US 10,698,314 B2  45  46  \  0~  ~  -w\  ;  ro  I  \__o  o  ~  W- I\  \  \  ~  -w\  ;  rs  I  \__o  o  ~  W- I\  -continued  ~j  s---{)-s  \__J  00  ~  W- I\  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 