Design and Fabrication of a Novel Microcontact Support Structure for Efficient Contact Surface Post-Mortem Characterization

P. Mahanta, F. Anwar, R.A. Coutu Jr.
Marquette University,
United States

Keywords: microcontact, reliability, microswitch

Summary:

Microelectromechanical systems (MEMS) ohmic switches have been widely investigated for cell phone, satellite, and radar systems as well as in automated test equipment (ATE) applications due to their low contact resistance (~1Ω), near-zero power consumption (~0W), very low insertion loss (~0.1dB@25 GHz), and high isolation (~35dB@10 GHz) [1]. Reliability associated with them is a matter of great concern for the applications where billions of lifecycles is a requirement and limits their widespread use by the industry. Microcontact surface tribology plays a critical role in determining their reliability and performance. In the past, actual MEMS devices, modified nanoindentors, scanning probe microscopes (SPM), and atomic force microscopes (AFM) have been used to study microcontact reliability and performance [2]. However, their performance regarding data collection is limited by very low cycle rates (10-100 Hz), atmospheric contamination, and complex contact surface post-mortem characterization [3]. Therefore, a fast, simple, and efficient test fixture is required to study the microcontact’s electrical, thermal, mechanical as well as chemical properties. In this work, a unique microcontact support structure has been fabricated using silicon on insulator (SOI) micromachining technique to facilitate the simultaneous measurements of contact resistance and contact force as well as simple and efficient contact surface post-mortem analysis associated with the microcontact. We fabricate fixed-fixed silicon (Si) beams with lengths varied from 350 µm to 500 µm, width 100 µm, and thickness about ~5 µm. A series of hemispherical contact bumps, ranging in size from 3-8 μm in diameter are formed on top of the beam by partial heating reflow at high temperature. Afterwards, a 300 nm gold (Au) metal contact layer with a 10 nm titanium (Ti) adhesion layer is sputtered deposited onto Si beam. This sputtered gold (Au) metal layer with Si is used as upper moveable contact. A cavity is formed on the backside of Si handle wafer using deep reactive ion etching (DRIE) process to position the force sensor on to the beam. This upper contact structure has been assembled with a bottom contact structure through a set of electroplated gold (Au) pillars. After a desired number of cycling operations, the microcontacts are separated to evaluate the contact surface wear through Scanning Electron Microscopy (SEM), X-ray photoelectron Spectroscopy (XPS), Auger, micro Raman, etc. In addition, the microcontact support structure will be used to characterize performance and reliability associated with wide range of other potential contact materials with engineered contact geometries that will facilitate reliable, robust microswitch designs for future direct current (DC) and radio frequency (RF) applications.