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2 818 CAN. J. CHEM. on the properties of the polymer materials and their response to the developers as well as the subsequent processing conditions. Present interest centers about the 1 km element size. As we approach the wavelength of the light used to form the resist image (~0.4 km), it is clear that diffraction effects should increase in importance. This has led to a strong interest in exploring the use of nm light to minimize diffraction. Printing equipment for this region of the spectrum is expected to be available in the near future. This paper presents an overview of the routes we have explored for deep uv resist technology. Resist design General cotzsiderations The first decision to be made is which wavelength region should be used for resist exposure. We believe that the high pressure mercury arc lamp that is currently used will continue to be the best exposure source for the near future. This implies that resist response should be optimized for the nm region. The choice represents a compromise between exposure at the shortest possible wavelength and a reasonable exposure time for a 12.7 cm diameter silicon wafer, allowing for the relatively low Hg lamp output at X < 300 nm. An important consideration is the match between the exposure source output spectrum and the absorption spectrum of the photoresist film which is usually km thick. t is important to have relatively uniform exposure throughout the thickness of the film in order to obtain vertical walls in the resist image. This suggests a low optical density, and we prefer to keep the optical density of the film below 0.3 for the range of significant source output. Conventional materials are nearly opaque (OD >1) for X < 300 nm, and it is impossible to obtain uniform exposure through the film thickness unless the film is very thin. This is the reason that new photoresists are needed. Sensitivity is generally defined as the amount of incident exposure light, in J cm-', that is required to give a satisfactory image after development. This does not take into account the match between the output and absorption spectra mentioned above. We prefer to measure sensitivity at a particular wavelength. The practical measure of sensitivity is simply the time it takes to make a resist image with a given exposure tool. Resist contrast is defined in a manner similar to that of conventional photography. One plots the amount of resist film thickness remaining after development versus the log of the exposure dose. This usually gives a curve which is fairly linear as the thickness approaches zero. Contrast (y) is defined as the slope in this region. 'The higher y is, the more the resist behaves as if it has a threshold exposure for development. This behavior is desirable because it indicates less sensitivity to stray light and/or diffracted light. High contrast is desirable for high resolution imaging. Conventional materials require exposures of mj ~m-~. For positive resists, it is unlikely that sensitivities significantly better than 50 mj cm-2 can be achieved. t should be noted that sensitivity is fundamentally measured in photons cm-2 and one will necessarily have to have more J cm-2 to have the same photon flux as the wavelength is decreased. Thus, deep uv lithography will require more light (joules), in a region of decreasing lamp output, to cause the same photochemical effect that would be observed with existing materials at conventional wavelengths. WAVELENGTH (fm) FG. 2. Absorption spectra of PMMA and P (M-OM) films, nominally 1 Km thick. The mole ratios of thc copolymers are given in parentheses. Resist mechanisms Negative resists are polymers having reactive groups that undergo photochemical crosslinking reactions either with themselves (e.g., cinnamate dimerization) or with a photosensitive small molecule (e.g., ethylenic polymer groups with bisazido compounds). n general, negative resists exhibit a greater tendency to swell during development than positive resists. The absorption of developer solvent plasticizes the film and limits the resolution that can be obtained to -2 km in -1 km thick films. For this reason we chose to design new positive resists for use in the deep uv region. 'There are two general mechanisms that are used in positive resist chemistry. One involves the degradation of a polymer to material of significantly lower molecular weight. This is essentially opposite to the change induced in a negative resist. f the molecular weight distribution is shifted sufficiently, one can then dissolve the irradiated regions. Poly(methy1 methacrylate), PMMA, is the best known example of this group (3). t is capable of extremely high resolution, using deep uv light, e-beam, X-ray, or ion beam exposure, but it is too insensitive to be practical. The low optical sensitivity is not surprising. PMMA has only pivalate ester chromophores, and they absorb weakly at 220 nm (Fig. 2). The combination of weak absorption and poor lamp output does not bode well for a reasonable exposure time. Sensitization is not possible because there is no low-lying excited state of the ester chromophore.

