Thin-Film Measurements in the Scanning Electron Microscope

Abstract

The scanning electron microscope is a specialized closed-circuit television system, which must be understood before the properties of thin films can be monitored using it. Therefore, the first part of this paper discusses fundamentals of scanning electron microscopy, with particular emphasis on signal generation caused by primary electron beam interaction with the object, collection, and amplification of the generated signal, and optimum display. Secondary electrons are the most frequently used information signal in conventional scanning electron microscopes. Images produced by the secondary electron signal show great depth of field, good shadow detail on very rough objects, are capable of high resolution (100 Å), and are unexcelled for showing topographical features of rough samples, particularly when stereo pairs are used. Secondary electron images can also show voltage contrast and magnetic contrast. Backscattered electron images show somewhat higher contrast, poorer resolution, poor shadow detail on rough objects, but can show contrast between areas of different atomic number. Images produced by electron-beam-induced currents in the sample can be used to locate p-n junctions (even when they lay beneath the surface), and to find crystallographic defects near the surface. Characteristic x-rays and Auger electrons can be used for chemical identification; the efficiency of generation of these signals is low, requiring larger beam currents for useful signals. Cathodoluminescent signals also promise to provide vital chemical information about samples, but these signals are weak, and often decrease as the exposure of the sample to electrons increases. The detailed understanding of scanning electron micrographs obtained using different information signals from the sample is not a trivial problem. The scientist must always remember that lighter parts of the image correspond to higher values of collected information signal, whatever that signal may be. Some pictures show contrast because the amount of information signal varies from place to place on the sample when the fraction of the signal that is collected remains constant, while other micrographs show contrast because although the amount of information signal is constant across the sample area of interest, the fraction of collected signal varies from place to place. Both of these effects can be illustrated with a micrograph of an integrated circuit, with bias applied between different electrodes. Sometimes more information is available in the video signal than can be displayed by a conventional television image, and improved display techniques are advisable. Deflection modulation (or Y-modulation) allows small signals superimposed on large signals to be seen clearly. Differentiation of the video signal is useful in many applications. Nonlinear operations on the signal, such as gamma correction and contrast expansion, can improve the presentation of desired information on the micrograph also. The scanning electron microscope can be quite useful in the study of thin films and thin-film structures. General surface topography can be determined, including surface roughness, voids, cracks, and often grain size. When voltage is applied across a region, electrical barriers such as p-n junctions, high-resistance regions, etc., can be detected readily. By pulsing the electron beam, carrier generation and recombination can be studied. Domains on magnetic samples and piezoelectric samples have also been studied. As the electron beam interacts with the sample, the properties of the sample may change. For example, bombardment of metal-oxide-semiconductor devices with voltage applied between metal and semiconductor can result in a stored charge in the oxide. Florescent light output from many materials decreases with bombardment. Resist materials are polymerized by electron bombardment, and very fine patterns can be defined by correctly steering the beam of a scanning electron microscope. Devices having dimensional tolerances considerably shorter than a wavelength of visible light have already been produced in this manner. In some applications, it may be desirable to monitor the size and orientation of single crystal formed during thin film formation. A recent method of producing Kikuchi-like patterns using a scanning electron beam may be useful in this regard. At present, the method is useful on large single crystals, but modifications of the method should make it useful for crystallites down to a few microns in size. The patterns show a wealth of fine detail, and should be useful for accurate measurements. The application of scanning electron microscopy to thin-film problems is in its infancy. Future work will use not only the techniques cited above, but also new techniques devised by thin-film scientists to produce the information they desire about the submicron structures they fabricate.