f- fAl 623 NETHACRYLANIDE COPOLYNER RESISTS FOR ELECTRON BEAN 1/1LITHOORPNY(U) CORNELL UNIV ITHACA NVY M NANASTE ET AL. 26 FED BE TR-2 N60114-SS-K-64?4

UNCLRSSIFIED F/6 t19 NL

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MICROCOPY RESOLUTION TEST CHART... NA' AU1UW1- Ml 'A~l

OFIEOF NAVAL nJRSEARCH

0 Contract NOOO 1 4-85-K-0474

(0 Technical Report No. 2

METHACRYLAM-IDE COPOLYMER RESISTS E E T _

FOR ELECTRON BEAM byLITHOGRAPHY SM 1 -L8sj3

Y. M. N. Namaste, S. K. Obendorf, and F. Rodriguez

Prepared for presentation at theSanta Clara Symposium on Microlithography h

Conference 63 1: Advances in Resist Technology and Processing II ISociety of Photo-Opt ical Instrumentation Engineers

Santa Clara, CA, March 9-14, 1986

Olin Hall, Cornell University .

School of Chemical Engineering rIthaca, NY 14853

Reproduction In whole or In part Is permitted for* 7, any purpose of the United States Government

*This document has been approved for publ ic release **

LII and sale; Its distribution Is unlimited

7 094

V'. .r - - •' -' t %-7 - ---W

Paper #631-13, SPIE Procs)

SPIE Santa Clara Symp. on jMicrolithography, Conf.631

Adv. Rebist. Tech •&Proc.I1

%March 10,11, 1986%r

Methacrylamide copolymer resists for electron beam lithography %

- L.M.N. Namast6. S.K. Obendorf, F. Rodriguez

Department of Textiles and Apparel. Department of Chemical EngineeringCornell University. Ithaca. New York 14853

Abstract " .

Polynethpcrylamide (PMAAm) and copolymers of MAAm with methyl methacrylate weresynthea d and evaluated for their applicability to electron beam lithography. Thesensiti V'6'of PMAAm has previously been reported as less than I MC/cml, with thermalstability at temperatures up to 330*C.' Despite these claims. further lithographicevaluation of this resist system is apparently absent from the literature. This researchwas conducted to further investigate the lithographic performance of these resists and to

determine their sensitivity using current definitions.

Using PHAAm homopolymer (Mw - 8.1 x 10'). with a 15 minute prebake at 2001C, thelithographic results were much poorer than expected. Patterns exposed to doses of10 vC/cm' or lower could not be developed using water as the developing solvent. forceddeveloping with Na.SiO, solution (pH-io) developed lower doses than water, but muchgreater thinning was observed. An unexposed thinning of 10% occurred when developingexposures of 15 uC/cm2 with water, and 40 VC/cm' with WaSiO, solution (20 KV). Swellingof the unexposed polymer and some adhesion problems were observed.

The high sensitivity previously reported for PMAAm' can not be attributed solely tochain scission efficiency (G3). which has been reported to be only 1.5 times that of PMMAI .

(Cs determined by 1-irradiation). An induction period In the dissolution of unexposedpolymer has also been sugggested as contributing to the sensitivity of this resist. Inthe present work, dissolution induction periods were observed witn laser interferometryfor the unexposed films, but the magnitude of these induction periods could not accountfor a large enhancement of sensitivity. Imide crosslink formation may have beenresponsible for the previously reported sensitivity of PMAAm.' In the present work. imide _.tformation was not observed, either after prebaking coated wafers at 180 to 240*C orheating of polymer solutions for 7 hours at SO0C. Apparently, the conditions for useful

for.ation of imide are difficult to reproduce.

3ecause of the problems encountered with the PMAAm homopolymer during lithography, aseries of MMA - MAAm copolymers were synthesized and evaluated as potential resists.These copolymers eliminated the problems of swelling and poor solubility of thehomopolymer, but sensitivity enhancement over that of PMMA was minimal.

Introduction

The search continues for a highly sensitive positive electron beam resist. Manyresearchers are presently working toward the goal of p'ducing a resist with sensitivityin the 1 uC/cm' range, which concurrently exhibits good thermal stability and reactive ionetch resistance.

Several years ago, workers in Japan reported on polymethacrylsmide (PMAAm) as apositive electron resist. They reported a sensitivity of about 0.6 VC/em' at anaccelerating voltage of 20 KV and thermal stability at temperatures up to 330C.Although this work is referenced In numerous review articles, there is apparently nofurther work reported on this resist since the original work was completed. This issurprising, given the excellent sensitivity originally reported.

