Focused-ion-beam (FIB) lithographic characteristics of low-pressure chemical-vapor-deposited Si3N4 resist have been studied using low-energy Ga+ -FIB exposure and CF4 reactive ion-etching (RIE) development. The resist thickness, Z(min), to minimize a substrate damage is determined by Monte Carlo (MC) simulation The Z(min) is approximately 190, 355, and 560 Angstrom for incident ion energies of 10, 30, and 40 keV, respectively. Ga+ FIB-exposed Si3N4 for CF4 RIE development demonstrates a negative-type resist which is dependent on resist thickness and dose. In the case of 30 keV Z(min) Si3N4, a pattern is formed initially at dose of about 5.70 x 10(15) ions/cm(2) (D-1), and a linewidth corresponding to a beam diameter of 0.36 mu m is obtained at dose of 4.73 x 10(16) ions/cm(2) (D-2). Above a critical dose of about 7.56 x 10(16) ions/cm(2) (D-3), which reaches a surface concentration of about 1.5 x 10(21) ions/cm(3) according to MC simulation,a dent begins to form from the center of pattern. The reason for the formation of dent is not clear at present but may be due to the decrease of resistance against CF4-reactive gas with the increase of the surface ion concentration up to a critical value. The values (D-1, D-2, D-3) decrease with decreasing resist thickness. As the incident energy increases, the threshold dose increases. The imaging contrast appear to be about 3.3-3.5. A 190-Angstrom-thick Si3N4 resist exposed by 10 keV FIB with a dose of 8.0 x 10(15) ions/cm(2) increases both the refractive index n, and the optical energy gap, E-op to about Delta n = 0.007 and Delta E-op = 0.07 eV. A clear pattern with a linewidth of 0.14 mu m is formed on Z(min )Si(3)N(4) resist by 40 keV FIB exposure.