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+20% bias. The resist film was baked (PEB) on a hot plate at the temperature shown in Table 1 for 60 seconds and developed in a 2.38 wt % TMAH aqueous solution for 30 seconds to form a hole pattern having a size of 23 nm. 

The resist pattern was evaluated. The size of 50 holes was 5 measured under CD-SEM (CG-5000, Hitachi, Ltd.), from which a size variation (3a) was computed and reported as CDU. The exposure dose that provides a hole pattern having a size of 23 nm is reported as sensitivity. 

The resist composition is shown in Table 1 together with 10 the sensitivity and CDU of EUV lithography. 

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It is evident from Table 1 that the positive resist compo› sitions within the scope of the invention exhibit satisfactory resolution and sensitivity and reduced CDU. When poly› mers containing recurring units (dl) to (d3) are used as the base resin, CDU is further reduced. 

Japanese Patent Application No. 2016-128886 is incor- porated herein by reference. 

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

The invention claimed is: 

1. A pattern forming process comprising the steps of coating a positive resist composition onto a substrate, baking to form a resist film, exposing the resist film to high-energy radiation, and developing the exposed resist film in a devel› oper, 

wherein the high-energy radiation is EB or EUV, wherein the positive resist composition comprises a base resin containing a polymer comprising recurring units (a) having the formula (a), and recurring units (bl) having a carboxyl group whose hydrogen is substituted by an acid labile group and/or recurring units (b2) having a phenolic hydroxyl group whose hydrogen is substituted by an acid labile group, an organic solvent and an acid generator, and an organic solvent, the polymer having a weight average molecular weight, of 1,000 to 500,000, 

wherein RA is hydrogen or methyl, R $^{1 }$is hydrogen or C$_{1 }$-C$_{4 }$ straight or branched alkyl, and R 2 is a single bond or methylene. 

2. A pattern forming process comprising the steps of coating a positive resist composition onto a substrate, baking to form a resist film, exposing the resist film to high-energy radiation, and developing the exposed resist film in a devel› oper, 

wherein the high-energy radiation is EB or EUV, wherein the positive resist composition comprises a base resin containing a polymer comprising recurring units (a) having the formula (a), recurring units (b 1) having a carboxyl group whose hydrogen is substituted by an acid labile group andior recurring, units (b2) having a phenolic hydroxyl group whose hydrogen is substituted by an acid labile group and recurring units of at least 

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