1.  Field of the Invention
The present invention relates generally to lithography photomasks, and particularly to optical photolithography mask blanks for use in optical photolithography systems utilizing vacuum ultraviolet light (VUV) wavelengths below 193 nm, preferably below 175 nm, preferably below 164 nm, such as VUV projection lithography systems utilizing wavelengths in the 157 nm region. 
2.  Technical Background
Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light below 193 nm provide benefits in terms of achieving smaller feature dimensions.  Such systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits with smaller feature sizes.  Current optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have progressed towards shorter wavelengths of light, such as the popular 248 nm and 193 nm wavelengths, but the commercial use and adoption of vacuum ultraviolet wavelengths below 193 nm, such as 157 nm has been hindered by the transmission nature of such vacuum ultraviolet wavelengths in the 157 nm region through optical materials.  Such slow progression by the semiconductor industry of the use of VUV light below 175 nm such as 157 nm light has been also due to the lack of economically manufacturable photomask blanks from optically transmissive materials.  For the benefit of vacuum ultraviolet photolithography in the 157 nm region such as the emission spectrum VUV window of a F2 excimer laser to be utilized in the manufacturing of integrated circuits there is a need for mask blanks that have beneficial optical properties including good transmission below 164 nm and at 157 nm and that can be manufactured economically. 
Photomask blanks used in such lithography systems are different from the other optical elements of the system such as lenses and mirrors in that the photomasks are generally very thin and play a unique part in the system in terms of providing a substrate for integrated circuit patterns that are projected through the system.  Patterns of the integrated circuits to be made are formed on the surface of photomask blanks, so that an image of the pattern on the photomask blank can be projected through the lithography system and printed on a surface of an integrated circuit semiconductor wafer.  Photomask blanks must meet very strict requirements for dimensional stability to avoid warping and shrinking and for optical properties such as high transmission in order to ensure the extreme accuracy required to form very fine integrated circuit patterns and inhibit the distortion there of. 
The present invention overcomes problems in the prior art and provides a means for economically manufacturing high quality improved photomask blanks that can be used to improve the manufacturing of integrated circuits with vacuum ultraviolet wavelengths.