In the present work, methacrylamide polymers are synthesized and evaluated with theintention of reproducing previous sensitometry and then improving on the resist system by%evaluating similar polymers and copolymers of methacrylamide. The factors influencing thesensitivity are Investigated, including chain-scisSion efficiency. prebake conditions.developing systems, and dissolution induction periods.

Experimental

Polymethacrylamide (PMAAm) homopolymers and methacrylamide - methyl methacrylate

copolymers (HAAm-MMA) were synthesized by free-radical polymerization, initiated withpotassium persulfate. The polymerizations were carried out in mixtures of water and

I

IIECUMlTV CLASSIFICATION OF THIS PAGE (Whe.n Oslo Engted)

REPOT DCUMNTATON AGEREAD INSTRUCTIONSREPOR DOCMENTTIONPAGEBEFORE COMPLETfl4O FORM1REPORT NUMBER 12. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

4. IL adSblh S. TYPE of' REPORT &PERIOD COVERED

METHACRYLAMIDE COPOLYMER RESISTS FOR ELECTRON Technical ReportBEAM LITHOGRAPHY 6. PERFORMING ORG. REPORT NUMBER

C. 7. AUJTHOR(@) 6. CONTRACT Oft GRANT NUMBER(@) C.

Y.M.N. Namaste, S.K. Obendorf, and F. Rodriguez N00014-85-K-0474

9. PERFORMING ORGANIZATION NAME AND ADDRESS III. PROGRAM ELEMENT. PROJECT. TASK___

Cornell University AREA a WORK UNIT NUMNERS

Ithaca, NY 14853

It. CONTROLLING OFFICE NMU AND ADDRESS 12. REPORT OATS

Chemistry February 28. 1986Office of Naval Research 12. NUMBER OFPAGES

Arlington. VA 7 .17-7I&. MONI1TORING AGENCY NAME A ADORESS5ildifterent from, Controling Office) 1S. SECURITY CLASS. (of this report)

Unclassified

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I?. DISTRmbUTION STATENEN r (at the abettedt sam~ed J &lot* 2p, it EIiente heov Report)

Abstracts Distribution List, 356A/413-2.For unlimited distribution and public release.

Ill. SUPPLEMENTARY NOTES

To b prsened a "Avanes i ReistTecholoy ad PrcesingIII, pat otohe presnte Caat SyAdvam n iResistTechoy nrd Procen Soc.o part of

Optical Instrumentation Engineers, March 9-14, 1986, Santa Clara, CA.

It. KEY WORDS (ConlInwo an foewer*@ ade it noceee7' ed 14100110r Y block6 number)

Poly(methacrylamide)IMicrol ithographyElectron beam resistMethacrylamide copolymer

AbEA

AD. TRACT (Coninueo etO ro* $odd it n.eoons410 Old identIr'b bFee a1chmbe)

~~Polymethacrylamide-(PMAAm) and copolymers of MAAi with methyl methacry-late were synthestzed and evaluated for their applicability to electron beamlithogr.pj...,- The sensitivity of PMAAm has previously been repo ted as less,~ ~ ?Q% than 1 JYM.with thermal stability at temperatures up to 330YC.1 epriethese claims, further lithographic evaluation of this resist system is appar-ently absent from the literature. This research was conducted to furtherinvestigate the lithographic performance of these resists and to determineA

DD I O:1 1473 EDITION or 1 NOV es is ObSOLETeSh4 102.F4)d601SECURITY CLASSIFICATION OF Bat'ALeueo mrnd

... %,rr.et-5& 5 eC.

SEURT CLASSIFICATION OF THIS PAGE ("o.n Do&. Ent.eed

Using PMAAm homopolymer (Mw = 8.1 x 106),-with a 15 minute prebake at200t , the lithographic .results were much poorer than expected. Patternsexposed to doses of 10 ,Z or lower could not be developed using water asthe developing solvent. Forced developing with Na 2 SiO 3 solution (pH = 10)developed lower doses than water, but much greater thinning was obser ed. An 4unexposed thinning o U0% occurred when developing exposures of 15 -m-with water, and 40 #Ccm2 with Na2SiO 3 solution (20 KV). Swelling of the un-exposed polymer and some adhesion problems were observed.

