Metrics of resolution and performance for CD-SEMs

The performance of scanning electron beam instruments such as CD-SEMs can be defined in terms of parameters such as the beam probe size, the spatial resolution, and the signal to noise ratio of the image. A knowledge of these quantities is important in verifying the fact that an instrument meets its specification, and subsequently for tracking and optimizing its performance during use. Analytical methods based on the power spectrum (2-D Fourier transform analysis) of images are now beginning to be used for these purposes but care must be exercised to ensure reliable and meaningful results. Two new methods are suggested which can offer more detailed information about the microscope performance while avoiding the pitfalls of the simpler technique. Code implementing these tests, written as a plug-in macro for the well known NIH Image program, is available on-line.