Realization of thermally stable transmissive optical elements for the EUV wavelength range

Abstract

This paper describes the successful combination of masking or beam shaping elements based on thin membranes with the thermally ultra-stable properties of the ULE [1] glass substrate material. These kind of elements can be used in transmission in the optical beam path of EUV systems. By applying a combination of new technologies, thin membranes can be directly manufactured on ULE substrates. The thermal expansion properties of the membrane based transmissive optical elements are dominated by ULE substrate. These elements enable a position stability of features in the range of 10-8/Kelvin for element sizes with a length of up to 150 mm. The combination of technologies enables also the structuring of features with critical dimensions in the range of 100 nm on the membrane. The features on the membranes are for example absorbing or phase shifting structures or even holes. Holes in the membrane with diameters down to 200 nm have been demonstrated using e-beam based technologies. These kind of transmissive optical elements could be used in EUV exposure systems, actinic mask inspection systems or other optical systems which are using light in the EUV wavelength range.