In recent years, with an increasing degree of integration in integrated circuits, processing of an ultrafine pattern composed of lines having a width not greater than half micron has come to be required in the manufacturing of semiconductor substrates such as very-large-scale integrated circuit. To satisfy such a requirement, the wavelength to be used in an exposure apparatus for photolithography is decreasing. Now, even use of, among far ultraviolet lights, an excimer laser beam (such as XeCl, KrF or ArF) having a short wavelength is under investigation. For the formation of patterns for lithography in this wavelength region, chemical amplification resists are typically employed.
When a KrF excimer laser is used as an exposure light source, a resin having small absorption mainly in a region of 248 nm and having poly(hydroxystyrene) as a basic skeleton is used as the main component so that the chemically amplified resist forms good patterns with high sensitivity and high resolution. It therefore becomes a superior system to a conventional naphthoquinone/novolac resin system.
When a light source having a shorter wavelength such as an ArF excimer laser (193 nm) is used as an exposure light source, on the other hand, a compound having an aromatic group essentially exhibits large absorption in a 193 nm region so that even the chemical amplification system does not produce sufficient results.
Resists for ArF excimer laser containing a resin having an alicyclic hydrocarbon structure has therefore been developed.
For example, Patent Document 1 and Patent Document 2 describe a composition containing a resin having a polycyclic acid-decomposable recurring unit and a non-acid-decomposable recurring unit. These resins are chemically amplified resists containing a protective group which is labile to an acid and dissociates in the presence of an acid. In spite of an eager demand for finer resist patterns, it has become difficult to achieve sufficient resist performances only by using such a protective group singly.
For example, Patent Document 3 therefore discloses a positive resist composition containing a resin having two recurring units having an acid-decomposable group.
As described in Patent Documents 4 to 6, a resist polymer obtained by introducing a crosslinking site, which dissociates in the presence of an acid, into the side chain of a resin containing a recurring unit having an acid dissociable protecting group is studied.
In this resist polymer, a crosslinking bond is cleaved by an acid catalyst, leading to improvement in dissolution contrast between an exposed area and an unexposed area. For the polymerization to obtain the polymer, a bifunctional monomer such as diacrylate is used and a crosslink reaction occurs in the side chain of a high molecular chain. As a result, the molecular weight distribution of the polymer thus formed is extremely large so that the polymer has poor solubility and the polymer tends to have an ultrahigh molecular weight. Even after decomposition with an acid, high-molecular-weight components sparingly soluble in an alkaline developer remain and existence of these foreign matters causes a problem, that is, an increase in development defects.
As described in Patent Document 5, when a crosslinked polymer having, in the high molecular side chain thereof, a crosslinking site having an acid-labile acetal skeleton is used as a resist polymer, it tends to have poor storage stability due to very high sensitivity to an acid.
There is proposed, on the other hand, a positive photosensitive resin having, in the high molecular main chain thereof, an acid dissociable structure (Patent Documents 7 and 8). Although such a proposal is made for overcoming the above-described problem of the side-chain crosslinking polymer, it does not produce a sufficient effect.
In addition, improvement in exposure latitude, focus latitude, and pattern collapse property is required more strongly than ever in order to satisfy the demand for finer resist patterns.    Patent Document 1: Japanese Patent Laid-Open No. 2003-167347    Patent Document 2: Japanese Patent Laid-Open No. 2003-223001    Patent Document 3: Japanese Patent Laid-Open No. Hei 11-119434    Patent Document 4: Japanese Patent Laid-Open No. 2005-37893    Patent Document 5: Japanese Patent Laid-Open No. 2001-98034    Patent Document 6: Japanese Patent Laid-Open No. 2000-214587    Patent Document 7: Japanese Patent Laid-Open No. 2006-91762    Patent Document 8: International publication WO05/085301 pamphlet