Low-temperature, contactless radio-frequency (RF) probing systems are necessary for characterizing sensors operating at liquid nitrogen or helium temperatures, and based on superconducting materials. The design and operation of the contactless RF probing systems relies on strong electromagnetic coupling that takes place between two different microwave transmission lines oriented in close proximity, but not in contact with each other, to ensure high thermal isolation. The goal of this work is to develop a reliable, easily constructed, less expensive, contactless RF probe for characterizing microwave integrated circuits (MICs) and devices embedded in sensors fabricated on conformal or non-planar substrates, at elevated or cryogenic temperatures.

The design and construction of the Contactless RF Probes with inverted microstrip lines. The inset illustrates an alternate embodiment of the inverted microstrip line with non-uniform strip conductor.
At elevated temperatures typically encountered in aircraft engine sensor development, the characteristics of the probe pads degrade, and the physical contact with the probe becomes less reliable. Replacement probes are expensive, and their procurement is time-consuming. Contactless RF probes capable of acquiring the S-parameters of miniature MICs and devices at elevated and cryogenic temperatures can overcome the limitations of conventional RF probes that require planarity to make good physical contact.

The simplest contactless RF probes are constructed from inverted microstrip lines fabricated on a dielectric substrate. Coaxial connectors are attached at one end of the microstrip lines to interface the probes with a precision network analyzer (PNA) for RF measurements. The opposite ends of the microstrip lines are terminated in an open circuit. Mounting holes are also provided in the dielectric substrate for attaching the probes to the x-y-z axis positioners located on a probe station. The distance traveled by the probes along each axis is precisely controlled by micrometers. The x- and y-axis micrometers control the longitudinal and transverse movement of the probes and enable locating the probes parallel and colinear with the input and output microstrip lines of the device under test (DUT). In addition, to assist the visual alignment of the probes, alignment marks are provided on the DUT substrate. The z-axis micrometer controls the distance of separation or the gap between the inverted microstrip line and the microstrip line of the DUT, which determines the coupling. To enhance coupling between the microstrip lines, the probes are lowered such that the gap dimension is small and on the order of a few tens of mils. DC bias pads are provided on the DUT substrate for applying a bias voltage to the active devices.

The probe design and operation is based on the RF characteristics of broadside coupled microstrip lines supported on two separate dielectric substrates. The probes are fabricated using well established, low-cost, step-and-repeat photolithography techniques.

The contactless RF probing system relies on proximity coupling between the probe transmission line and the input/output transmission lines of the device or circuit under test. A separation on the order of 0.05 in. (≈1.3 mm) between the probe and the device transmission lines is estimated to be adequate for excellent thermal isolation as well as strong coupling of RF signals in and out.

The contactless nature of the probes allows characterizing MICs and devices at elevated temperatures without damaging either the DUT or the probes. In addition, mounting the probes inside a vacuum chamber allows for characterization at cryogenic temperatures.

This work was done by Jennifer L. Jordan and Rainee N. Simons of Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-19148-1.