High-frequency internal IC signal sampling using electrostatic force microscopy

An Electrostatic Force Microscopy technique for non-contact internal function and failure analysis of high-speed integrated circuits is described. Internal circuit voltages are measured by sensing the local electrostatic force on a small micromachined probe that is held in close proximity to the circuit test point. The force acting on the tip of the probe varies with the square of the voltage difference between the tip and the circuit test point. Using this square law interaction, an amplitude modulation based frequency downconversion technique is employed to measure high frequency signals that can have a bandwidth much greater than the mechanical response of the probe. By applying an ultrafast electrical sampling pulse to the probe, it is able to sample up to 10 Gbit/s (OC-192) yielding voltage waveform and timing information. A force technique is applied to obtain absolute voltages without the need for complex calibration or precise probe positioning. Measurements on a microstrip thru-line are presented to demonstrate the capability of the probe. A probe position control system, based on controlling the capacitance gradient, is used to improve the probe spatial resolution and voltage sensitivity.