Oxime Sulfonate Technical Paper Technical Chemistry for Advanced Microlithography By Hitoshi Yamato, xime sulfonate compounds absorption of DNQ resist at 248 nm and Toshikage Asakura, are one of the important 193 nm is too high to transmit the light Yuichi Nishimae, chemistries as a photoacid throughout the resist to the bottom. As Akira Matsumoto, Ogenerator for advanced lithography an alternative technology, a chemically 2 Junichi Tanabe, application and practically used in mass amplified (CA) resist was developed. Jean-Luc Birbaum, production of semiconductor chips. This The CA resist is composed of polymer Peter Murer, chemistry is adjustable for various bearing acid-labile groups and photoacid applications like g-/h-/i-line, KrF and ArF Tobias Hintermann generator (PAG). Photolytic lithography. This paper describes an decomposition of PAG during light and Masaki Ohwa overview of oxime sulfonate chemistry exposure generates acids in the CA for semiconductor resist applications. resist. This acid acts as catalyst to facilitate de-protection reaction of ester Introduction group or acetal group of polymer, The development of semiconductor rendering polymer matrix alkaline- chips is making progress with astonishing soluble. For achieving desired resist speed, producing electronic apparatuses performance such as resolution, that are smaller, have higher speeds, depth-of-focus, line edge roughness and and offer better performance. This etching resistance, the acid-labile dynamic advancement is significantly polymers were intensively studied and attributable from the development of developed notably. For further photolithography technology. In an early improvement of the CA resist, a PAG era of integrated circuits (IC), a was the key ingredient. Several manufacturing lithography technique chemistries were extensively was introduced with a light source of investigated as the photoactive g-line wavelength (436 nm). For further component for the CA resists miniaturization of IC, the light source (e.g., iodonium sulfonate, sulfonium wavelength gets shorter and shorter sulfonate, diazomethane-disulfone, [i.e., i-line (365 nm), KrF (248 nm) and N-sulfonyloxyimido compounds, ArF (193 nm)]. Currently, ArF immersion nitrobenzyl sulfonate, etc.).3 Oxime is actively investigated as the most sulfonate chemistry was used due to its advanced technology, and EUV potential ability as latent acid and its (13.5 nm) is discussed as the next wide structure variation. generation lithography. When the g-line and i-line light sources were used, the resist comprised diazonaphthoquinone (DNQ) as a photosensitive component.1 However, after introduction of KrF laser, the DNQ resist cannot be employed for advanced lithography because the Oxime sulfonate

