A printability study for phase-shift masks at 193nm lithography

This paper reports on the mask quality printability for 6% attenuated phase-shift masks (PSM) using 193nm lithography. It is part of a systematic assessment of the impact of mask defects on the printed image. Our previous work [1-3] focused on defect printability of binary masks. Furthermore, an attenuated PSM is also used to print contact holes (CH), as it improves the process latitudes. In this paper we focus on the 2D quality of the CH on the mask. Due to mask-writer effects, the ideally square CH becomes rounded on the mask. The 2D mask quality determines the printed image, together with the exposure conditions and wafer stack. In a first section the influence of programmed mask defects on a 6% attenuated PSM is evaluated. The 100nm gatestyle design is based on the Defect Sensitivity Monitor (DSM of ASML MaskTools). We consider two different degrees of optical proximity correction (OPC), i.e., no OPC, and both serifs and scatter bars (SB). The mask contains programmed soft defects made from resist dots to mimic opaque defects such as bumps and spots. The wafers are exposed on an ASML PAS5500/950 scanner with 193nm conventional illumination (0.7sigma) using a numerical aperture (NA) of 0.63. The printing results are compared to aerial image simulations taking into account the actual mask contours. A comparison table, which classifies the defect based on the induced linewidth deviation, is drawn from it. The second section deals with 2D quality of CH on a 6% attenuated PSM. The design contains CH of various sizes and in various pitches. The CH are provided with serifs on the corners, thereby varying size and placement. These serifs are intended as a correction to the design to incorporate possible mask-writer-generated rounding of the ideally square CH. The purpose of this study is to evaluate the influence of such serifs in the design towards wafer printing. The wafers are exposed on an ASML PAS5500/1100 scanner with 193nm conventional illumination (0.6sigma) using a NA of 0.7. The accuracy of the aerial image simulation is examined using the experimental printability data.