3 CHANDROSS ET AL PPMA CH, 0-Scission -CHZ-L=CHZ+.L- + Further polymer degradation C H3 C H3 PMPK +CO + C~,--.~ol~mer degradation FG. 3. The mechanism proposed for the photodegradation of PMMA and the poly(viny1 ketone)^ SOzX FG. 4. The photochemical conversion of a naphthoqu~none diazide to an alkali-soluble carboxylic acid, as in convcntional pos~tive photoresist. X is usually an aryloxy group. The mechanism of PMMA photodegradation is thought to involve radical formation followed of the polymer backbone (Fig. 3). The same process can occur in poly- (vinyl alkyl ketone)^ which have also been used as resists. Because of their weak absorption of light (carbonyl chromophore, X nm, E = 25), sensitizers are required. The second type of positive resist has been used extensively for some time. t is a two-component system that operates via an ingenious mechanism.' The major component is a Novolak resin (phenol-formaldehyde) that is soluble in aqueous alkali. The minor component is a photosensitive hydrophobic compound, a substituted naphthoquinone diazide, that is photo- 'Reference 2, p chemically converted to an alkali-soluble indenecarboxylic acid (Fig. 4). The ability of the "solution inhibitor" to prevent developer attack is destroyed by light, and the irradiated portion of the film can be dissolved preferentially by aqueous alkali. These materials are optically dense below 300 nm, even in 0.5 km films, and thus they cannot be used for deep uv exposure. New PMMA resists Our work in this area is based on the hope that it would be possible to add small amounts (> 15 mol%) of photosensitive groups that would lead to backbone cleavage while maintaining the excellent filmforming and high resolution properties of PMMA itself. A known example of this approach is the incorporation of the acyloximino ester chromophore, devised by Delzenne et al. (4). They noted that irradiation of solutions of copolymers of MMA and 3-methacryloyloximino-2-butanone, in the 365 nm region (weak absorbance), led to a decrease in molecular weight. This seemed to be a good starting point because the acyloximino group has a strong absorption at 220 nm whose tail extends beyond 240 nm. Photolysis in the solid state is not necessarily the same as that in solution because of the cage effect of the matrix that favors the reformation of broken bonds, but one can expect some similarity. The absorption spectra of two of these copolymers, as well as that of the parent PMMA, are given in Fig. 2. The photodegradation envisaged for the copolymer is shown in Fig. 5. As expected, the photosensitivity observed increases with the amount of acyloximino monomer incorporated. The results are given in Table 1. A further increase in sensitivity can be obtained by the incor-

4 820 CAN.. CHEM. VOL. 61, 1983 kc- :' 1 bch.- F ] [-CH2- kch.- +CH~CN+CH,& CEO c=o tl C=O C=O, + Polymer degradation P (M-OM) 1 C-C / \ CH, FG. 5. The photodegradation proposed for P(M-OM). TABLE 1. Properties of copolymers of methyl methacrylate and 3-methacryloyloximino- 2- butanone Polymer Sensitivity" Mol. wt. Mol. wt/ (full output 1 kw Composition Mol. no. Hg-Xe lamp) PMMA~ P (M-OM') 97: P (M-OM) 94: P (M-OM) 84: P (M-OM) 63 : P (M-OM-cN") 69: 16: "Exposure required relative to PMMA; developed with methyl isobutyl ketone. "dupont, Elvacite 2010, "high molecular weight". '3-Methacryloyloximino-2-butanone. "Methacrylonitrile. TABLE 2. Photosensitization of P (M-OM) Sensitivity" E, (Sens.) (full output, kw) Polymer Sensitizer (%) (kcal rnol - ') Xe-Hg lamp) P (M- OM)" P (M-OM)" 11-tert-Butylbenzoic acid (10) - 0. P (M-OM)" N-acetylcarbazole (10) 68.3' 0.08 P (M-OM)" Benzophenone (10) 68.5" 0.25 "Relative exposure required, PMMA = 1, developed with methyl isobutyl ketone "P (M-OM) (94:6). 