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wherein 

[0017] in the formula (Ia), AR represents an aromatic group, and X$_{1 }$represents a group having a carbon number of 5 or more and being capable, of decomposing under the action of an acid, and 

[0018] in the formula (Al), m represents an integer of one of 1 and 2. 

[0019] (2) The positive resist composition as described in the item (1), wherein X$_{1 }$ has a tertiary carbon atom bonded to the oxygen atom in formula (Ia). 

[0020] (3) The positive resist composition as described in the item (1), wherein X$_{1 }$ has an alicyclic group. 

[0021] ( 4) A positive resist composition comprising: 

[0022] (A-2) a resin of which a solubility in an alkali developer increases under the action of an acid, the resin comprising a repeating unit represented by formula (lb) and a repeating unit represented by formula (A2); and 

[0023] (B) a compound capable of generating an acid upon irradiation with one of actinic rays and radiation: 

wherein 

[0024] in the formula (lb), AR represents an aromatic group and X$_{2 }$ represents one of a hydrogen atom and a hydrocarbon group, and 

[0025] in the formula (A2), A$_{1 }$represents a group contain› ing a group capable of decomposing under the action of the acid, and n represents an integer of one of 1 and 2. 

[0026] (5) The positive resist composition as described in the item (4) above, wherein X$_{2 }$ is a group capable of decomposing under the action of the acid. 

[0027] (6) The positive resist composition as described in the item (5), wherein X$_{2 }$ is a group having an alicyclic group and being capable of decomposing under the action of the acid. 

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[0028] (7) A pattern forming method comprising: 

[0029] forming a resist film from a positive resist compo› sition as described in the item (1 ); and 

[0030] exposing and developing the resist film. 

[0031] (8) The pattern forming method as described in the item (7), wherein the resist film is exposed with one of electron beam and extreme ultraviolet light. 

[0032] (9) A pattern forming method comprising: 

[0033] forming a resist film from a positive resist compo› sition as described in the item (4); and 

[0034] exposing and developing the resist film. 

[0035] (10) The pattern forming method as described in the item (9), wherein the resist film is exposed with one of electron beam and extreme ultraviolet light. 

[0036] According to the present invention, as regards the pattern formation by the irradiation of electron beam, KrF excimer laser light, EUV light or the like, a positive resist composition excellent in the sensitivity and resolving power and further excellent in the pattern profile, defocus latitude depended on line pitch, and dissolution contrast, and a pattern forming method using the composition can be pro› vided. 

[0037] The compounds for use in the present invention are described in detail below. 

[0038] Incidentally, in the present invention, when a group ( atomic group) is denoted without specifying whether sub› stituted or unsubstituted, the group includes both a group having no substituent and a group having a substituent. For example, an "alkyl group" includes not only an alkyl group having no substituent (unsubstituted alkyl group) but also an alkyl group having a substituent (substituted alkyl group). 

[1] Resins (A-1) and (A-2) which are insoluble or sparingly soluble in an alkali developer and of which solubility in an alkali developer increases under the action of an acid 

[0039] The resin of which solubility in an alkali developer increases under the action of an acid, contained in the positive resist composition of the present invention, is at least either a resin (A-1) comprising a repeating unit repre› sented by formula (Ia) and a repeating unit represented by formula (Al), or a resin (A-2) comprising a repeating unit represented by formula (lb) and a repeating unit represented by formula (A2). 

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[0040] In formula (Ia), AR represents an aromatic group, preferably a phenyl group (a hydroxyphenyl group as a phenyl group having a substituent), a naphthyl group or an anthranyl group. 

## DETAILED DESCRIPTION OF THE INVENTION 

