). Then the two dies ineach test coupon were separated manually by scribing andbreaking. The remaining photoresist was removed on one ofthe dies. The step heights of the remaining Parylene structureswere measured. The obtained step height data were used tocalculate the vertical etch rates of the photoresist maskinglayer.Patterned test coupons measuring 20 mm 10 mmand containing two identical calibration dies were carefullyseparated from wafers by manually scribing and breakingeach piece. Test coupons were etched for a fixed time of10 min under varying process conditions in plasma etching(PEII-A, Technics Plasma, Kirchheim, Germany), RIE (1000TP/CC, SemiGroup Texas, LLC, McKinney, Texas), andDRIE (PlasmaTherm SLR-770B, Unaxis Corporation, StPetersburg, FL) equipment. Process pressure, gas flow,power and etching chemistries were varied. DRIE modeenabled switched chemistry etching in which samples wereexposed to alternating cycles of (1) deposition of a C4F8-basedTeflon-like sidewall passivation layer and (2) etching in O2plasma. Deposition of the fluoropolymer layer protected thesidewalls from lateral etching. Table 1 summarizes the process3.3. Preparation of SEM samples by replica moldingAccurate determination of vertical etch rate, lateral etchrate, sidewall angle and the etch profile is possible onlyby examining cross-section samples under scanning electronmicroscopy (SEM). Preparation of Parylene samples for SEM4
J. Micromech. Microeng. 18 (2008) 045004E Meng et al(a)(b)(c)(d )Figure 6. Etch data for Parylene C samples (n 4) processed by RIE using an oxygen plasma at 200 W: (a) vertical etch rate, (b) lateraletch rate, (c) anisotropy and (d) the measured sidewall angle. Process pressure (150, 200 and 250 mTorr) and oxygen flow rate (40, 80 and120 sccm) were varied.Table 2. Parameters for DRIE recipes.Recipe parametersOxygen onlyOxygen argonSwitched chemistryO2 flow rate (sccm)C4F8 flow rate (sccm)Ar flow rate (sccm)Etch coil power (W)Etch platen power (W)Deposition coil power (W)Deposition platen powerEtch step time (s)Deposition time (s)Etch process pressure (mTorr)Deposition process pressure (mTorr)20, 60, 10000400, 80020N/AN/A600N/A13, 23N/A20, 60, 10000, 50, 10080020N/AN/A600N/A23N/A20, 60, 10035(40, during the deposition step)80020825110, 2032322is particularly challenging. In general, cleaving polymerfilms to produce cross-sections requires special techniques,such as focused ion beam (FIB) or cryogenic freezing, toprevent tearing or deformation of fine features. However,consistent cross-sections of Parylene films are difficult toobtain by cleaving frozen samples and FIB tools are bothrare and expensive. Accurate negative reproductions ofdelicate etched features can be obtained by replication,a method commonly employed in polymer microscopy[41]. Application of replica casting allowed preparationof cross sections of etched structures in a non-destructivemanner.The replication process is summarized in figure 2.Silicone rubber (Sylgard 184, Dow Corning, Midland, MI)was prepared (AR-250 Hybrid Mixer, Thinky Corp., Tokyo,Japan) with a 10:1 base-to-curing-agent ratio. The prepolymermix was poured onto the etched test coupons (figure 2(a)),degassed (V0914 vacuum oven, Lindberg/Blue, Asheville,NC), and cured at 65 C for 1 h (figure 2(b)). Each replica waspeeled from the etched master and cut into a suitable size forSEM imaging (figure 2(c)). Replicas were then cross sectionedwith a razor blade (figure 2(d)) and sputter-coated with Au,making the surface conductive for SEM viewing (figures 2(e)and (f )).5
J. Micromech. Microeng. 18 (2008) 045004E Meng et al(a)(b)(c)(d )Figure 7. Etch data for Parylene C samples (n 4) processed by RIE using oxygen plasma at 400 W: (a) vertical etch rate, (b) lateral etchrate, (c) anisotropy and (d) the measured sidewall angle. Process pressure (150, 200 and 250 mTorr) and oxygen flow rate (40, 80 and 120sccm) were varied.A vertical profile corresponds to A 1 and occurs when thereis no undercutting. The sidewall angle, θ , is measured asindicated in figure 4.3.4. Parylene C etch rate measurementReplications of etched 10 µm line features for each etchingmode are shown in figure 3. The cracks in the image arepresent in the sputter-coated Au layer and are possibly due toslight expansion in the silicone rubber under vacuum. ImageJ(v.1.34, National Institutes of Health) software facilitated themeasurement of individual feature dimensions used in thecalculation of vertical etch rate, lateral etch rate and anisotropy.The sidewall angle of etched lines was also obtained fromacquired SEM images using ImageJ. Four cross sections of10 µm line features were measured to obtain these parameters.The definitions for the measurements used to calculate eachparameter are defined in figure 4.Vertical etch rate, Rvertical , is defined ashRvertical (1)twhere h is the etched depth and t is the etch duration (10 minin all cases). Lateral etch rate, Rlateral , is defined as(10 µm a)/2Rlateral (2)twhere a is the width of the top of the etched line. The startingline width was 10 µm for all cases. Etch anisotropy, A, can byquantified by using the following definition [42]:RlateralA 1 .(3)Rvertical4. Results and discussionThe lateral and vertical etch rates, anisotropy, and sidewallangle of 10 µm wide Parylene C lines obtained by oxygenplasma removal in each etching mode as functions of appliedpower and process pressure were determined (figures 5–8). Inthe RIE and DRIE modes, the effect of varying the oxygenflow rate was also examined. All of the data presented in theseplots are displayed as mean SE where n 4. RepresentativeSEM images for Parylene C removal by oxygen-only plasmasin each of the different etching systems are presented infigure 9.As expected for plasma etching, the vertical and lateraletch rates were similar; in some cases, the lateral etch rate wasgreater than the vertical etch rate (figure 5). The etched profilesof Parylene C lines were decidedly isotropic. Changing theprocess pressure did little to affect the vertical etch rate butseemed to provide a slight improvement in sidewall angle. At400 mTorr and 200 W, the etched lines were damaged and onlytwo line samples were recovered. Since the sample size was6
J. Micromech. Microeng. 18 (2008) 045004E Meng et al(a)(b)(c)(d )Figure 8. Etch data for Parylene C samples (n 4) processed by DRIE using oxygen plasma: (a) vertical etch rate, (b) lateral etch rate, (c)anisotropy and (d) the measured sidewall angle. Process pressure (13 and 23 mTorr), oxygen flow rate (20, 60 and 100 sccm) and power(400 and 800 W) were varied.was removed prior to replication of etched line features forSEM viewing. In the particular case under examination here,the difference between anisotropy and sidewall angle implieseither reduced vert
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