Polymers in Electronics - American Chemical Society

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Downloaded by CORNELL UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch005

Deep UV Photolithography with Composite Photoresists Made of Poly(olefin sulfones) H. HIRAOKA and L. W. WELSH, JR. IBM Research Laboratory, San Jose, CA 95193 Poly(butene-1 sulfone), poly(methylcyclopentene sulfone), and a composite resist of poly(2-methyl pentene-1 sulfone) in a novolac resin are well known positive tone electron beam resists. These poly(olefin sulfones) do not have deep UV-sensitivity because they do not absorb UV-light down to 215 nm. When mixed with sensitizers, or used as a composite resist with novolac resins, they showed deep UV-sensitivity in a range of 100 to 250 mJ/cm ; they provided positive tone polymer patterns in straight wet development or in dry development, and negative tone polymer patterns, in which case exposed resist films with a novolac resin were first postbaked at 115°C for 15 min, and then developed in an alkaline developer. Their photosensitivity may be derived from charge transfer complex formations because of observed color change of a novolac resin solution upon addition of poly(olefin sulfone) solution with newly appearing UV-absorption bands, or by simple energy transfer from the light absorbing resin or from photosensitizers which resulted in an unzipping degradation of poly(olefin sulfones) as evidence by gaseous products formed. Poly(olefin sulfones) alone did not give any gaseous photoproducts upon exposure of 254 nm light, but in presence of sensitizers like nitropyridine N-oxide or in a matrix resin poly(olefin sulfones) degrade upon 254 nm irradiation, yielding SO and monomeric olefins. 2

2

Poly (olefin sulfones), like simple olefin sulfones or cyclic alkene sulfones, do not have any UV absorption down to 215 nm. This absence of UV absorption of poly(olefin sulfones) make them uninteresting for deep UV photolithographic applications. Several years ago, along with electron impact 0097-6156/ 84/ 0242-0055S06.00/ 0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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induced reactions, we reported vapor phase H g ( P ) photosensitized reactions of methyl sulfone, tetramethylene sulfone and butadiene sulfone w i t h 254 n m light irradiation, w h i c h resulted i n efficient elimination of S 0 to y i e l d ethane, cyclobutane, and butadiene, respectively 0 ) . I n absence of excited H g ( P ) vapor, these reactions d i d not occur because olefin sulfones do not have any U V absorption. T h e r m a l l y these compounds are fairly stable; methyl sulfone and tetramethylene sulfone b o i l at 2 3 8 ° and 2 8 5 ° C , respectively. C l e a r l y , the energy transfer f r o m the excited mercury atoms H g ( P ) to dissociative triplet states of o l e f i n sulfones t o o k place i n vapor phase, resulting i n efficient decomposition of olefin sulfones. W e also reported an electron beam induced degradation study of poly(cycloalkene sulfones) together w i t h p o l y ( o l e f i n sulfones) i n a dose range of 1 0 " c o u l / c m of electrons w i t h 25 k e V energy (2). T h e elimination of a methylene group was significant particularly w i t h poly(norbornylene sulfone) and i n a less extent w i t h poly(cyclopentene sulfone), giving rise to c a r b o n y l sulfide f o r m a t i o n ; w i t h poly (olefin sulfones) like poly(butene sulfones) u n z i p p i n g reactions took place, y i e l d i n g equal amounts of olefin and S 0 (3). Photosensitized degradation of poly(olefin sulfones) similar to the H g ( P ) photosensitized reactions of o l e f i n sulfones make them subject to photodegradation i n easily accessible wavelength regions. Almost all poly(olefin sulfones) have been reported only as positive tone electron beam resists (4). A s the only exception, poly(5-hexene-2-one sulfone) has been reported as a positive tone photoresist w i t h or without a photosensitizer, benzophenone (5). Because this polymer has a c a r b o n y l chromophore, its photosensitivity is clearly derived f r o m the polymer structure itself. W e w i s h to report first d r y developable photoresists w i t h p o l y ( o l e f i n sulfones) w i t h photosensitizers like pyridine N - o x i d e , and then we present our study o n composite resists made of poly(olefin sulfones) w i t h novolac resins or w i t h poly(p-hydroxystyrenes). 3

