John E. Maxon

Characterization of striae in ULE for EUVL optics and masks

Compositional striations in Cornings Ultra Low Expansion (ULE®) glass are thought to affect the surface roughness when the glass is polished. For EUV Lithography photomask blanks, it is important for the polished surface to be as smooth as possible. Therefore, since the compositional striations may impact photomask polishing, Corning has undertaken an effort to better characterize the striae and its impact on surface roughness, improve the fundamental understanding of its origin during boule formation, and develop methods and procedures to reduce its potential impact on polishing. This work has verified that striae can vary quite a bit throughout a single ULE glass boule. Characterization has shown that there are two main types of striae. These can be described as high frequency (secondary) striae and lower frequency (primary) striae. Due to the new understanding of the striae origin, two methods have recently been identified and used to greatly reduce or eliminate the high frequency striae component. Currently, new modeling efforts have helped identify potential process changes that may reduce the impact of the primary striae frequency. Experiments are in process to determine their effectiveness.

Corning ULE® glass can meet P-37 specifications

Corning ULE® glass is a binary SiO2 + TiO2 composition formed directly using a flame hydrolysis process. ULE possesses a very low thermal expansion range that can also be accurately adjusted for various applications including EUV photolithography. For ULE to be used for mask blanks and optics applications, it is also necessary that the material be capable of meeting stringent flatness and roughness specifications. For ULE, small compositional striations have been shown to affect the surface quality by inducing mid-spatial frequency roughness during polishing. Therefore, the main challenge has been to reduce mid-spatial frequency roughness to an acceptably low level by diminishing compositional striations present in the glass. Recently, a combination of predictive modeling and experimentation has resulted in a process that reduces striae to the levels needed for EUV masks and optics. These models have enhanced the fundamental understanding of the glass forming process, leading to process adjustments both in oscillation patterns and additional thermal treatments producing glass with improved striae characteristics. ULE masks with reduced striae have been polished to mid-spatial frequency roughness peak-to-valley levels of less than 8 nm. This sub-8 nm topography accounts for less than 20% of the total 50 nm flatness error budget allowable for EUVL masks. These results indicate that Cornings ULE product can meet the P-37 surface finishing specifications, and combined with ULEs superior CTE performance is positioned as the material of choice for EUV mask blanks.

Improvement of homogeneity in large optics made of Corning HPFS fused silica

In large fused silica blanks, typically greater than 225mm in diameter and 40mm thick, recent glass forming improvements have improved the homogeneity in Corning HPFS fused silica by 51%. Overall homogeneity peak to valley values improved from an average 1.4ppm to 0.69ppm. This paper describes the experimental results on homogeneity and birefringence resulting from these Forming improvements.