content_image/896199.jpg content_image/896195.jpg content_image/896196.jpg content_image/896197.jpg content_image/896198.jpg
polar group incorporated, they are still insufficient in precise control of acid diffusion. Their lithography performance is unsatisfactory when evaluated totally in terms of MEF, pattern profile and sensitivity. 

[0011] Patent Document 8 discloses a PAG having a lactam ring structure. Since this acid generator uses the nitrogen atom on the lactam ring as a part of the linker structure and is highly hydrophilic, the acid generator can be dissolved or leached out in water when the relevant resist material is processed by the immersion lithography using water. There is the desire to have a PAG which is fully controlled in acid diffusion and has satisfactory lipophilicity and solvent solubility. 

[0012] Patent Document 1: JP-A 2008-281974 

[0013] Patent Document 2: JP-A 2008-281975 

[0014] Patent Document 3: JP 4554665 

[0015] Patent Document 4: JP-A 2007-145797 

[0016] Patent Document 5: JP 5061484 

[0017] Patent Document 6: JP-A 2016-147879 

[0018] Patent Document 7: JP-A 2015-063472 

[0019] Patent Document 8: JP 5756672 (U.S. Pat. No. 8,609,891) 

[0020] Non-Patent Document 1: Journal of Photopolymer Science and Technology, Vol. 17, No. 4, p 587 (2004) 

[0021] While it is recently demanded to form resist pat› terns at a high resolution, a resist composition using a conventional PAG fails to fully suppress acid diffusion. As a result, lithography performance factors such as contrast, MEF and LWR are degraded. 

[0022] An object of the invention is to provide an onium salt and a resist composition comprising the same as a photoacid generator, the resist composition, when processed by photolithography using high-energy radiation such as KrF or ArF excimer laser, EB or EUV as the energy source, being reduced in acid diffusion and improved in exposure latitude, MEF, and LWR; and a pattern forming process using the resist composition. 

[0023] The inventor has found that a resist composition comprising an onium salt of specific structure as a photoacid generator is reduced in acid diffusion, improved in many lithography performance factors including EL, MEF, and LWR, and thus best suited for precise micropatterning. 

[0024] In one aspect, the invention provides an onium salt having the formula (1). 

<img src='content_image/896199.jpg'>

Herein Z is a monocyclic or polycyclic group having a lactam structure; Rf 1 to R:t4 are each independently hydro› gen, fluorine or trifluoromethyl, at least one of Rf 1 to R:t4 being fluorine or trifluoromethyl; R $^{1 }$is hydrogen or a C$_{1 }$-C$_{20 }$ monovalent hydrocarbon group which may contain a het- 

## CITATION LIST 

## DISCLOSURE OF INVENTION 

eroatom; R 2 is a C$_{1}$-C$_{20 }$ monovalent hydrocarbon group which may contain a heteroatom; L1, L 2 and L 3 are each independently a single bond, ether bond, ester bond, sulfonic acid ester bond, carbonate bond or carbamate bond; L 4 is an ether bond, ester bond, sulfonic acid ester bond, carbonate bond or carbamate bond; X 1 is a single bond or a C$_{1}$-C$_{40 }$ divalent hydrocarbon group which may contain a heteroa› tom; n $^{1 }$is an integer of O to 6, with the proviso that R $^{1 }$and R 2 may bond together to form a ring structure when n 1 is 1 and R 1 is not hydrogen, and two R 1 may bond together to form a ring structure when n$^{1 }$is an integer of at least 2 and R 1 is not hydrogen; and Q+ is an onium cation. 

[0025] The preferred onium salt has the formula (IA). 

<img src='content_image/896195.jpg'>

Herein Z, Rf$^{1 }$to Rf\ R1, R 2 , L $^{1 }$to L3 , X1, n $^{1 }$and Q+ are as defined above. 

[0026] Preferably the onium salt has any one of the formulae (la) to (le). 

<img src='content_image/896196.jpg'>

<img src='content_image/896197.jpg'>

<img src='content_image/896198.jpg'>

Herein Rf 1 to R:t4, R1, R 2 , L 1 to L 3 , X 1 , n 1 and Q+ are as defined above, X is -CH$_{2}$-, ----CH$_{2}$CH$_{2}$-, -0-, -S› or two separate -H, and n 2 is an integer of 1 to 4. 

[0027] More preferably, the onium salts having formulae (la) to (le) are represented by the formulae (laa) to (lec), respectively. 

