On the Evolution of Surface Morphology of Polysilicon Mems Structures During Fatigue
- PDF / 1,099,157 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 51 Downloads / 174 Views
ON THE EVOLUTION OF SURFACE MORPHOLOGY OF POLYSILICON MEMS STRUCTURES DURING FATIGUE S. M. ALLAMEH+, B. GALLYX, S. BROWNX, W.O. SOBOYEJO+ + The Princeton Materials Institute and The Department of Mechanical and Aerospace Engineering, Princeton University, 1 Olden Street, Princeton, NJ 08544 x Exponent Failure Analysis and Associates, 21 Strathmore Road, Natick, MA 01760
ABSTRACT This paper presents the results of an atomic force microscopy (AFM) study of the evolution of surface topology in notched polysilicon MEMS structures deformed under cyclic loading at room temperature. The in-situ and ex-situ AFM studies reveal changes in surface topology after cyclic actuation at a relative humidity of ~70%. These lead ultimately to large wavelength modulations close to the bottom of the notch, in the areas where the tensile stresses are maximum. This is in contrast with the wavelength of the surface modulations away from the notch, which remain relatively unchanged. The results are discussed in terms of possible chemical/surface processes that can occur in the presence of water vapor. INTRODUCTION In recent years, Micro-Electro-Mechanical Systems (MEMS) fabricated from polysilicon have been used extensively in a wide range of applications [1-8]. These include applications ranging from accelerometers to biological sensors [1-3]. Since fatigue damage can initiate and evolve under cyclic loading conditions, it has become important to develop a basic understanding of how cracks initiate and grow in polysilicon MEMS structures under cyclic loading conditions. Prior work on the fatigue of polysilicon has been done by Brown et al. [4-7] and Heuer et al. [8]. Recent work by Van Arsdell and Brown [9] has also shown that the fatigue of polysilicon is significantly affected by the presence of moisture. Hence, the fatigue of polysilicon appears be environmentally-assisted. The present study examines the possible evolution of surface topology that can occur in polysilicon MEMS structures during cyclic actuation. The evolution of surface topology is studied using in-situ AFM techniques. The observed evolution in surface topology is examined within the context of chemical surface processes and crack nucleation. MATERIAL The polysilicon MEMS structures that were used in this study, were supplied by Cronos Integrated Microsystems (formerly MCNC) of Raleigh-Durham, NC. The MEMS structures were fabricated in batch runs at Cronos. Details of the micromachining processing schemes are given in Ref. [2]. The polysilicon MEMS device that consists of a capacitive comb-drive attached to the end of a notched sample is presented in Fig.1. Capacitively-induced forces generated in the inter-digitating comb section of the cantilever are applied to the notched or un-notched constrained specimens within an area of ~ 10 µm x 20 µm. The devices were sealed under a topical SiO2 layer that was removed before actuation. The release process consisted of rinsing in acetone, dissolving topical SiO2 in concentrated (49.6%) hydrofluoric acid, rinsing in distilled w
Data Loading...
