Measuring Thermal Expansion Variations in ULE~(~R)Glass with Interferometry

摘要

Corning has focused its recent efforts on coefficient of thermal expansion (CTE) metrology improvements. Due to the unique environment required for EUVL technology, EUVL optics (and photomasks) require extremely uniform CTE properties, with targeted variations of less than Ippb/K. Until now, no practical metrology technique existed that could accurately verify if a material met such requirements due to the lack of precision. Corning has previously introduced the idea of measuring CTE in ULE~(~R)Glass using Phase Measuring Interferometry (PMI) by discovering the correlation between refractive index and CTE in ULE~(~R)Glass. However, refinement of the correlation was necessary. This paper focuses on the progress made towards that end, which has resulted in the ability to non-destructively measure peak to valley CTE variations to within 57 parts per trillion per degree Kelvin (ppt/K) at possible spatial resolutions in the micron range on thick or thin samples.