Dec.  27,  2018  3  US 2018/0373148 Al  [0022]  In another aspect, the invention provides a process  for forming a pattern comprising the steps of applying the  resist composition defined above onto a substrate, baking to  form a resist film, exposing the resist film to high-energy  radiation, and developing the exposed film in a developer.  [0023]  Typically, the high-energy radiation is i-line, KrF  excimer laser, ArF excimer laser, EB or EUV of wavelength  3 to 15 nm.  [0024]  The resist composition of the invention exhibits a  high sensitivity, high acid diffusion-suppressing effect, and  high resolution, and forms a pattern of good profile, dimen› sional uniformity, and reduced edge roughness after expo› sure. The resist composition is thus suited as fine pattern› forming material for the fabrication of VLSis and the  fabrication of photomasks by EB writing, and pattern form› ing material by i-line, KrF excimer laser, ArF excimer laser,  EB or EUV lithography.  [0025]  The resist composition, especially chemically  amplified resist composition is used not only in the lithog› raphy for semiconductor circuit formation, but also in the  formation of mask circuit patterns, micro-machines, and  thin-film magnetic head circuits.  [0026]  As used herein, 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 to m carbon atoms per group. As used  herein, the term "brominated" compound means a bromine› containing compound. In chemical formulae, Me stands for  methyl, and Ac for acetyl.  [0027]  The abbreviations and acronyms have the follow› ing meaning.  [0028]  EB: electron beam  [0029]  EUV: extreme ultraviolet  [0030]  Mw: weight average molecular weight  [0031]  Mn: number average molecular weight  [0032]  sity  Mw/Mn: molecular weight distribution or <lisper- [0033]  GPC: gel permeation chromatography  [0034]  PEB: post-exposure bake  [0035]  PAG: photoacid generator  [0036]  LWR: line width roughness  [0037]  CDU: critical dimension uniformity  [0038]  Base Resin  [0039]  The resist composition of the invention is defined  as comprising a polymer comprising recurring units having  the formula (a) as a base resin. For simplicity’s sake, the  units are referred to as recurring units (a) and the polymer is  referred to as polymer A.  [0043]  Examples of the monomer Ma are shown below,  but not limited thereto.  RA  and R  1  are as defined above.  Herein  RA,  R1, R 2 ,  X1, X 2 ,  m and n are as defined above.  [0042]  Monomer Ma may be synthesized, for example, by  reacting a compound having the formula (Mal) with a  compound having the formula (Ma2).  Herein  RA,  R1, R 2 , X 1 , X 2 , m and n are as defined above.  [0041]  Suitable monomers Ma from which recurring units  (a) are derived are those having the formula (Ma).  [0040]  Herein  RA  is hydrogen or methyl. R $^{1 }$is hydrogen or  an acid labile group. R 2  is a C$_{1 }$-C$_{6 }$ straight, branched or  cyclic alkyl group or a halogen atom other than bromine. X 1  is a single bond, phenylene group, or a C$_{1 }$-C$_{12 }$ straight,  branched or cyclic alkylene group which may contain an  ester moiety or lactone ring. X 2  is ---0-, ---O-CH$_{2}$ - or  -NH-, m is an integer of 1 to 4, and n is an integer of 0  to 3.  Advantageous Effects of Invention  DESCRIPTION OF EMBODIMENTS  Resist Composition  (a)  (Ma)  H  I  x2  +  cl~R 2 n  I\  1)  Br~  r--oR 1  (Ma2) 