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 Corning's ULE product can meet the P-37 surface finishing specifications, and
combined with ULE's superior CTE performance is positioned as the material of choice for EUV mask blanks.
Ultra-Low Expansion (ULE®) glass has been and continues to be a significant material for astronomical applications.
With a nominal composition of 7 wt. %TiO2 in SiO2, Corning Code 7972 ULE® has a mean room temperature
coefficient of thermal expansion (CTE) of 0 ± 30 ppb/°C with a typical CTE range of less than 15 ppb/°C, properties vital
to the manufacture of high resolution optics requiring extreme thermal stability. Combined with lightweighting
techniques developed at Corning during the past 30 years, ULE® has been successfully employed for numerous
monolithic and lightweight mirror applications including the 2.4 meter Hubble Space Telescope lightweight primary
mirror, the Airborne Laser (ABL) primary mirrors, and most recently the Discovery Channel Telescope 4 meter mirror
blank. ULE® maintains its strong candidacy for future ELT applications.
Recent challenges in mirror surface specifications and the development of alternative material choices calls for a
comparison with ULE®. The objective of this article is to review ULE® properties and manufacturing capabilities, and to
compare relevant material properties to those of alternative material options, thus allowing designers to properly execute
material selection. Finally, recent development efforts directed toward improving ULE® will be discussed.
Compositional striations in Corning's 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.
The EUVL industry has unique material requirements, which are being addressed. Implementation of metrology methods new to ULE Glass will be discussed along with material characteristics altered to meet the needs of EUVL. Metrology methods include multiple means of evaluating the striae, CTE and inclusions. Material characteristics have been altered to better meet the demands of the industry. The reduction in inclusion levels along with other improvements such as in the area of striae will be discussed here. Improvements of greater than 4x were achieved in these preliminary striae reduction trials.
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