To better understand the proximity effect of electron-beam lithography and its limitation on aspect ratios in a thick resist film (> 1.0 mu m), the simulation of isolated trench patterning has been performed using the Monte Carlo simulation software SELID (TM). Various key process parameters, including beam energy, exposure dose, substrate, acid diffusion length, and development time have been analyzed by means of the point-spread function, secondary electron distribution, energy distributions, and developed resist profiles. Exposure dose and development time are optimized to achieve vertical sidewalls, and the acid diffusion length has been adjusted to slightly change the top and bottom dimensions of the patterned trench. The simulation results show that increasing the beam energy significantly reduces the forward proximity effect and thereby increases the trench aspect ratio. In contrast, the substrate generally has only a minor effect in terms of the resist profile because its backscattering effect covers a long range up to 10.0 mu m from incident point (50 keV). The achievable maximum aspect ratio for a given resist thickness was found to scale approximately linearly with beam energy. The aspect ratio in a 1.0 mu m thick resist film increased from 3:1 to 20:1 as the beam energy was increased from 10 to 100 keV, and from 5:1 to 10:1 for a 4.0 mu m resist film as the beam energy was increased from 50 to 100 keV. Finally, experimental data using a 50 keV Leica e-beam writing tool is presented, and they agree with the results from our simulations. (c) 2006 American Vacuum Society.