Figure 1: Typical unstable contact resistance response: increasing at low (~10 mV) contact voltage. UNDERSTANDING AND CONTROL OF UNSTABLE CONTACT RESISTANCE IN RF MEMS GOLD-GOLD DIRECT CONTACT SWITCHES Linda L.W. Chow* and Katsuo Kurabayashi University of Michigan, Ann Arbor, USA *Currently with Intel Corporation, Chandler, U.S.A. Email: Linda.l.chow@intel.com ABSTRACT The implementation of direct contact RF MEMS switches is challenging owing to their unstable contact resistance and low power handling/delivery. This paper carefully studies RF MEMS switch contact behavior and proposes a new method to suppress its instability leading to device failure. Our study supports the hypothesis that MEMS contact switches fail primarily due to contact necking. Using the proposed method, we demonstrate the ability to keep MEMS switch contact resistance under ~0.05 ! in high-cycle cold-switching while a high contact current of >0.9 A is handled/delivered. INTRODUCTION Direct contact radio frequency microelectromechanical systems (RF MEMS) switches have been envisioned as ideal building blocks for Radio-On-Chip development [1], due to its ultra wide-band (DC to 100 GHz), ultra-low insertion loss (< 0.1 dB), high isolation (< -40 dB), and high linearity [2]. The contact material is mostly pure gold-gold for low ultra-loss consideration and ease/low-cost of fabrication compared to the highly precise control of dopants. However, in commercialization, there are two critical reliability challenges: unstable contact resistance, R C , and low power handling/delivery. Extensive research has been performed to study the contact behavior of the switches during cyclic switching for various parameters such as contact force [3-4], contact materials [5-6], apparent contact area [7], contact current [8], and contact voltage, V C , [9-10]. Nevertheless, the mechanisms governing the unstable R c still remains unclear. As plotted in Fig. 1, R C generally rises at V C lower than the softening voltage, V Softening , at which a sudden drop of the contact resistance occurs. And as in Fig 2, the R C drops when the V C is higher than the V Softening due to contact softening [10]. In this paper, we systematically study the contact mechanisms driving the R C increase, and propose self- healing control circuitry to achieve stable R C even in high power handling/delivery. HYPOTHESES We first propose 3 hypotheses to account for the switch failure mechanism: Hypothesis A (Fig. 3) suggests that contact asperities elongate and undergo mechanical necking during switch cycling. Hypothesis B (Fig. 4) proposes that they encounter surface hardening with dislocations built-up from mechanical cold work. And hypothesis C (Fig. 5) advocates that the contaminant film (C-film) gets thickened as hydrocarbons accumulate from the atmosphere on the increasing defect sites and reduces the contact asperity area/radius. Figure 2: Typical unstable contact resistance response: reducing at high (~200 mV) contact voltages. And contacts eventually get stuck. 978-1-4244-5763-2/10/$26.00 ©2010 IEEE 771