The Effects of Fast Secondary Electrons on Low Voltage Electron Beam Lithography

In 1981 Prof. Sir Alec Broers suggested that the spatial limit of direct writing electron beam lithography (DWEBL) would be limited to ~10 nm by the laterally scattered fast secondary electrons (FSE) even in atomically thin resist. Experiments and simulations have been carried out to quantify the contribution of FSE to the energy deposition that results in exposure of the resist over high beam energies. One possible solution to this restriction would be to use low energy electrons.

To examine Broers' hypothesis in low voltage electron beam lithography (EBL), studies in the low energy range on the effects of FSE were performed. Commonly used resists such as PMMA were employed, and the results were compared to those from conventional high voltage exposure. DWEBL was performed in a Schottky field emission gun scanning electron microscope (SEM), used in cathode-lens mode for low voltage operation. The exposure characteristics and sensitivity of the system at these energies have been investigated using Monte Carlo simulation methods. An improved model that describes electron energy losses and a parameterized point spread function of the electron energy distribution process in a solid material has been developed. Using the dose distribution of a source spread function, the patterning ability and ultimate resolution can be predicted correctly. Saturation doses were calculated at low energies, which would give a useful condition to target for routine exposure because it ensures the critical dimensions will not be affected by any random changes in beam intensity.