Search

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. [2015-0066]

This paper describes capacitive sensing stators for microelectromechanical systems (MEMS) suitably designed to minimize damping effects without worsening the capacitive variation per unit displacement. Such optimization can be exploited to maximize the readout signal in resonant systems. Two structures are designed with the aid of finite-element method models for the electrostatic domain, and with the aid of a deterministic integral model for damping predictions. The structures, fabricated in a 22- $\mu $ m-thick surface micromachining process, demonstrate a $3\times $ sensitivity improvement when used as resonant MEMS magnetometers.

The work presents a microelectromechanical system (MEMS) based magnetometer, targeting compass applications performance, which measures magnetic fields along an in-plane direction. The magnetometer is fabricated with the surface micromachining process used for consumer gyroscopes, accelerometers, and recently proposed out-of-plane magnetometers. The magnetometer is based on the Lorentz force principle, so to avoid the need for magnetic materials integration. It features an area of 282 μm×1095 μm, and it is wafer–wafer packaged at a nominal pressure (0.35 mbar) similar to the one used for gyroscopes. In agreement with theoretical predictions, operation is demonstrated both at-resonance and off-resonance: in both situations the measured resolution, normalized to unit bandwidth and applied Lorentz current, is about 120 nT mA/ Hz , but the maximum sensing bandwidth is extended from 4 Hz (at resonance) to 42 Hz in off-resonance mode, which copes with consumer specifications. Within magnetic fields of ±5 mT, the device shows measured linearity errors

The work presents a MEMS magnetometer for compass applications, which measures magnetic fields along an in-plane direction. The magnetometer is built with the surface micromachining process used for consumer gyroscopes, accelerometers, and recently proposed out-of-plane magnetometers. The magnetometer is based on the Lorentz force principle, it has an area of 282x1095 om 2 , and it is packaged at low pressure (0.25 mbar). Operation is demonstrated at- and off-resonance: in both situations the measured resolution per unit bandwidth and current consumption is about 120 nT·mA/√Hz, but the maximum sensing bandwidth is extended from 4 Hz (at resonance) to 42 Hz in off-resonance mode. Within magnetic fields of ±5 mT, the device shows measured linearity errors

Micro-Electro-Mechanical Systems (MEMS) accelerometers are micro-sized devices largely used for detecting accelerations in the consumer and automotive market. Both capacitive and resonant sensing have been successfully employed in these devices. In the present work, the focus is on a z-axis resonant accelerometer recently proposed in [1] and fabricated with the Thelma © surface-micromachining technique developed by STMicroelectronics [2] . After a full non-linear dynamic study, two possible optimized designs are studied through an optimization procedure. The goal of the work is to find a novel design for the z-axis resonant accelerometer which meets the linearity and the reliability requirements for MEMS accelerometers whitout losses in terms of sensitivity.

In this paper the effects of accidental impacts on polysilicon MEMS sensors are investigated within the framework of a three-scale finite element approach. By allowing for the very small ratio (on the order of 10−4) between the inertia of the MEMS and the inertia of the whole device, macro-scale analyses at the package length-scale are run to obtain the loading conditions at the sensor anchor points. These loading conditions are successively adopted in meso-scale analyses at the MEMS length-scale to detect where the stress level tends to be amplified by sensor layout. To foresee failure of polysilicon in these domains, as caused by the propagation of inter- as well as trans-granular cracks up to percolation, representative crystal topologies are handled in micro-scale analyses.

Micro-Electro-Mechanical Systems (MEMS) accelerometers are micro-sized devices largely used for detecting accelerations in the consumer and automotive market. Both capacitive and resonant sensing have been successfully employed in these devices.

This paper proposes a new design methodology enabling the co-design of MEMS with electronics while considering the influences of its surrounding package. The proposed solution has been validated on a case study of a MEMS accelerometer. We describe different options to simulate the MEMS accelerometer device with its package. Firstly, as a parametric MEMS+ component model based on high-order finite elements. Secondly, as an automatically generated reduced-order model in VerilogA format. Both models are fully compatible with standard electronic circuit simulators. The comparison between both models reveals that the VerilogA model shows similar accuracy (difference is smaller than 1%) while it is considerably faster in simulation time (by more than a factor of 1000). The proposed methodology can efficiently simulate the interaction between MEMS chip, package and electronic read-out circuit, which is an important part of a new approach of smart system design.

The design of a heterogeneous smart system is a major and multidisciplinary challenge. The next generation smart systems will include analog, digital and MEMS components described using different languages relying on parametric models at different abstraction level. The SMArt system Co-design (SMAC) FP7 ICT project aim is the overcome all the issues related to the mixed design of a smart system. Inside this project a step toward design tool integration has been made with the presented work. As a matter of fact, we present a case study for the definition of a complete flow for the design and use of MEMS devices inside standard microelectronics design tools

This paper reports the theoretical and experimental characterization of the dynamic behavior of torsional resonators that can be applied to inertial sensors. For the correct operation of the devices it is necessary to model the dynamic behavior of the electrostatically actuated torsional resonators both in the linear and nonlinear range. A complete analytical model is developed in this work including nonlinear terms in the electrostatic stiffness. This provides clear quantitative information about the available linear range of operation and opens the way to exploit the nonlinear range. The model is validated through comparison with experimental data on two 22 µm thick polysilicon resonators having different distances from the underlying electrodes.

