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Calculation and experimental determination of the structure transfer accuracy in deep x-ray lithography

Deep x-ray lithography with synchrotron radiation is the key microfabrication process in the LIGA technology. Micro-components with a height of some up to several mm can be manufactured with sub- precision. The pattern transfer accuracy is governed by technological constraints like thermal deformation of the mask as well as by various physical effects, e.g. diffraction, emission of photo- and Auger electrons, fluorescence radiation, radiation scattering and divergence of the synchrotron radiation beam. A computer program has been developed to investigate the significance of these effects to the dose distribution in the resist material, which in turn determines the lateral structure resolution and the quality of the resist structures. In this paper the models used for the calculation and the calculation procedure are introduced and the weight of the different contributions with respect to transfer accuracy is investigated. It is shown that beam divergence and diffraction are much less important than the image blur caused by photoelectrons. Fluorescence radiation emitted from the mask membrane or the substrate contributes to the dose deposition in the resist if mask membrane or substrate consists of high-atomic-number material. Scattering of radiation is negligible for resist layers which are less than some mm thick. The calculations are compared with measurement results for different substrate materials, synchrotron radiation sources and resist heights. A good agreement was found between calculated dose distributions and measured resist side wall profiles. This allows a partial compensation of the above-mentioned accuracy limiting effects early in the mask design phase.