Gallium-arsenide integrated circuit testing using electrooptic sampling

Gallium arsenide (GaAs) integrated circuits (IC's) and other III-IV compound semiconductors have demonstrated operation speeds greater than the time resolution of conventional test instruments used for their characterization. In addition, digital GaAs IC's such as static frequency dividers have demonstrated clock rates greater than 20 GHz, frequencies where circuit models are poorly refined and influenced by layout-dependent parasitics. Conventional test instruments using contact probes are limited to monitoring input and output test points of these circuits. Probing at internal nodes, however, would greatly ease analysis of the circuit's operation.;To address these issues, an electrooptic sampling system has been developed at Stanford to measure internal-node signals in GaAs IC's with picosecond time resolution. Infrared light is focused through the GaAs IC substrate to detect voltages via the electrooptic effect, and a laser system generating ultrashort light pulses repetitively samples the signal to achieve picosecond time resolution. To obtain realistic testing conditions, a microwave synthesizer for providing excitation to the IC is synchronized to the repetition rate of the optical pulses, allowing equivalent-time sampling of the IC response.;The electrooptic sampling system and its performance characteristics for IC testing are described, and a number of test results are presented. The system has 2 picosecond time resolution or a corresponding bandwidth greater than 100 gigahertz, and a voltage sensitivity of 70 microvolts per root Hertz. On digital IC's, propagation delays and switching signals of digital test circuits such as inverter chains, frequency dividers, and multiplexers have been measured. On microwave IC's, small-signal and large-signal response of microwave amplifiers have been measured. A technique to measure S-parameters of a circuit using the optical probe has been developed, allowing the measurement reference plane to be positioned on the IC and eliminating the need for physical calibration standards.