'Reference. "Reference 12. poration of methacrylonitrile (5). t is likely that this is primarily a consequence of polymer properties rather than a direct effect on the photochemical reaction. The most sensitive material was found to be a terpolymer of methyl methacrylate, the oximino methacrylate, and methacrylonitrile, in the ratio of 69: 16: 15. This polymer is more than 50 times as sensitive as PMMA to 240 nm light. When sensitized with tertbutylbenzoic acid, it requires an exposure of less than 30 mj cm-' and gives high resolution images. The limits of its use for optical printing have not yet been thoroughly explored. n addition to the use of benzoic acids as short wavelength sensitizers, the polymers can be made sensitive to longer wavelength (- 300 nm) light by the addition of photosensitizers such as N-acetylcarbazole (Table 2) (6). t appears that singlet energy transfer is involved. ndenone is another monomer that can be incorporated into PMMA by conventional free radical copolymerization (7). ndenone is prepared by in situ hydrolysis of the ethylene ketal just before polymerization because it polymerizes too readily to be stored. The high reactivity of the a,@-disubstituted ethylene is attributed to the presence of the antiaromatic cyclopentadienone moiety. The amount of indenone incorporated into the copolymer is controlled by its concentration in the polymerization solution. Absorption spectra of typical copolymers are shown in Fig. 6. t is clear that this system can be optimized for exposure in either the 280 or 250 nm region by appropriate choice of the indenone concentration. The amount actually incorporated is readily determined by optical spectroscopy. We assume that indenone reacts as an a,@-unsaturated ketone, and the mechanism postulated for photodegradation is shown in Fig. 7. t is analogous to that accepted for PMMA and

5 CHANDROSS ET AL assumes the normal Norrish a-cleavage of a carbonyl group. The copolynler is quite sensitive, requiring an exposure of only b J cm-' at 248 nm when 3% indenone is present. Sensitivity is proportional to absorption as long as the optical density of the film is low. A conlplete set of results is given in Table 3. 'The high sensitivity is explained on the basis of tlie steric 0.5 strain present in the cyclopentanone moiety. t is well known that the rate of a-cleavage increases in the order cyclohexanone < cyclopentanone < cyclobutanone. 'The analogous open chain copolynler incorporating phenyl vinyl ketone is far less sensitive (x80) than the indenone copolymer. A preliminary * 0.4 lithographic evaluation of poly(mma-co-indenone) indicated k that it is capable of high resolution (= pn) and that it has a V) z contrast similar to that of PMMA. W P J Solution inhibitor deep ultraviolet resist (8) a The conventional two-component resist system was dek P scribed in the introduction. t contains a large fractionof pheno o moieties that absorb strongly for A < 300 nm, a region of high absorption for the quinone diazide additive as well. There is a "minimum" in the resist absorption at 2260 nm where the 0.2 optical density is ~ 0. p.m-'. 5 Thus, in order to use a system like this for exposure at A < 300 nni, both a new matrix resin and a new photosensitive element are necessary. The choice of a resin seemed straightforward, based on what is known about the performance of the conventional material. 0.1 We require a film-forming polymer that will dissolve at a 3X reasonable rate in aqueous alkali, serve as a host for the photosensitive solution inhibitor, and not interfere in the photo WAVELENGTH (nm FG. 6. Absorption spectra of films of poly(methyl methacrylateco-indcnone), == 1 krn thick. Thc mol% of indenone is indicated for each curve. chemistry of the latter. We selected copolymers of methyl methacrylate and methacrylic acid (MAA) because they offer an easy route to adjusting the alkali solubility of the matrix. are optically transparent in the region of interest to us. The selection of a photochemical reaction for the solution inhibitor is far less obvious. We were led to the o-nitrobenzyl P-Cleavage Secondary radical Tertiary radical FG. 7. The mechanism postulated for the photodegradation of poly(rnethy1 methacrylate-co-indenone).

