Ion-beam exposure and patterning characteristics of an amorphous Se75Ge25 thin film which acts as a positive resist in focused-ion-beam (FIB) lithography have been studied using low-energy Ga+ ion sources below 30 keV. The a-Se75Ge25 resist exposed to 10 keV defocused-Ga+ ion beam with the dose of 5.0 X 10(14) to 9.3 X 10(15) ions/cm(2) results in increasing the optical absorption, which was also observed in the film exposed to an optical dose of 4.5 X 10(20) photons/cm(2). The absorption edge shift of 0.3 eV for the resist exposed to a dose of 9.3 X 10(15) ions/cm(2) at 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 keV Ga+ ion beam and above a dose of 1.4 X 10(15) ions/cm(2), a 590-Angstrom-thick resist film is completely etched by dipping for 10 s in 1:1:3 HNO3:HCl:H2O solution (25 degrees C), and then the etching rate is about 60 Angstrom/s. As the incident energy increases from 10 to 30 keV, the threshold dose decreases from 4.0 X 10(15) to 1.4 X 10(15) ions/cm(2) and 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 keV Ga+ FIB exposure with a multiscan diameter of 0.2 mu m and the above-mentioned development are employed, a resist pattern with a linewidth of about 0.225 mu m is obtained.