OPTICAL-PROPERTIES OF X-RAY-LITHOGRAPHY MASKS

The purpose of this paper is to present experimental results on optical properties of x-ray mask substrates relevant to x-ray lithography systems utilizing optical alignment between mask and wafer. Data on mask substrates of several materials including B-doped Si, SiC, polyimide, and diamond will be presented. In order to study the light scattering properties of the mask substrate materials, a novel light scattering measuring system was constructed which allows determination of the angular dependence of the transmitted, and reflected light from the mask membranes. The system discriminates between the forwardly transmitted, and specularly reflected light from the sample membranes by the use of perforated solid state light detectors. The detectors are placed on each side of the membrane to obtain simultaneous measurements of forward and back scattering, that otherwise may be affected by thin film interference effects. The data gives information on light scattering due to both the bulk material, and the surface roughness. For the case of B-doped Si membranes, the measured values are in good agreement with a light scattering model also described in this paper. The results shed light on the proper selection of x-ray mask substrate materials from the optical alignment point of view.