10 RADTECH REPORT NOVEMBER/DECEMBER 2007 developed.6 Figure 1 shows UV table 1 absorption spectrum, indicating sufficiently red-shifted absorption PAG ABC series profile over g-line (436 nm) with the PAG Photoacid generated λmax at 405 nm. This red-shifted profile Paper Technical A n-Propanesulfonic acid is attributable from the extended B n-Octanesulfonic acid conjugated system by combination of phenyl ring and quinoide structure of C p-Toluenesulfonic acid thiophene ring. With the negative tone model formulation, the PAG ABC series showed a good sensitivity not Several oxime sulfonate compounds, and corrosive and is a concern with only with i-line exposure, but also with aiming to g-/h-/i-line, KrF and ArF potential damage of the manufacturing g-line exposure. lithography, were developed. In equipment. Since sulfonic acids are As π-conjugation of an organic addition, a sensitization of oxime less volatile, less mobile and less molecule is extended, the solubility of sulfonate was investigated. These corrosive, they were therefore the molecule usually becomes lower. studies are described in this paper. preferred over hydrochloric acid in the However, PAG ABC series retained a resist applications. Though only a few good solubility above 15% in PGMEA as PAG for g-/h-/i-line Application sulfonic acid generators sensitive up to summarized in Table 2. The good Recently, there have been demands g-line were known, oxime sulfonate solubility of PAG minimized the risk of for broadband and i-line resist with high type PAG was investigated. This particle generation during resist storage sensitivity, high resolution and thickness. chemistry was adjusted to the target and improved formulation flexibility. DNQ resist has been widely used for applications and exposure wavelength Figure 2 displays a change of UV broadband and i-line applications so far, range by selecting from a wide range of absorption in acetonitrile solution upon but its performance does not satisfy such chromophores and acid moieties. exposure to 436 nm monochromatic new demands. The CA resist is suitable Berner et. al. reported oxime light. A distinctive absorption at 255 nm for these applications. Chloromethyl sulfonate as a latent acid catalyst emerged and the absorption above triazines with extended conjugation are for coating application5, but such 420 nm completely disappeared during known as suitable PAGs for g-/h-/i-line compounds do not have absorption exposure. The quantum yield of exposure.4 However, hydrochloric acid, in a longer wavelength. Aiming for photolysis, φd, was determined from photochemically generated from g-/h-/i-line application, IRGACURE® the absorption decrease at 436 nm, chloromethyl triazine, is highly volatile PAG ABC series (Table 1) was indicating that PAG A was 0.13. Similarly the amount of acid generated Figure 1 upon exposure was determined by the indicator dye method. In Figure 3, the decrease of the normalized absorption UV absorption of PAG A at 436 nm and the normalized molar acid concentration (relative to initial 1 molar PAG concentration) of the PAG A 0.01 g/l 0.8 photochemically generated acid were PAG A 0.50 g/l plotted against exposure time. It was 0.6 found that the decrease of absorption 0.4 at 436 nm (photobleaching) was equal Absorbance 0.2 to the generated acid concentration when both were normalized to the 0 190 240 290 340 390 440 490 initial molar PAG concentration. This suggests that photodecomposition Wavelength/nm of PAG ABC series quantitatively generates acid. Therefore, the quantum

yield of photolysis, φd, corresponds

NOVEMBER/DECEMBER 2007 RADTECH REPORT 11 polymer needed to be heated in the table 2 range of 120-130°C. The PAG ABC series was thermally Solubility in PGMEA stable up to 155°C in the neat form, PAG Solubility (%)

Technical Paper Technical but the stability was lowered to 140°C A 20 in a polymer matrix. This PAG was B > 30 stable enough for the application of C 17 acetal-type KrF resist. On the other hand, there is a risk in its application for ESCAP-type formulation in terms of thermal decomposition of PAG at PEB. The thermal stability of oxime directly to that for acid formation, φa, ESCAP-type polymer (i.e., co-polymer sulfonate was studied and a thermally thus φa/φd = 1. of PHS and tert-butyl methacrylate). stable KrF PAG D was developed.8 This The reported φa/φd ratios for oxime The acetal-type polymer can be sulfonates were found in the range of deprotected below 110°C at post-exposure compound possessed trifluoromethyl 0.6-0.8.7 Hence, it was concluded that baking (PEB), while the ESCAP-type group adjacent to oxime group, which the PAG ABC series provides good or superior photoacid generation efficiency, Figure 2 based on good acid conversion yield. The storage stability of PAG is one UV absorption of change of PAG ABC series of the critical requirements for CA upon exposure resist application because even a small 2.4 amount of acid can cause degradation 1.9 0 min of CA resist. The stability of the PAG 10 min 30 min ABC series was evaluated by monitoring 1.4 60 min 120 min acid concentration in the sample 240 min 0.9 stored at various temperature at Absorbance frequent intervals (Figure 4). No acid 0.4 formation was observed after more -0.1 200 300 400 500 600 than one-year in storage even at 40°C. Wavelength (nm) When the sample was stored at 4°C, the PAG ABC series was stable for more than five years. This evaluation is continuing. Figure 3 PAG for KrF Application When KrF excimer-laser (248 nm) Acid generation and UV change of PAG ABC series was introduced as the light source for upon exposure resist application, the conventional DNQ resist could not be employed due 1.2 1.2 to a high absorption of novolak polymer matrix and DNQ molecules. 0.8 0.8 Therefore, the CA resist, comprising [generated acid]/[loaded PAG] polyhydroxystyrene (PHS)-based 0.4 0.4 polymer and PAG, was developed for Normalized decrease of absorption at 436 nm Normalized decrease of absorption at 436nm KrF lithography. There are two 0 0 [generated acid]/[loaded PAG] [generated acid]/[loaded PAG] 0 100 200 300 400 polymer platforms with different Exposure time (min) acid-labile groups. One is acetal-type polymer (i.e., PHS partially protected with acetal group). The other is

