Scanning Electron-Acoustic Microscopy: Do You Know Its Capabilities?
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Electron-Acoustic Microscopy: Do You Know Its Capabilities? M. Urchulutegui
Introduction Characterization of materials usually requires microscopy techniques. Some of the most useful are based on a scanning microscope and involve scanning the sample surface with a focused beam (e.g., photons, electrons, ions, etc.). For example, photoacoustic microscopy uses a laser beam, acoustic microscopy uses an ultrasound beam, and scanning electron microscopy uses an electron beam. The interaction between the material and the beam produces a signal that can be used to generate a two-dimensional image. In scanning photoacoustic microscopy (SPAM),1-2 an intensity-modulated light beam is used to produce oscillations in the surface temperature of the sample. These oscillations induce changes in the pressure of a fluid in the photoacoustic cell as a consequence of the periodic heat conduction from the surface to the cell fluid. SuDsequently many materialcharacterization methods have employed the same philosophy as SPAM, using a modulated beam as an excitation probe. The breadth of such techniques is due to the large number of possible excitation 3 sources and signal detectors that have been proposed to probe the specimen response. In particular, scanning electron-acoustic microscopy (SEAM), also referred to as thermal wave microscopy, is a technique based on the utilization of a scanning electron microscope devel4 5 oped in 1980 ' and applied in recent years to material characterization. It can be considered an additional mode of scanning electron microscopy (SEM),
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which uses the generation of acoustic waves in the sample. Most reviews have concentrated on the application of SEAM to metals and semiconductors. However many other possibilities exist. A conventional scanning electron microscope can be converted into a scanning electron acoustic microscope by adding a beam-chopping system, a specimen stage that allows the detection of the signal and electronics for processing the output signal. This nondestructive technique allows the researcher to compare different scanning signals under the same experimental conditions. As the electron-acoustic signal depends on the experimental conditions of the scanning microscope and image contrast can be difficult to interpret, comparative studies are of great interest. For example, comparisons can be made between electron-acoustic images and secondary or backscattered electron images or cathodoluminescence images. Furthermore electron-acoustic microscopy has proven its ability to supplement information obtained by other techniques. This information depends on thermal, mechanical, and electronic properties of the material. It is a useful technique in the characterization of metals, semiconductors, ceramics, ferromagnetic and ferroelectric material and in the quality control of electronic devices. Scanning electron acoustic microscopy is based on a conversion of an electron6 beam-induced heat into sound. The related thermal-wave model based on the thermal-expansion mechanism was
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