Demonstration of non-invasive probing of CMOS devices with aluminum pads at frequencies up to 500 GHz

This paper demonstrates non-invasive probing measurement of transmission lines on CMOS chips from 100 MHz to 500 GHz. The surface of aluminum pads are covered with a natural oxide film, which usually needs to be penetrated by probe tips through extended skating. In this work, the oxide film is kept intact by reducing probe skating down to $10 \mu m$ using a precision-controlled probe station. This, in turn, allowed the use of extremely small $20 \mu m\times 15\mu m, 25-\mu m-$ pitch pads. The oxide film did not show significant resistance variations even after repeated probe touchdowns that would normally have worn out the pads. Stability of the measurement was investigated by comparing measured propagation constant in a wide frequency range, covering 1-mm coax, WR6, WR3, and WR2 bands. The propagation constant turned out to be continuous even at band crossings. The non-invasive probing could be particularly useful for characterizing CMOS passive devices, which do not require DC biasing.