ABSTRACT:A new instrument to accurately and verifiably measure mechanical properties across an entire MEMS wafer is under development. We have modified the optics on a conventional microelectronics probe station to enable three-dimensional imaging while maintaining the full working distance of a long working distance objective. This allows standard probes or probe cards to be used. We have proceeded to map out mechanical properties along a wafer column by the Interferometry for Material Property Measurement (IMaP) methodology. From interferograms of simple actuated cantilevers, out-of-plane deflection profiles at the nanometer scale are obtained. These are analyzed by integrated routines that extract basic mechanical properties such as Young's modulus and cantilever curvature. Non-idealities such as support post compliance and beam take off angle are simultaneously quantified. Verifiability is achieved by comparing properties at multiple voltages. When the non-idealities are properly taken into account, we find that Young's modulusE-161 GPa for polysilicon and is independent of wafer position. However, curvature and residual stress data correlate with wafer position.
Functional operation of RF MEMS resistive switches depends on dynamic characteristics of the cantilever contacts. Characteristics of these contacts, in turn, depend on parameters defining their shape and dimensions, material properties, boundary conditions, and actuation voltages. In this paper, a simple analytical model is presented and used to develop an understanding of the switch behavior. In addition, uncertainties corresponding to this model are also determined to quantitatively show the influence that various parameters defining the cantilever contact have on its dynamics which, in turn, influences performance of the RF MEMS switch. This performance can be optimized with the objective of achieving resonance frequency within, e.g., 1% of the desired value while constraining the nominal dimensions and finding the optimum set of uncertainties in these dimensions. Analytical results correlate well with the preliminary experimental characterization of the contacts.
Development of microelectromechanical system (MEMS) sensors for various applications requires the use of analytical and computational modeling/simulation coupled with rigorous physical measurements. This requirement has led to advancement of an approach that combines computer aided design (CAD) and multiphysics modeling/simulation tools with the state-of-the-art (SOTA) measurement methodology to facilitate reduction of high prototyping costs, long product development cycles, and time-to-market pressures while devising MEMS for a variety of applications. In this approach, a unique, fully integrated software environment for multiscale, multiphysics, high fidelity modeling of MEMS is combined with the optoelectronic laser interferometric microscope methodology for quantitative measurements. The optoelectronic methodology allows remote, noninvasive full-field-of view (FFV) measurements of deformations/motions (under operating conditions) with high spatial resolution, nanometer accuracy, and in near real-time. In this paper, both, the modeling environment (including an analytical process used to quantitatively show the influence that various parameters defining a sensor have on its dynamics — using this process dynamic characteristics of a sensor can be optimized by constraining its nominal dimensions and finding the optimum set of uncertainties in these dimensions that best satisfy design requirements/specifications) and the optoelectronic methodology are described and their applications are illustrated with representative examples demonstrating viability of the approach, combining modeling and measurements, for quantitative characterization of microsystem dynamics. These representative examples demonstrate capability of the approach described herein to quantitatively determine effects of dynamic loads on performance of selected MEMS.
Continued advances in microelectromechanical systems (MEMS) technology have led to development of numerous applications including, but not limited to: automotive, communication, information technology, deep-space, medical, safety, national security, etc. These developments are being made possible because of creative designs and novel packaging based on use of some of the most sophisticated analytical and experimental tools available today. These tools are also employed to overcome limitations due to inherent behavior of materials fabricated into miniature shapes subjected to extremely harsh operating conditions while satisfying very challenging specifications/requirements of their applications. Thermoelastic internal friction is present in all structural materials and has been found experimentally in miniature silicon resonators (e.g., microgyroscopes, accelerometers, as well as biological, chemical, and other sensors/actuators) that rely on vibrations of either sensing elements or application-specific elastic suspensions that resonate. Regardless of their applications, sensors are always designed to provide the most sensitive responses to the signals they are developed to detect and/or monitor. One way to describe this sensitivity is to use the Quality (Q) factor. Most recent experimental evidence indicates that as the physical sizes of sensors decrease (especially because of continued advances in fabrication, e.g., by surface micromachining) the corresponding Q-factors become more and more dependent on thermoelastic damping (TED). This form of damping depends on material properties such as coefficient of thermal expansion, thermal conductivity, specific heat, density, and modulus of elasticity. It is also related to such design/operating parameters as resonator dimensions and temperature. This paper reviews a theoretical analysis of the effects that thermoelastic internal friction has on the Q-factor of microscale resonators and shows that the internal friction relating to TED is a fundamental damping mechanism in determination of quality of high-Q resonators over a range of operating conditions. Furthermore, the analysis also shows that the Q of resonators can be critical to the development of modern sensors. Microscale resonators are often used as basic sensing elements in the modern micromachined sensors. These sensors are frequency-modulated devices and exhibit a change in output frequency that is related to measurements and/or control of a physical variable. Accuracy and precision of these measurements/controls are inherently dependent on the frequency stability of the sensor/device output. This, in turn, greatly depends on damping in the resonating element itself.
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