Investigation of Wear of Microelectromechanical Contacts
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ABSTRACT A test system has been assembled to examine factors controlling the reliability of microdevice switching. The system consists of a spherically tipped sputter-coated post driven by a piezoresistive actuator into a sputter-coated micromechanical membrane. Contact forces are calculated based upon the measured deflection of the micromechanical membrane, which is induced by post actuation. Membrane deflection is measured using white light interferometry. The known geometry of the membrane and probe tip permits the estimation of contact area and, hence, contact pressure. Preliminary testing of sputtered gold contacts resulted in localized melting, material transport to the membrane and failure of contact surfaces at relatively low contact pressures and current densities. Further testing will examine the factors governing the reliability of microdevice contacts, such as material properties, contact pressure and current density.
INTRODUCTION Many microdevice applications operate through the use of contacting layers or surfaces. The requirement for contact depends upon the particular microdevice. For applications such as microrelays or switches, contact provides signal transmission. For other devices such as optical switches or reflective and projection displays, contact provides a positive stop or detent for positioning part of the mechanical structure of the microdevice[1,2]. The need for positive stops for these applications is a consequence of the fundamental imprecision of microdevice manufacturing
and positioning; although small, microdevice dimensional tollerances (>5%) do not possess the same relative precision as larger manufactured products ( 10 V) as well as large current flows (> 30 mV). Common macro-relay failure modes involve spark erosion, corrosion and fretting. There is no reason to expect these phenomena to be less important on a microscopic scale. Indeed, given the sensitivity of microdevices to surface phenomena, there are good reasons to expect them to be significantly more important. The reliability of the mechanical contact remains an additional uncertainty inhibiting the commercial development of these microdevices. Many microdevices operate under high contact frequencies and therefore must be designed to reliably accumulate large numbers of cycles. Flat panel displays will contact at frequencies of at least 60 Hz, and optical switches will operate at 9 frequencies exceeding 100 kHz [1]. Over 5 years these frequencies correlate to more than 2 x 10 to 3 x 1012 accumulated contact cycles. These are substantially more contact cycles than are typically tested or expected for macroscopic contact applications. This effort addresses the development of test structures that will permit the exploration of fundamental factors controlling contact wear, including microrelays that conduct current.
117 Mat. Res. Soc. Symp. Proc. Vol. 605 ©2000 Materials Research Society
EXPERIMENT Several criteria were considered necessary for the development of an appropriate test structure. These included the follow
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