US 10,101,653 B2  108  107  cally, acetoxystyrene or acetoxyvinylnaphthalene is used  instead of hydroxystyrene or hydroxyvinylnaphthalene, and  after polymerization, the acetoxy group is deprotected by  alkaline hydrolysis, for thereby converting the polymer  product to hydroxystyrene or hydroxyvinylnaphthalene. For  alkaline hydrolysis, a base such as aqueous ammonia or  triethylamine may be used. Preferably the reaction tempera› ture is -20°  C. to 100°  C., more preferably 0°  C. to 60°  C.,  and the reaction time is 0.2 to 100 hours, more preferably 0.5  to 20 hours.  The base polymer should preferably have a weight aver› age molecular weight (Mw) in the range of 1,000 to 500,000,  and more preferably 2,000 to 30,000, as measured by GPC  versus polystyrene standards using tetrahydrofuran (THF)  solvent. With too low a Mw, the resist composition may  become less heat resistant. A polymer with too high a Mw  may lose alkaline solubility and give rise to a footing  phenomenon after pattern formation.  If a base polymer has a wide molecular weight distribu› tion or dispersity (Mw/Mn), which indicates the presence of  lower and higher molecular weight polymer fractions, there  is a possibility that foreign matter is left on the pattern or the  pattern profile is degraded. The influences of molecular  weight and dispersity become stronger as the pattern rule  becomes finer. Therefore, the base polymer should prefer› ably have a narrow dispersity (Mw/Mn) of 1.0 to 2.0,  especially 1.0 to 1.5, in order to provide a resist composition  suitable for micropatterning to a small feature size.  It is understood that a blend of two or more polymers 30  which differ in compositional ratio, Mw or Mw/Mn is  acceptable.  Other Components  With the sulfonium salt having formula (1-1) or iodonium  salt having formula (1-2) and the base polymer, defined  above, other components such as an organic solvent, sur› factant, dissolution inhibitor, and crosslinker may be  blended in any desired combination to formulate a chemi› cally amplified positive or negative resist composition. This  positive or negative resist composition has a very high  sensitivity in that the dissolution rate in developer of the  base polymer in exposed areas is accelerated by catalytic  reaction. In addition, the resist film has a high dissolution  contrast, resolution, exposure latitude, and process adapt› ability, and provides a good pattern profile after exposure,  and minimal proximity bias because of restrained acid  diffusion. By virtue of these advantages, the composition is  fully useful in commercial application and suited as a  pattern-forming material for the fabrication of VLSis. Par› ticularly when a chemically amplified positive resist com› position capable of utilizing acid catalyzed reaction is for› mulated, the composition has a higher sensitivity and is  further improved in the properties described above.  The resist composition may contain an acid generator  other than the sulfonium salt having formula (1-1) and  iodonium salt having formula (1-2). The other acid generator  is typically a compound (PAG) capable of generating an acid  upon exposure to actinic ray or radiation. Although the PAG  used herein may be any compound capable of generating an  acid upon exposure to high-energy radiation, those com- pounds capable of generating sulfonic acid, imide acid  (imidic acid) or methide acid are preferred. Suitable PAGs  include sulfonium salts, iodonium salts, sulfonyldiazometh› ane, N-sulfonyloxyimide, and oxime-O-sulfonate acid gen› erators. Exemplary acid generators are described in JP-A  2008-111103, paragraphs [0122]-[0142] (U.S. Pat. No.  7,537,880). The other acid generator is preferably added in  Suitable crosslinkers which can be used herein include  epoxy compounds, melamine compounds, guanamine com› pounds, glycoluril compounds and urea compounds having  substituted thereon at least one group selected from among  methylol, alkoxymethyl and acyloxymethyl groups, isocya› nate compounds, azide compounds, and compounds having  a double bond such as an alkenyl ether group. These  compounds may be used as an additive or introduced into a  polymer side chain as a pendant. Hydroxy-containing com› pounds may also be used as the crosslinker.  Of the foregoing crosslinkers, examples of suitable epoxy  compounds include tris(2,3-epoxypropyl)  isocyanurate,  In the positive resist composition, the dissolution inhibitor  is preferably added in an amount of  O  to 50 parts, more  preferably 5 to 40 parts by weight per 100 parts by weight  of the base polymer.  The dissolution inhibitor which can be used herein is a  compound having at least two phenolic hydroxyl groups on  the molecule, in which an average of from  O  to 100 mo! %  of all the hydrogen atoms on the phenolic hydroxyl groups  are replaced by acid labile groups or a compound having at  least one carboxyl group on the molecule, in which an  average of 50 to 100 mo!  % of all the hydrogen atoms on the  carboxyl groups are replaced by acid labile groups, both the  compounds having a molecular weight of 100 to 1,000, and  preferably 150 to 800. Typical are bisphenol A, trisphenol,  phenolphthalein, cresol novolac, naphthalenecarboxylic  acid, adamantanecarboxylic acid, and cholic acid derivatives  in which the hydrogen atom on the hydroxyl or carboxyl  group is replaced by an acid labile group, as described in  U.S. Pat. No. 7,771,914 (JP-A 2008-122932, paragraphs  [0155]-[0178]).  Exemplary surfactants are described in JP-A 2008- 111103, paragraphs [0165]-[0166]. Inclusion of a surfactant  may improve or control the coating characteristics of the  resist composition. The surfactant is preferably added in an  amount of 0.0001 to 10 parts by weight per 100 parts by  weight of the base polymer.  The organic solvent is preferably added in an amount of  100 to 10,000 parts, and more preferably 200 to 8,000 parts  by weight per 100 parts by weight of the base polymer.  Examples of the organic solvent used herein are described  in JP-A 2008-111103, paragraphs [0144]-[0145] (U.S. Pat.  No. 7,537,880). Exemplary solvents include ketones such as  cyclohexanone,  cyclopentanone  and methyl-2-n-pentyl  ketone; alcohols such as 3-methoxybutanol, 3-methyl-3- methoxybutanol, l-methoxy-2-propanol, and l-ethoxy-2- propanol; ethers such as propylene glycol monomethyl  ether, ethylene glycol monomethyl ether, propylene glycol  monoethyl ether, ethylene glycol monoethyl ether, propyl› ene glycol dimethyl ether, and diethylene glycol dimethyl  ether; esters such as propylene glycol monomethyl ether  acetate (POMEA), propylene glycol monoethyl ether  acetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl  3-methoxypropionate, ethyl 3-ethoxypropionate, t-butyl  acetate, t-butyl propionate, and propylene glycol mono-t- butyl ether acetate; and lactones such as y-butyrolactone,  which may be used alone or in admixture.  In the case of positive resist compositions, inclusion of a  dissolution inhibitor may lead to an increased difference in  dissolution rate between exposed and unexposed areas and  a further improvement in resolution. In the case of negative  resist compositions, a negative pattern may be formed by  adding a crosslinker to reduce the dissolution rate of  exposed area.  an amount of  O  to 200 parts, and more preferably 0.1 to 100  parts by weight per 100 parts by weight of the base polymer. 