Anelastic Creep Phenomena in Thin Metal Plated Cantilevers for MEMS
- PDF / 4,697,087 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 101 Downloads / 206 Views
Anelastic Creep Phenomena in Thin Metal Plated Cantilevers for MEMS Deborah J. Vickers-Kirby, Randall L. Kubena, Frederic P. Stratton, Richard J. Joyce, David T. Chang and Jinsoo Kim Sensors and Materials Laboratory, HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu, CA 90265, USA ABSTRACT During the past several years, we have developed high displacement sensitivity tunneling accelerometers using surface micromachining and metal electroplating techniques. These devices consist of a Au tunneling tip fabricated below a 1-2 µm thick metal cantilever beam of electroplated Ni or Au. A thin film of e-beam evaporated Au on the underside of the cantilever serves as the tunneling counter electrode. In operation, a 100mV bias is applied across the tunneling gap. A larger turn-on voltage is also applied between the cantilever and a control electrode, located on the substrate, to deflect the cantilever and maintain a constant tunneling current of 1 or 10 nA. Typical deflections of the end of 100 µm-long and 250 µm-long cantilevers are 0.5µm during operation. We have observed that the turn-on voltage decreases over time for most devices with a larger drop observed for the Au cantilevers. In all cases, the initial decay of the turn-on voltage was almost completely recoverable after the device was turned off for 24 hrs. This decay was not found to be strongly dependent on the magnitude of the tunneling current, but could be significantly reduced by pre-stressing the cantilever before operation. Finally, a vacuum anneal at 100 0 C influences the measured temperature dependence of the turn-on voltage. The observed effects appear to be consistent with fatigue and creep phenomena in the cantilevers. These effects are reversible at room temperature and are dependent on the stress and temperature history of the devices. A comparison is made between metal plated and all-Si structures. INTRODUCTION Fatigue or creep of thin metal layers used in microelectromechanical (MEMS) structures can have a significant effect on the observed device bias stability [1]. For many practical applications of MEMS devices, one requires a repeatable, known bias stability over both time and temperature, to enable calibration and compensation for environmental effects. The MEMS tunneling device described in this paper provides a unique vehicle for observation of the thin metal fatigue effects, in that it offers high sensitivity to displacements affecting the tunneling gap. A 1Å change in the tunneling gap produced by residual material stresses, or by fatigue or creep effects, results in roughly a factor of 2 times change in the observed tunneling current. In this study, we examined the bias stability of MEMS tunneling sensors comprised of 100 µm-long and 250 µm-long thin metal plated cantilever over a 1.5 µm tall tunneling tip [2]. The beam is deflected electrostatically by a control electrode that is located under the beam, which pulls the beam down into the tunneling position. Feedback control is then is used to maintain the effective 10Å tunneling gap.
Data Loading...
