Chapter 17
Design Considerations for 193-nm Positive Resists 1
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Robert D. Allen , I- Y. Wan , Gregory M. Wallraff , Richard A. DiPietro , Donald C. Hofer , and Roderick R. Kunz 1
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IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099 Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02173-9108
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Our approach to the design of positive, single layer resists for 193 nm lithography will be discussed. Phenolic resins, the archetype in positive photoresist materials, cannot be used at this wavelength due to optical opacity. Acrylic polymers combine the required optical transparency at 193 nm with easily tailored properties. With a design based on methacrylate terpolymers, we have recently developed a high resolution positive resist for 193 nm lithography with good imaging at both 193 and 248 nm. Our work has centered on gaining further insight into methacrylate polymer structure/property relationships, improving the imaging performance and finally increasing the etch resistance. Towards that end, we have employed a class of dissolution inhibitors for 193 nm resists that are combined with methacrylate polymers to provide 3-component resists. A family of 5B-steroid dissolution inhibitors that also increase etch resistance will be described. Imaging and etch performance of these resists will be disclosed, with particular emphasis on the impact of these steroid dissolution inhibitors on the thermal properties of the resist. These methacrylate chemically amplified resists show resolution capability below 0.25 micron, etch rates 20% higher than novolak resins, and dual wavelength (193/248 nm) imaging.
The explosive growth in performance of semiconductor devices has been fueled by advances in microlithography and photoresist technology. The current generation of advanced microprocessors and DRAM memory chips have critical dimensions approaching 0.5 microns and are printed using novolak-based mid-UV photoresists. Next generation devices will be produced with optical lithography at shorter wavelengths (ca. 250 nm, deep UV) combined with newer (chemically 0097-6156/95/0614-0255$12.00/0 © 1995 American Chemical Society
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amplified) photoresists. The technology path toward device generations beyond 0.25 micron is currently the subject of much discussion. ArF excimer (193-nm) lithography is a viable approach to extend optical lithography beyond 0.25 microns, but the resist technology develped for traditional DUV lithography is problematical at this 'deeper* UV wavelength. We will discuss the materials issues involved in the design of a positive resists for 193 nm lithography with regard to optical properties, resolution, photospeed and etch resistance. The design of positive (single-layer) resists for 193 nm lithography is a significant challenge. This emerging field of photoresist research has recently been reviewed. The imaging chemistry is quite similar to that practiced in traditional DUV lithography: photogeneration of a strong acid followed by acid catalyzed deprotection to render the exposed regions of the film soluble in aqueous base. The differences in resist design between traditional (248 nm) and 193 nm lithography are related to matters of optical transparency. Traditional DUV resists are based on hydroxystyrene polymers, phenolic resins with much improved optical properties at 248 nm than the structurally similar novolak resins. Hydroxystyrene polymers are extremely opaque at 193 nm, however. In fact, these resins are ideally suited for top-surface imaging at 193 nm. 111
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New polymer materials are required for 193 nm (single layer) resists, with high optical transparency at the exposure wavelength combined with properties that hydroxystyrene polymers (and few others) possess; 1) hydrophilicity (for good positive-tone development characteristics); 2) high T (130-170 °C), for good thermal properties and the latitude to perform higher post-expose bakes; 3) aromaticringsin high concentration (for good etch resistance), and 4) an easily blocked hydroxyl group (for incorporation of acid cleavable functionality). g
