1. Field of the Invention
The present invention relates to a resist composition and a method of forming a resist pattern that uses the resist composition.
Priority is claimed on Japanese Patent Application No. 2011-273759, filed Dec. 14, 2011, the content of which is incorporated herein by reference.
2. Description of Related Art
In lithography techniques, for example, a resist film composed of a resist material is formed on a substrate, and the resist film is subjected to selective exposure followed by development, thereby forming a resist pattern having a predetermined shape on the resist film. A resist material in which the exposed portions of the resist film become soluble in a developing solution is called a positive-type, and a resist material in which the exposed portions become insoluble in a developing solution is called a negative-type.
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have led to rapid progress in the field of pattern miniaturization. Typically, these pattern miniaturization techniques involve shortening the wavelength (and increasing the energy) of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are starting to be introduced in the mass production of semiconductor elements. Furthermore, research is also being conducted into lithography techniques that use an exposure light source having a shorter wavelength (and a higher energy level) than these excimer lasers, such as extreme ultraviolet radiation (EUV), electron beam (EB), and X-ray.
Resist materials for use with these types of exposure light sources require lithography properties such as a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity to these types of exposure light sources. As a resist material that satisfies these conditions, conventionally a chemically amplified resist composition has been used, which includes an acid generator component that generates acid upon exposure, and a base component that exhibits changed solubility in a developing solution under the action of acid.
Numerous compounds have already been proposed for the acid generator component for chemically amplified resist compositions, including onium salt-based acid generators, oxime sulfonate-based acid generators, diazomethane-based acid generators, nitrobenzylsulfonate-based acid generators, iminosulfonate-based acid generators, and disulfone-based acid generators.
Resins (base resins) are typically used as the base components of chemically amplified resist compositions.
For example, in an alkali developing process where an alkali developing solution is used as a developing solution, a chemically amplified resist composition for forming a positive-type resist pattern typically contains an acid generator component and a resin component that exhibits increased solubility in an alkali developing solution under the action of acid. If the resist film formed using this resist composition is selectively exposed during formation of a resist pattern, then acid is generated from the acid generator component within the exposed portions, and the action of this acid causes an increase in the solubility of the resin component in an alkali developing solution, making the exposed portions soluble in the alkali developing solution. As a result, by performing alkali developing, the unexposed portions remain as a pattern, resulting in the formation of a positive-type pattern.
As the resin component, a resin that exhibits increased polarity under the action of acid is typically used. When the polarity of the resin is increased, the solubility in an alkali developing solution increases. On the other hand, when the polarity is increased, the solubility in an organic solvent decreases, and therefore if a solvent developing process that uses a developing solution containing an organic solvent (an organic developing solution) is employed instead of the alkali developing process, then within the exposed portions of the resist film, the solubility in the organic developing solution decreases relatively, meaning that during the solvent developing process, the unexposed portions of the resist film are dissolved in the organic developing solution and removed, whereas the exposed portions remain as a pattern, resulting in the formation of a negative-type resist pattern. This type of solvent developing process that results in the formation of a negative-type resist pattern is sometimes referred to as a negative-type developing process (for example, see Patent Document 1).
Currently, resins that contain structural units derived from (meth)acrylate esters within the main chain (acrylic resins) are widely used as base resins for chemically amplified resist compositions designed for use in ArF excimer laser lithography or the like, as they exhibit excellent transparency in the vicinity of 193 nm (for example, see Patent Document 2). Here, the term “(meth)acrylate ester” is a generic term that includes either or both of the acrylate ester having a hydrogen atom bonded to the α-position and the methacrylate ester having a methyl group bonded to the α-position. The term “(meth)acrylate” is a generic term that includes either or both of the acrylate having a hydrogen atom bonded to the α-position and the methacrylate having a methyl group bonded to the α-position. The term “(meth)acrylic acid” is a generic term that includes either or both of acrylic acid having a hydrogen atom bonded to the α-position and methacrylic acid having a methyl group bonded to the α-position.
In order to improve the lithography properties and the like, the base resin typically includes a plurality of structural units. For example, in the case of an aforementioned resin component that exhibits increased polarity under the action of acid, typically, a base resin is used that contains a structural unit having an acid-decomposable group that decomposes under the action of the acid generated from the acid generator component, resulting in increased polarity, and also contains a structural unit having a polar group such as a hydroxyl group, and a structural unit having a lactone structure and the like.
One known technique for further improving the resolution is a lithography technique known as liquid immersion lithography (hereafter also referred to as “immersion exposure”), in which exposure (immersion exposure) is conducted in a state where the region between the objective lens of the exposure apparatus and the sample is filled with a liquid (an immersion medium) that has a larger refractive index than the refractive index of air.
By using immersion exposure, it is considered that higher resolutions equivalent to those obtained using a shorter wavelength light source or a higher NA lens can be achieved using the same exposure light source wavelength, with no reduction in the depth of focus. Furthermore, immersion exposure can be conducted using existing exposure apparatus. As a result, it is expected that immersion exposure will enable the formation of resist patterns of higher resolution and superior depth of focus at lower costs, and in the production of semiconductor elements, which requires enormous capital investment, immersion exposure is attracting considerable attention as a method that offers significant potential to the semiconductor industry, both in terms of cost and in terms of lithography properties such as resolution.
Immersion lithography is effective in forming patterns having all manner of shapes. Further, immersion exposure is capable of being used in combination with super-resolution techniques such as phase shift methods and modified illumination methods that are currently under investigation. Currently, techniques using an ArF excimer laser as the exposure source are the most actively researched immersion exposure techniques. Further, water is mainly being investigated as the immersion medium.
In recent years, the addition of a photoreactive quencher to a chemically amplified resist composition has also been proposed (for example, see Patent Documents 3 and 4). A photoreactive quencher is a salt formed from an anion and a cation, which, prior to exposure, has a quenching action that traps acid generated from the acid generator or the like via an ion exchange reaction, but which decomposes upon exposure, resulting in a loss of the quenching action. Accordingly, when a resist film formed using a chemically amplified resist composition containing such a photoreactive quencher is subjected to exposure, in the exposed portions, the photoreactive quencher loses its basicity relative to the acid generated from the acid generator or the like, whereas in the unexposed portions, the photoreactive quencher traps the acid, thereby suppressing the diffusion of acid from the exposed portions into the unexposed portions, resulting in improved lithography properties.