Thermal- and acoustic-wave techniques in scanning electron microscopy

Abstract

Since their introduction in 1980, thermal- and acoustic-wave techniques utilizing electron-beam excitation, denoted in the following as scanning electron acoustic microscopy (SEAM), have developed to include methods in the realm of scanning electron microscopy (SEM), giving additional and important information on material parameters compared with other SEM techniques. However, the SEAM method still has shortcomings, both theoretically and experimentally. New theories have to consider various principal sound-generation mechanisms, especially for semiconductors, ceramics, and ferromagnets. Furthermore, they must include three-dimensional and time-resolved calculations. From experimental evidence there is obviously the need for additional consideration of nonlinear signal generation. The theoretical discussion has to be supported by experiments; both phase analysis of the SEAM signal with respect to the electron-beam wave form and evaluation of the temporal SEAM behaviour are important for revealing information about the specimen. With special detectors, in situ experiments can be carried out for varying process parameters, as shown for the investigation of steel sheets. The SEAM performance has to be compared to other SEM modes by simultaneous experiments, especially for applications to semiconductors. Finally, extension to gigahertz frequencies and use of tomographic methods should increase the importance of SEAM in future.