Development of ZrSiO attenuated phase shift mask for ArF excimer laser lithography

We have reported that Zirconium Silicon Oxide (ZrSiO) film is one of the most promising materials for attenuated phase shift mask (att.PSM) for ArF excimer laser lithography. In this paper, we report on practical mask characteristics of ZrSiO att.PSM through its fabrication process. Optical constants (refractive index n / extinction coefficient k) of ZrSiO sputtered films vary continuously according to Ar/O-2 now ratios. A reasonable solution to improve spectroscopic property including transmittance at inspection wavelength can be obtained by a bi-layer structure, consisting of two films having different optical constants, that is absorptive film (AF) and transmissive film (TF). By selecting a pair of appropriate optical constants and adjusting thickness of each layer, we developed the bi-layer structure suitable for optical and other required properties. ZrSiO films are etched by chlorine-based gases, especially AF (bottom layer) dry etching using BCl3 gas has high selectivity to quartz substrate. By optimizing dry etching conditions, cross sectional profile has been attained to over 80 deg, moreover no remarkable residues and edge roughness can be seen. These masks are confirmed to have sufficient tolerance to conventional cleaning process by monitoring the change of transmittance or reflectance curve. Consequently, both transmittance and phase shift through fabrication process approached the required specification, +/-0.3% and +/-2deg, respectively. Inspection or measurement tools for conventional masks are also applicable. In addition, ZrSiO att.PSM is proved to have sufficient durability for ArF excimer laser irradiation.