We report on the development of an e-beam lithography process for the fabrication of x-ray masks for nanolithography with minimum feature size down to 65 nm. The process is based on accelerating voltages below 50 kV, with a single layer of 0.55 mum thick PMMA. Particular emphasis is addressed to the study and the control of the various variables affecting the ultimate practical resolution, i.e., the developer concentration, the accelerating voltage, the exposure latitude, the reproducibility of the results and the effect of the beam diameter. Monte Carlo simulation was used to separate the effect of forward- and backscattering. Dense and reproducible 65 nm-resolution arrays of gold electroplated lines have been achieved at 50 kV on silicon nitride membranes. Some of the fabricated nanomasks have been successfully replicated by synchrotron radiation lithography.