High-precision (<1ppb/°C) optical heterodyne interferometric dilatometer for determining absolute CTE of EUVL materials

Abstract

We have developed an optical-heterodyne-interferometric dilatometer tailored to meet EUVL requirements. It has the advantage of providing absolute coefficient of thermal expansion (CTE) measurements. The design of the dilatometer has been optimized to yield high-accuracy and reproducibility of measurements by means of consideration of uncertainty factors and their contributions. A prototype is constructed and we have evaluated it. To test the capabilities of the dilatometer, we measured the CTEs of various materials and CTEs ranging from parts per million per degree Celsius (ppm/°C) to parts per billion per degree Celsius (ppb/°C). All the measurements were successful, and we found that our dilatometer can handle a wide variety of materials, including EUVL low thermal expansion materials (LTEMs). Subsequently, a more detailed evaluation of the reproducibility of CTE measurements for titanium-doped silica glass was performed. The static reproducibility (σ) was 0.80 ppb/°C or better for a change of 1 ppb/°C in the target. The dynamic reproducibility, in other word resetability was ±0.85 ppb/°C or better. Regarding measurement accuracy, our data is comparing with those obtained with the AIST dilatometer. From the first results, the CTE difference between AIST and ASET was 1.7 ppb/°C. We continue to improve accuracy of measurement. As a test of capability of our dilatometer, we made a CTE characterization for material development. It showed typical CTE character of LTEMs. We feel confident that our dilatometer will be useful for the measurement of the CTEs of EUVL-grade LTEMs.