The goal of this research is to make possible the accurate mapping of the high-frequency dielectric (or impedance) properties of materials ranging from device wafers and polymeric substrates (in-line process control) to biological materials, such as human skin, at multiple depths on a fine scale. The conductive coaxial needles will be configured on a fixed-grid array and be of lengths varying from ~50-250 microns. The resulting constructs also provide a new technology to affordably produce RF probes with small separations. The reduction of probe costs (from few thousand dollars per probe to few dollars per probe) has the potential to transform RF-test protocols and enable affordable wafer level probing at a fine scale. Local integration of the electronics provides a low-noise measurement solution, but requires significant miniaturization and therefore motivates a transmission line approach with minimum leakage and cross-talk.

Intellectual Merit - Existing methods for high-frequency material characterization will be refined in scale and sample density by the proposed microsystem. Unlike current techniques that provide single point measurement in the range of 100's of microns, the proposed MEMS-based approach will enable a large number of measurements (while enabling multi-point sampling via the fixed needle matrix). This research represents a new merging of MEMS and microwave-suitable sensing techniques for impedance measurements.

Broader Impacts - The technology addressed in this research will impact several areas of test, measurement and systems design ranging from materials characterization to detection of impurities in wafer scale processing. The probe architectures will be suitable for high frequency metrological characterization of micron-scale devices, such as emerging mm- and sub-mm-wave transistor technologies. The fabrication techniques for producing integrated micro coaxial transmission lines will facilitate the development of 3-D microwave and mm-wave systems for sensing and communications, while simultaneously integrating sensing and packaging functions of the material. Finally, the ability for fine-scale material characterization will aid research in many materials-related areas including nano-particle thin-films, lubricants, fuels and other fluids. The research provides new opportunities for research fellows in our active training programs (Bridge to Doctorate and Alfred P. Sloan Foundation Doctoral Fellowship Program). The research outcomes will also be integrated into a new graduate level sequence at the University of South Florida that forms the core curriculum for our training grants.

Project Report

This work presents the development of an in-plane vertical micro-coaxial probe using bulk micromachining technique for high frequency material characterization. The coaxial probe was fabricated in a silicon substrate by standard photolithography and a deep reactive ion etching (DRIE) technique. The through-hole structure in the form of a coaxial probe was etched and metalized with a diluted silver paste. A co-planar waveguide configuration was integrated with the design to characterize the probe. The electrical and RF characteristics of the coaxial probe were determined by simulating the probe design in Ansoft’s High Frequency Structure Simulator (HFSS). The reflection coefficient and transducer gain performance of the probe was measured up to 65 GHz using a vector network analyzer (VNA). The probe demonstrated excellent results over a wide frequency band, indicating its ability to integrate with millimeter wave packaging systems as well as characterize unknown materials at high frequencies. Intellectual Merit: As a result of using the vertical micro-coaxial probe to characterize materials at high frequencies and to transmit signals in millimeter wave packaging systems, the probe device demonstrated its excellent dual capabilities for a variety of future commercial application. Implementing it in high frequency multi-level packaging systems as interconnects/transitions, as well as integrating it with unknown materials to determine their permittivity, this work introduced a useful and cost effective silicon based two-fold application developed for the first time. By designing the micro-coaxial probe in a silicon substrate, the compatibility with other high frequency RF modules and such is highly increased. Broader Impact: The probe configuration is designed to suit high frequency metrological characterization of micron-scale devices. The fabrication techniques used for producing the micro coaxial transmission lines facilitate the development of 3-D microwave and mm-wave systems for sensing not just the microwave material but also the characteristics of biological tissues, soil, and food material.

National Science Foundation (NSF)
Division of Electrical, Communications and Cyber Systems (ECCS)
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Usha Varshney
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Florida International University
United States
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