Micro-Contacts Testing using a Micro-Force Sensor Compatible with Biological Systems

Abstract

This paper presents the performance and reliability testing of microelectromechanical systems (MEMS) switches by using a micro-force sensor which was originally designed/used to conduct mechanical testing of biological cells. MEMS switches are key components for radio frequency (RF) applications due to their extremely low power consumption and small geometries over conventional technologies. However, unstable electrical contact resistance severely degrades the performance and reliability of such micro-switches. Therefore, our focus is to improve the performance and reliability of "cold" switched micro-contacts by using novel contact materials and engineered micro-contact surfaces. The contact metallurgies considered in this work are "similar" thin film combinations of Au, and composite Au/CNT. The nonengineered switch consists of a metallic hemispherical bump and a planar sheet as upper and lower contacts, respectively. On the other hand, the engineered switches have 2D pyramid structure in lower contacts while having a hemispherical bump at upper contact. Hemisphere on planar, Au-Au, contact pairs resulted in initial contact resistance (R C ) values of ~0.1Ω (F C = 200µN) that linearly increased to ~1.0Ω after ~10×10 6 cycles and then failed open (~10.0Ω) at ~20×10 6 switching cycles. The AuAu/CNT composite, hemisphere on planar contact pair showed similar R C performance with extended reliability (~40×10 6 switching cycles) when the composite film was integrated into the lower planar contacted. Upper hemisphere on the 2D pyramid, Au-Au, contact pairs resulted in initial R C values of ~0.9Ω (F C = 200µN) that linearly decreased to ~0.5Ω at >10×10 6 cycles (not failed). This work suggests that the combination of engineered lower contacts and composite materials can significantly improve the performance and reliability of micro-switches.

Design Guideline for a Cantilever-Type MEMS Switch with High Contact Force

“…The problem is well understood by the MEMS community. The operation of microscopic contacts is thoroughly investigated using indenters [29][30][31], scanning probe microscopes [32,33], and homemade facilities [34][35][36][37][38]. It is demonstrated that the contact resistance rapidly decreases with increasing the force and then reaches saturation.…”

Novel Test Fixture for Characterizing MEMS Switch Microcontact Reliability and Performance

“…In addition, this test setup is capable of measuring micromachined membrane force versus deflection behavior, as well as sensing micro force exerted by biological cell movements. A force sensor integrated with a piezoelectric actuator can be used to apply known, calibrated forces in μN ranges onto the membrane for verifying its spring behavior [14,15].…”