10  US 8,968,979 B2  9  denotes a C$_{3}$ -C$_{12 }$ cyclic hydrocarbon group or a bridged  cyclic hydrocarbon group, which may contain hydroxyl,  -0- or -S-, R  11 , R  $^{12 }$and R $^{13 }$are each independently  hydrogen, fluorine or trifluoromethyl, at least either one of  R 11 ,  R $^{12 }$and R 13  contains at least one fluorine atom. The  subscript a is 1 or 2, r, s, t, u and v are numbers in the range:  0<r<l .0, 0<ss0.8, 0<ts0.8, 0<us0.2, and 0<vs0.2.  Also provided is a chemically amplified positive resist  composition comprising (A) an organic solvent, (B) a base  resin comprising the polymer of any one of the foregoing  embodiments, (C) a photoacid generator, and (D) a basic  compound.  In another aspect, the invention provides a process for  forming a pattern, comprising the steps of applying the posi› tive resist composition of any one of the foregoing embodi› ments onto a substrate to form a resist coating; heat treating  the resist coating, then exposing it to high-energy radiation,  EUV or electron beam through a photomask; and optionally  heat treating the exposed resist coating, then developing with  a developer.  In the prior art, chemically amplified positive resist com› positions use a base resin comprising repeat units having a  In the disclosure, the singular forms "a," "an" and "the"  include plural referents unless the context clearly dictates  otherwise. The notation (Cn-Cm) means a group containing  from n tom carbon atoms per group. The abbreviation Me  stands for methyl.  When used in the micropatterning technology, especially  ArF lithography, the resist composition of the invention  exhibits a very high resolution capability. The composition is  thus useful in precise micropatterning. When used in the EB  image writing for processing of masks, the resist composition  exhibits a high resolution of fine size pattern and sensitivity  sufficient to comply with high-accelerating-voltage EB irra› diation, and high etch resistance. The composition is also  useful in mask production.  Then the inventors sought for a resin comprising repeat  units containing carboxylic acid protected with an acid labile  group which can provide a sufficiently high resolution to  enable micropatterning and also impart a high sensitivity  because of very high reactivity of the acid labile group with  the acid generated by the acid generator. In a first aspect, the  invention provides a positive resist composition comprising a  polymer comprising repeat units having the general formula  (1) or (2).  In the event where the acidic substituent group protected  with an acid labile group is carboxylic acid, a positive resist  composition using the same is favorable in processing of a  fine size resist pattern, but has a drawback of low sensitivity  because oflow reactivity with acid of the acid labile group.  For example, the chemically amplified resist composition  using t-alkyl-protected polymethacrylate has a lower sensi› tivity than the composition using acetal-protected polyhy- droxystyrene. Since the chemically amplified resist compo› sition using t-alkyl-protected polymethacrylate is  low  reactive, the post-exposure baking (PEB) step for promoting  the reaction with acid to facilitate deprotection must be at a  higher temperature. Differently stated, for the chemically  amplified resist composition using t-alkyl-protected poly› methacrylate, the acid generated by the acid generator must  be a very strong acid such as perfluoroalkanesulfonic acid.  However, it is also known that the reactivity with acid of the  acid labile group ( e.g., t-alkyl) for protecting carboxylic acid  is very low as compared with the reactivity with acid of the  acid labile group ( e.g., acetal) for protecting hydroxyphenyl.  That is, the reaction with acid for deprotection oft-alkyl› protected polymethacrylic acid is very weak as compared  with the reaction with acid for deprotection of acetal-pro› tected polyhydroxystyrene. As a result, a chemically ampli- fled positive resist composition using t-alkyl-protected poly› methacrylate has a very low sensitivity upon patterning.  The potentially alkaline-soluble acidic substituent group  which is protected with an acid labile group reacts with the  acid which is generated by an acid generator upon exposure to  high-energy radiation such as ArF excimer laser radiation,  EUV or EB whereby the substituent group turns into an  alkaline-soluble substituent group. In this way, the exposed  regions are dissolved in an alkaline developer, yielding a  positive resist pattern.  It is known that in the microlithogra- phy of a positive resist pattern, carboxylic acid surpasses  hydroxyphenyl as the acidic substituent group protected with  an acid labile group. This is because the carboxylic acid  resulting from deprotection by reaction with acid has a higher  acidity than hydroxyphenyl, offering a higher alkali solubility  to enable micropatterning.  In a further aspect, the invention provides a process for  forming a pattern, comprising the steps of applying the posi› tive resist composition of any one of the foregoing embodi› ments onto a substrate to form a resist coating; heat treating  the resist coating, then exposing it to high-energy radiation or  electron beam through a photomask; and heat treating the  exposed resist coating, then developing with a developer. In  one embodiment, the exposing step is performed by immer› sion lithography including holding a high refractive index  liquid having a refractive index of at least 1.0 between the  resist coating and a projection lens. In another embodiment, a  protective film is formed on the resist coating, and the expos› ing step is performed by immersion lithography including  holding a high refractive index liquid having a refractive  index of at least 1.0 between the protective film and a projec› tion lens.  In a still further aspect, the invention provides a photomask  blank comprising a chromium compound film and a coating  of the resist composition ofany one of the foregoing embodi› ment. A pattern is formed by heat treating the photomask  blank defined above, subjecting the resist coating to pattern› wise exposure to high-energy radiation through a photomask  or patternwise exposure to high-energy beam, optionally heat  treating the exposed resist coating, and developing it with a  developer.  Advantageous Effects oflnvention  DESCRIPTION OF EMBODIMENTS  potentially alkaline-soluble acidic substituent group which is  protected with a protective group ( or acid labile group) which  will be eliminated under the action ofacid. Exemplary poten› tially alkaline-soluble acidic substituent groups include  5  hydroxyphenyl groups and carboxylic acids. The acid labile  groups for protecting the former include acetal and tert-bu› toxycarbonyl groups, and the acid labile groups for protecting  the latter are often tertiary alkyl groups. A typical base resin  comprising repeat units containing hydroxyphenyl is polyhy- lO droxystyrene. Typical base resins comprising repeat units  containing carboxylic acid include poly(methacrylic acid)  and poly(acrylic acid). These resins in which some or all  acidic substituent groups are protected with the above-men- 15 tioned acid labile groups are used as the base resin in chemi› cally amplified positive resist compositions. 