Ultra-high accuracy thermal expansion measurements with PTB's precision interferometer

Demands on dimensional stability and on the detailed knowledge of thermal expansion properties of 'high-tech' materials are growing considerably. One application is the further development of photolithography towards EUV—lithography where it is very important to know the thermal expansion properties of the substrates used for the masks but also for the mirrors used as main optical components. This paper describes the recent state of length/thermal expansion measurements with PTB's precision interferometer. The measurements are based on the observation of the absolute length of gauge-block-shaped or cylindrically material samples as a function of temperature using phase stepping interferometry. As reported earlier, in the precision interferometer alignment errors and errors involving the assignment of the sample's position within the camera pixel array are reduced to a negligible level [1, 2]. As a further improvement, in this paper corrections for the temperature-dependent effect of platen flexing are suggested. Measurement examples are shown to demonstrate CTE-measurement capabilities with uncertainties in the range of 10^-9 K^-1 or less.