VOLTAGE CONTRAST

Abstract
In recent years, a lot of research has been carried out into voltage contrast which is now widely used in both failure analysis and the debugging of innovative integrated circuits. The aim of this paper is to review past developments in this field and its present and future ability to keep pace with the progress in microelectronics. Since secondary electrons are involved in this phenomenon, a brief summary of their properties is drawn up. The two physical effects produced, i.e. the local field effects and the secondary energy shifts, are then examined. They give rise respectively to : i) the qualitative voltage contrast pictures easily seen inside any standard scanning electron microscope, and ii) the quantization of local voltages using the electron beam as an electrical probe where, however, a spectrometer is required. Then, dynamic voltage contrast is examined and the conditions for extending this method to normally working integrated circuits are considered. On the one hand, the resultant testing of conductors buried under insulator offers new and very important capacities. On the other hand, some limitations in the very high frequency domain are discussed. Electron beam testing must not, by definition, change the working conditions of the Integrated Circuit (IC). The physical conditions of the disruption of ICs by electron beam irradiations are presented. The use of the electron beam as an active probe is then discussed. Electron beam testing techniques and their implementation on nowadays ICs are then briefly reviewed. So, both stroboscopic observations and local voltage quantization are available today. Other probing techniques are discussed which can be reckoned on as complementary methods, especially in the case of very high speed circuits such as HEMTs. In conclusion, voltage contrast is now of a mature age, but the extension towards future microelectronics also presupposes an extension in the domain of in situ testing methods and techniques.