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Though the idea of scanning electron microscopy (SEM) was born in Berlin by M. Knoll in 1935, and by M. von Ardenne for the transmission (STEM) mode in 1938, the first commercial scanning electron microscopes based on the work of C. W. Oatley and coworkers at Cambridge University started in 1965. Since that time SEM has grown to an established technique of surface imaging. The main advantages are a resolution of 1-10 nm, the large depth of focus, and the numerous types of electron-specimen interactions that can be used in imaging and analyzing modes. The range of applications covers materials science, biology and medicine, industrial research, semiconductor inspection including electron-beam lithography and metrology.
The early development of SEM at Cambridge in the late 1960's and early 1970's was done using low accelerating voltages. However the decreasing gun brightness of thermionic cathodes with decreasing voltage demanded to work in conventional SEM with acceleration voltages in the range 5-50 keV. The development of commercial field emission guns started by Coates and Welter in 1970 and of Schottky emission guns in the last ten years allows us to operate in the low-voltage range of 0.5-5 keV in a useful, reproducible manner. Low-voltage scanning electron microscopy (LVSEM) has the main advantage of a lower electron range, with the information more concentrated on thin surface layers. The charging of insulating specimens can be avoided in many cases. Inspection of semiconductor devices in the production line and for control also demands low electron energies to avoid radiation damage effects.
A correct understanding and interpretation of images and analyses requires a good knowledge of the electron-specimen interactions causing and influencing the signal intensities. These interactions in LVSEM are often quire different from those in conventional SEM. Textbooks about SEM primarily cover the high-voltage range from 5-50 keV, and the special problems to be considered in LVSEM are spread among journals and conference proceedings. It is the aim of this tutorial to outline and summarize these differences.
A review of the most important imaging and analyzing electron-beam instruments and methods is presented in the Introduction together with a summary of the advantages of LVSEM and of electron-specimen interactions. Chapter 2, on electron optics and instrumentation, demonstrates the influence of lens aberrations and design on electron-probe formation and discusses the special detector systems and strategies necessary to make the best use of electron-specimen interaction. The elastic and inelastic scattering processes described in Chapter 3 are responsible for the electron diffusion, and influence the electron range, depth of information, and emission of secondary (SE) and backscatter (BSE) electrons. The dependence of SE yield and the backscattering coefficient on electron energy, surface tilt and material, as well as the angular and energy distributions, are summarized in Chapter 4. Charging and radiation damage effects discussed in Chapter 5 can cause imaging artifacts, but the latter can also be used for electron-beam lithography. Chapter 6 describes the different types of image contrast and the differences from the conventional SEM modes due to the influence of the electron-specimen interactions discussed before. The use of low-energy electrons offers a more effective application of electron-spectroscopic methods by energy and electron-spin analysis, which need in all cases an ultrahigh-vacuum specimen chamber and are mentioned in Chapter 7. The tutorial is completed by an extensive bibliography that offers a good base for further studies.
Acknowledgements
A large part of the results, diagrams, and images in this book were measured and calculated by research students from our laboratory during the last few years. I offer my appreciation to R. Bongeler, U. Golla, M. Kassens, B. Schindler, and R. Senkel, and to K. Brinckmann and M. Silder for the artwork of this book.
L. Reimer
February 1993
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