Effect of feature size, pitch, and resist sensitivity on side-lobe and ring formation for via hole patterning in attenuated phase-shift masks

Attenuated Phase Shift Masks (att PSM) have become very popular in the industry for printing contact holes. Higher transmission att PSM generally tends to give a better depth of focus and exposure latitude. However, the main drawbacks of using higher transmission masks are side lobes, printing of unnecessary patterns and resist erosion. The side lobbing is strongly dependent on the feature size, pitch, coherency of exposure radiation, illumination type and the transmission of the mask being used. Along with these factors, the other most important factor is the resist contrast. In this paper the effect of pitch, feature size, and resist sensitivity were evaluated on side lobes and rings formation for via holes designed down to 180 nm. Six different pitches were studied (1:1 to 1:5). Two different types of resists were used and the mask transmission used for the study was 8%. Simulations were carried out using PROLITH 3D version 7.1 from KLA Tencor while the experimental verifications were done at Nikon 248 nm step and scan tool. The experimental results were found in accordance with the simulation data. The effect of NA & (sigma) have also been studied on resolution, exposure latitude and depth of focus.