6  US 8,114,570 B2  5  and sulfonyloxyimides. These compounds are sensitive to  high-energy radiation such as UV, deep-UV, EUV, electron  beam, x-ray, excimer laser, gamma-ray and synchrotron  radiation and generate sulfonic acids having the general for› mula (la) in response to high-energy radiation, so that they  are useful as photoacid generators in chemically amplified  resist compositions.  BRIEF DESCRIPTION OF DRAWINGS  DESCRIPTION OF THE PREFERRED  EMBODIMENTS  The singular forms "a," "an" and "the" include plural ref› erents unless the context clearly dictates otherwise.  The notation (Cn-Cm) means a group containing from n to  m carbon atoms per group.  Photoacid Generator  The photoacid generators of the invention are compounds,  typically sulfonium salts, iodonium salts, oxime sulfonates  R $^{1 }$is a divalent C$_{1 }$-C$_{20 }$aliphatic group which may contain  a substituent group having a heteroatom such as oxygen,  nitrogen or sulfur, or may form a cyclic structure with RO.  Suitable substituent groups on R  1  include hydroxy, alkoxy,  aryloxy, fluoro, chloro, bromo, iodo, carboxyl, alkoxycarbo› nyl, amido, carbonyl, acyloxy, benzoyloxy, and ether-bond› ing oxygen. Illustrative examples of  R  1  include methylene,  ethylene, 1,3-propylene, 1,2-propylene, isopropylidene, 1,4- butylene, 1,6-hexylene, and those of the following structural  formulae wherein the broken line denotes a bonding site.  Illustrative examples of RO include hydroxy, methoxy,  ethoxy, n-propoxy, isopropoxy, cyclopropyloxy, n-butoxy,  sec-butoxy, isobutoxy, tert-butoxy, n-pentyloxy, cyclopenty› loxy, n-hexyloxy, cyclohexyloxy, n-octyloxy, n-decyloxy,  n-dodecyloxy, 1-adamantyloxy, 2-adamantyloxy, bicyclo  [2.2.1 ]hepten-2-yloxy, 1-adamantanemethoxy, 2-adaman- tanemethoxy,  phenoxy,  4-methoxyphenyloxy,  4-tert-bu› tylphenyloxy, 4-biphenyloxy, 1-naphthyloxy, 2-naphthyloxy,  10-anthranyloxy, 2-furanyloxy, 2,2,2-trifluoroethoxy, 1, 1, 1,  3,3,3-hexafluoro-2-propyloxy, and 2,2,3,3-tetrafluoropro› poxy. Inter alia, hydroxy, methoxy, cyclohexyloxy, and  1-adamantanemethoxy are preferred.  In formula (la), RO is hydroxyl or a substituted or unsub- stituted, straight, branched or cyclic C$_{1 }$-C$_{20 }$organoxy group,  typically alkoxy group. R  1  is a divalent C$_{1 }$-C$_{20 }$ aliphatic  group, typically alkylene or alkenylene group, which may  contain a substituent group having a heteroatom such as oxy- gen, nitrogen or sulfur. Alternatively, R $^{1 }$forms a monocyclic  or polycyclic structure with RO. Suitable substituent groups  on RO include hydroxy, alkoxyl, aryloxy, fluoro, chloro,  bromo, iodo, carboxyl, alkoxycarbonyl, amido, carbonyl, and  ether-bonding oxygen. When RO is an alkoxy group, one or  more hydrogen atoms on the alkoxy may be substituted by  aryl groups.  Herein RO is hydroxyl or a substituted or unsubstituted,  straight, branched or cyclic C$_{1 }$-C$_{20 }$organoxy group, R $^{1 }$is a  divalent C$_{1 }$-C$_{20 }$aliphatic group which may contain a substitu› ent group having a heteroatom such as oxygen, nitrogen or  sulfur, or R $^{1 }$may form a mono- or polycyclic structure with  RO.  substrate to form a resist coating, heat treating the resist  coating, applying on the resist coating a protective coating  which is insoluble in water and soluble in an alkaline devel› oper, exposing the coated substrate to high-energy radiation  through a photomask and a projection lens, while holding  water between the coated substrate and the projection lens,  and optionally heat treating, and developing the exposed coat› ing with a developer.  [ 15] A pattern forming process comprising the steps ofapply› ing the resist composition of any one of [9] to [12] onto a  substrate to form a coating, heat treating the coating, image› wise writing with an electron beam, optionally heat treating  the coating, and developing it with a developer.  [ 16] A pattern forming process comprising the steps ofapply› ing the resist composition of any one of [9] to [12] onto a  substrate to form a coating, heat treating the coating and  exposing it to high-energy radiation through a photomask,  and heat treating and developing the exposed coating with a  developer, wherein the process further comprises the step of  applying a protective coating on the resist coating, and the  exposure step is performed by immersion lithography with a  liquid having a refractive index of at least 1.0 held between  the protective coating and a projection lens.  Since the photoacid generators have a carboxyl group,  carboxylate structure or lactone structure in addition to the  acyloxy group at ~-position relative to the sulfonate, they  enable to adequately control acid diffusion and are fully com› patible with resins and other components in resist composi› tions. The photoacid generators that generate these sulfonic  acids perform well without raising problems during the  device fabrication process including coating, pre-baking,  exposure, post-exposure baking, and developing steps. The  leach-out of sulfonic acids in water during theArF immersion  lithography is minimized. The influence of water left on the  wafer is minimized, restraining defects from forming. In the  disposal of resist-containing waste liquid after the device  fabrication, acyloxy groups at ~-position are hydrolyzable  under basic conditions so that the sulfonic acids are trans› formed into less accumulative compounds oflower molecular  weight. In the disposal by combustion, the sulfonic acids are  more combustible because of a low degree of fluorine substi› tution.  ADVANTAGEOUS EFFECT OF INVENTION  FIG.1  is a diagram showing the $^{1}$H-NMR/DMSO-d$_{6 }$ spec› trum of PAG-A in Synthesis Example 14.  FIG.  2  is a diagram showing the $^{19}$F-NMR/DMSO-d$_{6 }$spec› trum of PAG-A in Synthesis Example 14.  FIG.  3  is a diagram showing the $^{19}$F-NMR/CDC1$_{3 }$ spec› trum of PAG-B in Synthesis Example 15.  FIG.  4  is a diagram showing the $^{1 }$H-NMR/CDC1$_{3 }$ spectrum 55  of PAG-B in Synthesis Example 15.  o::::  err:: err::  Cw):::: 0 o::::  ({):::: [{):::: Cw)::::  ROC(=O)R $^{1}$-----COOCH$_{2}$CF$_{2}$SO$_{3 }$ -w  (la) 