Development and application of ultrafast scanning electron microscopy

Abstract

Scanning electron microscopes (SEMs) can capture detail on the single nanometer length scale through the interaction of a tightly focused electron beam with a sample, but this impressive spatial resolution is not matched with a capability to resolve dynamic processes on the ultrafast time scale. A variety of processes occur at nanosecond and faster time scale, and at spatial scales out of reach of conventional light optical microscopes, for example in nanoscale solid state devices and nanomechanical resonators. An imaging tool combining high spatial and temporal resolution is therefore required. In recent years, some research groups have worked on a technique to add ultrafast imaging to the capabilities of a SEM, building on concepts developed for transmission electron microscopy. In so-called ultrafast scanning electron microscopy (USEM), the combination of a pulsed laser and a pulsed electron beam enables the formation of movies capturing dynamics much faster than possible with a conventional SEM. Dynamics are initiated with femtosecond laser excitation of the sample and probed with electron beam pulses arriving with tightly controlled delay. The temporal resolution of this pump-probe scheme is determined by the laser and electron pulse duration. Secondary electrons, emitted from the top few nanometer of the sample, are collected and used to construct ultrafast movies. The aim of this thesis is to further develop the technique by making multiple improvements in our implementation, gaining additional insight into the contrast mechanism of USEM, and exploring new applications.