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.