Study on voltage contrast in SEM

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Abstract
Voltage contrast in the SEM is a contactless technique for the study of potentials and potential distributions. Its applications include the examination of static and dynamic voltage signals on semiconductor devices, the direct observation of grain boundaries on polycrystalline semiconductors, and the detection of acoustic wave signals on a piezoelectric material. The quality of a recorded micrograph and the minimum detectable voltage are both limited by the noise level associated with the electron–beam and the detector system. In general, the electron–beam current must be increased to apoint at which the noise level in the collected signal is acceptable. A relationship between the minimum detectable voltage and the beam current has been established for any specific recording condition. The theory assumes a Maxwellian energy distribution of the secondary electrons. Furthermore, the collector system is assumed to act like a simple energy filter which accepts only secondary electrons having an energy higher than Ef. The theoretical results also indicate that the highest detection sensitivity can be achieved when the collector system is designed so tht Ef is around 2.6 eV for an ’’average’’ metal. A relationship between the electron beam current and the expanded SNR caused by a voltage difference has been obtained experimentally. There was a satisfactory agreement between theoretical predictions and experimental results. The experiment was conducted in an SEM which is equipped with a standard scintillator–photomultiplier detector.

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