Proximity effects can be reduced when exposing x-ray mask membranes by increasing the electron-beam accelerating voltage. Forward scattering in the resist is reduced and exposure due to backscattered electrons is also reduced since the more energetic electrons are able to pass through the electroplating base and thin mask membrane. In this paper, a detailed study of the proximity effect at high electron-beam voltages for additive process x-ray mask fabrication will be discussed. The main contributors to the proximity effect at high beam voltages were found to be backscattering from the x-ray mask membrane, fast secondary electron production by the incident beam, backscattering from the Au/Cr plating base, and finite beam size. The proximity effect was found to still be significant at 75 kV; but, greatly reduced compared to the measured effect at 50 kV. A Monte Carlo simulation program which included the effects of fast secondary electron production was used to analyze the experimental results. Simulated radial absorbed energy distributions were found to agree well with experimental data from x-ray mask structures. The use of proximity correction parameters, determined from the Monte Carlo data, in a proximity correction program predicted that correction will be necessary at 75 and 100 kV. The reduction in proximity effect gained by using 100 kV was found to be quite small compared to the reduction gained by using 75 kV instead of 50 kV. © 1991.