Semiconductor on glass photocathodes for high throughput maskless electron beam lithography

Abstract

Previous results obtained from negative electron affinity (NEA) photocathodes have shown high brightness (1×108 A/cm2sr at 3 kV), energy spreads as low as 50 meV at room temperature, uniform sensitivity to red and infrared light over cm2 areas, picosecond-scale switching, and low emission noise. These properties make possible a number of new electron beam tools, including a high-throughput multibeam maskless wafer exposure tool. Simulations show that up to 10 μA of current at 50 kV can be delivered to the wafer in such a system with a total spot diameter of 70 nm. The main obstacle to the use of NEA photocathodes in lithography instruments is the issue of cathode stability and lifetime. An ultrahigh system vacuum was built to activate and evaluate NEA photocathodes in a demountable system that includes areas with poorer vacuum. In this system a low quantum efficiency photocathode (<0.1%) was maintained at constant emission up to 190 nA with no discernible decay. A higher-efficiency cathode provided a brightness estimated at 2.2×107 A/cm2sr at 5 kV. Lifetime measurements on this cathode showed no measurable effect from resist exposure, and stable emission of 375 nA for 15 h, followed by a steady decay of ∼1.5%/h with no re-cesiation. Higher current results (3.75–1.3 μA) show a decay rate of ∼6%/h without re-cesiation. The use of cesium in a feedback loop was shown as an effective way to stabilize emission. Furthermore, the decay is limited to an area 2.6 mm in diameter, as the rest of the cathode is shielded by nonevaporable getter material. This result indicates that the source lifetime can be extended hundreds of times by simply moving the cathode to expose a fresh area. A moveable NEA photocathode in combination with a cesium source in a feedback loop promises to be a highly reliable, long-lifetime electron source.