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Since the quaternary ammonium salt contains an iodine atom or atoms in the molecule, it has substantial EUV absorption. Upon EUV exposure, it generates secondary electrons, followed by energy transfer to an acid generator, leading to sensitization. This establishes a high sensitivity and low acid diffusion, succeeding in improving both LWR or CDU and sensitivity. 

The quaternary ammonium salt may be synthesized, for example, by neutralization reaction of a quaternary ammo› nium hydroxide with hydriodic acid. Also commercially available products are useful as the quaternary animonium salt. 

From the standpoints of sensitivity and acid diffusion suppressing effect, the quaternary ammonium salt is prefer› ably present in the resist composition in an amount of0.001 to 50 parts, more preferably 0.01 to 20 parts by weight per 100 parts by weight of the base polymer to be described below. 

The quencher may contain a quencher other than the quaternary ammonium salt. The other quencher is typically selected from conventional basic compounds. Conventional basic compounds include primary, secondary, and tertiary aliphatic amines, mixed amines, aromatic amines, heterocy› clic amines, nitrogen-containing compounds with carboxyl group, nitrogen-containing compounds with sulfonyl group, nitrogen-containing compounds with hydroxyl group, nitro› gen-containing compounds with hydroxyphenyl group, alcoholic nitrogen-containing compounds, amide deriva› tives, imide derivatives, and carbamate derivatives. Also included are primary, secondary, and tertiary amine com- pounds, specifically amine compounds having a hydroxyl, ether, ester, lactone ring, cyano, or sulfonic acid ester group as described in JP-A 2008-111103, paragraphs [0146]› [0164], and compounds having a carbamate group as described in JP 3790649. Addition of a basic compound may be effective for further suppressing the diffusion rate of acid in the resist film or correcting the pattern profile. 

Quenchers of polymer type as described in U.S. Pat. No. 7,598,016 (JP-A 2008-239918) are also useful as the other quencher. The polymeric quencher segregates at the resist surface after coating and thus enhances the rectangularity of resist pattern. When a protective film is applied as is often the case in the immersion lithography, the polymeric 

