Voltage Measurement in the Scanning Electron Microscope

This chapter discusses the mechanism of voltage contrast, describes the contrast linearization schemes for measuring voltage in the scanning electron microscope (SEM), and outlines the techniques used for recording periodic waveforms at the higher frequencies using the sampling methods. Voltage contrast in the SEM is widely used for the semiconductor device and integrated-circuit investigations. This contrast is the basis of currently available electron-beam testing systems. The contrast signal is small compared to the video signal, and its magnitude varies nonlinearly with the specimen potential. By means of electron energy analysis and suitable circuitry, the contrast is linearized and the voltage changes, as low as 0.5 mV, are measured. Some schemes of linearization include the floating-specimen feedback scheme and the retarding-potential analyzer feedback scheme. The minimum measurable voltage is dependent on the noise in the measurement signal, the specimen, and the type of analyzer used. The three types of analyzers used in the voltage measurement schemes are the retarding-potential analyzer, the cylindrical-sectoral analyzer, and the cylindrical-mirror analyzer.