Lifetime and reliability results for a negative electron affinity photocathode in a demountable vacuum system

Abstract

Negative electron affinity photocathodes may have useful applications as electron sources for high-throughput microlithography [A. Baum et al., J. Vac. Sci. Technol. B 15, 2707 (1997)]. However, the nature of such a system has raised questions about the lifetime and reliability of a cathode during operation. In this article, we report on the lifetime and reliability of cathode operation under various conditions applicable to lithography. To perform these measurements, a 632 nm laser was focused onto a spot smaller than 10 μm in diameter on the back surface of the cathode (active area 0.5–2.0 μm thick). The emitted electrons were accelerated to 5 kV to form a magnified image of the cathode on a phosphor screen 1 m away. The 1/e lifetime of the cathode was measured as a function of the cathode current, which turned out to be an inverse relationship. Additionally, a wafer coated with SAL-601 resist was substituted for the phosphor screen to determine if resist outgassing induced by exposure affected operating lifetime. It was found that the cathode had a lifetime (75 h at 165 nA) that was the same as that obtained without the wafer under similar conditions. Lifetime was also found to be a function of initial level of cesiation of the surface and cesium levels during activation. In particular, when the cathode was initially overcesiated, the Faraday cup current (in the plane of the phosphor) was found to be stable to 2% for up to 3 days at 200 nA, indicating that the cesium level that optimizes lifetime is not necessarily the same level that optimizes quantum efficiency.