Image Sharpness Measurement in Scanning Electron Microscopy - Part I
Michael T. Postek, Andras Vladar
This study introduces the idea of the sharpness concept in relationship to the determination of scanning electron microscope (SEM) perfomance. Scanning electron microscopes are routinely used in many manufacturing environments. Fully automated or semiautomated SEMs as metrology instruments are used in semiconductor production and other forms of manufacturing where the ability to measure small features with the smallest possible errors is essential. It is felt that these automated instruments must be routinely capable of 5 nm (or better) resolution at or below 1 kV accelerating voltage for the measurement of nominal 0.18?35 ?m size parts of the integrated circuits. Testing and proving on a day-by-day basis that an instrument is performing well is not easy, but, understandably, is an industry need and concern. Furthermore, with the introduction of fully automated inspection and metrology instrumentation, not only does an appropriate, easy to obtain or manufacture sharpness measurement sample have to exist but also an objective and automated algorithm must be developed for its analysis. Both of these have been the object of a study at the National Institute of Standards and Technology (NIST), and the fundamentals are discussed in this paper and the computer-based automated analysis in a companion paper (Part II; Vladar et al. 1998). The method described in these papers is based on the analysis of the frequency domain representation of the SEM image and can also be used to check and optimize two basic parameters -- focus and astigmatism -- of the primary electron beam as related to a measure of image sharpness. The application of this technique to check regularly the resolution of the SEM in quantitative form will also be discussed.
astigmatism, electron beam diameter, focus, Fourier transform, scanning electron microscopy, sharpness