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Backside probing of flip-chip circuits using electrostatic force sampling

We present a noncontact probing technique for measurement of high-frequency voltage waveforms from the backside of a flip-chip mounted integrated circuit. The signals are accessed by mechanical thinning and focused ion beam milling through the backside. Internal circuit voltages are measured by sensing the local electrostatic force on a small micromachined probe held in close proximity to the circuit measurement point. The instrument currently has a 3 GHz bandwidth and a capacitive loading on the test point of less than 1 fF. The output waveforms from ring oscillator flip-chip test circuits are presented.