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Although Scanning Electron Microscopes (SEM) have improved greatly over the last decade the techniques usually employed to measure their performance have not changed significantly in half a century. In particular, describing the imaging performance of an SEM by a single number - its 'resolution' - provides no useful information about its real world imaging capabilities nor about any of the factors that might limit that usefulness of the SEM for tasks such as metrology. The Contrast Transfer Function (CTF) discussed here analyses the way in which the SEM processes signal components of different spatial frequencies. The resultant plot provides information on the noise limited spatial resolution limit, predicts how this will vary with noise level, and provides a powerful general diagnostic capability. This type of measurement, which has become standard practice for transmission electron microscopes, can be performed using the public domain software package IMAGE-J, is rapid, and requires only a specimen offering a broad and flat Fourier spectrum. The capabilities of this approach are demonstrated by a number of examples.

The physical dimensions of nanoscale objects are an important indicator of their functionality. However, measuring feature size from a SEM image is difficult not only because of fundamental considerations, such as the nature of beam interactions and the information transfer properties of the microscope, but because the magnification of the SEM image from which a measurement will be made is completely uncalibrated and additionally is subject to local distortions and variations. Nano-gauges fabricated by electron beam lithography - one or two dimensional structures on the size scale of the objects of interest - provide a local length standards within the image field from which the relative size of features can be accurately determined. In order to provide an absolute measurement of size the dimensions of the nano-gauge structure must themselves be calibrated against some primary standard. Because there are no convenient standards of appropriate scale available we propose that this can be done using a moire fringe technique to bridge the gap between the nanoscale and common length standards such as ruled diffraction gratings.

A scanning ion microscope (SIM) is analogous to a scanning electron microscope (SEM) but utilizes a beam of helium ions, with energy of 10 to 25 keV , instead of electrons. The SIM potentially offers several advantages for device critical dimension metrology as compared to the more familiar CD-SEM. These include a high brightness source which is sub-nanometer in size, an enhanced secondary electron yield, restricted beam penetration, and superior image contrast and information content. Possible problems include pervasive positive charging, ion implantation, and a lack of detailed experimental and theoretical knowledge about low energy ion interactions with solids. Comparison of line profiles across structures made by electron induced and ion induced secondary electrons show that there are some significant differences between them which arise from the different modes of interaction in the two cases. As a result the algorithms employed for line width determination will require revision in order to produce data which is consistent with CD-SEM data.