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 ind ependently 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 p umping systems and photoemitter surface protection after activation. Here w e propose to use a high-power 257 nm laser in combination with the relative ly high work function and low quantum efficiency of gold films to produce p ractical multicathode electron sources for electron beam lithography. Gold films have the offsetting advantages that their photoemission characteristi cs are relatively reproducible and stable even in contaminating environment s. 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 q uantum efficiency of approximately 10(-4) at 257 nm, producing photocurrent s greater than 1 mu A at a laser power of under 200 mW. The quantum efficie ncy is very reproducible and relatively stable under a variety of environme ntal 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 approxima tely 0.2 eV. The results are in good agreement with a straightforward exten sion of existing photoemission models for bulk material. (C) 1998 American Vacuum Society.