Brightness evaluation of pulsed electron gun using negative electron affinity photocathode developed for time-resolved measurement using scanning electron microscope

Abstract

Temporal changes in carrier relaxations, magnetic switching, and biological structures are known to be in the order of ns. These phenomena can be typically measured by means of an optical-pump & electron-probe method using an electron microscope combined with a pulsed electron source. A photoemission-type pulsed electron gun makes it possible to obtain a short-pulsed electron beam required for high temporal resolution. On the other hand, spatial resolution is restricted by the brightness of the pulsed electron gun used in electron microscopes when a low brightness electron source is used and an irradiation current larger than a certain value is required. Thus, we constructed a prototype pulsed electron gun using a negative electron affinity (NEA) photocathode for time-resolved measurement using a scanning electron microscope (SEM) with high spatiotemporal resolution. In this study, a high-speed detector containing an avalanche photodiode (APD) was used to directly measure waveforms of the pulsed electron beam excited by a rectangular-shape pulsed light with a variable pulse duration in the range of several ns to several μs. The measured waveforms were the same rectangular shape as incident pulsed excitation light. The maximum peak brightness of the pulsed electron beam was 4.2×107 A/m2/sr/V with a pulse duration of 3 ns. This value was larger than that of the continuous electron beam (1.6 × 107 A/m2/sr/V). Furthermore, an SEM image with image sharpness of 6.2 nm was obtained using an SEM equipped with a prototype pulsed electron gun at an acceleration voltage of 3 kV.