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D r y Developable Composite Photoresists w i t h P o l y ( Q l e f i n Sulfones) Pyridine N-oxide and its derivatives have been studied as p h o t o c r o s s l i n k i n g agents f o r polymers l i k e p o l y s t y r e n e , p o l y ( m e t h y l methacrylate) and others (6). T h e y also act as crosslinking agents for novolac resins (7). P y r i d i n e N - o x i d e has its m a x i m u m U V - a b s o r p t i o n at 265 n m i n ethanol, while p-nitropyridine N - o x i d e has its U V absorption peaks at 332 and 234 n m w i t h almost equal intensity i n ethanol. W e have found that p o l y ( l - b u t e n e sulfone) w i t h p-pyridine N - o x i d e as a sensitizer yielded positive tone polymer patterns i n thermal development after U V light exposure through a c o n v e n t i o n a l photo-mask w i t h a cut-off w a v e l e n g t h of 300 nm. Poly(l-butene sulfone) was m i x e d w i t h p-nitropyridine N - o x i d e (20 w t % i n solid) and dissolved i n nitromethane; the films were spin-coated onto silicon wafers, and prebaked at 1 0 0 ° C for 15 m i n . A f t e r U V exposure, w i t h a m e d i u m pressure mercury lamp w i t h a n approximate dose of 100 m J / c m , polymer images were developed b y heating 2

Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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Deep UV

Photolithography

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at 1 0 0 ° C for 7 m i n . T h e developed patterns are shown i n F i g . 1. P y r i d i n e N - o x i d e and benzophenone were also used as sensitizers; p - n i t r o p y r i d i n e N - o x i d e , however, gave clear deeper images. Poly(2-methyl-l-pentene sulfone) can replace p o l y ( l - b u t e n e sulfone) for positive tone photo-images; the resulted images, however, were i n f e r i o r to p o l y ( l - b u t e n e sulfone). D i f f e r e n t f r o m p o l y ( l - b u t e n e sulfone), poly(2-butene sulfone) w i t h a n aromatic bisazide sensitizer yielded negative tone photo-images; here the sensitizer acted as a crosslinking agent. T h e mass spectroscopic analysis of the gases formed i n t h e r m a l development of the U V exposed p o l y ( l - b u t e n e s u l f o n e ) / p y r i d i n e N - o x i d e revealed o n l y 1-butene and S 0 as the products, w h i c h indicated d e p o l y m e r i z a t i o n of the polymer initiated by energy transfer f o r m the sensitizer. These photosensitized poly(olefin sulfones) are not suitable for dry etching processes, and they are not reactive i o n etching resistant. Resists made of poly(olefin sulfones) and novolac resins w h i c h w i l l be described next are C F plasma etch resistant w i t h reasonable photosensitivities.


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Composite Photoresists M a d e of P o l y ( Q l e f i n Sulfones) and N o v o l a c Resins M a t r i x resists consisting of p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) and novolac resins have been reported as positive tone electron beam resists (8). UV absorption spectrum of the matrix resist of p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) i n V a r c u m , w h i c h is cresol-formaldehyde novolac resin, is s h o w n i n F i g . 2 along w i t h the spectra of the polysulfone and V a r c u m ; clearly new absorption peaks appear i n the near U V region; the absorption intensity per unit f i l m thickness remains unchanged for the m a i n absorption at 280 n m . T h i s is also revealed by color changes w h e n two separate solutions of poly(olefin sulfone) and novolac resin dissolved i n A Z - t h i n n e r , w h i c h is 2-ethoxyethyl acetate, are m i x e d together. T h e similar color change and new U V absorption peaks appear in composite resists of poly(olefine sulfones) with p o l y ( p - h y d r o x y s t y r e n e s ) , as shown i n F i g . 3. These results indicate some complex formations between poly(olefin sulfones) and the resins, w h i c h may affect the energy transfer of absorbed U V light f r o m the host resin to polysulfones. T h e photo-energy transfer f r o m the resin to polysulfones c a n be substantiated by a mass spectroscopic analysis of gaseous products f r o m the composite resists exposed to 254 n m ; the polysulfone alone does not give any gaseous products because it does not absorb U V light at this wavelength. Figure 4 shows the mass spectra of the gaseous products formed u p o n 254 n m exposure of the composite resist of p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) i n V a r c u m resin. T h e energy transfer f r o m the resin to the polysulfone caused the m a i n chain scission of the polysulfone, giving rise to 2 - m e t h y l - l - p e n t e n e and S 0 formation. F i g u r e 5 shows the wide b a n d E S C A data of the composite resist before and after U V light exposure i n air. R e d u c e d intensities of sulfur signals and increased intensity of the oxygen signal after U V light 2



