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The growing packing density of integrated circuits calls, to an increasing extent, for the testing of the functioning of the individual circuits of ICs. If a mechanical prober is used for this purpose, the resulting capacitive loading of the circuit is liable to alter its performance. It is shown in the present work that the electron beam represents an ideal nonloading and nondestructive probe which can be finely focused and positioned on measuring points within the circuit under test. A modified scanning electron microscope allows the recording of waveforms within a circuit with a voltage resolution of the order of 10-100 mV and a time resolution of less than 1 ns. The efficiency of the technique was demonstrated by measuring the internally derived clock pulses and the voltage distributions of a digit-line of a 4096-bit MOS RAM and comparing the results with computer simulations.