Audrey Berthelot

Analysis of Mode-Split Operation in MEMS Based on Piezoresistive Nanogauges

Microelectromechanical system (MEMS) sensors based on nanoscale piezoresistive sensing elements (nanogauges) can have mechanical modes either related only to micrometric springs or related also to nanogauge constraints. Due to the different impact that fabrication process imperfections have on these two kinds of modes, their correlation can be poorer from part to part than in sensors based on capacitive readout. In this context, this paper compares the correlation between two modes in MEMS with and without nanogauges. Experiments show a ± 30% relative variation in the modes difference over 26 samples of the former type, which is more than 3.5 times more than what observed on similar structures with no nanogauges. A theoretical model identifies the sources of this fluctuation (local etching and height nonuniformities), and predicts the behavior and improvements using different springs design.

Large full scale, linearity and cross-axis rejection in low-power 3-axis gyroscopes based on nanoscale piezoresistors

This work presents in-plane and out-of-plane Coriolis rate gyroscopes based on nano-scale piezoresistive readout and using an eutectic bonding between the bottom wafer, where the sensor is formed, and the cap wafer, where routing and metal pads are fabricated. The gyroscopes feature a novel design with a central levered sense frame, to maximize the device symmetry and compactness. The position of the piezoresistive nano-gauges along the lever system optimizes the scale-factor. Operation on a ± 3000 dps full-scale-range (FSR) demonstrates quite competitive performance, with a linearity error lower than 0.25% and a cross-axis rejection 50× better than state-of-the art consumer gyroscopes.

Investigation of the fatigue origin and propagation in submicrometric silicon piezoresistive layers

The work investigates fatigue damage accumulation in a 250-nm thick Silicon layer that can be integrated in surface micromachining processes to realize piezoresistive sensing elements, as an alternative to capacitive detection. The investigation is done through a suitably designed structure which combines a 20-μm-thick layer, used to apply cyclic stresses, to the nanometric layer, where stress accumulation is obtained. Fatigue results are compared to previous works on micrometric Silicon and put in the context of the current theories about the origin and propagation of fatigue in micro- and nano-machined Silicon.

High-damped accelerometer based on squeeze-film damping and piezoresistive nanogauge detection for vibrating environments

This paper reports the conception, the fabrication and the electrical testing of an overdamped silicon accelerometer dedicated to vibrating environments. Squeeze-film damping effects are implemented to drastically reduce the mechanical component bandwidth while keeping a satisfactory resolution. Mechanical filtering of vibrations above 3 Hz is expected to strongly lower the mechanical overcharge and the fatigue of the structure that may occur when the MEMS is exposed to large parasitic vibrations and shocks. Such a low-pass system has not been reported yet, to the best of our knowledge.

P-type silicon nanogauge based self-sustained oscillator

This paper reports self-sustained motion of a low frequency MEMS resonator that leans on tiny p-type silicon piezoresistive nanowires, as a result of Thermal Piezoresistive Back Action (TPBA). In this device, a velocity dependent force arises from physical coupling between mechanics and electronic transport in small conductive silicon beams because of self-heating. Up to date, only damping rate increase has been reported for p-doped silicon beams based MEMS resonators. So far, most papers required n-doped silicon beams to allow self-sustained oscillation. Yet, this paper demonstrates self-sustained motion using p-doped silicon nanobeams as TPBA actuators under a constant bias voltage. The quality factor (QF) of the resonator increases from 28000 under vacuum to at least 1.8×106 for DC-bias voltage down to 950 mV. Self-oscillation is observed for bias voltage at 1,11 V. TPBA modeling accounts for the experimental results and attributes a major contribution for the amplification efficiency to the nanobeams thermal time constant along with the nanoscale size effects.

MEMS reliability study in shock environments through numerical and experimental investigations

This paper reports a novel method to evaluate and improve the reliability of mechanical stops during design and validation phases of MEMS (Micro ElectroMechanical System) in shock environments. Firstly, inplane stop contact behavior is modeled through both steady-state and dynamic mechanical FEM (Finite-Element Modeling) to validate physics package and to extract nonlinear stiffness and stress distribution as functions of contact force applied on a cylinder-to-plane Hertz contact type. Then, the transient response of a MEMS including stops behavior is modeled with a lumped impact element approach which allows to compute contact force as a function of applied half-sine shock parameters. Finally, several shock tests have been performed on numerous devices embedding previously modeled stops to evaluate experimental survival rate. Fitting experimental data to numerical results combined with Weibull theory exhibits a good compliance which allows to estimate silicon Weibull parameters respectively at 0.7 GPa, 1.1 GPa and 4 for threshold stress, average stress and Weibull modulus.

Tuning the Anti-Phase Mode Sensitivity to Vibrations of a MEMS Gyroscope

This paper proposes a stiffness correction method to improve the resilience to vibration of a dual-mass MEMS gyroscope with a particular focus near the resonance frequency of the anti-phase drive mode (fDa), i.e., its operational mode. Because of its balanced shape, this operating mode is ideally insensitive to vibrations. However, fabrication imperfections generates a residual sensitivity to parasitic vibrations that can disturb normal operation of the sensor. This work shows that the application of a DC voltage (Vtr) at the drive actuation electrode enables to decrease this sensitivity by a factor of at least 30 because of the stiffness tuning of the dual-mass structure. Experiments are performed to confirm this assumption and an efficient stiffness correction method is proposed to improve device operation.

Ultra-compact and highly sensitive pressure sensor based on nano-gauge detection and cointegrated with inertial sensors

This paper reports for the first time the experimental pressure response of an ultra-compact and highly linear absolute pressure sensor based on suspended piezoresistive nano-gauges with mechanical amplification and embedded low voltage self-test. Nano-gauges being suspended and structurally protected from the external environment, it can be used in high temperature and harsh environments. Unidirectional and amplified stress on the nano-gauges allows drastic miniaturization (0.12mm2 footprint for a 1 bar structure). In addition, the M&NEMS technology used for the manufacturing enables its direct cointegration with inertial sensors.

Suspended piezoresistive silicon nanogauges bridge for mems transduction: Spurious signal rejection capability

This paper reports a resistor bridge made of two suspended piezoresistive silicon nanowires and shows that such a transduction system is particularly suitable for sensing mechanical motion in MEMS. Thanks to their small cross section, silicon nanogauges are extremely sensitive to stress caused by MEMS motion. Because of its sensitivity to some spurious signals, like temperature variations or substrate internal stresses, a nanogauge resistor alone does not provide a signal that meets usual specification requirements for static sensors like accelerometer, magnetometer or pressure sensor. Here, we show, for the first time, the capability of suspended nanogauges bridges to reject temperature signal changes by a factor larger than 1000, over a wide temperature range [-40°C, 150°C], when compared to a nanogauge alone.