1. 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

2. (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 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.