12 RADTECH REPORT NOVEMBER/DECEMBER 2007 stabilizes this molecule. PAG D was stable up to 188°C even in a polymer Figure 4 matrix and more than 200°C in a neat form as shown in Figure 5. Therefore, Storage stability of PAG A in neat form

this PAG can be employed in both Paper Technical 100 Magnification of Y axis acetal- and ESCAP-type formulations. 1.0 ] (%) 80 0.8 0.6 O O 4?40˚C 0.4 OS OS 60 r.t. 0.2 N N O O 40?40˚C 0.0 F F 40 0 50 100 150 200 250 300 F F F F O O 20 PAG D Generated acid]/[Loaded Generated A PAG As can be seen from the UV [ 0 0 50 100 150 200 250 300 absorption spectrum in Figure 6, Time (Week) PAG D had an absorption valley around 248 nm and showed a relatively low absorption at the wavelength of KrF lithography. Such a low absorption combined with high sensitivity is a Figure 5 highly desired property, because a low-overall absorbance ensures uniform DSC curves of PAG D exposure throughout the whole thickness of the resist. Compared to neat other PAGs typically used for KrF lithography such as triphenyl-sulfonium salt (TPS) and diazodisulfone (DAS), with PHS PAG D showed balanced behavior with respect to a number of Heat flow application-relevant properties as 0 50 100 150 200 250 KrF PAG as summarized in Table 3. Temp (C) PAG for ArF Application With the polymer development for ArF formulation, a methacrylate unit, bearing aliphatic cyclic group such as adamantyl, was incorporated as a Figure 6 acid-labile group to improve etch resistance and transparency at 193 nm. UV absorption of PAG D As a result, PAG with a strong acid 1 release was required, because the 0.8 PAG D 0.01g/l cleavage of ester bonding of the polymer matrix (e.g., 2-methyl- 0.6 PAG D 0.50g/l adamantyl methacrylate) was a key 0.4 reaction during the lithography process. Absorbance 0.2 The oxime sulfonate compounds with a strong acid such as perfluoroalkyl- 0 190 240 290 340 390 440 490 sulfonate were investigated. Usually such compounds were found to not be Wavelength / nm so stable because the perfluoroalkyl- sulfonate is a good leaving group. For

NOVEMBER/DECEMBER 2007 RADTECH REPORT 13 instance, an attempt was made to isolated as a pure material. In the case C4F9 synthesize the PAG ABC series with of PAG D analogue with trifluoro- OOS O Rf perfluoroalkylsulfonate, but the methanesulfonate, the pure compound N -C F PAG E 3 7 target compound was degraded was obtained, but it was not stable in -C F H PAG F Rf 4 8 -C F H PAG G Technical Paper Technical the presence of amine in solution. during the purification and not 6 12

Long perfluroalkyl group with table 3 oxime sulfonate

Screening results of PAG D A fine-tuning of the structure has 1) 2) 3) posi PAG Abs248 Solubility Td in PHS E0 improved the stability of oxime sulfonate 2 (wt/v%) (wt/v%) (C) (mJ/cm ) with perfluoroalkylsulfonate, resulting in PAG D 0.091 25 188 1.73 PAG EFG series compounds.9 The key cf. structure was a long perfluoroalkyl TPS 0.329 1.4 >200 0.27 group next to oxime sulfonate, which DAS 0.019 ND 142 5.06 stabilized the molecule. 1) 0.01 g/l in AcCN As shown in Figure 7, PAG EFG 2) in PGMEA 3) Determined by DSC with equal amount of PHS series was as stable as TPS in the presence of triethanolamine in PGMEA stored at 40°C, while PAG D Figure 7 analogue with trifluormethane- sulfonate degraded to approximately Storage stability of PAG G in the presence of amine 40% after 28 days. This result in PGMEA at 40˚C suggests that the PAG EFG series is applicable for resist formulation. 1.2 The UV absorption spectra of PAG 1.0 PAG G EFG series and TPS at equivalent 0.8 TPS nf weight percentage in acetonitrile are 0.6 PAG D 0.4 presented in Figure 8. The absorbance concentration 0.2

