US 10,725,377 B2  9  10  diffusion during the exposure step. When the composition is  coated as a resist film and processed to form a pattern, the  resist film exhibits a very high resolution during pattern  formation, and forms a pattern with minimal LER and  minimal line width variation relative to dose changes and  pattern layout dependency. Owing to the recurring units of  formula (B 1 ), the adhesion of the resist film to a processable  substrate is improved, and the solubility of the resist film in  alkaline developer is controlled.  The pattern forming process using the negative resist  composition can form a resist pattern with minimal LER  while maintaining a high resolution. The invention is best  suited for a micropatterning process, typically KrF, EUV or  EB lithography.  FIG.  1  is a diagram showing  $^{1}$H-NMR  spectrum of  Compound Q-1 in Example 1-1.  FIG.  2  is a diagram showing  $^{1}$H-NMR  spectrum of  Compound Q-2 in Example 1-2.  FIG.  3  is a diagram showing 19F-NMR spectrum of  Compound Q-2 in Example 1-2.  BRIEF DESCRIPTION OF DRAWINGS  DESCRIPTION OF EMBODIMENTS  The abbreviations and acronyms have the following  meaning.  EB: electron beam  EUV: extreme ultraviolet  Mw: weight average molecular weight  Mn: number average molecular weight  Mw/Mn: molecular weight distribution or dispersity  GPC: gel permeation chromatography  PEB: post-exposure bake  PAG: photoacid generator  LER: line edge roughness  CDU: critical dimension uniformity  It is understood that for some structures represented by  chemical formulae, there can exist enantiomers and diaste- reamers because of the presence of asymmetric carbon  atoms. In such a case, a single formula collectively repre› sents all such isomers. The isomers may be used alone or in  admixture.  The negative resist composition of the invention is  defined as comprising (A) a sulfonium compound and (B) a  base polymer.  Component (A) is a sulfonium compound having the  formula (A).  Negative Resist Composition  (A) Sulfonium Compound  (A)  In formula (A), R1, R 2  and R 3  are each independently a  C$_{1 }$-C$_{20 }$straight, branched or cyclic monovalent hydrocarbon  group which may contain a heteroatom. Examples of the  monovalent hydrocarbon group include alkyl groups such as  methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl,  n-pentyl, t-pentyl, n-hexyl, n-octyl, n-nonyl, n-decyl, cyclo› pentyl, cyclohexyl, 2-ethylhexyl, cyclopentylmethyl, cyclo› pentylethyl, cyclopentylbutyl, cyclohexylmethyl, cyclo› hexy lethy 1, cyclohexy lbuty 1, norborny 1, tricyclo [ 5 .2 .1. 0 2 • 6 ]  decanyl, adamantyl, and adamantylmethyl, and aryl groups  such as phenyl, naphthyl, and anthracenyl. In these hydro› carbon groups, one or more hydrogen may be replaced by a  moiety containing a heteroatom such as oxygen, sulfur,  nitrogen or halogen, or a moiety containing a heteroatom  such as oxygen, sulfur or nitrogen may intervene between  carbon atoms, so that the group may contain a hydroxy  moiety, cyano moiety, carbonyl moiety, ether bond, thioether  bond, ester bond, sulfonic acid ester bond, carbonate bond,  carbamate bond, lactone ring, sultone ring, carboxylic anhy› dride or haloalkyl moiety.  In formula (A), p and q are each independently an integer  of O to 5, and r is an integer of O to 4. Each of p, q and r is  preferably 0, 1 or 2 for ease of synthesis and availability of  reactants.  When p is 2 to 5, two adjoining groups R 1  may bond  together to form a ring with the carbon atoms to which they  are attached. When q is 2 to 5, two adjoining groups R $^{2 }$may  bond together to form a ring with the carbon atoms to which  they are attached. When r is 2 to 4, two adjoining groups R 3  may bond together to form a ring with the carbon atoms to  which they are attached.  Examples of the sulfonium compound having formula (A)  are given below, but not limited thereto.  30  As used herein, the singular forms "a," "an" and "the"  include plural referents unless the context clearly dictates  otherwise. "Optional" or "optionally" means that the sub› sequently described event or circumstances may or may not  occur, and that description includes instances where the  event or circumstance occurs and instances where it does  not. The notation (Cn-Cm) means a group containing from 35  n to m carbon atoms per group. In chemical formulae, Me  stands for methyl, Ac stands for acetyl, and the broken line  designates a valence bond. 