William Rogers Rosch

Hardness, elastic modulus, and fracture toughness bulk properties in Corning calcium fluoride

Knoop and Vickers hardness, Youngs Modulus, and fracture toughness measurements were performed on Cornings Code 9575 calcium fluoride in various orientations. Other commercially available sources of calcium fluoride were also measured for comparison of properties. Knoop hardness and elastic properties exhibited a dependence on orientation while no such dependence was observed for Vickers hardness and fracture toughness. The results also indicated that these physical properties were not dependent on the source of the material

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.

Study of CaF2 samples using DUV birefringence measurement and x-ray diffraction techniques

In this paper we report measurement results of optical lithography grade calcium fluoride samples using deep ultraviolet (DUV) birefringence and X-ray diffraction methods. Linear birefringence maps of a variety of calcium fluoride samples were generated from measurements at both optical lithography wavelengths (157 nm, 193 nm and 248 nm) and at 632.8 nm. Comparing the respective wavelength results for birefringence in certain samples showed significant differences in birefringence patterns observed at 157 nm and 633 nm for a light beam propagating along the [111] crystal axis. Such differences cannot be explained from the dispersion of stress birefringence at those wavelengths. Our interpretation is that the discrepancy in the birefringence patterns observed at 157 nm and 633 nm is due to crystal defects in those calcium fluoride samples. The crystal quality of those calcium fluoride samples was subsequently determined by X-ray diffraction techniques. The results obtained from both birefringence and X-ray data substantiate each other qualitatively for judging the crystal quality of calcium fluoride samples.

Improvements in CaF2 material properties for next-generation microlithography applications

The key requirements of CaF2 lens blanks are transmission, birefringence and optical homogeneity. While it is obvious that high transmission and resistance to fluence- and time-dependent darkening is critical, the subtleties of single crystals, compared to isotropic glasses, play an important role in the implementation of CaF2 in microlithography steppers. The existence of intrinsic birefringence has caused stepper manufacturers to employ various crystallographic orientations in their lens designs. This fact means that crystal growth and annealing processes must be optimized accordingly. Small-scale defects, known as dislocations and sub-grain mis-orientation, manifest themselves as larger-scale defects known as slip and mosaic. These imperfections may impact birefringence and a critical homogeneity parameter known as residual rms. Residual rms is directly related to the concentration of asymmetric defects in CaF2 crystals, such as those aforementioned. In the present paper, the author will report on progress made at Corning to meet the stringent requirements of both 193nm and 157nm systems.