Evaluating the potential of alternating phase shift masks using lithography simulation

This paper quantifies the expected gain in the process window of 150nm structures printed with DUV for alt. PSM vs. COG masks and HT PSM. Most of the analysis was performed for dense lines and isolated lines using lithography simulation. Alt. PSM show an increase of dose latitude by 9% and an improved DOF by 0.2 mu m for dense lines. For isolated lines the real advantage is seen in the increase of DOF by 0.7 mu m. Furthermore it will be demonstrated, that alternating PSM can improve the imaging performance of contacts significantly over competitive techniques. Chromeless PSM may push the ultimate resolution limit. However to vary the linewidth three adjacent quartz edges must be used, since two phase edges are instable in defocus. A phase shifting region needs to exceed a minimum width in order to enhance the contrast of the aerial image of the whole feature. Experimental data and simulations show that the required minimum phase-shifter width for an isolated line is in the region of 400nm. Simulation and experiment show, that 90 degrees phase edges are very sensitive to defocus and neighbouring patterns. Using a 3D mask simulator, correction values for etch depth and parameters for a lateral underetch were determined in order to achieve intensity balancing for alt. PSM for various feature sizes.