These four taken-for-granted characteristics that are present by default in DUV (248 nm) resists need to be painstakingly designed into single-layer resists for 193 nm lithography, where phenolic resins cannot be used. In light of their excellent optical transparency (see Figure 1), methacrylate polymers are (to date) the new paradigm for 193 nm resist design. In this case, gaining high resolution imaging can be accomplished, as can etch resistance. Building a resist with both excellent etch resistance combined with good imaging characteristics is the challenge. This paper discusses our resist design with special emphasis on modification of a high resolution first generation 193 nm resist (Version 1) to gain enhanced etch resistance. Control and balancing of the separate factors of T , hydrophilicity, etch resistance, and imaging properties (simultaneously) will be discussed. 131
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EXPERIMENTAL Methacrylate polymers described here for use in 193 nm resists were prepared by free radical solution polymerization. Molecular weights were controlled by inititator structure and concentration, polymerization solvent and temperature, and through the use of chain-regulating additives. Conversions of monomer to polymer 151
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257 Design Considerations for 193-nm Positive Resists
are dependent on polymerization conditions, but were typically between 80-100%. Polymers were isolated by precipitation into hydrocarbon solvents,filteredand dried at elevated temperatures for 24 hours. For example, methyl methacrylate (MMA) (200 grams, 2 moles), t-butyl methacrylate (TBMA) (100 grams, 0.70 moles) and methacrylic acid (50 grams, 0.58 moles) were charged to a 2 liter round bottom flask with a magnetic stir bar. Unstablized (inhibitor-free) tetrahydrofuran was added as the polymerization solvent (1200 grams). Finally, the polymerization initiator bis-azoisobutyronitrile (AIBN) was added (1.5 grams, 0.009 moles). The polymerization reactor was fitted with a reflux condenser. The polymerization mixture was repeatedly degassed with nitrogen/vacuum cycles. The reaction was carried out by heating to reflux (67 °Q and was allowed to proceed for 24 hours under a nitrogen blanket using a Firestone valve. The polymerization mixture was then cooled to room temperature, at which time the viscous polymer solution was diluted with approximately 400 grams of THF. The polymer was isolated by precipitation into hexane, via dropwise addition, into a large excess of the rapidly stirred non-solvent. The precipitated polymer was filtered, washed, and dried in a vacuum oven at elevated temperature for 1-2 days. Collected yield of 90% was found (310 grams of polymer was produced) with a molecular weight (M^, ) of 75,000 g/mole, a polydispersity of 2.2 and a T = 150 °C. g
Resists solutions are prepared by dissolving the polymer of interest into propylene glycol methylether acetate (PGMEA), then adding the other components of the formulation (dissolution inhibitors, photoacid generators). The PAG used in all resist compositions reported here was bis-(t-butyl phenyl)iodonium triflate (TBIT) (Figure 2). Loading of TBIT in the resist was typically between 1 and 2 wt% (vs. resist solids). Resists were processed as follows: resist solutions were spin-coated onto HMDS-primed wafers, post-apply baked (PAB) above 100 °C for 1 min, exposed at 193 nm (SVGL Micrascan 193 prototype, NA= 0.50) or DUV (GCA XLS "lotus" lens, NA= 0.48), post-expose baked (PEB) above 100 °C for 1 minute, developed in 0.01-0.05N TMAH (tetramethylammonium hydroxide) (Version 1), and in 0.02-0.13N TMAH (Version 1.5 and Version 2). Exposed wafers were immersion developed for 20-60 seconds.
RESULTS AND DISCUSSION We use a building block approach in resist design, employing different monomers to tailor polymer properties. The use of methacrylate monomers facilitates this approach, as these are easily incorporated into the polymer because the copolymerization characteristics are largely unaffected by the structure of the ester group. Using this methodology, we developed a high speed, aqueous developing positive tone resist several years ago for direct-write printed circuit board lithography. The approach taken for materials design used acid-labile methacrylate polymers as a polymer platform. This is also a class of materials with the required optical transparency at 193 nm and with an easily tailored structure. These versatile i61
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250
275 300 325 WAVELENGTH (nm)
Figure 1. Optical absorbance spectrum of poly(hydroxystyrene) vs. acrylic polymer.
