US 2020/0133122 Al  2  Apr. 30, 2020  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.  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).  CITATION LIST  [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)  DISCLOSURE OF INVENTION  [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).  [0026]  Preferably the onium salt has any one of the  formulae (la) to (le).  Herein Z, Rf$^{1 }$to Rf\ R1, R 2 , L $^{1 }$to L3 , X1, n $^{1 }$and  Q+  are as  defined above.  (lA)  (la)  (le)  (1)  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- 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. 