0 Copyright 1995 American Chemical Society
JUNE 1995 VOLUME 11, NUMBER 6
Letters Selective Copper Chemical Vapor Deposition Using Pd-Activated Organosilane Films Stephen J. Potochnik,*?tPehr E. Pehrsson,t David S. Y. Hsu,B and Jeffrey M. Calved Gas JSurface Dynamics Section, Code 61 74, Center for Bio /Molecular Science and Engineering, Code 6900, and Surface Chemistry Branch, Code 6170, Naval Research Laboratory, Washington, D.C. 20375-5340 Received September 26, 1994. In Final Form: January 30, 1995@ Conductive; adherent copper films were deposited selectively on diamond substrates using ligating aminosilane self-assembledfilms and a Pd-based catalyst. Copper chemical vapor deposition (CVD) was Pa of (hexafluoroacety1acetonato)performed at 444-456 K in a cold-walled chamber using 4 x copper(1)-trimethylvinylsilane mixed 1:l (dv) with Hz carrier gas. In the absence of the Pd catalyst, only isolated copper particles deposited on hydrogenated or aminosilane-coated diamond. The Pd catalyst enhanced selectivity for Cu deposition by a factor of lo3 to lo4. Copper patterns with feature sizes to 1 pm were formed by lithographically patterning an aminosilanefilm on diamond using U V (193nm) radiation and a contact mask prior to catalyst deposition and copper CVD. The Pd catalyst also enhanced Cu deposition on aminosilane-coatedSi(100)and quartz substrates. Treating substrates with octadecylsilane or aminosilane self-assembled films without the bound Pd catalyst reduced copper deposition compared to Si(100) native oxide or hydrogenated diamond surfaces.
Introduction A process has been developed for achieving selective copper chemical vapor deposition (CVD) on diamond substrates using ligating aminosilane self-assembled films and a Pd-based catalyst. The process (Figure 1) involves five steps: (1) surface oxidation; (2) aminosilane film attachment; (3)film patterning; (4) Pd catalyst application; (5) copper CVD. Substrate oxidation generates reactive surface species, such a s hydroxyl groups, that allow covalent attachment of organosilane films. A ligating aminosilane film provides complexation sites for a n oligomeric Pd-based catalyst. In subsequent copper CVD, copper deposits readily from (hexafluoroacety1acetonato)copper(1)-trimethylvinylsilane (hfac-CUI-tmvs) on aminosilane-coated surfaces treated with the oligomeric Pd ~~~~~
~~
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t ASEENRL Post-doctoral Fellow. Gadsurface Dynamics Section. 0 Surface Chemistry Branch. 1' Center for BioAUolecular Science and Engineering. Abstract published in Advance ACS Abstracts, May 1, 1995. @
complex. Copper does not deposit readily on hydrogenated or aminosilane-coated surfaces. To create a pattern of surface reactivity for selective copper CVD, the aminosilane film can be patterned with lithographic techniques prior to Pd catalyst application and copper CVD. Self-assembled organosilane films have been applied to a wide variety of substrates, including Si, SiOz, Al203, metals (Al, W, Pt), polymers (polyethylene, epoxy, polyesters), and diamond.l-' Organosilanes of the general form where R is a n organohctional group and (1)Ulman, A.An Introduction to Ultrathin Organic Films;Academic Press, Inc.: New York, 1991;p 237. (2)Calvert, J. M.; Pehrsson, P. E,; Dulcey, C. S.; Peckerar, M. C. Mater. Res. SOC.Symp. h o c . 1992,260,905. (3) Potochnik, S. J.; Hsu, D. S. Y.; Calved, J. M.; Pehrsson, P. E. Mater. Res. SOC.Symp. Proc. 1994,337,429. (4) Calvert, J. M. J. Vac. Sci. Technol., B 1993,11, 2155. (5)Dressick, W.J.;Calved, J. M. Jpn. J.Appl. Phys. 1993,32,5829. (6) Perkins, F.K; Dobisz, E. A.; Brandow, S. L.; Koloski, T. S.;Calvert, J. M.; Rhee, K W.; Kosakowski, J. E.; Manian, C. R. K.J. Vac. Sci. Technol. B 1994,12,3725. (7)Manian, C. R. K.; Perkins, F. K.; Brandow, S. L.; Koloski, T. K.; Dobisz, E. A.; Calved, J. M. Appl. Phys. Lett. 1994,64, 1.
