FIELD-EMISSION CHARACTERISTICS OF THE SCANNING TUNNELING MICROSCOPE FOR NANOLITHOGRAPHY

We present a systematic study of the performance of scanning tunneling microscope (STM)-based, low energy electron beam lithography, using s imulations of field emission from STM tips, emphasizing realistic cond itions of tip geometry and operation. We calculate the potentials and electric field for a hemispherical model emitter in an axially symmetr ic system. Emission current density at the tip is calculated using the Fowler-Nordheim equation, and current density at the sample is obtain ed by calculating trajectories of emitted electrons. We characterize t he beam diameter at the sample as a function of emitter radius, tip-sa mple bias, emission current, resist thickness, and tip work function. The beam diameter is primarily affected by the tip-sample gap, increas ing at larger gaps, characteristic of high bias and large tip curvatur e. For optimal tip radius the beam diameter increases linearly with bi as from approximately 2 nm at 5 V to 25 nm at 50 V. Beam diameter is n early independent of emission current over the range 0.05-50 nA. Diele ctric resist films cause an increase in beam diameter due to increased tip-substrate gap. Beam diameter is very sensitive to tip work functi on, increasing dramatically for low work function tips. Tips comprised of asperities on flat surfaces produce significantly smaller beams co mpared to ''standard'' tips of the same emitter radius. However, for l ow bias (<15 V) beam diameter becomes insensitive to tip geometry. We compare these simulations to selected experimental results to evaluate the limitations to performance and assess the feasibility of routine sub-10 nm structure fabrication using STM-based low energy electron be am lithography. (C) 1996 American Vacuum Society.