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The great advantage of the atomic force microscope and related technologies (tunneling current, near field optical, tips that oscillate vertically or laterally, etc.) has been simplicity of design. Vertical deflection of a reference tip on a cantilevered probe is detected by light scattering and used to shift the sample in the z direction to restore tip position. The output is generally in the form of surface images rather than measurements with high dimensional accuracy. Atomic scale resolution is achievable but lower magnification imaging is more difficult.Piezo devices used for x-y and z axis control have a limited range of motion and are noisy, nonlinear and subject to creep. Piezo displacement is not simply a function of voltage, or of the time integral of current, but depends upon many factors including rate and distance of motion, and internal resistance and capacitance in the device. It is possible in principle to calibrate such a system using a reference grid standard, but even this is highly dependent on scanning speed (different in the x, y, z directions) and will not provide quantitative z information.

Solid state lithium-diffused silicon detectors are commonly used as energy-dispersive X- ray detectors. These devices will, however, produce electrical pulses with a height proportional to the energy of electrons (and other particles) as well. We have used a Si(Li) detector with conventional electronics (amplifier, multi-channel pulse-height analyzer) to measure the energy spectrum of electrons backscattering from pure elements with atomic numbers from 12 (Mg) to 92(U).Since the electron beam striking the target also generates X-rays (characteristic and continuum) that are measured by the detector along with the backscattered electrons, and since it is not possible to stop the X-rays from reaching the detector while allowing the electrons to pass, we measured the electron spectrum by difference. Figure 1 shows superimposed, the spectra (for a copper target) of X-rays plus electrons (upper curve) and X-rays only (lower curve), the latter obtained with the use of a magnetic electron trap in front of the detector.

For the high percentage of scanning electron microscopes equipped with x-ray analytical capability, a common mode of operation is the x-ray “dot image” to describe the distribution on the specimen surface of an element of interest. As with the conventional microprobe, this is done by displaying a pulse or dot on the display tube whenever an x-ray with an energy (or wavelength) corresponding to the element of interest is detected. By photographically integrating many of these dots, an image that correlates with other specimen images (e.g. backscattered or secondary electrons) is produced. When very many dots are recorded, the image may take on a continuous-tone appearance but, in general, it is possible to make only very rough qualitative estimates of relative intensity in different areas because the human eye does a very poor job of estimating dot density. Edges or other sudden changes are more easily recognized than gradual variations.