Patterning process and resist composition

A pattern forming process comprising the steps of:

• applying a resist composition onto a substrate, the resist composition comprising a polymer comprising recurring units of at least one type having a hydroxyl group substituted with an acid labile group selected from recurring units (a1) or (a5) of the general formulae (1) or (5), an acid generator, and an organic solvent or comprising a polymer comprising recurring units of at least one type having a hydroxyl group substituted with an acid labile group selected from recurring units (a1) or (a5) of the general formulae (1) or (5) and recurring units of at least one type selected from recurring units of the general formulae (e1) to (e3) and an organic solvent,
• prebaking the composition to form a resist film,
• exposing the resist film to high-energy radiation,
• baking, and
• developing the exposed film in an organic solvent-based developer to form a negative pattern wherein the unexposed region of film is dissolved away and the exposed region of film is not dissolved,
• wherein R1 and R18 each are hydrogen or methyl, R2 and R19 each are a single bond or a straight, branched or cyclic C1-C6 alkylene group which may contain an ether or ester radical, and R3 and R20 each are an acid labile group,
• wherein R24 and R28 each are hydrogen or methyl, R21 is a single bond, phenylene, —O—R33, or —C(═O)—Y—R33, Y is oxygen or NH, R33 is a straight, branched or cyclic C1-C6 alkylene group, alkenylene or phenylene group, which may contain a carbonyl (—CO—), ester (—COO—), ether (—O—) or hydroxyl radical, R22, R23, R25, R26, R27, R29, R30, and R31 are each independently a straight, branched or cyclic C1-C12 alkyl group which may contain a carbonyl, ester or ether radical, or a C6-C12 aryl, C7-C20 aralkyl, or thiophenyl group, Z0 is a single bond, methylene, ethylene, phenylene, fluorophenylene, —O—R32—, or —C(═O)—Z1—R32—, Z1 is oxygen or NH, R32 is a straight, branched or cyclic C1-C6 alkylene group, alkenylene or phenylene group, which may contain a carbonyl, ester, ether or hydroxyl radical, M− is a non-nucleophilic counter ion, e1, e2 and e3 are in the range:
• 0≦e1≦0.3, 0≦e2≦0.3, 0≦e3≦0.3, and 0<e1+e2+e3≦0.3.