:ovi- 200 °C) g
Bile acid esters (52?-Steroids) were used in early (pre-chemically amplified) DUV resists by Reichmanis and co-workers in the early 1980's. Photo-induced deprotection of o-nitro benzyl esters created carboxylic acids in the exposed portion of the film. Extensive studies of substituent effects demonstrated the possibility of good dissolution inhibition, but photospeeds were quite slow. O'Brien and co-workers examined t-butyl cholate as a dissolution inhibitor for novolak resins, an example of a three-component chemically amplified resist. " The central idea in this work was the search for an inhibitor with little or no absorbance at 248 nm, so as to afford a dilution in the optical density of novolak resins at this DUV wavelength. 1111
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We were intrigued at the possibility of a dissolution inhibitor with combined etch resistancefromthis steroid family of alicyclic compounds. We prepared a variety of these compounds (see Figure 6) and investigated structure/property relationships
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Design Considerations for 193-nm Positive Resists 263
OR R=Me, t-Bu H0
V
H
OH
Ursocholate Esters (ME-2) 0
Me
II
*0R R =Me, t-Bu R=H,acyl Uthocholate Esters (ME-3; TB-3; TB-4; TB-5)
Figure 6. Steroid dissolution inhibitor structures.
with attention paid to solubility, dissolution inhibition in methacrylate terpolymers, dissolution promotion after exposure and imaging performance. The impact of manipulation of structure in the 52?-steroid family on dissolution properties was extreme (see Table I). Both passive (methyl esters) and active (t-butyl esters) steroids were examined in an aqueous base soluble methcrylate polymer used in our version 1 resist. Films were developed in 0.1 Ν TMAH before and after exposure to 25 mJ/cm of 254 nm filtered light. Unexposed and exposed dissolution rates were measured, and inhibition (decrease in dissolution rate by adding 25% inhibitor) and the dissolution rate ratio (R/R ) were calculated. Methyl esters of cholic, ursocholic and lithocholic acid demonstrated a strong substituent dependence on dissolution properties. The cholate ester compound is by far the least efficient inhibitor, with a percent inhibition of only 5 %. Simply removing one hydroxyl group improves the inhibition by a factor of 5 (methyl ursocholate) (ME-2) and further removal of the second hydroxyl to yield methyl Uthocholate (ME-3) improves the inhibition by a further factor of 3, to 83.5%. 2
0
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Table I. Alicyclic Etch/Dissolution Inhibitors
active
passive
Dissolution Inhibitor
Unexposed Percent Dissolution Inhibition Rate(p/min.)
Exposed Dissolution RateOi/min.)
R/R(0)
none(K2)
4.14
0
26.0
6.3
ME-1
3.91
5.5
17.5
4.5
ME-2
3.00
27.5
11.4
3.8
ME-3
0.68
83.5
8.6
12.6
TB-1
2.6
38
23.0
8.8
TB-3
0.84
80.0
27.0
32.3
TB-4
0.53
87.2
25.0
47.2
TB-5
0.25
94
35.0
140
Developer, 0.1 ON TMAH 25% Inhibitor loading
Expose dose, 25 mJ/cnf, 254 nm
Acid-cleavable "active" (t-butyl) esters were prepared. The substitution trends are similar to the passive compounds. Tert-butyl cholate is a very poor inhibitor, while t-butyl Uthocholate is a much better inhibitor (see Table I). The exposed dissolution rate is much less sensitive to the alicyclic substitution pattern (not the case in the "passive" compounds). As a result, R/R is ca. 4 times higher for t-butyl Uthocholate (TB-3) than for t-butyl cholate (TB-1). 0
Further enhancements in dissolution inhibitor were realized through hydroxyl substitution of t-butyl Uthocholate. Two compounds are shown in the table (TB-4 and TB-5), which have substantially improved dissolution inhibition properties. These compounds inhibit the dissolution rate of the Version 1 terpolymer by close to 90% and maintain a high exposed dissolution rate. TB-5 is highly soluble in methacrylate polymers and in PGMEA. Loadings as high as 50% (wt) were achieved without detectable phase separation with this compound.
