Biagio De Masi

Mechanical characterization of epitaxial polysilicon in MEMS

Publisher Summary The fast-growing application of thin polysilicon films in microelectromechanical systems (MEMS) requires an accurate evaluation of mechanical properties by means of micromechanical experimental testing techniques. To evaluate the elastic Young modulus and the fracture strength of epitaxial polysilicon used in MEMS two new types of on-chip tests have been designed: a pure tension test and a single-edge-notched tension test, both performed using electrostatically actuated devices. The pure tension tests on 20 specimens shows a low dispersion and gave a Young modulus for the silicon of 143 GPa. A total of 92 notched specimens were tested up to failure. The experimental results, supported by finite element simulations, allow to accurately computing the tensile limit stress of polysilicon. The major issues related to this research are a new electrostatic actuator; the possibility to measure through direct tensile tests at the scale of micron the Young modulus of epitaxial silicon and the tensile limit stress; the low dispersion of experimental results.

Micro-Scale Simulation of Impact Rupture in Polysilicon MEMS

The issue of Micro Electro Mechanical Systems (MEMS) mechanical reliability is becoming increasingly important. This implies that sufficiently accurate methodologies to determine the mechanical response of MEMS subject to various loading conditions must be developed.

Threshold Shock Sensor Based on a Bistable Mechanism: Design, Modeling, and Measurements

We analyze and test a microelectromechanical systems (MEMS) shock sensor that switches from a first stable state to a second stable state when subjected to an acceleration exceeding a fixed value. The transition between the two states is obtained because of the bistability properties of specific elastic beams. These are fabricated with a curved initial shape and change their configuration due to the contact force transmitted by an inertial mass sensing the acceleration. No power supply is needed (the device is passive) and the state of the sensor can be detected whenever required in several ways, e.g., by inserting a weak link that is broken during the snap-through of the elastic beam and modifies the electrical resistance between two external contacts. In this paper, focusing on a specific realization with a 1000-g (where g is the acceleration of gravity) threshold, we discuss its modeling and validate it with the experimental data. Experiments are in very good agreement with the numerical simulations, and show that the detection method is stable and robust. Finally, we discuss possible sources of uncertainties and propose a novel optimized design.

A study of adhesion forces in thick epitaxial polysilicon under dynamic impact loading

The work presents a structure and a method for the in-line characterization of impacts and adhesion phenomena between MEMS moving and fixed parts: the focus is on the monitoring of an inertial proof mass motion when colliding with a mechanical stopper. Through such measurements, one can evaluate the energy balance during impact events. The work analyzes the adhesion force evolution after a number of impact cycles comparable or larger than shocks in a 5-year operation. Results obtained on two different specimens show growing and then stabilizing adhesion forces of on average 170 nN, under impact cycles with about 500 fJ energy loss. No marked dependence on the specimen area is obtained. The possibility to change and track the impact kinetic energy is also demonstrated.

Optimization of Lorentz-force MEMS magnetometers using rarefied-gas-theory

We review the design of Lorentz force-based magnetometers to be employed in MEMS inertial measurement units. Taking into account the constraints of an industrial MEMS technology already used for accelerometers and gyroscopes, it has been recently shown that standard designs have intrinsic limitations. E.g. in the classical magnetometer operated at resonance where two parallel current carrying springs are connected by a central shuttle on which sensing parallel plates are mounted, the sensitivity does not depend on the number of plates and is limited to typical values around 1aF/(μT mA) at 1mbar. In this paper two solutions have been investigated: springs have been used for both actuation and sensing, with no sensing plates; exploiting better knowledge of rarefied gas dynamics, new stators have been designed. The combination of these factors has increased the sensitivity to 4.5aF/(μT mA) at 1mbar as predicted by numerical models and verified in experiments.

An on-chip experimental assessment Of Casimir force effect in micro-electromechanical systems

The influence of Casimir force on the mechanical behaviour of micro-electro-mechanical systems (MEMS) has deserved much attention in the recent scientific literature. This paper reports the outcomes of an experimental test, carried out on a micro-structure which reproduces the essential features of real-life MEMS. The aim of the experiment was to evaluate the effect of Casimir forces between two parallel plates, separated by a sub-micron gap. The results of the tests are critically evaluated by comparison with theoretical predictions, account taken of some peculiar features of Casimir forces for silicon slabs in real MEMS. The obtained results suggest that Casimir force could play an important role for micro and nano devices.