0 1995 American Chemical Society 0743-7463/95/2411-1841$09.00/0
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1842 Langmuir, Vol. 11, No. 6, 1995
Silane Film
I
,si..,.
3) Pattem
UV Radiation
1
.
1
1
Film
4) Apply
Catalyst 5) Copper CVD
____)
Figure 1. Schematic of the process for patterned, selective copper CVD on diamond. Xis a halide or alkoxide group, react with surface hydroxyl groups to form a covalently-bonded R-terminated fi1m.l Self-assembled films are well-suited to area-selective control of surface chemistry, because they can be patterned using a variety of techniques, including U V radiation, X-rays, and electron beam sources, such as the scanning tunneling microscope (STM).4-8 Selective deposition of materials on patterned organosilane films has been demonstrated to 20-nm geometries using electroless m e t a l i ~ a t i o n . The ~ ~ ~inherent resolution in these selfassembled films is defined by molecular dimensions, which are on the order of nanometers for the aminosilanes used here.g Several organometallic CUI precursors, including hfacCUI-tmvs, deposit high-purity copper films selectively at 423-473 K.9-14 These precursors deposit copper via a disproportionation reaction (1)
-+
2hfac-Cu1-tmvs(g) Cuo(s)
Cu"(hfac),(g)
+ 2tmvs(g) (1)
which proceeds much more readily on metals than on insulating substrates. Selectivity is apparently governed by substrate electronic properties that control the electron transfer necessary for disproportionation of adsorbed prec~rsor.~~-~~ Several investigators have recently presented techniques for controlling selective CVD processes by reducing surface reactivity on certain regions of a substrate. Jain used methylchlorosilanes to enhance selectivity e t al.l1JZ (8)Schoer, J. K.; Ross, C. B.; Crooks, R. M.; Corbitt, T. S.; HampdenSmith, M. J. Langmuir 1994, 10, 615. (9) Reynolds, S. K.; Smart, C. J.; Baran, E. F.; Baum, T. H.; Larson, C. E.; Brock, P. J. Appl. Phys. Lett. 1991, 59, 2332. (10)Shin, H. K.; Chi, K. M.; Hampden-Smith, M. J.; Kodas, T. T.; Paffett, M. F.; Farr, J. D. Chem. Mater. 1992,4, 788. (11)Jain, A.; Farkas, J.; Chi, K. M.; Hampden-Smith, M. J.; Kodas, T. T. AppZ. Phys. Lett. 1992, 61, 2662. (12) Jain,A.;Kodas, T. T.;Jairath, R.;Hampden-Smith, M. J. J. Vac. Sci. Technol., B 1993, 11, 2107. (13) Norman, J. A. T.; Muratore, B. A.; Dyer, P. N.; Roberts, D. A,; Hochberg, A. K. J. Phys. N 1991, I , C2-273. (14)Norman, J. A. T.; Muratore, B. A,; Dyer, P. N.; Roberts, D. A,; Hochberg, A. K.; Dubois, L. H. Mater. Sci. Eng. 1993, B17,87. (15) Cohen, S. L.; Liehr, M.; Kasi, S. Appl. Phys. Lett. 1992, 60, 50. (16)Cohen, S. L.;Liehr, M.; Kasi, S.AppZ. Phys.Lett. 1992,60,1585.
in Cu CVD on W-patterned Si02 substrates. The methylchlorosilanes apparently passivated reactive hydroxyl groupsto reduce unwanted Cunucleation on SiOz. Schoer e t al.8 used octadecyl mercaptan self-assembled monolayers to inhibit copper deposition on gold surfaces. The monolayers were patterned with a STM to obtain selective Cu CVD on gold. The process presented here is the first report to our knowledge that achieves selective Cu CVD by preferentially promoting deposition using self-assembled film technology, rather than passivating intrinsically active surfaces. Diamond provides a unique model surface for selective copper CVD studies. Undoped diamond has a wide band gap (5.5 eV) and thus is essentially in~u1ating.l~ Hydrogen-terminated diamond serves as a paradigm of hydrocarbon polymers or highly-ordered self-assembled alkane monolayer surfaces. This selective CVD process was also applied to silicon and quartz substrates and should provide selectively enhanced copper deposition on any dielectric or semiconducting substrate that can be modified with self-assembled films and can withstand the CVD processing conditions.
