The characteristics of an amorphous Se75Ge25 thin film as a positive-type resist in focused-ion-beam lithography

In order to study the applicability of a-Se75Ge25 thin film resists for focused-ion-beam (FIB) lithography, the ion-induced characteristics of a thin film which acts as a positive-type resist for irradiation by a FIB and the micropatterning by a low-energy FIB conventionally utilized for mask-repair are investigated. The ion resist parameters of the resist are analytically calculated by using the LSS (Lindhard-Scharaff-Schiott)-LNS (Lindhard-Nielsen-Scharaff) theory for a variety of ions and for a broad range of incident energies. A numerical simulation by Monte Carlo (MC) method is also applied to complement the limit of the analytical results and to obtain sufficient information related to energetic ion-scattering events in the resists. The results show that the resist thicknesses, Z(min), to minimize substrate damage are similar to 273, similar to 582, and 1180 Angstrom for incident energies of 10, 30, and 80 keV, respectively. The irradiation of the resist by a 10-keV defocused Ga+ beam with a dose 9.3x10(15) ions/cm(2) results in a large shift of the absorption edge of about 0.3 eV, which is thought to be due to an increase in disorder and, therefore, can be expected to produce high etching selectivity. When both a 30-keV FIB exposure with a multiscan diameter of 0.2 mu m and a wet-etching development by dipping for 10 s at 25 degrees C are employed, a clear pattern with a linewidth of about 0.225 mu m is obtained and the imaging contrast appears to be about 2.5.