The high sensitivity previously reported for PMAAm1 cannot be attributedsolely to chain scission efficiency (Gs), which has beo reported to be only1.5 times that of PMMA 2 (Gs determined by y-irradiation). An inductionperiod in the dissolution of unexposed polymer has also been suggested ascontributing to the sensitivity of this result., In the present work, dis-solution induction periods were observed with )aser interferometry for theunexposed films, but the magnitude of these $1duction periods could not ac-count for a large enhancement of sensitivd. Imide crosslink formation mayhave been responsible for the previously reported sensitivity of PMAAm.1 Inthe present work, dissolution induction periods were observed with laserinterferometry for the unexposed films, but the magnitude of these inductionperiods could not account for a large enhancement of sensitivity. Imidecrosslink formation may have been responsible for the previously reportedsensitivity of PMAAm.1 In the present work, imide formation was not obser-ved, either after prebaking coated wafers at 180 to 240'C or heating of poly-,.mer solutions for 7 hours at 80'C. Apparently, the conditions for usefulformation of imide are difficult.td reproduce.-- Because of the problems encountered with the PMAAm homopolymer during

lithography, a series of MMA-MAAm copolymers were synthesized and evaluatedas potential resists. These copolymers eliminated the problems of swellingand poor solubility of the hompolymer, but sensitivity enhancement over thatof PMMA was minimal.REFERENCES1. Matsuda, S., Tsuchiya, S., Honma, M., Hasegawa, K., Nagamatsu, G. and

Asano, T. Polym. Eng. Sci., 17, No. 6, 410-413 (1977).2. O'Connor, D.J., "Radiation Degradation of Polymethacrylamide," Doctoral

Thesis, City University of New York, 1984.

1 .

S --..%....- .-.

-2-

Table I - Polymer synthesis

TemperHAAm NNA Water Acetone Initiator ature Time Recovery

Polymer (g) (g) (ml) (ml) (g) ('C) hrs.) (g)

P AAU .:.W(low MV) 25 -- 65 130 0.05 60 4 17

PMAAm(high MW) 25 -- 22.5 65 0.018 55 7 17 pP(MMA-co-21% MAAW) 7 18 22.5 65 0.018 55 2 2.8

P(MMA-co-49% MAAm) 15 10 22.5 65 0.018 55 2.25 1..1

P(MMA-co-71% MAA) 20 5 22.5 65 0.016 55 1 1.6

acetone. Details or the polymerizations are listed in Table I. Two molecular weights ofthe PMAAm homopolymer were synthesized, and three Compositions or the MAAm-MMA copolymerswere synthesized. The homopolymers synthesized are soluble in formamide, and in aqueoussolutions or acetic acid and urea. The calculated compositions and measured intrinsicviscosities or the MAAm-MMA copolymers are presented in Table II. The copolymercompositions have been confirmed using IR spectroscopy.

Table II - Copolymer compositions and viscosities

Mol. Fraction MAAs Intrinsic Viscosity(calculated) [n], dl/g 30*C Solvent

1.00 2.5 Acetic acid(glacial)

0.71 2.8 Acetic acid(O.TOT wt. fraction)

0.49 3.1 Acetic acid .,.,(glacial)

0.21 0.7 Chloroform t .

Intrinsic viscosities or the PMAAm homopolymers, determined in 8 M urea at 30*C. are0.405 and 0.90 dl/g. Using a relationship reported by Titkova* for 8 N urea, theseviscosities correspond to weight-average molecular weights (Mw) of 2.3 x 10' and 8.1 x . %10. The higher molecular weight PMAAm is not soluble in water at any temperature. The %.%

lower molecular weight PMAAm Is soluble in water only when heated to 90*C. but this %polymer reprecipitates upon cooling. In the present work. the best solvent identified forthe homopolymer is a 40% aqueous solution of acetic acid. which Is used for casting the--"homopolymer films.

Matsuda and coworkers' reported on a PMAAm polymer that is soluble in water at elevatedtemperatures and does not reprecipitate upon cooling. The intrinsic viscosity of this ,. s--"polymer Is reported to be 1.25 dl/g In water at 30*C. Using a relationship reported forpolyacrylamide. the authors calculated a viscosity average molecular weight (Mv) of 2.4 x -101.