Experimental Methods In the current work, selective copper CVD was studied on undoped synthetic high-pressure, high-temperature (HPHT) single-crystaldiamonds (Sumitomo Electric Carbide)sawed and mechanically polished within 3" of the (100) face. Substrates were cleaned with sequential ultrasonic agitation in acetone, l,l,l-trichloroethane, and methanol, boiled sequentially in 3:l HC1/HN03 and 3:2H z S O m 0 3 , and then cleaned in 1:l:lHF/ HNOdacetic acid at 295 K. Diamond surfaces were then hydrogenated in a microwave plasma using 1300 Pa and 400 sccm Hz for at least 20 s at 1073 K. Long Hz-plasma treatments (e.g.,120 min)etched and smoothed the diamond surfaces, which gave sharper LEED patterns than the as-received surfaces.18 X-ray photoelectron spectroscopy ( X P S )showed that diamond surfaces contained approximately 1atom % 0 and 99 atom % C after Hz-plasma treatment. Hydrogenated diamond surfaces do not readily adsorb contaminants from the air and provide a well-characterized starting surface for further modification. To allow attachment of the aminofunctional silane films to diamond, hydrogenated surfaces were oxidized in a radio frequency plasma using 130 Pa and 250 sccm 0 2 for 2-4 min at 200 W. Plasma oxidation generated sufficient reactive surface moieties for attachment of the silane film^.^,^ The identity and distribution of reactive oxygen-containing species on these surfaces, however, is unknown and under investigation. Hydroxyl species,which provide reactive sites for organosilane film attachment on Si, SiOz, and related substrates,' have been identified on oxidized diamond surfa~es.l~-~l Amino-functional self-assembled films were deposited by immersing oxidized substrates in a 1%(v/v)aqueous solution of [(3-(2-aminoethyl)amino)propylltrimethoxysilane(EDA, from Huls America) with 1 mM acetic acid or a 1%(v/v) solution of m,~-(((aminoethyl)amino)methyl)phenethyltrimethoxysilane (PEDA,from Huls America)in 95:4(v/v)CH@H/&O with 1mM acetic acid at 295 K for 20 min. After coating, substrates were rinsed in deionized (DI) HzO or CH30H, dried in a stream of filtered Nz, and baked for 5 min at 493 K to cure the films. Ellipsometry and atomic force microscopy (AFM)measurements
suggest that EDAzZand PEDAZ3form disordered films of roughly (17) Stoneham, A. M. In The Properties of Natural and Synthetic Diamond; Field, J. E., Ed.; Academic Press, Inc.: San Diego, CA, 1992; P 3. (18)Thoms, B. D.: Owens, M. S.; Butler, J. E.; Spiro, C. A .. p p l . Phys. Lett. 1994, 65, 2957. (19) Sappok, R.; Boehm, H. P. Carbon 1968, 6, 573. (20) Ando. T.: Yamamoto. K.: Ishii.. M.:. Kamo,. M.:. Sato,. Y. J. Chem. SOC.,Faraday. Trans. 1993; 89, 3635. (21) Evans, S. In The Properties ofNatura1 and Synthetic Diamond; Field, J. E., Ed.; Academic Press, Inc.: San Diego, CA, 1992; p 181. (22) Brandow, S. L.; Dressick, W. J.; Marrian, C. R. K.; Chow, G. M.; Calvert, J. M. J. EZectrochem. SOC.,in press.
