133  US 9,250,518 B2  134  in contrast. When a negative tone pattern is formed using an  organic solvent, the improvement in the contrast of exposed  portions leads to an improvement in the rectangularity of  negative pattern. Onium salts including sulfonium salts, iodo› nium salts and ammonium salts of an a-non-fluorinated sul› fonic acid and a carboxylic acid are highly effective in con› trolling the diffusion of an a-fluorinated sulfonic acid, imidic  acid and methidic acid. This is because the onium salt result› ing from salt exchange is less mobile due to a higher molecu› lar weight. In the event that a hole pattern is formed by  negative tone development, since acid is generated in many  regions, it is very important to control the diffusion of acid  from the exposed area to the unexposed area. The addition of  onium salts including sulfonium salts, iodonium salts and  ammonium salts of an a-non-fluorinated sulfonic acid and a  carboxylic acid as well as the amine quencher defined herein  is very important from the aspect of acid diffusion control.  The attachment of an acid generator to the polymer main  chain is effective in restraining acid diffusion, thereby pre› venting a reduction of resolution due to blur by acid diffusion.  Also roughness (LER or LWR) is improved since the acid  generator is uniformly distributed.  The base polymer for formulating the positive resist com› position comprises recurring units (al) and/or ( a2) having an  acid labile group as essential components and additional 25  recurring units (b), (c), (d), (e), (fl), (f2) and/or (f3) as  optional components. A copolymerization proportion of units  (al), (a2), (b), (c), (d), (e), (fl), (f2) and (f3) is: 0sal<l.0,  0sa2<1.0,  0<al+a2<1.0,  0sbs0.9,  0scs0.9,  0sds0.8,  0ses0.8, 0sfls0.5, 0sf2s0.5, and 0sf3s0.5; preferably  0sals0.9, 0sa2s0.9, 0.lsal+a2s0.9, 0sbs0.8, 0scs0.8,  0sds0.7, 0ses0.7, 0sfls0.4, 0sf2s0.4, and 0sf3s0.4; and  more preferably  0sals0.8, 0sa2s0.8,  0.lsal+a2s0.8,  0sbs0.75, 0scs0.75, 0sds0.6, 0ses0.6, 0sfls0.3, 0sf2s0.3,  and 0sf3s0.3. Note al+a2+b+c+d+e+fl+f2+f3=1.0.  For the base polymer for formulating the negative resist  composition, an acid labile group is not necessarily essential.  The base polymer comprises recurring units (b ), ( c ), ( d), ( e ),  (fl), ( f2) and ( f3) in a copolymerization proportion: 0<bs 1.0,  0scs0.9, 0sds0.8, 0ses0.8, 0sfls0.5, 0sf2s0.5, and  0sf3s0.5; preferably 0.2sbsl .0, 0scs0.8, 0sds0.7, 0ses0.7,  0sfls0.4, 0sf2s0.4, and 0sf3s0.4; and more preferably  0.3sbsl.0,  0scs0.75,  0sds0.6,  0ses0.6,  0sfls0.3,  0sf2s0.3, and 0sf3s0.3. Note b+c+d+e+fl+f2+f3=1.0.  These polymers may be synthesized by any desired meth› ods, for example, by dissolving one or more monomers  selected from the monomers to form the recurring units (al),  (a2), (b ), ( c ), ( d), ( e ), (fl), (f2) and (f3) in an organic solvent,  adding a radical polymerization initiator thereto, and effect› ing heat polymerization. Examples of the organic solvent  which can be used for polymerization include toluene, ben› zene, tetrahydrofuran, diethyl ether and dioxane. Examples of  the polymerization initiator used herein include 2,2’-azobi› sisobutyronitrile (AIBN), 2,2’-azobis(2,4-dimethylvaleroni› trile ), dimethyl 2,2-azobis(2-methylpropionate ), benzoyl  peroxide, and lauroyl peroxide. Preferably the system is  heated at 50 to 80°  C. for polymerization to take place. The  reaction time is 2 to 100 hours, preferably 5 to 20 hours.  When hydroxystyrene or hydroxyvinylnaphthalene is  copolymerized, an alternative method is possible. Specifi- cally, acetoxystyrene or acetoxyvinylnaphthalene is used  instead of hydroxystyrene or hydroxyvinylnaphthalene, and  after polymerization, the acetoxy group is deprotected by  alkaline hydrolysis as mentioned above, for thereby convert› ing the polymer product to polyhydroxystyrene or hydroxy- polyvinylnaphthalene. For alkaline hydrolysis, a base such as  aqueous ammonia or triethylamine may be used. The reaction  temperature is -20°  C. to 100°  C., preferably 0°  C. to 60°  C.,  and the reaction time is 0.2 to 100 hours, preferably 0.5 to 20  hours.  The polymer used in the resist composition should prefer- ably have a weight average 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 as a 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 multi-component polymer has a wide molecular weight  distribution or dispersity (Mw/Mn), which indicates the pres› ence oflower 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 multi-component copolymer  should preferably have a narrow dispersity (Mw/Mn) of 1.0 to  2.0, especially 1.0 to 1.5, in order to provide a resist compo› sition suitable for micropatterning to a small feature size.  It is understood that a blend of two or more polymers which  differ in compositional ratio, molecular weight or dispersity  is acceptable.  The basic compound of formula (1) is advantageously used  in a chemically amplified positive or negative resist compo› sition having an acid generator added thereto. Specifically,  the basic compound is added to the polymer serving as a base  resin, which may be further combined with any desired com- ponents including an organic solvent, dissolution inhibitor,  surfactant, crosslinker and the like to formulate a 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 polymer in exposed areas is acceler- ated by catalytic reaction. In addition, the resist film has a  high dissolution contrast, resolution, exposure latitude, and  process adaptability, 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 ofVLSis. Par› ticularly when an acid generator is incorporated to formulate  a chemically amplified positive resist composition capable of  utilizing acid catalyzed reaction, the composition has a higher  sensitivity and is further improved in the properties described  above.  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.  Any conventional basic compounds may be added along  with the basic compound of formula (1) for further suppress› ing the diffusion rate of acid in the resist film or correcting the  pattern profile. Addition of a surfactant may improve or con› trol the coating characteristics of the resist composition.  Suitable conventional basic compounds include primary,  secondary, and tertiary aliphatic amines, mixed amines, aro› matic amines, heterocyclic amines, nitrogen-containing com› pounds with carboxyl group, nitrogen-containing compounds  with sulfonyl group, nitrogen-containing compounds with  hydroxyl  group,  nitrogen-containing  compounds  with  hydroxyphenyl group, alcoholic nitrogen-containing com› pounds, amide derivatives, imide derivatives, and carbamate  derivatives. Also included are primary, secondary, and ter-