Scanning electron microscopes and solid-state transmission electron detectors are widely available and generally easy to use, making the collection of imaging techniques referred to as Scanning Transmission Electron Microscopy in a Scanning Electron Microscope (STEM-in- SEM) more accessible today than ever before. These techniques are well suited to a host of transmission imaging applications including nanoparticle metrology, imaging beam-sensitive materials, grain texture studies, and defect analyses, for example. In this tutorial, we will describe how to obtain both qualitative and quantitative information from different transmission detectors including secondary electron conversion devices, different solid-state detectors, and a recently-developed programmable STEM (p-STEM) detector.
The first part of the presentation will briefly describe some of the pros and cons of low energy STEM-in-SEM imaging (i.e., with primary electron energies <30 keV). Different transmission imaging modes that can be accessed in a conventional SEM, and the types of information that can be gleaned from those modes will also be described. The importance of acceptance angle control will be emphasized. To that end, and because most commercially available STEM-in- SEM detectors enable only a few acceptance angle options, a straightforward and economical mask/aperture system [5] that can be adapted to most transmission detectors will be described. We briefly demonstrate how this approach can be used with a rudimentary solid-state STEM detector to obtain different imaging modes, and to quantify electron scattering distributions (i.e., diffraction patterns, Fig. 1b) for material systems amenable to this type of signal collection.
The second part of the presentation will emphasize an on-axis, pixelated detector for programmable scanning transmission electron microscopy (p-STEM). The apparatus includes a digital micromirror device (DMD) that enables imaging and diffraction in a single detector by directing different parts of the signal to different sensors. A CMOS digital camera is used to image the diffraction pattern, and a photomultiplier tube (PMT) synchronized with the microscope scanning system is used to generate a real space image by integrating (pixel-by-pixel) a portion of the diffraction pattern defined by the user. In effect, the DMD replaces the objective aperture in a transmission electron microscope (TEM) or an annular detector in a scanning transmission electron microscope (STEM). However, the DMD has the distinct advantage that arbitrary user-definable patterns (i.e., virtual apertures) can be programmed to it in real time, meaning that conventional imaging modes can be used to obtain quantifiable contrast and that non-conventional imaging modes can easily be explored. Detector operation is demonstrated by showing qualitative and quantitative information obtained from diverse samples including conventionally thinned (electropolished) stainless steel, oriented Au and Si films, and 2-dimensional materials. We will also show how the detector can be used to obtain information about the microscope including beam convergence angle and electron column alignment.
The third part of the presentation will present alternative and emerging imaging methods. For example, we will demonstrate how ptychography can be implemented in an SEM, how thermal diffuse scattering can be used as a local temperature probe, and different ways to ascertain sample strain.