Titkova' reported that PMAAs with Mw < 6.8 x 10' is soluble In water at 25'C. and thatPMAAm with 6.8 x 10' ( Mw < 6.0 x 101 is soluble In water at 80-90*C. 'the process being

reversible, and PMAAm with Mw above 6.0 x 101 Is Insoluble In water at any temperature.The results of the present work agree with those of Titkova: The polymer synthesized with tMw - 2.3 x 10' fits into the second category, being soluble in water above 80*C. and thepolymer with Mw - 8.1 x 10. fits into the third category delineated by Titkova. being

Availability Codes

Avag andIorD is se

* ~Ir &X .71

co-yorkers' with a reported molecular weight (MY) of 2.4 x 10' would not be expected to

- ..

remain dissolved In water when cooled to room temperature based o the ork of Ttkova. nd

Infrared spectra of the PMAAm homopolymer and copolymers were measured with a VPerkin-Elmer 683 IR Spectrophotoaeter. Approximately 1 pm thick films of the resists werespin-coated onto 3-in. diameter silicon warers. and transmission through the res~5t andwater was measured using a blank wafer in the reference beam. Differential scanningcalorimetry was conducted with a Perkin-Elmer OSC-4. with a scanning rate of 10*C/in. ina nitrogen atmosphere.

For lithographic evaluations, films of each resist material were spin-coated onto3-in. diameter silicon wafers. A solution of 40% acetic acid was used for casting thePMAAs homopolymer and the 71% MAAs copolyzer of ItA. The 49 HAAm copolymer was cast fromglacial acetic acid. and the 21% MAAm copolymer from 2-ethoxyethanol (cellosolve). PHM'A(for reference) was cast from chlorobenzene. The copolymer films were prebaked at 200Cfor 15 minutes. The PHAAm homopolymer was prebaked at 180. 200. 220. and 2401C for 15minutes for an analysis of the effect of prebake conditions on lithographic performance.Film thicknesses were measured with a Tencor alpha step.

Lithographic test patterns were produced with a Combridge EBHF-II-150 using an

accelerating voltage of 20 KY. Contrast and thinning were determined by measuring filmthicknesses after developing 20 um wide lines exposed at a series of 16 doses. Developingwas conducted in stagnant Solutions. terminated with an appropriate non-solvent, followedby drying with a stream of nitrogen. A 15 minute postbake at 150-C was used.

Dissolution rates were measured using a laser interferometer film thickness monitor(LIfTM)' for preliminary optimization of developing systems prior to lithographicevaluation. LIFTH was also used to study prebake effects on the PMAAm homopolymer byobserving solubility rate ratios and swelling behavior. Dissolution induction periodswere also observed with laser Interrerometry.

Results

The P-Am hosopolymers synthesized are not soluble in water at room temperature, andtherefore films are cast from solutions in 40% acetic acid In water. in this respect, the ".procedures used in the present work differ from the previous work of Matsuda andco-workers,' who used only water for casting the PMAAm films.

Under all experimental conditions, swelling of lithographic patterns was observed forthe high molecular weight (Mu -- 8.1 x 10) homopolymer. which limited the resolution ofthe lithographic patterns to about 1 um (Figure 1). Swelling was not alleviated by .altering the developer pH. or by increasing the prebake temperature. The lower molecular V h

weight PMAAm homopolymer (Mw - 2.4 x 10') also exhibits severe swelling as well as-::: adhesion problems.

- ~ %

'. *4. . 4..- .?.:~.?

'

figure 1. SEN photomicrograph of lithographic patterns written in PHAAm homopolymer.Electron dose is 20 VC/cm', and the developer Is Na.SiO, solution (pH - 10). The film hasbeen prebaked at 220C for 15 minutes.

e 4 "

t -. ' "

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Figure 2. Thinning curves for PIAAO...---Mw - 8 a 10"; prebaked at 220"C. Ordinate Figure 3. Contrast curves for PMAAU . %.axis shows the thickness of the unexposed lomopolymer. 200eC prebake. p,film remaining at the time which ther--

dose shown on the abscissa has been

completely developed.

The thinning behavior of PMAAm depends greatly on the developer. Using a 220C prebake"- "'"""

0.0,

and developing with pure vater 10 of the unexposed background was developed in order tocompletely develop a 20 Orm dde line exposed at 15 3.C/em (Figure 2). Because or thelimited solubility in water, resist exposed at doses lower than 10 uC/E is not developed"-"

ilth pure water, regardless or developing time. Increasing developing time does notdevelop lowner doses, but only increases thinning of the unexposed flm, presumably by

hextraction o" lover molecular edght fractions of the polymer. A rough calculation offinal molecular eight after exposure to 10 uC/c1 of 20 Key electrons yields a Ofnal

molec ular weight within an order of magnitude of the threshold molecular weight forsolubility in water at room temperature (Mw - 6.8 x 10*) reported by Titkova, et al.'