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Design Considerations for 193-nm Positive Resists
Mixtures of the steroids with the Version 1 methacrylate terpolymers affords (Version L5) resists with relatively good imaging properties and enhanced etch resistance. Figure 7 shows 0.35 micron images printed in the Version 1.5 resist as a function of PEB temperature. Note that a high PEB (130 °C) causes degradation in the imaging resolution, while a much lower PEB (104 °C) provides better imaging quality. The requirement for sharply lower PEB in this Version (1.5) of the resist appears to be a function of the T of the steroid/methacrylate mixture. The thermal properties of these mixtures are dominated by the plasticizing effect of the steroid. For example, TB-5 is slow to crystallize, melts at ca. 50 °C., then shows a strong glass transition below -10 °C (see Figure 8). At the relatively high steroid loadings employed, significant T suppression (plasticization) is quite likely. In our experience, post-expose baking at or above T when using a triflic acid generator causes image degradation apparently due to acid diffusion. In order to gain meaningful reduction in etch rates by increasing the steroid loading, overplasticization of the resist occurs. If one could raise the T of the resin, and its intrinsic etch resistance, the steroid could be added in somewhat lower concentration. This T increase would allow for increased loading of the steroid before overplasticization becomes a problem. g
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E F F E C T O F DISSOLUTION INHIBITOR ON G L A S S TRANSITION RESIST: V1.5 - POLYMER / INHIBITOR/PAG, 100 / 33 / 2.5 POLYMER Tg -165° PROCESS: 1.130°C PAB FOR 1 min 2. EXPOSE - (193-nm, 0.5 NA) 3. POST-EXPOSURE BAKE 4. DEVELOP IN 0.05 Ν TMAH, 20 s
Figure 7. Imaging of Version 1.5 three-component resist.
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DSC
Temperature (°C) Tg = -11.7°C (Plastïcîzer)
Tm = 50 °C (Solubility)
Figure 8. DSC thermogram of a pure steroid dissolution inhibitor.
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Design Considerations for 193-nm Positive Resists 267
ALLEN ET AL.
Version 2 resist was born of this concept. The steroid dissolution inhibitor TB-5 is added to a tetrapolymer of isobornyl methacrylate (IBMA) (or adamantanemethyl methacrylate) and MMA-TBMA-MAA (Figure 9). The IBMA in this case raises T (to over 200 °C) and imparts increased etch resistance. Figure 10 shows the impact of the steroid on the thermal properties of this high T tetrapolymer, through the use of thermomechanical analysis (TMA) on spin-coated resist formulations. Note the strong plasticization of this resist as a function of steroid loading. Note also the strong impact of PAB temperature on resist thermal properties. m
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The increased Tg of the IBMA tetrapolymer/TB-5 mixture allows for an increase in PEB temperature and a corresponding increase in imaging quality. High quality 0.25 micron features have been printed in 0.6 microns of Version 2 resist, exposed with the SVGL 193 nm step-and-scan prototype. Figure 11 shows 248 nm imaging results at 0.5 micron resolution in this Version 2 resist. Etch resistance of Version 2 resist is far better than Version 1 or Version 1.5. The presence of alicyclics in both the polymer and dsissolution inhibitor produced chlorine etch rates only slightly faster than novolak resins. Version 2 resist has achieved etch rates as low as 1.2 times that of novolak, in a resist formulation with good imaging quality. This is perhaps the first example of the combination of etch resistance and imaging quality in anon-phenolic, 193 nm-transparent photoresist. ^ - ^ - C H — C^j 2
CH
CH
3
^ CH — C ] — £ c H — C
^CH —C ]
2
2
2
C=0
c=o
I OCH
ο
MMA
I
I
3
• Etch Resistance •High Tg • Commercially Available • Acid Cleavable
Mechanical Properties * Hydrophilicity
c=0
I
C H ~ C —-CH 3
I
CH
3
MAA • Aqueous Developing
1
IBMA
cm
3
3
TBMA • Deprotection
Figure 9. Methacrylate tetrapolymer structure used in Version 2 resist.
SUMMARY The evolutionfroma high quality imaging resist with little etch resistance (Version 1), to a resist which combines imaging quality with etch resistance (Version 2) was described. Integral to this transformation was the introduction of a three component resist. Alicyclic dissolution inhibitorsfromthe 52?-steroid family, when combined with methacrylate tetrapolymers and an iodonium triflate photoacid generator, form three-component resists with the appropriate balance of
Version 1
Version 2
ΤΜΑ: A Measure of Softening Temperature of 193 nm Resist Films
es 00
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ALLEN ET AL.
