Stability improvement at high emission densities for gold thin film photocathodes used in advanced electron beam lithography

Multi-electron beam lithography has been proposed as a promising approach to achieve high throughput for mask writing and direct wafer writing. Laser driven photocathodes represent an attractive candidate for multiple beam, high brightness sources. Thin film gold photocathodes that can be handled in air are of particular interest because of their potential for practical sources. In this article we present a study of the degradation mechanisms that change photocurrent yield for thin film gold photocathodes. Two general degradation mechanisms were studied: microstructural changes of the gold thin film and surface reactions. Observed microstructural changes included loss of gold coverage of the sapphire substrate, gold grain growth and an increase in surface roughness. A titanium adhesion layer was shown to stabilize coverage and proved stable to 700 degreesC by in situ transmission electron microscopy (TEM) experiments on planar sections of the Au/Ti/sapphire thin film stack photocathode. Reactions at the surface included physisorption and chemisorption of species and the subsequent reactions that occur when they diffuse to the laser illumination/electron emission site. X-ray photoelectron spectroscopy detected multiple monolayers of hydrocarbons on the surface of the Au following deposition and exposure to air. A product layer at the illumination sites as thick as 28 nm was detected using scanning electron macroscopy, atomic force microscopy and TEM following long term (1000-1200 min.) photoyield stability tests. Photoyield versus time plots for all the Au/Ti/sapphire cathodes were similar with an initial high photoyield, a drop to a minimum, then a rise to a second maximum followed by a slow 2%/h decay. Desorption of weakly bound physisorbed and chemisorbed species and growth of a reaction product layer within the electron emission zone are proposed as mechanisms to explain the photoyield behavior. Similar photoyield stability behavior was observed when Pt/Ti/sapphire photocathodes were tested. The photoyield degradation mechanisms observed in these tests are likely to be active on all thin film transmissive photocathodes subjected to high intensity illumination and will add to changes which occur when more reactive thin film photocathodes are used such as cesium telluride. Stability was improved to less than 1%/h for 16 h within the region of slow decay following the initial transient by illuminating the emission site on the cathode with high intensity UV light with a high O-2 partial pressure. Emission current densities in excess of 6.0 A/cm(2) were measured with a Faraday cup. Given these results, this work suggests that higher current densities and stability can be achieved. (C) 2001 American Vacuum Society