New phase-shifting mask structure for positive resist process

One of the problems in applying the phase-shifting method in the positive resist process is the resist bridge generated at the phase-shifter edge. This problem has occurred in the past because the light intensity decreased to zero due to the interference at the phase-shifter edge. In order to solve this problem, we propose a new phase-shifting mask structure containing an intermediate phase-shifter. This intermediate phase-shifter will change the phase of the light by 90 degrees and will be placed at a peripheral edge of the conventional phase-shifter on the transparent substrate. The effect of this mask structure is demonstrated. A 0.3 micrometers lines and spaces pattern is successfully resolved without resist bridge, and the DOF at a 0.35 micrometers lines and spaces pattern is 1.2 micrometers wide. It is also demonstrated that this mask structure is effective on patterns such as LOCOS.