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

Microlithographic systems rely on precision alignment and a high-level of dimensional stability to achieve required performance. In critical applications, immunity to thermally induced dimensional changes is achieved by the use of low coefficient of thermal expansion (CTE) materials such as ULE® in components such as reflective optics and machine structures. ULE® has an expansion coefficient (α) that is typically in the 0 ± 30 ppb K range and it may be engineered to achieve a specific value. A highaccuracy 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 uncertainty U(α)<1 ppb K (k=2) in the 273-373K (0-100°C) temperature range. This effort is aimed at developing techniques for performing this measurement. Fundamental Issues with Measurement of Dilatation The fundamental issue with making a dilatation measurement is distinguishing between the dimensional changes of the sample and that of the instrument structure. Dimensional changes of any part of the metrology loop (including the frame or sensors) is indistinguishable from changes of the sample. The aim of the design is to separate/eliminate or minimize the influence of undetected dimensional changes in the metrology loop not directly attributable to the sample. These undesirable changes may be mechanical, thermal or optical in origin. The following figures illustrate some current high-accuracy techniques. The metrology loop is identified along with practical realizations of some of these configurations. Uncertainty Analysis Requirements Definition SPIE Conference 2002 Ultra-High Accuracy Measurement of the Coefficient of Thermal Expansion for Ultra-Low Expansion Materials Double Michelson Dilatometer Michelson/Modified Michelson Dilatometer Fabry-Perot Dilatometer Comparison of Current High-Accuracy Methods Timeline Challenges • Dilatation metrology with uncertainty less than 5 parts in 10 for 10K temperature change – Spurious metrology loop displacement – Interferometer nonlinearity – Effects of optical contacts – Temperature dependent changes in phase changes on reflection – Sample alignment stability – CTE variation – Thermal gradients • Temperature metrology with an uncertainty of 0.2K over 10K step • Sample heating and cooling – Part time constant – Thermal stability of structure