Mask blank, resist pattern forming process and chemically amplified positive resist composition

A chemically amplified positive resist composition comprising a base polymer containing a polymer comprising phenolic hydroxy group-containing repeat units having the formula (A1) and repeat units containing a carboxy group protected with an acid labile group, having the formula (A2), an organic solvent, a photoacid generator, and a quencher, wherein a weight ratio of the photoacid generator to the quencher is less than 6/1,

• wherein RA is hydrogen, fluorine, methyl or trifluoromethyl,
• X1 is a single bond, *—C(═O)—O— or *—C(═O)—NH—, * designates a point of attachment to the carbon atom in the backbone,
• A1 is a single bond or a C1-C10 saturated hydrocarbylene group in which some constituent —CH2— may be replaced by —O—,
• R1 is halogen, an optionally halogenated C2-C8 saturated hydrocarbylcarbonyloxy group, optionally halogenated C1-C6 saturated hydrocarbyl group, or optionally halogenated C1-C6 saturated hydrocarbyloxy group,
• a1 is an integer satisfying 0≤a1≤5+2a3−a2, a2 is an integer of 1 to 3, and a3 is an integer of 0 to 2,
• wherein RA is hydrogen, fluorine, methyl or trifluoromethyl,
• X2 is a single bond, *—C(═O)—O—X21—, phenylene group, or naphthylene group, the phenylene group and naphthylene group may be substituted with an optionally fluorinated C1-C10 alkoxy moiety or halogen, X21 is a C1-C20 aliphatic hydrocarbylene group, phenylene group or naphthylene group, the aliphatic hydrocarbylene group may contain at least one moiety selected from an optionally fluorinated C1-C10 alkoxy moiety, hydroxy, ether bond, ester bond and lactone ring, * designates a point of attachment to the carbon atom in the backbone,
• RB and RC are each independently a C1-C10 hydrocarbyl group which may contain a heteroatom, RB and RC may bond together to form a ring with the carbon atom to which they are attached,
• R2 is each independently halogen, cyano group, C1-C5 acyl group, C1-C5 alkoxy group, C1-C5 fluorinated alkyl group, or C1-C5 fluorinated alkoxy group,
• R3 is each independently a C1-C10 hydrocarbyl group which may contain a heteroatom,
• b1 is an integer of 0 to 2, b2 is an integer of 0 to 2, b3 is an integer of 0 to 5, b4 is an integer of 0 to 2, b1 and b2 are not equal to 0 at the same time.