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Langmuir, Vol. 11, No. 6,1995 1843
Figure 2. Scanning electron micrographs of C(100) surfaces after copper CVD for 24 min at 446 K. Surface treatments prior to Cu CVD: (a, left) H2-plasma treated, (b, middle) oxidized and EDA-coated, and (c, right) Pd catalyst applied to EDA film. Isolated Cu particles are visible in (a). The features in (b) are pits in the C(lO0) surface caused by H~plasmaetching. monolayer or submonolayercoverage on native oxide surfaces of Siusing these depositionconditions. Organosilanechemisorption on oxidized diamond surfacesis not well understood and is under investigation. The amine functionality in EDA and PEDA provides complexation sites for subsequent attachment of the Pd-based catalyst, and PEDA requires a lower photochemical dose for lithographic patterning with W radiation than EDA.4*5 The oligomeric PdCb2--based catalyst,5pNwhich catalyzes electroless deposition of Ni and Co, was deposited on the ligating amino-functional silane films from aqueous solution. The chemical and structural properties of the Pd-catalyzed surface that is active for Cu CVD are under investigation. This selective deposition process was also performedon native oxide surfacesof Si(100)(p-type, 1- 10Qcm)and Si02substrates, which were cleaned and hydroxylated with sequential ultrasonic agitation in toluene and methanol, immersed for 30 min in 1:l HCVCHJOH, rinsed with DI H20, immersed for 30 min in concentrated H2S04, rinsed with DI H20, and then immersed for 5 min in DI H2O at approximately 360 K prior to film attachment. The aminosilane films and the Pd-based catalyst were applied using the samemethodsas those used for diamond. To determine if alkylsilane films limited copper deposition on Si in a manner similar to how octadecylmercaptan films limited deposition on A u , ~an octadecylsilanefilm was applied to Si by immersing a cleaned Si(lO0)native oxide substrate in a 1%(v/v) solution of octadecyltrichlorosilane(OTS, from Huls America) in toluene for 7 min at 295 K under an argon atmosphere. Copper depositions were performed for 12-24 min at 444456 K and a total chamber pressure of 8 x Pa using (hexafluoroacetylacetonato)copper(I)-trimethylvinylsilane mixed 1:1(v/v)with H2carrier gas. The pressure at the substratesul'face Pa, however, because the gas mixture was higher than 8 x was directed at the sample surface with a 12-mmdiameter glass
doser tube during deposition. Details of the cool-wall, lowPa)have been pressure CVD chamber (base pressure = 4 x copper reported e l ~ e w h e r eFor . ~ each ~~~ ~ ~ ~deposition experiment, four to seven individually-processedsubstrates were mounted with silver paint on a 1 x 1cm2Si wafer to allow comparisons of deposition selectivity under identical CVD conditions. The wafer was mounted in the chamber on a Pt foil that was heated resistively, and sample temperature was monitored with a PtPt(lO% Rh) thermocouple spot welded to the Pt foil. The hfacCUI-tmvs precursor (SchumacherCo.)was purified with several freeze/pump/thaw cycles and heated to approximately 320 K. Immediately prior to Cu deposition,with the doser turned away (23) M o r , C. N.;Turner, D. C.; Georger,J. H.; Peek, B. M.; Stenger, D. A. Lungmuir 1994,10,148. (24) Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calabrese, G. S.; Calved, J. M. J. Electrochem. Soc. 1994,141,210. (25) Hsu, D. S. Y.; Turner, N. H.; Rerson, K. W.; Shamamian, V. A. J. Vac. Sci. Technol., B 1992, 10, 2251. (26) Hsu, D. S. Y.; Gray, H. F. Appl. Phys. Lett. 1993,63, 159.
fromthe sample, the sample was heated in vacuum to the desired deposition temperature in about 10 min. Sample temperature was subsequently maintained in approximately 1sccm of flowing H2 and a chamber pressure of 4 x 10-3 Pa for 10 min. Then the precursor was metered into the H2 stream until the total chamber Pa and the doser was immediately pressure increased to 8 x rotated to a position 3 mm in front of and normal to the sample for copper deposition. After 12-24 min, the precursor flow was stopped and the sample temperature was maintained in flowing H2 for approximately 5 min. The sample was then cooled at 10 Wmin to 350 K and maintained at this temperature for 30 min to allow unreacted precursor to desorb. Then the H2 flow and sample heating were turned off.
