Performance of dilatometer for determining absolute CTE of EUVL LTEMs

Abstract

An optical heterodyne interferometric dilatometer, which was specially designed to meet the requirements of extreme ultraviolet lithography (EUVL), has been developed. It employs an interferometer to measure the absolute coefficient of thermal expansion (CTE), which means that it directly measures the change in sample length. An examination of its capabilities revealed that it can handle a wide variety of materials with a coefficient of thermal expansion (CTE) ranging from ppm/°C to ppb/°C. In addition, it was found that the reproducibility of CTE measurements ( σ) on low-thermal-expansion materials (LTEM) for EUVL was less than 1 ppb/°C. Thus, it was concluded that this dilatometer would be useful for the precise measurement of the CTEs of EUVL-grade LTEMs.