Dilute aqueous solutions of sodium silicate (Na,SIO,) are forced developers for thePMAAm resist, and do not exhibit a solubility limit as observed with water, as even theunexposed resist is soluble in these solutions. Thinning occurs to a much greater extentwith sodium silicate solutions than with water. Ten percent thinning occurs fordevelopment of patterns exposed at 0 UC/cm', using a sodium silicate solution with a pHof 10 (Figure 2). This thinning behavior in the forced developing regime does notrepresent any Improvement over that exhibited by PMMA."

Adequate contrast is obtained when developing PMAA% with pure water. A contrast (Y) of2.0 is measured when developing patterns exposed to a dose of 13 UC/cm' in water for 0.5minute (Figure 3). Increasing developing time reduces the contrast and does not developpatterns exposed at lower doses (Figure 3).

Using prebake temperatures in the range of 180 to 24OC. no significant differences Inlithographic performance are observed. Swelling occurs when using any of these prebaketemperatures, and the contrast Is practically independent of prebake temperature. Prebake *temperature, over the experimental range, does not affect the solubility limit experienced JAwith aqueous developing, nor the degree of thinning by forced developing with sodiumsilicate solutions.

V-

%

IF on. . -

-5-rp

Previous workers reported a sensitivity of 0.6 pC/m for PHAAu with an initialmolecular weight (Mv) Ot 2.4 1 10% Using a 15 minute prebake at 2001C and an acceleratingvoltage of 20 KV.' Although the criterion for sensitivity determination vas notpublished. this is clearly a greater sensitivity than that determined in the presentwork. What factors could be responsible for the high sensitivity previously reported forPhAAm? The chain scission efficiency (Gs). determined with gamma irradiation, has been

reported to be 2.05. or about 1.5 times that of PHNA.' This modest increase in G-scissioncould not be responsible for the nearly 100-fold increase in sensitivity over that ofPMMA.

It has been proposed that the sensitivity o this resist may be enhanced by an

induction time during dissolution. Using a laser interferometer file thickness monitor(LIFTH)I. induction periods are observed during dissolution of unexposed files o PMAAm

and the hAAm-MMA copolymer containing 21% HAAm. The copolymer provides an excellentexample of an induction period which is seen as the initial flat portion of the intensitycurve that occurs before the characteristic sinusoidal output created by the interferencepattern during dissolution (Figure 4). The observed induction periods are not stronglyaffected by exposure to vacuum or aging. In the case of the PMAAm homopolymer. theinduction period is actually a period of severe swelling, occurring prior to dissolution.

Since these induction periods are absent during dissolution of the exposed films, thepnenomena would tend to increase the solubility rate differential between exposed andunexposed polymer, thus Increasing the sensitivity. However, the magnitude of these

induction periods are such that about 0.05 um of exposed resist (20 ,C/cml) would be

expected to dissolve during the induction period of the unexposed resist. The magnitudeof this effect would not account for the high sensitivity reported for PMAAm resist. The

concurrent swelling of the unexposed PHAAm homopolymer also limits any contribution of the

induction period to enhancement of sensitivity for that system.

FFLIL

z

% g - --~1 FILM DISSOLVIN.G . . % "

U

o 5 .0 i5 20

TIME (mill-)

Figure 4. Dissolution induction period exhibited with poly(MMA-21% MAl).

The most likely cause of the previously attained sensitivity reported by Matsuda and

co-workers' is the formation of intermolecular igide crosslinka. It Was reported that 62%

imidization occurred after heating a 7% aqueous solution o PMAAm for 7 hours at 60"C' or

80"C.' Imide formation was also reported for prebaking on the wafer at 200-250"C.'Crosslinking has been shown to enhance sensitivity by Increasing the initial molecularweight of resists and reducing thinning of unexposed resist.' Presumably because of the

imide bridges, the highly selective aqueous developing systems were able to develop the

relatively large exposed areas without much concurrent thinning of the unexposed resist.

Under the experimental conditions o the present work, however, conversion of the amide toimide is not observed. After heating an aqueous solution of the law molecular weightPKAAm (Mw - 2.4 x 10") for 7 hours at 809C, infrared spectral analyses do not indicate theformation of imide. Also, inidization is not detected after prebaking PMAAe on siliconwafers at 180 to 2401C. Imide tormation with PMAAm apparently is difficult to control.