6 822 CAN.. CHEM. VOL TABLE 3. Photosensitivity of poly(methy1 methacrylate-co-indenone)" an( Film OD Exposure requircd (J/cn12) lndenone content at 280 nm at 248 nm at 280 nm at 248 nm 3% % % CH-O- R -- - ii -- "Developed with methyl isobutyl ketone saturated with water. 0 -NTROBENZYL CHOLATE FG. 8. Absorption spectrum of o-nitrobenzyl cholate in a P(MMA-MAA) film, -1 km thick. ester rearrangement for two reasons. This photochemistry is well known for the facile conversion of carboxylic acid esters to the free acids, and the absorption spectrum of the nitrobenzene chromophore seemed well suited to our purpose (Fig. 8). The o-nitrobenzyl ester rearrangement functions as a photochemical switch to liberate the alkali-soluble carboxylic acid (Fig. 9). We thought it would be advantageous to choose the R group to be as large as possible so that it would occupy a substantial volume fraction of the resist film. Such a species would then function as an amplifier for the effect of the change in the small photosensitive moiety. Our first choice was cholic acid. The free acid is very soluble in aqueous alkali FG. 9. Photochemical reaction of an o-nitrobenzyl ester to liberate thc parent acid. FG. 10. Structure of o-nitrobenzyl cholatc. (==SO0 g L-), while the aliphatic esters are insoluble. t is far more effective than anything else we have studied such as decanoic acid, fluorenone-3-carboxylic acid (a photosensitizer), and various phthalic acid half-esters. ortho-nitrobenzyl cholate (Fig. 10) was synthesized by the reaction of sodium cholate with o-nitrobenzyl chloride in aqueous alcohol. The poly(mma-co-maa) was prepared by conventional free-radical polymerization. The molecular weight and composition of the resin have an effect on the overall sensitivity; material having a ratio of 75:25 and a molecular weight of ca is quite satisfactory. The solution inhibitor is present at ca. 20% by weight. The developer that works best is an aqueous solution of Na2C03 (10% by weight). The resist films are spin coated conventionally from cyclopentanone solution. The absorption spectrum of a typical film is shown in Fig. 8. t is clear that this resist is well suited for exposure in the 260 nm region where the sensitivity is ~ 0. J 2 cm-2 for a typical system. Sensitivity depends on a number of factors (inhibitor concentration, baking time and temperature before exposure, developer composition, and procedure) that must be studied further to allow the choice of a practical material. We have made contact prints that demonstrate 1 km resolution (Fig. 1 ). The unexpected result that came out of our initial studies was the unusually high contrast (y > 5) of this resist system. The analogous conventional system has a contrast of 52.5; the PMMA systems show y < 2. There is no way to predict or rationalize contrast, but we can say that this system comes closer to "threshold exposure" behavior than any other organic photoresist. We believe that the clue to understanding the high contrast lies in the unusual development characteristics ob-

7 CHANDROSS ET AL. 823 work required to optimize any resist system after its basic chemistry is shown to be promising. This investment should become more attractive as imaging equipment for this region of the spectrum becomes available. Optical lithography is a wellknown, attractive technology, and we hope that it can be used to make high-density integrated circuit devices having feature sizes less than one micrometer. FG. 11. Km Lines and spaccs contact-printed in o-nitrobenzyl cholate/p (MMA-MAA) photoresist. The spacc in the light portion of the figure at the bottom is Km. served for these films. The exposed regions are swollen but not dissolved directly by the alkaline developer. They dissolve subsequently in the water used to rinse the film. This may be a reflection of "salting out" the ionized resin polymer by the alkaline developer. The swelling does not seem to have a deleterious effect on the resolution that can be obtained. More work has been done on this system, and the reader is referred to recent presentations (9, 10). Summary The work described here demonstrates that one can design new resist systems for the deep ultraviolet. There is a lot of. J. KOSAR. Light sensitive systems. John Wiley and Sons, nc., New York, NY W. S. DEFOREST. Photoresist: materials and processes. McGraw-Hill Book Co., New York, NY B. J. LN. BM. J. Res. Dev. 20, 213 (1976). 4. G. A. DELZENNE, U. LARDON, and H. PEETERS. Eur. Polym. J. 6, 933 (1970). 5. E. RECHMANS and C. W. WLNS, JR. ACS organic coatings and plastics chemistry preprints. 43, 243 (1980). 6. E. RECHMANS, C. W. WLKNS, JR., and E. A. CHANDROSS. J. Electrochem. Soc. 127, 2514 (1980). 7. R. L. HARTLESS and E. A. CHANDROSS. J. Vac. Sci. Technol. 19, 1333 (1981). 8. E. RECHMANS, C. W. WLKNS, JR., and E. A. CHANDROSS. J. Vac. Sci. Technol. 19, 1338 (1981). 9. E. RECHMANS, C. W. WLKNS. JR., and E. A. CHANDROSS. 161st Meeting. The Electrochemical Society, Montreal, Canada, May 9-14, Abstract No C. W. WLKNS, JR., E. RECHMANS, and E. A. CHANDROSS. 161st Meeting. 'The Electrochemical Society, Montreal, Canada, May 9-14, Abstract No M. ZANDER. Ber. Bunsenges. Phys. Chem. 72, 1161 (1968). 12. J. G. CALVERT and J. N. PTS. Photochemistry. John Wiley, New York, NY p. 298.