Variations in Secondary Electron Emission from MgO Characterized by Secondary Electron Spectroscopy

Abstract

The recent developments of the in-lens field emission scanning electron microscopes make it possible to image specimen surfaces with subnanometer resolution by collecting high resolution type I secondary electron (SE) signals. SE images with a resolutiori better than 1 nm have also been obtained in scanning transmission electron microscope (STEM) instruments. A problem related to the correct interpretation of these high resolution images is the lack of knowledge on the energies of the collected electrons contributing to the images. On the other hand secondary electron spectroscopy (SES) on clean surfaces has been developed for several years and biased SE imaging of several deposits on metallic and semiconductor crystal surfaces has been demonstrated. The combination of ultra-high resolution SE imaging with SES and Auger electron spectroscopy (AES) can be realized in pur UHV STEM instrument (MIDAS). The description of the performance of the MIDAS is reported elsewhere. We report here some recent results on SE imaging of MgO smoke crystals, in the MIDAS, with energy analysis of the collected secondary electrons.Strong variations of SE signals from different MgO crystals have been observed and the intensity of the collected secondary electrons from some MgO cubes increases under electron beam irradiation. Figure 1 shows a SE image of several MgO cubes, revealing that some of the crystals (indicated by A) have much higher intensity than others (indicated by B) and bright diffuse patches on the dark crystal can be seen (indicated by D). We have used SES to study the variations in SE emission under electron irradiation. The fact that the velocity distribution of the emitted secondary electrons may depend strongly on the surface morphology of the samples studied makes the interpretation of these SE spectra complicated. In order to avoid this problem and other possible complications arising from the detector geometry we acquired SE spectra from a fixed flat area of a single crystal MgO shown in figure 2. The sample was biased at -30 V and the resolution of the SE spectrum is about 1 eV. Figure 3a shows the SE spectra, taken from the exit surface of the crystal. Spectrum 1 was obtained from a poorly emitting MgO cube as that shown in figure 2 (all the spectra were obtained from the circled area). Spectra 2, 3 and 4 were obtained after about 10, 20 and 40 minutes electron irradiation respectively. In order to study spectral shape variations the spectra were normalized so that they all had the same peak intensity. The normalized data were shown in figure 3b.