A Probe Technology For 100+ GHz Integrated Circuits With Aluminum Pads

By Amr M. E. Safwat, Mike Andrews, Leonard Hayden, K. Reed Gleason, and Eric Strid
Cascade Microtech, Inc.

Abstract
Wafer probes for aluminum pads with 110 GHz performance and low repeatable contact resistance are demonstrated. These probes are based on an existing membrane probe card technology that routinely achieves less than 0.2 ohms contact resistance in production environments.

We demonstrate a ground-signal-ground (GSG) probe with less than 0.1 ohms contact resistance over 5000 contact cycles and less than 0.05 ohms variation during a 5-hour single-contact test interval, both on aluminum pads. 1.8 dB maximum attenuation and 12 dB minimum return losses at both the coaxial connector and the probe tips are achieved from 0 to 110 GHz.

Introduction
There is a growing interest in probing Si-based RFICs and test structures with aluminum pads. Probing RFICs with aluminum pads is significantly more difficult than probing ICs with gold pads. This is because a thin layer of aluminum oxide (about 60 angstroms thick) naturally forms on the aluminum surface that impairs the electrical contact between the tips of the probes and the aluminum pads. Any measurement that is sensitive to a series resistance will be affected by contact resistance variations. Such measurements include inductor Q measurements and long characterization tests that require repeatable device contact for time periods beyond a few minutes.

Conventional probes use tungsten tips to penetrate the aluminum oxide layer. Unfortunately, the tungsten itself also oxidizes and the aluminum oxide, after only a few contacts, accumulates on the tips of the probes, significantly increasing the contact resistance and hence resulting in measurement deterioration. As a remedy, frequent cleaning of the probe tips is obligatory. Worse, the operator is usually unsure of the precision of the measurements, due to the nonrepeatable nature of the contact resistance.

Recently probe card developments based on polyimide membrane technology showed superior performance on aluminum pads. In this paper, and based on this technology, we present a new single-port RF probe for aluminum pads that has a superior performance compared to conventional probes.

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