228  US 8,968,979 B2  227  (PEB) for 60 seconds, and puddle developed with a 2.38 wt%  tetramethylammonium hydroxide aqueous solution for 30  seconds, forming a 1: 1 line-and-space pattern and a 1: 10  isolated line pattern. During the PEB, an optimum tempera› ture for each resist composition was employed.  was resolved at the optimum exposure dose. The edge rough› ness of a 100-nm line-and-space pattern was measured by  SEM.  For a particular resist composition, an optimum PEB tem› perature for EB writing, sensitivity, and resolution are  reported in Table 15.  TABLE 15  Example  Comparative  Example  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  2  3  4  5  6  Optimum  PEBtemp.,  CC.  100  100  100  100  100  100  90  100  100  100  100  100  100  90  100  100  90  100  100  100  90  90  90  90  120  120  110  120  120  110  Sensitivity,  µC/cm 2  Pattern profile  12  rectangular  10  somewhat rounded  11  rectangular  11  rectangular  11  rectangular  12  rectangular  10  rectangular  12  rectangular  11  rectangular  10  rectangular  10  rectangular  14  rectangular  10  rectangular  10  somewhat rounded  10  rectangular  11  rectangular  10  rectangular  12  rectangular  13  rectangular  8  somewhat rounded  9  somewhat rounded  10  rectangular  11  rectangular  9  rectangular  16  slightly tapered  16  slightly tapered  14  rectangular  18  tapered  18  tapered  16  tapered  Resolution,  µm  0.075  0.070  0.050  0.055  0.055  0.050  0.055  0.055  0.060  0.050  0.065  0.055  0.055  0.050  0.055  0.055  0.055  0.055  0.060  0.055  0.055  0.055  0.050  0.050  0.090  0.090  0.090  0.085  0.085  0.085  As seen from Table 15, the resist compositions within the  scope of the invention exhibit a high sensitivity and high  resolution as compared with the comparative resist composi› tions. The optimum PEB temperature for the inventive resist  compositions is lower than that for the comparative resist  compositions, indicating that the inventive resist composi› tions have a higher sensitivity. Examples 3 to 24 exhibit better  resolution than Examples 1 and 2, indicating that copolymer› ization of indene or acenaphthylene units provides a higher  contrast.  Evaluation of ArF Excimer Laser Lithography  Each of positive resist compositions (Examples 25 to 50  and Comparative Examples 7 to 12) was spin coated on a  silicon wafer having an antireflective coating (ARC-29A,  Nissan Chemical Industries, Ltd.) of78 nm thick and baked at  110°  C. for 60 seconds, forming a resist film of 120 nm thick.  The coated wafer was exposed by means of an ArF excimer  laser stepper (Nikon Corp., NA 0.85), post-exposure baked  Table 16 tabulates the test results (maximum resolution,  mask fidelity and profile) of the inventive and comparative  resist compositions.  The patterned wafer was observed under a top-down scan- ning electron microscope (TDSEM). The optimum exposure  (Eop) was defined as the exposure dose (mJ/cm 2 )  which  provided a 1: 1 resolution at the top and bottom of a 80-nm 1: 1  line-and-space pattern. The maximum resolution of the resist  was defined as the minimum line width (on-mask size, in  increments of 5 nm) of a 1: 1 line-and-space pattern that was  resolved and separated at the optimum exposure, with smaller  values indicating better resolution. The 1: 10 isolated line  pattern at the optimum exposure was also observed for deter› mining an actual on-wafer size of the isolated line pattern  with an on-mask size of 140 nm, which was reported as mask  fidelity ( on-wafer size, a larger size being better). The pattern  was visually observed to see whether or not its profile was  rectangular.  Example  Optimum  Maximum  Mask  PEBtemp.,  Eop,  resolution,  fidelity,  CC.  mJ/cm 2  nm  nm  Pattern profile  25  90  37  70  88  rectangular  26  90  34  70  90  rectangular  27  90  36  70  91  rectangular  28  90  37  70  91  rectangular  29  90  34  70  92  rectangular  TABLE 16 