The Practical Implementation of Voltage Contrast as a Diagnostic Tool

The various modes of voltage contrast, their basic theory of operation, and the techniques and equipment necessary for their practical implementation in a small multipurpose scanning electron microscope (SEM) are described in detail. The voltage contrast modes considered include: static, dynamic, quasistatic, clock stretching, synchronous, stroboscopic, and full quantitative. Details of SEM mounts and signal interconnection schemes both internal and external are presented. Techniques for mounting dual inline packages and ceramic chip carriers (C3) have proven particularly useful and are easily adaptable to most SEM types. The selection of the proper operating parameters necessary to achieve successful voltage contrast operation and micrographs is presented in detail. For example, investigations of dynamic range, voltage sensitivity, and the little described floating ground levels were performed in the case of static voltage contrast. Similarly, the effects of changing frequency and device ground level were explored for the synchronous voltage contrast modes. Experimental devices ranged from simple transistors where the bias and signal flows could easily be understood to more complex integrated circuit chips with their multiplicity of current paths and dynamic signal patterns. The extension of the theory, fixturing and operation to even more complex devices such as LSI and VLSI circuits is discussed.