In recent years, the trend toward micro-scale pattern rule has been increasing with the trend toward large-scale integration and high-speed of LSI. The trend toward a shorter wavelength of the exposure light source lies behind it. For example, it has become possible to mass-produce DRAM (dynamic random-access memory) of 64M-bit (processing dimension is 0.25 μm or less) by the wavelength shortening from mercury lamp i-line (365 nm) to KrF excimer laser (248 nm). Furthermore, in order to realize the production of DRAM's having integration degrees of 256M and 1G or greater, a lithography using ArF excimer laser (193 nm) has been studied on a full scale, and a 65 nm node device has been studied by a combination with a high NA lens (NA≧0.9). Although the use of F2 laser having a wavelength of 157 nm had been named as a candidate for the production of the next 45 nm node devices, the application was postponed by many problems represented by cost increase of scanner, change of optical system, low etching resistance of resist, and the like. As an alternative to F2 lithography, proposed was ArF immersion lithography. Now, the development is going on toward its early introduction.
As a resist suitable for such exposure wavelength, “chemically amplified resist material” attracts much attention. This contains a radiosensitive acid generator (hereinafter referred to as “photoacid generator”), which generates an acid by radiation irradiation (hereinafter, referred to as “exposure”), and is a pattern-forming material that forms a pattern by making a difference in solubility between the exposed portion and the unexposed portion through a reaction using the acid generated by the exposure as a catalyst.
As the photoacid generator used for such chemically amplified resist material, onium sulfonates, such as iodonium sulfonate and sulfonium sulfonate, sulfonic acid esters, N-imidosulfonate, N-oximesulfonate, o-nitrobenzylsulfonate, trismethanesulfonate of pyrogallol, and the like are known.
The acids generated from these photoacid generators upon exposure are alkanesulfonic acids, arylsulfonic acids, and partially or entirely fluorinated arylsulfonic acids, alkanesulfonic acids, and the like.
Of these, acid generators that generate partially or entirely fluorinated alkanesulfonic acids have a sufficient acid strength against deprotection reactions of protective groups that are difficult in deprotection, and therefore many of them have been put into practical use. As the examples, it is possible to mention triphenylsulfonium trifluoromethanesulfonate, triphenylsulfonium perfluoro-n-octanesulfonate, and the like.
However, in the case of triphenylsulfonium trifluoromethanesulfonate, the acid to be generated becomes a sufficiently strong acid, and the resolution performance as a photoresist becomes sufficiently high, but it has a defect of high mask dependency as a photoresist due to low boiling point of the acid and due to long diffusion length of the acid. Furthermore, in the case of triphenylsulfonium perfluoro-n-octanesulfonate, it has a sufficient acidity and is almost appropriate in terms of acid boiling point and diffusion length. Therefore, it attracts much attention in recent years. However, in the case of considering environmental problems, radiosensitive acid generators having such perfluoroalkylsulfonyl structure are generally low in combustibility, and human body accumulation is also suspected. Thus, there is a proposal of limiting the use in a report (Non-patent Publication 1) by the US ENVIRONMENTAL PROTECTION AGENCY.
Under such background, there have been the developments of acid generators that generate partially or entirely fluorinated alkanesulfonic acids and that have characteristics of having a sufficient acidity, of being appropriate in terms of acid boiling point and diffusion length, and of having less burden on the environment. Thus, there have been the developments of alkoxycarbonylfluoroalkanesulfonic acid onium salts as acid generators, such as triphenylsulfonium methoxycarbonyldifluoromethanesulfonate (Patent Publication 1), (4-methylphenyl)diphenylsulfonyl t-butoxycarbonyldifluoromethane sulfonate (Patent Publication 2), or triphenylsulfonium (adamantan-1-ylmethyl)oxycarbonyldifluoromethanesulfonate (Patent Publication 3).    Patent Publication 1: Japanese Patent Application Publication No. 2004-117959    Patent Publication 2: Japanese Patent Application Publication No. 2002-214774    Patent Publication 3: Japanese Patent Application Publication No. 2004-4561    Non-patent Publication 1: Perfluorooctyl Sulfonates; Proposed Significant New Use Rule [Oct. 18, 2000 (Volume 65, Number 202)]
Then, there have been reported resist compositions containing these acid generators and pattern-forming methods using such resist compositions.