Patterning process, resist composition, and acetal compound

A resist composition comprising a polymer, an acid generator, and an organic solvent,

• said polymer being dissolvable in a developer, adapted to form a negative pattern, and comprising recurring units (a1) and/or (a2) having an acid labile group-substituted hydroxyl group represented by the general formula (1) shown below, and recurring units derived from a monomer having an adhesive group selected from the group consisting of hydroxyl, cyano, carbonyl, ester, ether, lactone, carboxyl, and carboxylic anhydride,
• said developer being selected from the group consisting of 2-octanone, 2-nonanone, 2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone, methylcyclohexanone, acetophenone, methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate, amyl acetate, butenyl acetate, isoamyl acetate, phenyl acetate, propyl formate, butyl formate, isobutyl formate, amyl formate, isoamyl formate, methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate, amyl lactate, isoamyl lactate, methyl 2-hydroxyisobutyrate, ethyl 2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate, benzyl acetate, methyl phenylacetate, benzyl formate, phenylethyl formate, methyl 3-phenylpropionate, benzyl propionate, ethyl phenylacetate, and 2-phenylethyl acetate,
• wherein R1 and R4 are each independently hydrogen or methyl, R2 is a straight, branched or cyclic, di- to penta-valent aliphatic hydrocarbon group of 1 to 16 carbon atoms which may contain an ether or ester radical, R3 and R5 each are an acid labile group, with the proviso that when R2 contains an adamantane ring, an acid labile group of acetal form having the general formula (2):
• wherein R6 is a straight, branched or cyclic, monovalent hydrocarbon group of 1 to 10 carbon atoms, R7 and R8 are each independently hydrogen or a straight, branched or cyclic, monovalent hydrocarbon group of 1 to 10 carbon atoms, R7 and R8 may bond together to form an aliphatic hydrocarbon ring with the carbon atom to which they are attached, and R9 is a straight, branched or cyclic, monovalent hydrocarbon group of 1 to 15 carbon atoms is excluded from R3, m is an integer of 1 to 4, a1 and a2 are numbers in the range:
• 0≦a1<1.0, 0≦a2<1.0, and 0<a1+a2<1.0.