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Figure 1.


D r y developed images of p o l y ( l - b u t e n e s u l f o n e ) / p - n i t r o p y r i d i n e

N-oxide.


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Deep UV

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Photolithography

Poly(2-methyl-1pentene sulfone) in Brominated poly (p-hydroxystyrene)


Poly(2-methyl-1pentene sulfone) in Brominated poly (p-hydroxystyrene)

^ B r o m i n a t e d poly . ^(p-hydroxystyrene)

Poly(2-methyl-1 -pentene sulfone)

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Figure 3. U V absorption poly (p-hydroxystyrene). S0

C 2

spectra

of

PMPS

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brominated

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4 8

C3

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5

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C H 6

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M/e

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(64

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I.I, 18 II

BACKGROUND

Figure 4. Gaseous products formed i n 2 5 4 n m irradiation of PMPS i n V a r c u m resin.


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exposure i n air are clearly demonstrated i n the E S C A data. T h e change of the C core level signal, shown i n F i g . 6, indicates formation of c a r b o n y l carbons at 289 e V and of carbons attached w i t h hydroxy groups at 287 e V after a prolonged exposure to U V light. T h e charge transfer complex absorption bands described above are not strong enough to be practically useful. F o r lithographic image generation, a quartz mask has to be used so that even the b o t t o m part of the resist films absorbs 254 n m U V light, and then this energy is to be transferred to cause the degradation of the polyolefin sulfone. Because of this energy transfer only the most readily degradable polysulfone c a n be used as a composite photoresist; p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) yielded far better images than p o l y ( l - b u t e n e sulfone), although their electron beam sensitivity is the same (4). T h e polymer patterns can be made either i n positive mode or i n negative mode depending u p o n the processing technique as described next. T h e fact that negative tone patterns were obtained i n a K O H - b a s e d developer after heating at 1 3 0 ° C for 5 m i n indicates presence of trapped polymeric free radicals after the U V exposure w h i c h initiated crosslinking u p o n the postbake. Negative tone patterns are superior i n quality than those i n positive tone.


l s

Figure 7 shows photo-images obtained w i t h the composite resist made of p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) and V a r c u m ; positive tone patterns are shown o n the left, and negative tone patterns o n the right. T h e resists films were spin-coated onto s i l i c o n wafers f r o m a n A Z - t h i n n e r solution of p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) (16.5 w t % i n solid) and V a r c u m , a n d prebaked at 1 0 0 ° C for 20 m i n . T h e U V exposures were carried out w i t h a medium pressure mercury lamp w i t h a dose of 200 m J / c m of 254 n m light; it can be reduced to 100 m J / c m for f u l l development. F o r positive tone polymer patterns, the resist films were developed i n a K O H - b a s e d developer without post-bake after the U V exposure. F o r negative tone p o l y m e r patterns, the resist films were developed after post-bake at 1 3 0 ° C for 5 m i n , f o l l o w i n g the U V exposure. W e speculate that during this post-bake process, polymer free radicals generated i n the m a i n chain scission undergo cross-linking w i t h the novolac resin. There are no f i l m thickness loss observed after complete development w i t h an adequate U V dose. W i t h o u t the polysulfones, the novolac resin alone yielded very poor quality negative tone images w i t h very t h i n remaining f i l m thickness. T h e w a l l profiles of the negative tone patterns are s h o w n i n F i g . 8; v e r t i c a l profiles or slightly undercutting profiles are obtained. T h e r e l a t i o n of the remaining f i l m thickness after complete development versus 254 n m dose is shown i n F i g . 9; (a) for the positive tone images, and (b) for the negative tone images w i t h a composite resist of p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) and V a r c u m resin. T h e similar results are obtained w i t h a composite resist made of b r o m i n a t e d poly(p-hydroxystyrene) and p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) w i t h an increased photosensitivity for negative tone images. 2

