Measurement of local potentials using the scanning electron microscope, and its application in the failure analysis of micro-electronic circuits
This thesis describes the development of an electron beam test system based on a conventional scanning electron microscope (SEM), and its application for the determination of voltage and timing information for detailed local study of individual components within micro-electronic circuits. The project showed how the SEM may be operated at low voltages (less than 5kV), to maximise Secondary Electron (SB) emissions which carry the voltage information and control both the primary electron penetration and energy deposition within a specimen. The conventional SE imaging system was used to display voltage distribution over a relatively large area of any specimen. However, only qualitative measurements are possible, as the detector is not able to discriminate between changes of SE yield and energy distribution, and cannot suppress localised field effects which alter the SE emission's energy profile and angular distribution, causing measurement error. To achieve quantitative voltage measurements, SE emission was sensed using the Voltage Measurement Electron Controller (VMEC). This additional equipment has several electrodes, each generating electrostatic fields, one of which acts as a SE energy filter detecting changes to their energy distribution as the surface potential alters. The electrostatic field generated by the electrode nearest the specimen increases the collection efficiency of the detector, and suppresses the action of the local fields (which alter the energy and angular distribution of the emitted SB's). These effects were investigated. Extra equipment was then interfaced to the SBM, including a set of beam blanking plates and their control unit, for the rapid switching of the primary beam, a signal processing unit for noise reduction, image capture and image subtraction, and data analysis. The analytical capability of this equipment was demonstrated via its ability to display and measure the voltage levels of various local sites across a simple quad NAND gate device. The EBT system which had been developed was used for the analysis of two commercial micro-electronic products, making both voltage and timing measurements from metal conductor tracks with widths as narrow as 1.5 micrometres.
Scanning electron microscopy (SEM), Micro-electronic circuits, Electron beam test system
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