To improve the depth of focus (DOF) of isolated lines, attenuated assist feature (AAF) technique has been proposed; AAFs having more than 20% transmittance were located around an isolated line. In this mask, the transmittance and phase shift angle of AAF as well as its position and width have effects on lithographic performance. In particular, the phase shift angle has strong effect on focus latitude. The performances of two AAF masks (65% transmittance/28 degree phase shift and 40% transmittance/54 degree phase shift) were evaluated by using an NA equals 0.6, sigma- in$//$sigma$-out$/ equals 0.42/0.7, i-line stepper. The focus latitude of 0.3 micrometer isolated line became flat around the best focus position with 28 degree phase shift AAFs. In conclusion, we can obtain wide DOF for isolated lines by selecting optimum phase shift angle of AAF.
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