High-precision ( < 1ppb/℃) Optical Heterodyne Interferometric Dilatometer for Determining Absolute CTE of EUVL Materials

摘要

We have developed an optical-heterodyne-interferometric dilatometer tailored to meet EUVL requirements. The key feature is that it can measure the absolute coefficient of thermal expansion (CTE). The design of the dilatometer has been optimized to yield highly accurate, reproducible measurements by taking into consideration uncertainty factors and their contributions. A prototype was constructed and evaluated. To test its capabilities, we measured the CTEs of various materials, which had values ranging from ppm/℃ to ppb/℃. All the measurements were successful, and we found that our dilatometer could handle a wide variety of materials, including low-thermal-expansion materials (LTEMs) for EUVL. 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/℃ or better for a change of 1 ppb/℃ in the target. The dynamic reproducibility, or in other words the resetability, was ±0.85 ppb/℃ or better. Regarding measurement accuracy, our results are comparable to those obtained with the AIST dilatometer. From the first results, the CTE difference between AIST and ASET was 1.7 ppb/℃. 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.