US 2007/0072121 Al  2  Mar. 29  2007  -continued  (Al)  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.  [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:  [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.  [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.  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.  [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.  [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.  [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.  DETAILED DESCRIPTION OF THE  INVENTION  [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).  [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.  (lb)  AR  I  ---f CH - CH- - I  C=O  I  OX2  (A2)  (Resin (A-1))  AR  I  ---fCH-CH- - 1  c=o  I  OX1  (la)  (Al) 