Backside probing of flip–chip circuits using electrostatic force sampling

A non-contact scanning probe microscopy technique for measuring high-frequency voltage waveforms from the backside of a flip-chip mounted integrated circuit is presented. The signals on interconnects on the frontside of the die are accessed by mechanical thinning and focused ion beam milling through the backside of the die. A scanning force microscopy micromachined probe is placed inside the focused ion beam hole so that it is in close proximity to the circuit measurement point. Internal circuit voltage waveforms are measured by using the scanning probe in a non-contact mode and sensing the local electrostatic force on the tip of the probe. 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 measured.