Barbara Simoni

A Resonant Microaccelerometer With High Sensitivity Operating in an Oscillating Circuit

A new micromachined uniaxial silicon resonant accelerometer characterized by a high sensitivity and very small dimensions is presented. The devices working principle is based on the frequency variations of two resonating beams coupled to a proof mass. Under an external acceleration, the movement of the proof mass causes an axial load on the beams, generating opposite stiffness variations, which, in turn, result in a differential separation of their resonance frequencies. A high level of sensitivity is obtained, owing to an innovative and optimized geometrical design of the device that guarantees a great amplification of the axial loads. The acceleration measure is obtained, owing to a properly designed oscillating circuit. In agreement with the theoretical prediction, the experimental results show a sensitivity of 455 Hz/ ( g being the gravity acceleration) with a resonant frequency of about 58 kHz and a good linearity in the range of interest.

A high sensitivity uniaxial resonant accelerometer

A new micro-machined uniaxial silicon resonant accelerometer characterized by a high sensitivity is presented. The geometrical setting is analytically optimized in order to improve the sensitivity and the linearity of the sensor. The high level of sensitivity is obtained at relatively low quality factors and keeping the dimensions very small, thanks to an innovative and optimized geometrical design of the device. The proposed accelerometer has been fabricated and the first experimental measurements are presented in the paper.

Two-Scale Simulation of Drop-Induced Failure of Polysilicon MEMS Sensors

In this paper, an industrially-oriented two-scale approach is provided to model the drop-induced brittle failure of polysilicon MEMS sensors. The two length-scales here investigated are the package (macroscopic) and the sensor (mesoscopic) ones. Issues related to the polysilicon morphology at the micro-scale are disregarded; an upscaled homogenized constitutive law, able to describe the brittle cracking of silicon, is instead adopted at the meso-scale. The two-scale approach is validated against full three-scale Monte-Carlo simulations, which allow for stochastic effects linked to the microstructural properties of polysilicon. Focusing on inertial MEMS sensors exposed to drops, it is shown that the offered approach matches well the experimentally observed failure mechanisms.

A new biaxial silicon resonant micro accelerometer

A new biaxial silicon resonant accelerometer characterized by a high sensitivity and a low cross-axis sensitivity is presented in this paper. The device allows for the simultaneous measure of acceleration acting along two different axes using two couples of resonating slender beams linked to two couples of flexible beams and to a proof mass. The conceptual scheme used for the biaxial resonant accelerometer is similar to the one applied for the uniaxial resonant accelerometer reported in [1]–[3]. Experimental results demonstrate a differential sensitivity of 201 Hz/g around a resonance frequency of 84 kHz.

A new two-beam differential resonant micro accelerometer

A novel uniaxial micro-machined resonant accelerometer is presented. The device working principle is based on the stiffness variations of a beam which is fully clamped to the substrate on one side and clamped to a seismic mass on the other side. A movement of the seismic mass, induced by an external acceleration, causes either a compressive or a tensile stress on the beam, inducing a variation of its stiffness. This variation results in a change of the resonance frequency of the beam. The accelerometer is arranged in a differential structure, with two beams built in such a way that their changes in the resonance frequency have opposite sign. This solution allows obtaining a doubled sensitivity with the same area and allows reducing the non linear behavior. First experimental results show that the device has an overall differential sensitivity Δfres/g ≈ 450 Hz/g in the linear range of operation, with an overall area occupation lower than (500 µm)2.

Two-scale vs three-scale FE analyses of shock-induced failure in polysilicon MEMS

Shock-induced failure of polysilicon MEMS is investigated by adopting a multi-scale approach. To understand the capability of this approach and to assess its accuracy, we compare the failure forecasted through two-scale and three-scale simulations. In the first case we model the response of the device to the shocks at the package level (macroscopic scale) and at the sensor level (mesoscopic scale). In the latter case we also allow for micro-structural features of the polysilicon film constituting the movable parts of the MEMS, so as to track the failure mode. Focusing on a commercial off-the-shelf uniaxial accelerometer subject to drops, results of the three-scale approach show that the micro-cracking leading to failure is confined inside a rather narrow region close to the anchor points. Outcomes of the two-scale approach correctly match this evidence, provided an appropriately defined failure criterion for the anisotropic polysilicon film is adopted. Moreover, the time to failure predicted by the two approaches well agree. Therefore, while the three-scale approach furnishes much insights on the failure mode, the overall response of the sensor appears to be correctly (from an industrial perspective) estimated by the far simpler and more economic two-scale simulations.

A multiscale-stochastic finite element approach to shock-induced polysilicon MEMS failure

The effects of mechanical shocks on polysilicon MEMS accelerometers are here investigated within the frame of a multi-scale finite element approach. To accurately model MEMS dynamics and possible failure events, three length-scales are explored: macroscale, characterized by stress waves propagating inside the package and eventually impinging upon sensor anchors; mesoscale, characterized by forced vibrations of the whole sensor; microscale, characterized by possible nucleation and propagation up to percolation of trans- and/or inter-granular cracks in highly stressed regions of the sensor. Focusing on microstructural features, we show that the morphology of the polysilicon film constituting the movable parts of the sensor does affect MEMS failure. Account taken of brittleness of polysilicon at room temperature, a Monte Carlo methodology is employed to assess the links between failure mode and: the orientation of the axes of elastic symmetry of each FCC silicon grain; the trans-granular strength and toughness anisotropy; the network of grain boundaries (GBs); the mechanical properties of GBs.