Jul.  18,  2019  1  US 2019/0219920 Al  RESIST COMPOSITION, METHOD OF  FORMING RESIST PATTERN, POLYMERIC  COMPOUND, AND COPOLYMER  TECHNICAL FIELD  DESCRIPTION OF RELATED ART  [0001]  The present invention relates to a resist composi› tion, a method of forming a resist pattern, a polymeric  compound, and a copolymer.  [0002]  Priority is claimed on Japanese Patent Application  No. 2016-197492, filed Oct. 5, 2016, and Japanese Patent  Application No. 2017-176005, filed Sep. 13, 2017, the  contents of which are incorporated herein by reference.  [0003]  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 of the resist film  become insoluble in a developing solution is called a nega› tive-type.  [0004]  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 miniaturization  techniques involve shortening the wavelength (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 used in mass production. Furthermore,  research is also being conducted into lithography techniques  that use an exposure light source having a wavelength  shorter (energy higher) than these excimer lasers, such as  electron beam (EB), extreme ultraviolet radiation (EUV),  and X ray.  [0005]  Resist materials for use with these types of expo› sure 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.  [0006]  As a resist material that satisfies these conditions,  a chemically amplified composition is used, which includes  a base material component that exhibits a changed solubility  in a developing solution under the action of acid and an  acid-generator component that generates acid upon expo› sure.  [0007]  For example, in the case where the developing  solution is an alkali developing solution (alkali developing  process), a chemically amplified positive resist which con› tains, as a base component (base resin), a resin which  exhibits increased solubility in an alkali developing solution  under action of acid, and an acid generator is typically used.  If a resist film formed using such a resist composition is  selectively exposed at the time of forming a resist pattern, in  exposed areas, acid is generated from the acid generator  component, and the polarity of the base resin increases by  the action of the generated acid, thereby making the exposed  areas of the resist film soluble in the alkali developing  solution. Thus, by alkali developing, the exposed portions of  the resist film are dissolved and removed, whereas the  [0016]  Further, for improving lithography properties, for  example, there has been proposed a chemically amplified  resist composition which uses, as a base component, a  polymeric compound containing a structural unit having an  acid decomposable group with a specific structure (an acid  decomposable group containing a tertiary carbon atom hav› ing bonded thereto an aliphatic hydrocarbon group contain› ing an ether bond) (see, for example, Patent Literature 3).  [0015]  For example, there has been disclosed a resist  composition in which a polymeric compound having a  specific acid dissociable, functional group has been used to  enhance the reactivity to acid, and improve solubility in a  developing solution (see, for example, Patent Literatures 1  and 2).  [0014]  In particular, when a resist film is exposed with  EUV (extreme ultraviolet) or EB (electron beam), in the  resist material, control of acid diffusion becomes a problem.  Conventionally, for controlling acid diffusion, various meth› ods of changing the design of the base resin has been  proposed.  [0013]  In the formation of a resist pattern, the behavior of  acid generated from the acid-generator component upon  exposure is one of the factors which has large influence on  the lithography properties.  [0012]  As onium salt acid generators, those which have an  onium ion such as triphenylsulfonium in the cation moiety  are mainly used. Generally, as the anion moiety for onium  salt acid generators, an alkylsulfonate ion or a fluorinated  alkylsulfonate ion in which part or all of the hydrogen atoms  within the aforementioned alkylsulfonate ion has been sub› stituted with fluorine atoms is typically used.  [0011]  As the acid-generator component used in a chemi› cally amplified resist composition, various kinds have been  proposed. For example, onium salt acid generators such as  iodonium salts and sulfonium salts; oxime sulfonate acid  generators; diazomethane acid generators; nitrobenzylsul› fonate acid generators; iminosulfonate acid generators; and  disulfone acid generators are known.  [0010]  For example, in the case of a resin composition  which exhibits increased solubility in an alkali developing  solution by the action of acid, a structural unit containing an  acid decomposable group which is decomposed by the  action of acid generated from an acid generator component  and exhibits increased polarity. Further, a structural unit  containing a lactone-containing cyclic group or a structural  unit containing a polar group such as a hydroxy group is  used in combination.  [0009]  In general, the base resin used for a chemically  amplified resist composition contains a plurality of structural  units for improving lithography properties and the like.  [0008]  On the other hand, when such a base resin is  applied to a solvent developing process using a developing  solution containing an organic solvent ( organic developing  solution), the solubility of the exposed portions in an organic  developing solution is decreased. As a result, the unexposed  portions of the resist film are dissolved and removed by the  organic developing solution, and a negative resist pattern in  which the exposed portions of the resist film are remaining  is formed. Such a solvent developing process for forming a  negative-tone resist composition is sometimes referred to as  "negative-tone developing process".  unexposed portions of the resist film remain as patterns,  thereby forming a positive resist pattern. 