This difficulty has been encountered with other crosslinking resist systems,*' and

severely limits the process latitude of a resist system. Further work to reproduce and

control the imidization reactions of PMAAm would be useful.

• I,; .

. .i * , . . . .

-6

IBecause of the various difficulties encountered with the PMAAU homopolyer. a series ofcopolymers with methyl methacrylate were synthesized. Copolymers of MAAS with styrene.barium acrylate and methylmethacrylate have been studted previously. but the compo3itionsor these were limited to copolymers containing at least 90% methacrylaside.' Dissolution...rate measurements with the MMA-MAAs copolymers using laser interferometry Indicated that r .- .the greatest solubility rate ratios were exhibited by the copolymer containing

rapproximately 21 Sol % MAAS. In contrast to the MAAS homopolymer. this copolymer did not 41,~exhibit evere swelling problems when writing 1 uam patterns. Using a 15 minute prebake at200*C an developing with 2-ethoxyethanol. adequate contrast (Y - 2.1 at 20 mC/cal) isobserved with this copolymer (Figure 5). Unfortunately, this copolymer resist is subjectto considerable thinning and in this way Is only a marginal Improvement over PMMA (Figure6].

Conclusions

The sensitivity Of the PMAAm homopolymer resist is about 15 saC/ce'. where sensitivityI

is defined as the incident electron dose that can be completely developed with less than10% thinning of unexposed resist. This sensitivity has been observed using prebaketemperatures ranging from 180 to 20*C and an accelerating voltage of 20 KV. Patternsexposed at doses less than 10 iaC/ca* could not be developed with water, but dilutesolutions of sodium silicate developed lower doses (with extensive thinning). Swellingand adhesion problems limited the resolution of the PMAAm resist. The previously reportedhigh Sensitivity of PMAAx' was most likely due to the formation of imide crSlInk3. Suchimide formation is not observed in the present work. imide rormation is difficult tocontrol with this resist system, which seriously limits the process latitude of the

resist.

to.- - - -- -- -- -

- - -08 0.. *p.. Io

0 0-~

-,O6U 0 0 0 4

0 04j

02 0

0 \

25 10 20 6 to to so

INCSOENT DOSE (pC/C...i ICIOENT DOSE (pC/Cm'i

Figure 5. Contrast curve for Figure 6. Thinning curve forP(MMA -21% MIAWm, prebaked 15 minutes at P(MMA -21% MAAm). The film was prebaked200OC, and developed with 2-ethoxyethanol. 15 minutes at 2000C. and developed with

2-ethoxyethanol.

Acknowledgment

The authors thank the office of Naval Research for funding this research.

------

References

M. atsuda. S. . Tauchipa. S.. Honma. M.. Hasegawa, K., Nagamatsu. G. and Asano. T.;Polym. Eng. Sdi.. 17. No. 6. 410-1413 (1977).

2. O'Connor. D.J. * "Radiation Degradation of Polymethacrylamide." Doctoral Thesis. CityUniversity of New York. 1984.

3. Matsuda. S.. Tsuchiya. S. , Honma. M. . Wagamatsu. G. U.S. Patent 04. A21,936. assignedto Matsushita Electr ic Industrial Co..* Inc.. 3Osa4 and Fuji Chemicals IndustrialCo. L td.. To kyo. October 24. 1978.

4. Titkova. L.V.. Prokopova, E.. Sedlacek, .B. Petrus, V..* Dusek, K..* and Bohdanecky. M..

Envir. Polym. J.. 114, 1145 (1978).

5. Hatzakis. M.. J . Vac. Set. Technol.. 12, No. 6. 1276-1279 (1975).

6. Rodriguez. F.,. Krasicky. P.D. . Groele. R.J. , Solid State Technol.. 28(5). 125 (1985).

7. Roberts. E.D.. Am. Chem. Soc. Div. Org. Coat. Plast. Chem. Pap.. 35, 281 (1975).

B. Moreau. W..* Merrit, D.,* Moyer. W.. H atzakis, M..* Johnson. D..* Pederson, L.. J . Vac.

Sci. Technol., 16. 1989 (1979).

9. Namast6. Y.M.N.. Obendorf. S.K.. Rodriguez. F.. J. Appl. Polym. Sci., 30. 4631-146141(1985).

. 'a. a. %

! -k : , -, .-. 1%j

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