Normalized PAG of PAG EFG series at 193 nm was 0.0 about one third of that of TPS. The 0102030 time/day low absorption of PAG EFG facilitated a better resist profile as compared to TPS. The upper limit of PAG concentration in a given resist was a function both of PAG solubility and Figure 8 absorption. Thus, the low absorption and good solubility of the PAG EFG UV absorption of PAG EFG series series means increased formulation flexibility for ArF resist. 1.4 By monitoring the remaining 1.2 PAG G amount of PAG by HPLC analysis 1 PAG F PAG E 0.8 during exposure at 193 nm, the TPSnf 0.6 quantum yield of PAG G was estimated Absorbance 0.4 as compared with TPS and BPI 0.2 (di-tertbutylphenyliodonium salt). The 0 result is summarized in Table 4. PAG G 190 240 290 340 390 gave the highest quantum yield (0.27), Wavelength / nm which was superior to TPS by 46%. On the other hand, the quantum yield of BPI was quite low, 0.06. The iodonium

14 RADTECH REPORT NOVEMBER/DECEMBER 2007 chromophore was not suitable for ArF lithography from the viewpoint table 4 of photospeed. Quantum yield of PAG in acetonitrile ArF immersion lithography is PAG Quantum yield in solution

becoming the major technology of the Paper Technical next generation for 45 nm node. Water PAG G 0.27 was employed as the immersion TPSnf 0.18 medium due to its high-refractive BPInf 0.06 index (n=1.44) and high transparency at 193 nm. The demanded properties for PAG in immersion application are non-leaching to water and contribution to reduce immersion specific defects. Figure 9 The evaluation of PAG leaching from ArF model formulation to water Contact angle of water on ArF resist revealed that TPS had significant 90 leaching behavior, 2-9 x 10-11mol/cm2. On the other hand, no leaching of PAG 85

EFG series was observed (below 80 detection limit: < 2.6 x 10-12mol/cm2) 75 due to non-ionic property. Water droplets remained on the 70 surface of resist after exposure and Contact Angle / Degree 65 stage scanning was one of the causes for the immersion specific defects. 60 w/o PAG TPS PAG G PAG E PAG F Hydrophobicity of the resist surface was found to play an important role to reduce the immersion specific defects. In addition, the high hydrophobic property of the surface enabled high-stage scan speed, leading to high Figure 10 throughput.10 Figure 9 displays the contact angle of water on ArF model Sensitivity of PAG D in combination with sensitizers formulation including various PAGs. at i-line

The addition of PAG EFG series 100.0100.0 increases the hydrophobicity of resist ) surface, while the addition of TPS 2 80.080.0 O decreases the hydrophobicity. PAG 60.060.0 EFG series can contribute to reduce 40.040.0 the defects and increase the scan O speed. These results suggest that Sensitivity (mJ/cm2) Sensitivity (mJ/cm 20.020.0 PAG EFG is suitable for ArF 0.00.0 N immersion lithography. DBA ABP AC

O O Sensitization of Oxime

N N O O Sulfonate O As described, the use of Sensitizer photolithography with light source of g-/h-/i-line was extensively studied besides the front-end process of

NOVEMBER/DECEMBER 2007 RADTECH REPORT 15 This paper was originally published Figure 11 in the Journal of Photopoly. Sci. Technol. 20 (5), 637-642 (2007). ◗ Sensitivity of PAG in combination with DBA at i-line

Technical Paper Technical >100 >100 Reference 1. For example, R. Dammel, Diazonaphthoquinone-based Resists, 100 DBA content pp. 9-96, SPIE Optical Engineering against PAG

) Press, Washington, D. C., 1993. 2 80 2. H. Ito, J. Polym. Sci. A: Polym. Chem., 0% 60 41 (2003) 3863-3870. 10% 3. a) J. V. Crivello, Adv. Polym. Sci., 62 40 20% (1984) 1-48. b) M. Shirai, M. Tsunooka, Prog. Polym. Sci., 21 (1996) 1-45.