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In a l l these studies, U V light i n a region f r o m 2 5 0 to 3 0 0 n m has been used to degrade poly(olefin sulfones) via energy transfer f r o m the host resin i n the case of composite resists w i t h V a r c u m , and f r o m sensitizers i n the case of


HIRAOKA A N D WELSH

Deep UV

Photolithography

>

J


(a) Original

/

2p

2s

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540

445

350

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Figure 5. W i d e band E S C A data of matrix resist films, P M P S i n V a r c u m .

C

1 s

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(a) Original

(b) UV-lrradiated in Air

J 291.0

288.5

286.0

283.5

281.0

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Figure 6. Change of carbon core level signals of P M P S i n V a r c u m .


P O L Y M E R S IN E L E C T R O N I C S


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Figure 7. D e e p U V p o l y m e r patterns obtained w i t h a matrix resist of p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) and V a r c u m resin; positive tone images o n the left and negative tone images o n the right.

Figure 8.

Negative tone resist patterns o b t a i n e d w i t h a matrix resist of

p o l y ( 2 - m e t h y l - l - p e n t e n e sulfone) and V a r c u m resin.



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HIRAOKA AND WELSH

Deep UV

Photolithography

254 n m Dose, m J / c m

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2

Figure 9. R e l a t i o n s of r e m a i n i n g f i l m thickness versus 254 n m dose; (a) p o s i t i v e tone patterns, (b) negative tone patterns with a p o l y ( 2 - m e t h y l - l - p e n t e n e s u l f o n e ) / V a r c u m resist.


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dry developable photoresist described earlier. The nature of the energy transfer is not clear; it is probably through complexes between the polysulfones and the resins described above, or through dissociative triplet states as in Hg( P) photosensitized reactions of simple olefin sulfones. 3


Literature Cited 1. Hiraoka, H., Presented at the 167th ACS National Meeting at Los Angeles, California, May 1974; Abstract No. Phys. 137. 2. Hiraoka, H., Presented at the First Chemical Congress of the North American Continent, Mexico City, Mexico, November 1975; Abstract No. 60. 3. Recently a different view of electron beam induced degradation of poly(olefin sulfones) has been presented, particularly in a very low electron dose range, by J. Pacansky, R. Kroeker, E. Gipstein, and A. Gutierrez, The Electrochemical Society 1982 Spring Meeting, Montreal, Canada, May 1982; Abstract No. 273. 4. Bowden, M. J. and Thompson, L. F. J. Appl. Poly. Sci. 1973, 17, 3211; Gipstein, E.; Moreau, W.; Chiu, G.; Need, O. J. Appl. Poly. Sci. 1977, 21, 677. 5. Himics, R. J.; Kaplan, M.; Desai, Ν. V.; Poliniak, E. S. Tech. Papers, Soc. Plastics Eng. Mid-Hudson Sec., October 13-15, 1976. 6. Decout, J. L.; Lablanche-Combier, Α.; Loucheux, C. Photographic Sci. and Eng. 1980, 24, 255. 7. Hiraoka, H. IBM Tech. Discl. Bull. 1982, 24, 5756. 8. Bowden, M. J.; Thompson, L. F.; Fahrenholts, S. R.; Doerries, E. M. J. Electrochem. Soc. 1981, 128, 1304 RECEIVED September

2, 1983


Polymers in Electronics - American Chemical Society

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