Ultra-High Accuracy Measurement of the Coefficient of Thermal Expansion for Ultra-Low Expansion Materials

摘要

Microlithographic systems rely on precision alignment and a high-level of dimensional stability to meet their performance requirements. In critical applications, immunity to thermally induced dimensional changes is achieved by the use of ultra-low linear coefficient of thermal expansion (hereafter referred to as CTE and denoted by α) materials such as ULE~(~R) in components such as reflective optics and machine structures. The CTE of ULE~(~R) may be varied within limits during manufacture and is typically in the 0 +- 30 ppb K~(-1) range. A high-accuracy determination of the CTE is essential for both process control and for providing an essential input to the design of such systems for error budgeting purposes. Currently, there is a need for CTE determination with an expanded uncertainty U(α)(k=2) <1 ppb K~(-1) in the 273-373 K (0-100℃) temperature range. A survey of the state-of-the-art of high-accuracy absolute measurement of CTE is presented along with a discussion of the significant error sources in each of the current techniques. Metrology techniques, sample design and instrumentation are described along with uncertainty estimates for representative instruments. The design philosophy and prospects for a new instrument that satisfies the above mentioned need are described.