Positive resist composition

1. A positive resist composition comprising (a) a resin that is decomposed by the action of an acid to increase solubility in an alkali developing solution, contains a structural unit having a group represented by formula (X) shown below, has a weight average molecular weight of not more than 5,000, and contains an acid decomposable group in an amount of not more than 40% based on the sum total of a number of the acid decomposable group and a number of an alkali-soluble group not protected with the acid decomposable group, and (b) a compound that generates an acid upon irradiation of an actinic ray or radiation:

in formula (X), R1 and R2, which may be the same or different, each represent a hydrogen atom or an alkyl group having from 1 to 4 carbon atoms; R3 and R4, which may be the same or different, each represent a hydrogen atom or an alkyl group; Z represents a phenyl group or an alicyclic group; and m represents an integer of from 1 to 20.

2. The positive resist composition as claimed in claim 1 , which further comprises a nitrogen-containing basic compound.

3. The positive resist composition as claimed in claim 1 , which further comprises a surfactant.

4. The positive resist composition as claimed in claim 1 , wherein the compound that generates an acid upon irradiation of an actinic ray or radiation comprises a compound that generates a sulfonic acid upon irradiation of an actinic ray or radiation.

5. The positive resist composition as claimed in claim 1 , wherein the compound that generates an acid upon irradiation of an actinic ray or radiation comprises two or more compounds that generate an acid upon irradiation of an actinic ray or radiation and at least one of the compounds that generate an acid upon irradiation of an actinic ray or radiation is a compound that generates a carboxylic acid upon irradiation of an actinic ray or radiation.

6. The positive resist composition as claimed in claim 1 , which further comprises a solvent.

7. The positive resist composition as claimed in claim 6 , wherein the solvent comprises propylene glycol monomethyl ether acetate.

8. The positive resist composition as claimed in claim 7 , wherein the solvent further comprises propylene glycol monomethyl ether.

9. The positive resist composition as claimed in claim 1 , which is suitable irradiation with an actinic ray or radiation selected from an electron beam, an X-ray and an EUV beam.

10. A pattern formation process comprising preparing a resist film with the positive resist composition as claimed in claim 1 , irradiating the resist film with an actinic ray or radiation and developing the irradiated resist film with a developing solution.

11. The pattern formation process as claimed in claim 10 ,

wherein the actinic ray or radiation is one of an electron beam, an X ray and an EUV beam.

12. The positive resist composition as claimed in claim 1 ,

wherein Z represents a group represented by the formula:

or a group represented by the formula:

wherein R5 represents an alkyl group, which may have a substituent, an aryl group, which may have a substituent, or an aralkyl group, which may have a substituent; and

n represents an integer of from 1 to 5.

13. The positive resist composition as claimed in claim 12 ,

wherein Z represents a group represented by the formula:

wherein R5 represents an alkyl group, which may have a substituent, an aryl group, which may have a substituent, or an aralkyl group, which may have a substituent; and

n represents an integer of from 1 to 5.

14. The positive resist composition as claimed in claim 1 ,

wherein the resin has a molecular weight distribution of from 1.0 to 1.6.