Electron scattering of low-Z high-density materials in X-ray mask patterning

The scattering properties of low-Z high-density materials are discussed, through Monte Carlo simulation, in view of their utilization as membranes for X-ray masks. The interplay between low atomic number and high atomic density is discussed and a comparison with silicon is carried out, both in case of bulk targets and membranes. The low Z causes smaller proximity effects, but the decisive factor relative to silicon is constituted by the higher mechanical stiffness, which, in principle, allows to greatly reduce membrane thickness. However, we show that, in spite of the more favourable intrinsic scattering properties, the choice of these materials, in actual systems, is not always a real advantage. In the analysis of the single-layer resist process for X-ray mask fabrication, no significant difference relative to Si is found in the two limits of low and high energy. On the contrary, the choice of low-Z high-density materials is found to be advantageous at intermediate energy. In particular, we demonstrate that, while in the case of low-Z high-density materials, 0.15 μm resolution is successfully obtained already at 30 keV, in case of Si, this same resolution is only reached at e-beam energies of at least 40 keV.