Design Considerations for 193-nm Positive Resists269
RESIST: V2.0 - THREE-COMPONENT RESIST WITH IMPROVED ETCH RESISTANCE POLYMER -16% WEIGHT ISOBORNYL MOIETY INHIBITOR - CHOLIC ACID ESTER DERIVATIVE PHOTOACID - BIS (t-BUTYL PHENYL) IODONIUM TRIFLATE TOTAL RESIST 32% WEIGHT ALICYCLIC CARBON ETCH RESISTANCE: 1.2 χ NOVOLAC IN HIGH-DENSITY Cl PLASMA (Helicon) IMAGING: 0.48 NA DUV STEPPER 2
500-nm FEATURES
Figure 11. Imaging results of Version 2 resist exposed at 248 nm.
hydrophilicity, glass transition, alicyclic carbon, acid cleavable protecting groups and transparency at 193 nm. A high speed, aqueous developing 193 nm single layer resist with a combination of chlorine etch resistance approaching novolak resin and good imaging quality resulted from this approach. ACKNOWLEDGMENTS The authors thank Hoa Truong and Monica Barneyfromthe IBM Almaden Research Center for help with material characterization and polymer synthesis, respectively, and Deanna DownsfromMIT Lincoln Laboratory for help with photoresist formulation and processing. Dr. Roger Sinta, Shipley Company, is gratefully acknowleged for his description of the TMA analysis of resistfilms.This work was supported by the Advanced Lithography Program of the Advanced Research Projects Agency.
LITERATURE CITED 1. R. D. Allen, G. M. Wallraff, D. C. Hofer, R. R. Kunz, S. C. Palmateer and M. W. Horn, Microlithography World, 21, Summer 1995. 2. M. A. Hartney, R. R. Kunz, D. J. Ehrlich, and D. C. Shaver, Proc. SPIE, 1262, 119 (1990).
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3. R. R. Kunz, R. D. Allen, W. D. Hinsberg and G. M . Wallraff, Proc. SPIE, 1925, 167(1993); R. D. Allen, et al. J. Photopolym. Sci. Tech., 6(4) 575 (1993); R. D. Allen, G. M . Wallraff, W. D. Hinsberg, L. L. Simpson, and R. R. Kunz, In "Polymers for Microelectronics", ACS Symposium Series 537, Thompson, L. F., Willson, C. G. and Tagawa, S. Eds., ACS, Washington, D.C., 1994, pp. 165-177. 4. Y. Kaimoto, K. Nozaki, S. Takechi and N. Abe, Proc. SPIE, 1672, 66 (1992). 5. G. Odian, "Principles of Polymerization", Third Edition, Wiley, New York, 1991. 6. M . Endo, et al., IEDM Tech. Digest, 45, December (1992). 7. K. Nakano, K. Maeda, S. Iwasa, J. Yano, Y. Ogura, E. Hasagawa, Proc. SPIE, 2195, 194 (1994). 8. R. D. Allen, G. M . Wallraff, R. A. DiPietro, D. C. Hofer, and R. R. Kunz, J. Photopolym. Sci. Technol., 7(3), 507(1994). 9. T. Ushirogouchi, N. Kihara, S. Saito, T. Naito, K. Asakawa, T. Tada, and M . Nakase, Proc. SPIE, 2195, 205(1994). 10. R. D. Allen, G. M . Wallraff, R. A. DiPietro, D. C. Hofer, and R. R. Kunz, Proc. SPIE, 2438 (1995) in press. 11. E. Reichmanis, C. W. Wilkins, D. A. Price, E. A. Chandross, J. Electochem. Soc. 130, 1433 (1983); E. Reichmanis, et al., J. Polym. Sci. Polym. Chem. Ed. 21, 1075 (1983). 12. M . J. O'Brien, J. Polym. Eng. Sci. 29, 846 (1989). RECEIVED September 15, 1995
