FINITE ELEMENT ANALYSIS OF THERMAL CHARACTERISTICS OF MEM SWITCHES X. Yan, N. McGruer, G. Adams1, S. Majumder2 Electrical and Computer Engineering Department, Northeastern University, Boston, MA 02115 1 Mechanical Engineering, Northeastern University, Boston, MA 02115 2 Radant Technologies, Inc., Stow, MA The goal of this project is to understand the thermal characteristics of MEM switches. Excessive heating is known to limit their current-carrying capacity. Finite element models of the heating of a microswitch due to current flow are compared with experimental results. 1 2 3 a Side View Beam Gate Source Drain Contact Bump Beam Drain Top View A MEM Switch The free end of the beam overhangs above the gate. If a voltage is applied between the gate and the source, the beam is pulled down, the contact bump makes contact with the drain trace and a current can flow from drain to source. Close-up of Contact Region (The thin dark line in the circle is the contact bump. ) Contact Surface (1: Apparent contact area; 2: load bearing area; 3. Real contact area.) Finite Element Model of 1/4 Switch Modeled Contact Surface a = 0.1µm a = 0.5µm a = 0.2µm Red: highest temperature Blue: contact temperature Dash dot dot: a = 0.1 µm Dash: a = 0.2 µm Solid: a = 0.5µm Side View T = 300K V applied T = 300K V = 0V T = 300K Top View Meshed Model and Boundary Conditions Side View of Temperature Contours with Different Contact Radii Highest Temperature and Contact Temperature at Different Currents

(relative to contact center) Micrographs of drain (the lower contact), after removal of the cantilever beam. Light spots show changes in the gold drain metal due to contact between the drain and the upper contact on the cantilever beam. The area of contact increases at higher currents, indicating partial collapse of the contact asperities at high temperatures. I = 0.07A I = 0.21A I = 0.35A The graph on the left shows that the switch resistance increases with current. The black lines are experimental results. The colored lines are modeling results (from bottom up, the contact radius decreases from 0.3 µm to 0.2, 0.16, and finally 0.1 µm). Experimental Results Table 1. Comparison of experimental results and simulation predictions of drain metallization failure. Melting Point (relative to contact center) Melting Current Melting Voltage Simulation Results (From bottom up, a decreases from 0.3µm to 0.2, 0.16, and finally 0.1µm.) 0.45 V 3m - 5m Experiments 0.35 A 0.4 - 0.48 A 0.4m - 2.6m Simulations 0.45 V Summary Simulations show that a larger contact area results in the hottest spot being located further away from the contact center, as the larger contact spot presents a less constriction and that the highest temperature occurs in the thin film drain trace rather than at the contact interface. SEM pictures of drain traces verified that the drain melts at high currents. The model correctly predicts the switch voltage at which the drain trace melts, but underestimates the switch resistance, and therefore overestimates the failure current.