Results and Discussion Copper CVD was performed for 12-24 min at 444-456 K on diamond, Si, and Si02 substrates that received various treatments prior to mounting in the CVD chamber. Scanning electron microsopy (SEM) images and composition analysis with X-ray photoelectron spectroscopy (XPS) showed that copper deposited selectively on surfaces that were modified with the aminosilane films and the Pdbased catalyst. Deposition selectivity was qualitatively reproducible over the range of deposition conditions used in this preliminary study. Figure 2 shows SEM images of Ha-plasma etched, EDAcoated, and Pd-activated C(100)substrates after copper CVD for 24 min at 446 K. Only isolated Cu particles were detected on the hydrogenated and EDA-coated surfaces. The enhancement in copper deposition due to the Pd catalyst is dramatic; a continuous copper film formed on the Pd-catalyzed substrate. The selectivity enhancement due to the Pd catalyst was estimated by comparing copper particle coverages, which correlate to nucleation densities, on the hydrogenated, EDA-coated, and Pd-catalyst-treated diamond substrates. The Cu particle density on the smoothed and hydrogenated diamond (Figure 2a), which received a 120-min H2-plasma treatment prior to Cu CVD, was approximately 8 x lo6 particles/cm2. In a few isolated regions on this surface, Cu particles formed in distinct lines. The cause of this ordered deposition is under investigation and may be due to surface structures originating from crystal growth or surface polishing. In contrast, a hydrogenated diamond surface that received only a 20-sH2-plasma treatment had a particle density of 2 x lo7particles/cm2. Nucleation of copper particles apparently occurred at surface defects, which are removed by the H2-plasma treatment. Copper
1844 Langmuir, Vol. 11, No. 6, 1995
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to over 90 atom % and Cu concentrations decreased on these surfaces. The OTS-coated Si sample contained less Cu than the untreated Si surface. Hence, the OTS film apparently reduced copper deposition on Si. This blocking effect of the OTS film is qualitatively similar to the findings of Schoer et al.,s who found that octadecyl mercaptan monolayers excluded Cu deposition from a gold surface, although additional experiments are required to better quantify the effects of alkylsilane films in Cu CVD. Maximum selectivity in Cu CVD may be obtained by using 20 the aminosilane films and the Pd catalyst in combination with OTS or related films. Oo 20 40 60 ao 100 120 140 On the Si surface treated with EDA and the Pd catalyst, Si was not detected with XPS after Cu CVD or after 35 s of sputtering (Figure 312). Scanning electron microscopy images recorded prior to sputtering revealed a Cu film on this surface similar to the Cu film on diamond shown in Figure 2c. The Cu film was thick enough and continuous enough to obscure the Si substrate from detection by XPS. An estimate of the XPS detection depth for Si beneath a Cu film27showed that the Si substrate was covered with at least 6 nm of Cu prior to sputtering. In addition, copper was the dominant species detected with XPS throughout the first 200 s of sputtering of the Pd-catalyzed Si surface. A sputtering rate estimate of 0.1-0.2 nmfs for Cu using this instrument indicates that sputtering for 250 s removed approximately 25 to 50 nm of Cu. Thus, the sputtering profile (Figure 3c) is consistent with the Cu film thicknesses of 50-60 nm measured by profilometry (Tencor Alpha Step 250) for similar films deposited in 24 min a t 444-447 K. The high oxygen concentration (-10 atom %) in the subsurface region of the Pd-catalyzed Si surface may have been due to 0 incorporated in the Cu film during deposition or oxidation of the granular Cu film by exposure to air after Cu CVD. In addition, since Cu-Si mixtures oxidize readily,28,29 residual 0 2 or HzO in the XPS chamber may have reacted with the exposed sample surface during profiling and analysis. Sputtering T h e ( 6 ) Since the aminosilane films and Pd catalyst yield Figure 3. Composition depth profiles of Si(100)surfaces after selectively enhanced copper deposition, various lithoCu CVD for 24 min at 447 K. Surfaces treatments prior to Cu graphic patterning techniques can provide area-selective CVD: (a)Si native oxide with no additionaltreatment; (b)OTScoated; (c) Pd catalyst applied to EDA film. Legend: 0 , Si; ., control of surface reactivity for Cu CVD. Hence, a patterned aminosilane film was formed on a C(100)surface Cu; 0, 0; 0, C; 0, Pd. prior to catalyst application and Cu CVD by exposing a PEDA-coated diamond to 1.1J/cm2 of 193-nm radiation particle coverage on the EDA-coated surface (Figure 2b) from an ArF excimer laser through a contact mask was less than 1x lo6particles/cm2. The EDAfilm reduced (Figure 1, step 3). Irradiation of the unmasked PEDA copper deposition on diamond. The film may hinder cleaved the amino-functional moieties that are necesadsorption, mobility, or reaction of the precursor on the sary for catalyst attachment from the s u r f a ~ e Figure .