Sensitivity (mJ/cm 20% 20 4. G. Buhr, R. Dammel, C. R. Lindley, Polym. Mater. Sci. Eng. 61 (1989) 0 269-77. PAG D PAG F 5. G. Berner and W. Rutsch, US patent 4540598. 6. T. Asakura, H. Yamato, M. Ohwa, J. Photopolym. Sci. Technol. 13 (2000) 223-30. semiconductor manufacturing. They electron transfer from the sensitizers to 7. M. Shirai, M. Tsunooka, Bull. Chem. are aimed for new fields like MEMS the oxime sulfonate molecule, but Soc. Jpn. 71 (1998) 2483-2507. (Micro Electro Mechanical Systems), further studies are necessary for the 8. a) H. Yamato, T. Asakura, A. dielectric layer, buffer layer of mechanism elucidation. Another oxime Matsumoto, M. Ohwa, Proc. SPIE semiconductor chip, display sulfonate compound, PAG F, was also 4690 (2002) 799-808. b) T. Asakura, H. Yamato, A. Matsumoto, P. Murer applications, etc. PAG ABC series was studied with respect to sensitization by and M. Ohwa, J. Photopolym. Sci. used for these applications, which DBA as shown in Figure 11. This PAG Technol. 16 (2003) 335-346. has red-shifted absorption profile, showed a good sensitivity at i-line in 9. a) H. Yamato, T. Asakura, T. leading to the direct excitation at a combination with DBA. It is worthwhile Hintermann and M. Ohwa, Proc. SPIE 5376 (2004) 103-114. b) T. Asakura, longer wavelength. As an alternative to mention that anthracene compounds H. Yamato, T. Hintermann and approach, the sensitization ability of work well as the sensitizer for oxime M. Ohwa, Proc. SPIE 5753 (2005) oxime sulfonate compounds was sulfonate compounds. 140-148. c) H. Yamato, T. Asakura, and M. Ohwa, Proc. SPIE 6153 (2006) investigated. 61530F/1-61530F/9. PAG D had no significant absorption Conclusion 10. S. Owa, H. Nagasaka, Y. Ishii, K. The oxime sulfonate compounds at 365 nm. It did not give any sensitivity Shiraishi and S. Hirukawa, Proc. SPIE have great potential as PAGs and 5754 (2005) 655-668. with the negative tone formulation they are adjustable for various upon i-line exposure. To this demands such as high thermal and —Hitoshi Yamato, Toshikage formulation, various sensitizes with storage stability, red-shifted absorption Asakura, Yuichi Nishimae, Akira 50 wt% against PAG were added Matsumoto, Junichi Tanabe, Jean- profile, etc. Several oxime sulfonate and their sensitivity at i-line was Luc Birbaum, Peter Murer, Tobias type PAGs for g-/h-/i-line, KrF and measured, respectively. The result Hintermann and Masaki Ohwa are ArF lithography applications have with the Technology Center of the sensitivity was presented in been developed. They are currently Electronic Materials, Coating Effects Figure 10. Amino-coumarin (AC), Segment, Ciba Specialty Chemicals, used in the mass production of aminobenzophenone (ABP), and Amagasaki, Japan. semiconductor chips. anthracene (DBA) compounds had an effect on the sensitization of PAG D, Acknowledgments and triggered the decomposition of The authors would like to PAG, giving photosensitivity of the express sincere thanks to Midori formulation at i-line. Among them, Masaki, Yuka Kotake, Mika Kuno 9, 10-dibutoxy-anthracene most and Rumi Yamashita for their effectively sensitized PAG D. The experimental support. sensitization mechanism assumed an

16 RADTECH REPORT NOVEMBER/DECEMBER 2007