In the process of producing a semiconductor device such as IC and LSI, microprocessing by lithography using a photoresist composition has been conventionally performed. Recently, the integration degree of an integrated circuit is becoming higher and formation of an ultrafine pattern in the sub-micron or quarter-micron region is required. To cope with this requirement, the exposure wavelength also tends to become shorter, for example, from g/i line to KrF excimer laser light.
In the lithography using KrF excimer laser light, it is very important to satisfy high sensitivity, high resolution, good pattern profile, good iso/dense bias and sufficient depth of focus at the same time, and this problem needs to be solved.
As regards the resist suitable for a lithography process using KrF excimer laser light, electron beam or EUV light, a chemically amplified resist utilizing an acid catalytic reaction is mainly used from the standpoint of elevating the sensitivity and in the case of a positive resist, a chemical amplification resist composition containing, as main components, an acid generator and a phenolic polymer that is insoluble or sparingly soluble in an alkali developer but becomes soluble in an alkali developer by the action of an acid (hereinafter simply referred to as a “phenolic acid-decomposable resin”), is being effectively used.
With respect to such a positive resist, several resist compositions using a phenolic acid-decomposable resin obtained by copolymerizing an acid-decomposable acrylate monomer have been heretofore known. Examples thereof include a positive resist composition disclosed in Patent Document 1.
However, a carboxylic acid is produced in the exposed area and therefore, the dissolution rate for a developer becomes excessively high, giving rise to a problem that when a resist pattern is actually formed on a substrate, the pattern top is thinned and a pattern in an ideal rectangular form is not formed. In order to solve this problem, for example, Patent Document 2 has proposed a polymer having a repeating unit capable of reducing solubility of the polymer in an alkali developer, such as methyl methacrylate and styrene.
However, when a polymer containing a repeating unit having very high hydrophobicity, such as styrene, is used for a positive resist composition, the solubility in an alkali developer excessively decreases, as a result, a defect may be readily produced.
In the case of using a repeating unit having relatively low hydrophobicity, such as methyl methacrylate, the above-described development defect can be hardly generated, but, on the other hand, the plasma etching resistance is insufficient and selective etching becomes difficult.    Patent Document 1: U.S. Pat. No. 5,561,194    Patent Document 2: Japanese Patent No. 3116751