100 GHz wafer probes based on photoconductive sampling

The authors fabricated optoelectronic wafer probes with both free-space and fiber-optic input, and they adapted microwave error correction techniques to allow calibrated measurements with the new probes. Photoconductive switches on the probe tip define stimulus pulses and sampling intervals, and signals are transferred to and from the wafer under test by coplanar waveguide transmission lines and plated contact bumps. Vector error correction eliminates the need for time gating to separate the input and reflected pulses, while enhancing accuracy. Since probe flexure under contact significantly disturbs alignment of free-space beams, fiber-optic input yields the most precise measurements. The authors demonstrate calibrated, on-wafer measurements of the complex reflection coefficient S/sub 11/ at frequencies up to 100 GHz.<>