1. Field of the Invention
The present invention relates to a negative resist composition and a patterning process using the same.
2. Description of the Related Art
In recent years, as an integrated circuit progresses toward a high integration, a further finer patterning is required. Especially, when a resist pattern of 0.2 μm or less is formed, as a usual manner, a chemically amplified resist composition wherein an acid generated by an exposure light acts as a catalyst, from which a high sensitivity and a high resolution can be obtained, has been used. As the exposure light source, a high energy beam such as an ultraviolet beam, a far-ultraviolet beam and EB has been used, and especially, an exposure method by using EB or EUV has been drawing attention as the method that the finest pattern is expected among the methods currently under consideration.
In a resist composition, there are a positive type in which an exposed part is dissolved and a negative type in which an exposed part remains as a pattern. They are selected based on the ease of its use and depending on the required resist pattern. A chemically amplified negative resist composition usually contains a polymer which is alkaline-soluble and is made alkaline-insoluble by action of an acid, and an acid generator generating an acid by decomposition with an exposure light, and, in addition, usually contains a basic compound to control diffusion of the acid generated by the light exposure.
Many negative resist compositions using a phenolic unit as an alkaline-soluble unit constituting the polymer that is soluble in an aqueous alkaline developer have been developed, especially for exposure to a KrF excimer laser beam. These have not been used for an ArF excimer laser beam because a phenolic unit does not transmit the light when the exposure light having a wavelength of 150 to 220 nm is used. However, in recent years, these have been drawing attention again as a negative resist composition for exposure to EB and EUV, which are exposure methods to obtain a further finer pattern, as reported in Japanese Patent Laid-Open (kokai) No. 2006-201532, Japanese Patent Laid-Open (kokai) No. 2006-215180, and Japanese Patent Laid-Open (kokai) No. 2008-249762.
As a further finer pattern is required, negative resist compositions using a hydroxystyrene unit which is the typical structure of the phenolic unit have been improved many times. However, as a pattern size is miniaturized considerably like 0.1 μm or less, conventional materials cannot have solved the problems that a so-called bridge (a finely-filamented resist layer remained between pattern lines of a fine pattern) is easily caused and an undercut occurs at the substrate's interface and thus patterns easily collapse.
In addition, as for so-called substrate dependence of a pattern (a pattern profile change near the substrate due to a material of a substrate to be processed), with a miniaturization of a pattern to be obtained, even a small profile change has become a problem. Especially, when pattern forming is done by using a chemically amplified negative resist on a chromium oxynitride, the outermost surface material of a photomask blank, in processing of the photomask blank, a so-called undercut (a cleavage formed at the contact part of the resist pattern with the substrate) is caused, but conventional materials could not solve it sufficiently.
Further, in the case of the resist elaboration as mentioned above, examples of characteristics to be demanded for resist compositions are not only a higher resolution which is a fundamental performance of a resist, but also a higher etching resistance. This is because a progressive miniaturization of patterns necessitates to progressively decrease thicknesses of resist films. Known as one technique for obtaining such a higher etching resistance, is a method, which also has been disclosed in Japanese Patent Laid-open (Kokai) No. 2008-249762, to introduce a compound: a polycyclic compound such as indene and acenaphthylene, which includes an aromatic ring(s) and a non-aromatic ring(s) having a carbon-carbon double bond conjugated with the aromatic ring, into a polymer having a hydroxystyrene unit as a subsidiary component.
In turn, as polymers for positive resists, it has been proposed to use a polymer having only an indene skeleton such as described in Japanese Patent Application Laid-open (kokai) No. 2004-149756, while another method has been proposed in Japanese Patent Application Laid-open (kokai) No. 2006-169302 to use a unit having an acenaphthylene skeleton by combining it with a hydroxystyrene derivative.
In Japanese Patent Laid-Open (kokai) No. 2008-249762, a polymer comprising a hydroxystyrene unit and a chlorostyrene unit was used as a styrene derivative unit, while a miscible system of triphenyl sulfonium-2,5-dimethylbenzene sulfonic acid and triphenyl sulfonium-2,4,6-triisopropylbenzene sulfonic acid was used as an acid generator. By using this material containing a crosslinker which forms cross-linkages among polymers by the aid of the acid as a catalyst to make the polymer insoluble in a developer, a line-and-space pattern of 60 nm with few undercuts and without a bridge has been obtained. However, pattern formation of 50 nm or less was difficult because of occurrence of a bridge.