7973 is a titanicate-containing silicate low expansion glass that is specifically designed to meet masking requirements and as an optical substrate for ultraviolet lithography applications. 7973 has a similar composition to ULE® 7972 glass and is also made using the same vapor deposition process.One of the major design changes required in the development of lithography technology from 193nm to 13.4nm processes is the stepping light group from refraction to reflection. In reflective optics, the substrate material should be purely passive. The incident light should be reflected by multiple layers of optical coatings and photomask without any mechanical or optical deformation due to the underlying substrate. In order to minimize the deformation caused by small temperature changes and to achieve strict ultraviolet lithography specifications, the coefficient of thermal expansion (CTE) of the substrate must be close to zero and strictly controlled near zero within the expansion range. An extremely low coefficient of thermal expansion is required to reach parts per billion per degree Celsius (ppb/C).