Abstract

We have developed and characterized a novel, amplified, high-impedance circuit probe which utilizes a polyimide film as the substrate for a probe tip and photoconductive sampling gate. While the utility of freely-positionable photoconductive probes has been substantiated for both high- and low-impedance testing of electronics¹-³, the invasiveness (in terms of both field disturbance and charge extracted), mechanical flexibility, and sensitivity at low light levels limit the usefulness of instruments based on these devices. The probe described in this work has many advantages over conventional, unamplified probes using semiconductor substrates. This is due to the low permittivity of the strong, but flexible, polyimide film employed, the optical transparency of the polyimide, and the low capacitance and high resistance of the JFET source follower connected to the output of the sampling gate. For instance, while still maintaining a single-picosecond temporal resolution, the amplified polyimide probe disturbs the fields surrounding a circuit less than a typical probe, while providing two orders of magnitude improvement in sensitivity and the ability to convert current to voltage (and thus measure absolute voltages). In addition, problems associated with extracting excessive amounts of charge from a circuit, as well as with optical heating and scattering of the probe light, are reduced because only very small laser intensities are required to gate the signal being measured.

© 1996 Optical Society of America