Broadband optoelectronic wafer probing

Optoelectronic S-parameter measurements offer the bandwidth needed to characterize today´s state-of-the-art transistors, but have not yet achieved the throughput or accuracy provided by a vector network analyzer with microwave probes. In this paper, we discuss a new approach in which movable optoelectronic probes, calibrated by testing simple standard devices, are stepped around the wafer to provide accurate, high-throughput S-parameter measurements. Sub-picosecond laser pulses drive photoconductive switches on the probe tips, to generate electrical stimulus pulses and define sampling intervals, and signals are transferred to and from the wafer under test by coplanar waveguide transmission lines and plated contact bumps. The probes provide electrical pulses as short as 3 psec (FWHM), while maintaining a broadband 50 ohm termination to ensure stability of the device under test. Since probe flexure under contact significantly disturbs alignment of free-space beams, fiber-optic input is used to improve reproducibility. Analysis by vector error correction in the frequency domain removes systematic errors and separates the incident and reflected pulses without subjective time-window gating. We have demonstrated precise measurement of the complex reflection coefficient S ₁₁ at frequencies up to 175 GHz. Noise simulations have been performed to investigate the effect of various system parameters on the measurement uncertainty and useful bandwidth for S-parameter tests