Ion-beam exposure and patterning characteristics of an amorphousSe75Ge25thin film which acts as a positive resist in focused-ion-beam (FIB) lithography have been studied using low-energyGa+ion sources below 30 keV. Thea-Se75Ge25resist exposed to 10 keV defocused-Ga+ion beam with the dose of5.0×1014to9.3×1015 ions/cm2results in increasing the optical absorption, which was also observed in the film exposed to an optical dose of4.5×1020 photons/cm2.The absorption edge shift of 0.3 eV for the resist exposed to a dose of9.3×1015 ions/cm2at 10 keV is about twice that of photo exposure. These large shifts could be estimated as due to an increase in disorder, considering a decrease in the slope of the Urbach tail and a broad pattern of x-ray diffraction. For the exposure of a 30 keVGa+ion beam and above a dose of1.4×1015 ions/cm2,a 590-Å-thick resist film is completely etched by dipping for 10 s in 1:1:3HNO3:HCl:H2Osolution (25 °C), and then the etching rate is about 60 Å/s. As the incident energy increases from 10 to 30 keV, the threshold dose decreases from4.0×1015to1.4×1015 ions/cm2and then the imaging contrasts appear to be about 0.5 and 2.5, respectively. The decrease of the threshold dose with increasing the exposure energy is evidence that a predominant factor in the FIB exposure characteristics is the energy transfer rather than the implanted ions themselves. When 30 keVGa+FIB exposure with a multiscan diameter of 0.2 μm and the above-mentioned development are employed, a resist pattern with a linewidth of about 0.225 μm is obtained.