A new out-of-plane resonant micro-machined accelerometer has been designed, modelled and fabricated. The sensing principle is based on the variation of the electrostatic stiffness of two torsional resonators mechanically coupled with an inertial proof mass. The accelerometer, fabricated by the ThELMA® surface micro-machining process of STMicroelectronics, constitutes a further step of a research focussing on the design of in-plane and out-of-plane resonant micro accelerometers. Preliminary electrostatic measures of the torsional resonators response have been compared with the theoretical predictions.

Fringe-field capacitive sensing has a potential interest for the inertial MEMS community as it decouples the sensing direction from the pull-in direction, and it is therefore intrinsically immune from this issue. This paper presents a MEMS accelerometer based on fringe-field sensing, that exploits a surface micromachining process with very thin planar interconnections (

The present work concerns the operating principle and a thorough experimental characterization of a new polysilicon resonant micro accelerometer for out-of-plane measurements, fabricated using an industrial surface micromachining technique. This device is characterized by differential resonant sensing, obtained from the variation of the electrostatic stiffness of two torsional resonators under the application of an external acceleration. The sensitivity, defined as the differential shift in resonance frequencies per gravity unit (lg = 9.8 m s−2), is of about 10 Hz g−1when operated at a DC bias of 1.5 V only. Over an acceleration range larger than 10 g, the deviation from linearity is lower than 1% and the cross-axis rejection is larger than 34 dB. The resonators temperature coefficients of frequency, in the order of −29 ppm C−1, are matched within about 0.1%, resulting in linear offset drifts against temperature lower than 5 mg up to 95 C in absence of any digital compensation.

The effect of packaging on the impact-carrying capacity of micro electro-mechanical systems (MEMS) is investigated, with specific reference to a translational accelerometer. By exploiting the small ratio between the masses of MEMS and package/die (typically 10 − 3 or less) a decoupled two-scale, finite element approach is adopted: at the package/die length-scale the dynamics of whole device after the impact against a flat target surface is studied; at the sensor length-scale the response of the MEMS to the drop-induced loading is investigated, and MEMS details where the stress state can exceed the tensile strength of polysilicon are identified. Two drop orientations are considered, here termed bottom and top; in the first case, package and die strike the target with their bottom surfaces; in the second case, they fall upside-down, and strike the target with their top surfaces. By comparing the simulation outcomes in terms of maximum attained tensile stress, it results that package does not always lead to benefits in term of capability of the studied sensor to sustain drops. In the bottom drop configuration, e.g. MEMS failure may be triggered by the package.

A 30 µg/sqrt(Hz) resolution MEMS based low-g 3-axis accelerometers targeted for emerging automotive application is presented. The device is composed by 2-dice in a single package: the sensing element and the processing IC. The sensing element is realized with an epitaxial micro-machining process (ThELMA™) while the IC uses a 0.5 μm BICMOS process. The IC amplifier is able to detect capacitive changes as low as 10 aF (10·10−18 F) and the device is able to provide both an analog and a digital output. Moreover it has very good linearity and very low cross talk between the sensing axes thanks to a design that fits very well the ST micro-machining process ThELMA. Finally, the proposed interface accelerometer is factory-trimmed to ensure repeatable performance without production-line adjustments in the end product

A parametric numerical study aimed at understanding the influence of the average grain size on the mechanical behaviour of polysilicon films was addressed in this paper. A 2D geometrical model of the polycrystal coupled with a finite element (FE) procedure was employed. To simulate inter-granular and trans-granular crack propagation a cohesive crack model was used. The results were finally analyzed using the Weibull weakest link approach.

We investigate the shock-induced stress state and possible failure mechanisms in polysilicon MEMS sensors. In case of accidental drop events, we aim at highlighting the links between drop features, like drop height and impact angles, and the location of the failing detail of the device. Taking into account the small inertial contribution of the sensor to the whole package dynamics, we adopt a decoupled multi-scale numerical approach, where macro-scale analyses (at die length-scale) are used to define the acceleration records to be adopted as loading in meso-scale (at sensor length-scale) simulations. We show that a commonly adopted indicator to assess drop severity is not able to take in due account the details of the impact event and possible very localized failures at the sensor level.

In this paper, we report on feedback interferometric measurements on a micromachined gyroscope and on a micromachined linear accelerometer. Characterization has been performed for different values of pressure and of other parameters using a laser diode. Resonance frequencies and quality factors have been measured. Moreover, hysteresis and other nonlinear phenomena on specific samples have also been detected. The proposed method is based on optical injection and represents an efficient alternative to the standard electrical measurements, which actually shows some limitations for bare prototype testing.

This paper reports the theoretical and experimental characterization of a new z-axis silicon resonant micro accelerometer fabricated by the THELMA© surface micromachining technique, characterized by differential sensing and very small dimensions. The working principle of this device is based on the variation of the electrostatic stiffness of two torsional resonators. This work is a prosecution of the research on resonant accelerometers published in [1,2].

A 2D geometrical model for polycrystals was developed in this paper by means of a Voronoi tessellation in which each crystal is assumed to be elastic anisotropic. An implicit–explicit FE dynamic code coupled with an automatic procedure for the introduction of cohesive interface elements with cohesive traction-jump softening laws was used in order to simulate intergranular and transgranular fracture. Accidental drop simulations were performed with the principal aim to capture the maximum acceleration and to simulate local rupture phenomena in MEMS. In order to reduce the excessive problem size, a simplified, decoupled global–local three level multi-scale approach was used.