Amorphous Se 75Ge 25 Resist Profile Simulation of Focused-Ion-Beam Lithography

The two- and three-dimensional (2D and 3D) development profiles of an amorphous Se75Ge25 thin film, which acts as a positive resist in focused-ion-beam (FIB) lithography, are simulated using a simplified development model. For the Monte Carlo (MC) simulation, both the depth and the lateral distance in resist are divided into discrete cubic pixels of 25 ×25 ×25 Å, and then the energy densities deposited by the exposure of a FIB with a diameter of 0.2 µm, an incident energy of 80 keV and a fixed dose of 1.1 ×1015 ions/cm2, are summed up in these pixels. For the development condition of the HCl+HNO3+H2O solution in the ratio 1:1:3 at 25°C, the exposed region in the Se75Ge25 resist is markedly etched for 2T to 3T, in which T represents a normalized development time for applying the various doses and energies and corresponds to 2.1 s under the above-mentioned exposure conditions. For 3D simulation, the developed pattern widths are 0.200, 0.220 and 0.225 µm for 2T, 4T and 6T, respectively. The slope of developed walls is nearly constant throughout the entire depth range, and the development rate is decreased compared with the 2D case. Under this exposure condition, 4T was chosen as a reasonable development time.