The role of electron scattering in x-ray reflection-masks

Electron scattering phenomena, occuring in the pattering of tungsten/carbon reflection-masks for X-ray projection lithography, are theoretically investigated. A bi-layer resist subtractive process is analysed by Monte Carlo simulation and proximity-effect evaluation. Results are utilised in order to calculate resist development profiles. Electron-beam energy (20 to 100 keV), mask substrate material (silicon and diamond) and pattern density are considered as process variables. In the presence of a bulk substrate, the better intrinsic scattering characteristics of diamond, with respect to silicon, result in higher-contrast patterns starting from intermediate energies. Only minor improvements are obtained by going to the higher electron energies. A resolution of 0.25 μm is achieved at 50 keV with bulk-diamond substrates. The use of a membrane, allowed by the favourable mechanical properties of diamond, is proposed in order to improve resolution during the lithographic stage of the mask-fabrication process. A resolution of 0.15 μm is demonstrated at 75 keV with diamond supporting membranes.