Photoemission from gold thin films for application in multiphotocathode arrays for electron beam lithography

Photoemission is a promising approach to electron sources for electron beam lithography because of the ease with which various shapes or arrays of independently modulated sources can be fabricated. However, most high-quantum-efficiency photoemitters are extremely sensitive to even partial monolayers of contamination, and therefore require some combination of differential pumping systems and photoemitter surface protection after activation. Here we propose to use a high-power 257 nm laser in combination with the relatively high work function and low quantum efficiency of gold films to produce practical multicathode electron sources for electron beam lithography. Gold films have the offsetting advantages that their photoemission characteristics are relatively reproducible and stable even in contaminating environments. It is possible, therefore, to prepare and handle them in air as well as operate them in less demanding vacuum environments. It is shown that a back-illuminated 15 nm gold film on a quartz or sapphire substrate exhibits a quantum efficiency of approximately 10(-4) at 257 nm, producing photocurrents greater than 1 mu A at a laser power of under 200 mW. The quantum efficiency is very reproducible and relatively stable under a variety of environmental and operational conditions. Slow changes, by as much as a factor of 3, over time periods of several weeks following sample preparation have been observed, consistent with variations of the gold work function of approximately 0.2 eV. The results are in good agreement with a straightforward extension of existing photoemission models for bulk material. (C) 1998 American Vacuum Society.