200  US 9,703,193 B2  199  boxy!, ether bond, ester bond, sulfonic acid ester bond,  carbonate bond, lactone ring, sultone ring and carbox› ylic anhydride.  6.  The resist composition of claim  4,  further comprising  a photoacid generator other than the onium salt.  7.  The resist composition of claim  6  wherein the other  photoacid generator has the formula ( 4) or  (5):  group which may contain a heteroatom, R 600  is a  straight, branched or cyclic C$_{1 }$-C$_{30 }$ divalent hydrocar› bon group which may contain a heteroatom, any two or  more ofR 400 , R $^{500 }$and R$^{600 }$may bond together to form  a ring with the sulfur atom to which they are attached,  L is a single bond or a straight, branched or cyclic  C$_{1 }$-C$_{20 }$ divalent hydrocarbon group which may contain  a heteroatom, X 1 , X 2 , X 3  and X 4  are each indepen› dently hydrogen, fluorine or trifluoromethyl, with the  proviso that at least one of X1, X 2 , X 3  and X 4  is a  substituent group other than hydrogen.  8.  The resist composition of claim  4,  further comprising  an amine compound.  9.  The resist composition of claim  4,  further comprising  a compound having the formula  (6) or (7):  ( 4 (6)  (7)  wherein R 151 , R 152  and R 153  are each independently  hydrogen, halogen exclusive of fluorine, or a straight,  branched or cyclic C$_{1 }$-C$_{40 }$ monovalent hydrocarbon  group which may contain a heteroatom, any two or  more ofR  151 , R  $^{152 }$and R $^{153 }$may bond together to form  a ring with the carbon atom to which they are attached,  R $^{154 }$is a straight, branched or cyclic C$_{1 }$-C$_{40 }$monova› lent hydrocarbon group which may contain a heteroa› tom, and M+ is an onium cation.  10.  The resist composition of claim  4,  further comprising  a surfactant which is insoluble or substantially insoluble in  water and soluble in alkaline developer, and/or a surfactant  which is insoluble or substantially insoluble in water and  alkaline developer.  11.  A pattern forming process comprising the steps of  applying the resist composition of claim  4  onto a substrate,  pre baking to form a resist film, exposing a selected region of  the resist film to KrF excimer laser, ArF excimer laser, EB  or EUV, baking, and developing the exposed resist film in a  developer.  12.  The pattern forming process of claim  11  wherein the  developing step uses an alkaline aqueous solution as the  developer, thereby forming a positive pattern in which an  exposed region of the resist film is dissolved away and an  unexposed region of the resist film is not dissolved.  13.  The pattern forming process of claim  11  wherein the  developing step uses an organic solvent as the developer,  thereby forming a negative pattern in which an unexposed  region of the resist film is dissolved away and an exposed  region of the resist film is not dissolved.  14.  The pattern forming process of claim  13  wherein the  organic solvent is at least one solvent selected from the  group consisting of 2-octanone, 2-nonanone, 2-heptanone,  3-heptanone,  4-heptanone,  2-hexanone,  3-hexanone,  diisobutyl ketone, methylcyclohexanone, acetophenone,  methylacetophenone, propyl acetate, butyl acetate, isobutyl  acetate, pentyl acetate, butenyl acetate, isopentyl acetate,  propyl formate, butyl formate, isobutyl formate, pentyl  formate, isopentyl formate, methyl valerate, methyl pen› tenoate, methyl crotonate, ethyl crotonate, methyl propi› onate, ethyl propionate, ethyl 3-ethoxypropionate, methyl  wherein R$^{400 }$and R$^{500 }$are each independently a straight,  branched or cyclic C$_{1 }$-C$_{30 }$ monovalent hydrocarbon  wherein  R: 1 \  R:11’1, R:11’ 2 , R’c1, R’c 2  and R’c3 are each  independently fluorine or a straight, branched or cyclic  C$_{1 }$-C$_{40 }$ monovalent hydrocarbon group which may  contain a heteroatom, or a pair ofR:11’$^{1 }$and R:11’ 2 , or R’c1  and R’c 2  may bond together to form a ring with the  carbon atom to which they are attached and any inter› vening atoms,  R’d  is a straight, branched or cyclic  C$_{1 }$-C$_{40 }$ monovalent hydrocarbon group which may  contain a heteroatom,  wherein R 100 , R 200  and R 300  are each independently a  straight, branched or cyclic  C$_{1 }$-C$_{20 }$ monovalent hydro› carbon group which may contain a heteroatom, any two  or more of R 100 , R $^{200 }$and R$^{300 }$may bond together to  form a ring with the sulfur atom to which they are  attached, x- is an anion selected from the formulae  (4A) to (4D):  (4B)  (4C)  (4D)  (5)