Noninvasive probing of high frequency signal in integrated circuits using electrostatic force microscope

Abstract

We report a noninvasive high speed voltage sampling technique by using atomic force microscope. This technique is based on the mixing of interaction forces between a conductive probe-tip and a device interconnect line. Mixing phenomena allow measurements of signals with frequencies far above the mechanical resonance frequency of the cantilever. To increase the sensitivity and stability of the signal detection in ambient environment, we employed a delayed pulse sampling and lock-in amplifier detection of the small electrostatic force resulting from a high frequency signal on integrated circuits. The probing pulse is first modulated by a low frequency sinusoidal voltage using a double-balanced diode mixer. The junction between probe-tip and integrated circuit interconnection line works as the second mixer which mixes the probing voltage with the high speed waveform in the interconnection line. This technique can sense voltage signals even through a passivation layer of a 0.5 μm in thickness. We sampled an 8-bit vector signal repeating at 50 MHz through a passivation layer by using this technique.