29  US 2007/0072121 Al  Mar. 29, 2007  Thereafter, the dissolution rate at each exposure dose was  measured by using an aqueous 2.38 mass% tetramethylam› monium hydroxide (TMAH) solution to obtain a sensitivity  curve. The exposure dose when the dissolution rate of the  resist was saturated in this sensitivity curve was defined as  the sensitivity and also, the dissolution contrast  (y value)  was calculated from the gradient in the straight line part of  the sensitivity curve. As the  y  value is larger, the dissolution  contrast is more excellent.  [0188]  The results are shown in Table 4.  2.  The positive resist composition as claimed in claim 1,  wherein X$_{1 }$ has a tertiary carbon atom bonded to the oxygen  atom in formula (Ia).  3.  The positive resist composition as claimed in claim 1,  wherein X$_{1 }$ has an alicyclic group.  4.  A positive resist composition comprising:  (A-2) a resin of which a solubility in an alkali developer  increases under the action of an acid, the resin com- TABLE 4  Com osition  Evaluation  Sulfonic Acid  Carboxylic Acid  Basic Compound Other Component Sensitivity  Resin Generator  Generator  (0.003 g)  Example 21  A-10  B-2 (0.065 g)  C-2 (0.005 g)  D-1  Example 22  A-3  B-3 (0.07 g)  C-4 (0.005 g)  D-3  Example 23  A-7  B-4 (0.07 g)  C-7 (0.004 g)  D-2  Comparative  C-1  B-2 (0.065 g)  D-1  Example 3  (0.001 g)  (mJ/cm 2 )  y  value  W-1  2.5  8.5  W-1  3.0  9.5  W-1  2.5  9.0  W-1  3.0  5.0  [0189]  It is seen from Table 4 that in the characteristic  evaluation by the irradiation with EUV light, the positive  resist composition of the present invention exhibits high  sensitivity and high contrast and is excellent, as compared  with the composition of Comparative Example.  [0190]  The present application claims foreign priority  based on Japanese Patent Application (JP 2005-285022)  filed Sep. 29 of 2005, the contents of which is incorporated  herein by reference.  prising a repeating unit represented by formula (lb) and  a repeating unit represented by formula (A2); and  (B) a compound capable of generating an acid upon  irradiation with one of actinic rays and radiation:  What is claimed is:  1.  A positive resist composition comprising:  (A-1) a resin of which a solubility in an alkali developer  increases under the action of an acid, the resin com› prising a repeating unit represented by formula (Ia) and  a repeating unit represented by formula (Al); and  (B) a compound capable of generating an acid upon  irradiation with one of actinic rays and radiation:  (la)  AR  I  ---f CH - CH-,- I  c=o  I  OX1  (Al)  wherein  in the formula (Ia), AR represents an aromatic group, and  Xl represents a group having a carbon number of 5 or  more and being capable of decomposing under the  action of an acid, and  in the formula (lb), AR represents an aromatic group and  X$_{2 }$ represents one of a hydrogen atom and a hydrocar› bon group, and  in the formula (A2), A$_{1 }$ represents a group containing a  group capable of decomposing under the action of the  acid, and n represents an integer of one of 1 and 2.  wherein  5.  The positive resist composition as claimed in claim 4,  wherein X$_{2 }$ is a group capable of decomposing under the  action of the acid.  6.  The positive resist composition as claimed in claim 5,  wherein X$_{2 }$ is a group having an alicyclic group and being  capable of decomposing under the action of the acid.  (lb)  AR  I  ---fCH-CH-,- 1  c=o  I  OX2  (A2)  in the formula (Al), m represents an integer of one of 1  and 2. 