<title>Deep-UV attenuated phase-shift mask for a quarter-micrometer photolithography</title>

Abstract

A set of deep UV attenuated phase shift masks (PSMs) was fabricated to clarify the correlation between PSM optical characters and the printing performance. A SiNx single-layer film was adopted as a phase shifter which works as an attenuator simultaneously. We fabricated nine PSM test masks in which both phase shift and transmittance were varied. In these masks, phase shift ranges from 176 to 205 degrees and transmittance ranges from 6.4 to 13.1%. The target feature size of the experiment was set to that of 256 M DRAM. Line/space patterns under 0.25 micrometer and the contact holes under 0.3 micrometer have been evaluated. The evaluation was made using an exposure-defocus window analysis from measured aerial images. Each mask was compared by the calculated depth of focus (DOF) at 10% exposure latitude. The experimental results show that shifter optical characters do not play an important role in case of line/space patterns. However, the phase shift error leads to serious reduction of DOF for hole patterns. In using high transmittance PSM, mask bias is inevitable to print under -0.30 micrometer hole patterns because of the side-lobe peak. To obtain the maximum DOF, mask bias should be decided with meticulous care.