~~~ surface. In contrast, particle density for uniform Cu 4 shows optical micrographs of Cu features deposited deposition like that on the Pd-activated surface (Figure on PEDA-coated diamond after film patterning, Pd2c) is > 1O1O particles/cm2. Thus, selectivity enhancecatalyst application, and Cu CVD for 24 min a t 444 K. ments of a t least lo3 to lo4 were achieved with the Pd Figure 4b shows resolution of the smallest features (1 catalyst. pm) on the contact mask; smaller features should be Composition depth profiles (Figure 3) of three Si(100) attainable with higher resolution masks or other patnative oxide surfaces that received different treatments terning technique^.^-^,^^ prior to copper CVD also show the dramatic selectivity enhancement achieved with the Pd catalyst and the ability The copper film resistivities of approximately lOpQ-cm, of a n OTS film to reduce copper deposition. Copper CVD as measured by a four-point probe, were higher than that was performed for 24 min a t 447 K on clean Si native of bulk copper (1.67 pQ.cm) but compare well with values oxide, OTS-coated Si, and EDA-coated Si that was treated for similar thin CVD-deposited copper Scotch with the Pd catalyst. Depth profiles of each surface were then acquired using alternating cycles of X-ray photo(27) Seah, M. P. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; John Wiley and Sons: Chichester, 1983; p 211. electron spectroscopy (XPS,Surface Science Laboratories (28) Harper, J. M. E.; Charai, A,; Stolt, L.; DHeurle, F. M.; Fryer, SSX-100)and sputtering for 5-30 s with 3-keV Ar+ ions. P. M. Appl. Phys. Lett. 1990, 56, 2519. Compositions were estimated from XPS peak areas using (29) Stolt, L.; Charai, A.; DHeurle, F. M.; Fryer, P. M.; Harper, J. M. E. J. Vac. Sci. Technol. A 1991, 9, 1501. software provided with the instrument. The Si (Figure (30) Calvert, J. M.; Calabrese, G. S.; Bohland, J. F.; Chen, M. S.; 3a) and OTS-coated Si (Figure 3b) surfaces contained 11 Dressick,W. J.;Dulcey, C. S.;Georger, J. H.; Kosakowski, J.;Pavelcheck, and 4 atom % Cu, respectively, after Cu CVD. During the E. K.; Rhee, K. W.; Shirey, L. M. J. Vac. Sci. Technol. B 1994, 12, first 50 s of sputtering, Si concentrations increased rapidly 3884.
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Cu from diamond, Si, or Si02 surfaces. Film thickness estimates show that copper deposited at 2-3 n d m i n on Pd-activated surfaces at 444-447 K in our chamber. By analogy to other selective deposition results using hfacCUI-tmvs and similar CUIprecursors at 433-453 K and 1-70 Pa,9-14 we expect high-purity, high-conductivity copper films can be deposited on Pd-activated surfaces at rates of more than 100 n d m i n . In summary, a process has been developed for selective chemical vapor deposition of adherent, conductive Cu films on diamond, Si, and SiO2. An oligomeric Pd catalyst, attached to the substrate through a ligating aminosilane self-assembled film, activates the surface for copper deposition. Lithographic patterning of the aminosilane film allows selective deposition of Cu on the patterned, Pd-catalyzed surface. In contrast, octadecylsilane or aminosilane films without the Pd catalyst can reduce Cu deposition compared to H-terminated diamond or Si(100) native oxide surfaces. Since these surface modification and patterning techniques are broadly applicable, this process may be useful for enhancing copper deposition on other substrates that can be fhctionalized with selfassembled films and can withstand the CVD process conditions.
Figure 4. Optical micrographs of patterned Cu features deposited on a C(100) surface: (a, top) “2 x 8” identifies the 2-pm-wide Cu lines separated by 8-pm-wide spaces in the lower right quadrant of image; (b, bottom) features in lower half of image are 1-pm-wide Cu lines separated by 2-pm-wide spaces.
Acknowledgment. The authors gratefblly acknowledgeM. Chen, W. Dressick, C. Dulcey, D. Ma, M. Owens, and L. Troilo for technical assistance, and W. Gladfelter and J. Butler for useful discussions. Funding for this work was provided by the Office of Naval Research (ONR) under the Chemical Vapor Processing Program and Molecular Engineering Accelerated Research Initiative. S.J.P. also wishes to acknowledge the American Society for Engineering Education and the Technology Directorate of ONR for a postdoctoral fellowship at the Naval Research Laboratory.
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