Proc. SPIE. 4980, Reliability, Testing, and Characterization of MEMS/MOEMS II
KEYWORDS: Microelectromechanical systems, Microscopes, Resonators, Silicon, Scanning electron microscopy, Ion beams, Finite element methods, Photomicroscopy, Material characterization, Nonlinear response
For reliable MEMS device fabrication and operation, there is a continued demand for precise characterization of materials at the micron scale. This paper presents a novel material characterization device for fatigue lifetime testing. The fatigue specimen is subjected to multi-axial loading, which is typical of most MEMS devices. Polycrystalline silicon (polysilicon) fatigue devices were fabricated using the MUMPS process with a three layer mask process ground plane, anchor, and structural layer of polysilicon. A fatigue device consists of two or three beams, attached to a rotating ring and anchored to the substrate on each end. In order to generate a sufficiently large stress, the fatigue devices were tested in resonance to produce a von Mises equivalent stress as high as 1 GPa, which is in the fracture strength range reported for polysilicon. A further increase of the stress in the beam specimens was obtained by introducing a notch with a focused ion beam. The notch resulted into a stress concentration factor of about 3.8, thereby producing maximum von Mises equivalent stress in the range of 1 through 4 GPa. This study provides insight into multi-axial fatigue testing under typical MEMS conditions and additional information about micron-scale polysilicon mechanical behavior, which is the current basic building material for MEMS devices.
Optical Micro-Electro-Mechanical Systems (Optical MEMS, or MOEMS) comprise a disruptive technology whose application to telecommunications networks is transforming the horizon for lightwave systems. The influences of materials systems, processing subtleties, and reliability requirements on design flexibility, functionality and commercialization of MOEMS are complex. A tight inter-dependent feedback loop between Component/ Subsystem/ System Design, Fabrication, Packaging, Manufacturing and Reliability is described as a strategy for building reliability into emerging MOEMS products while accelerating their development into commercial offerings.
We present a full factorial study of the effect of relative humidity and voltage on the oxidation of surface-micromachined poly-silicon wiring and electrodes. Our system consists of 500 nm thick poly-Si wires and electrodes insulated from the substrate wafer by 600 nm of Si-rch Si<SUB>x</SUB>N<SUB>y</SUB>, fabricated using a surface-micromachinging process. In dry ambients, oxidation or damage to the bottom poly-Si layer (the Poly0 layer) in MicroElectroMechanical Systems (MEMS) devices occurs so slowly that little can be learned in a timely manner, even when stressing the electrodes at electric fields close to dielectric breakdown. We observe however that in ambient with elevated relative humidity the Poly0 wires and electrodes anodically oxidize within a short period of time when operated at moderately large voltages. Only the most positively biased poly-Si structures oxidize, and we describe the anodic oxidation and association volume expansion as a function of a number of accelerating factors including relative humidity and voltage. A threshold is observed in relative humidity bot not in voltage.
While considerable press has been given to characterization of mechanical properties of MicroElectroMechanical Systems (MEMS) as related to reliability, environmental robustness, and lifetimes studies, characterization of electrical properties of MEMS have not been widely published. In this paper we present an examination of electrical properties (surface and substrate leakage currents, sheet resistance, substrate contact resistance and interlayer contact resistances) of polysilicon thin films used in surface micromachined MEMS test structures. Environmental and electrical overstress conditions that affect leakage have been studied. Two test structures have been used to independently study surface and substrate leakage currents at different levels of humidity (0% to 80% RH) and applied voltage (100 to 150 volts). Both static and lifetime studies have been conducted. Significant differences in surface and substrate leakage lifetime characteristics are observed, suggesting different failure mechanisms for these two important electrical phenomena in MEMS reliability.