F. Tounsi

Sensitivity and power modeling of CMOS mems single axis convective accelerometers

In this paper, we present 3D finite element modeling and simulation of a CMOS MEMS single axis convective accelerometer. We describe the sensor architecture and present a sensor geometry model to be used in 3D FEM simulations. Differences between 3D and previously published 2D simulations are discussed. This work investigates 3D effects which give the opportunity to better predict not only sensor sensitivity but also power dissipation. Experimental sensitivity values and 3D FEM ones are compared for two different sensor geometries and two different heater temperatures. For a prototype having a heater-cavity border distance of 340?m and a heater length of 230?m, maximum sensitivity point is obtained for detectors localized at a distance of 125?m from heater center. This distance should be moved to 90?m if a 50?m heater length is used. So, detectors should be placed closer to the heater than the usually used mid distance. Moreover, optimal detectors location shifts closer to the heater as heater length shrinks. We also show that if heater length is reduced by 80% (from 230 to 50?m), then both electrical power and sensitivity decrease by 63% and 55%, respectively. So, best efficiency is obtained for shorter heaters. In addition, detectors length decrease is found to have a significant effect on sensitivity, with an increase of 58% and 87% using heater lengths of 230?m and 50?m, respectively. Here, detectors length decreased from the total side bridge length to a fraction of this length equals to 2.5%. Optimal length is obtained when detectors are implemented on the same side bridge fraction as that used to implement the heater on the central bridge.

A MEMS-based shifted membrane electrodynamic microsensor for microphone applications

In this paper we present a multidisciplinary modeling of a MEMS-based electrodynamic microsensor, when an additional vertical offset is defined, aiming acoustic applications field. The principle is based on the use of two planar inductors, fixed outer and suspended inner. When a DC current is made to flow through the outer inductor, a magnetic field is produced within the suspended inner one, located on a membrane top. In our modeling, the magnetic field curve, as a function of the vertical fluctuation magnitude, shows that the radial component was maximum and stationary for a specific vertical location. We demonstrate in this paper that the dynamic response of the electrodynamic microsensor was very appropriate for acting as a microphone when the membrane is shifted to a certain vertical position, which represents an improvement of the microsensors basic design. Thus, a proposed technological method to ensure this offset of the inner inductor, by using wafer bonding method, is discussed. On this basis, the mechanical and electrical modeling for the new microphone design was performed using both analytic and Finite Element Method. Firstly, the resonance frequency was set around 1.6u2009kHz, in the middle of the acoustic band (20u2009Hz – 20u2009kHz), then the optimal location of the inner average spiral was evaluated to be around 200µm away from the diaphragm edge. The overall dynamic sensitivity was evaluated by coupling the lumped elements from different domains interfering during the microphone function. Dynamic sensitivity was found to be 6.3u2009μV/Pa when using 100u2009µm for both gap and vertical offset. In conclusion, a bandwidth of 37.6u2009Hz to 26.5u2009kHz has been found which is wider compared to some conventional microphones.

Macro model analysis of a single mass 6-DOF Inertial Measurement Unit system

In this paper, a preliminary analysis of a 6-axis micromachined Inertial Measurement Unit (IMU) structure is performed. We firstly give a review of current approaches found in literature for 6-axis IMU sensor technology. Then, we present and analyze the proposed single mass macro model structure. We mainly investigate the simultaneous detection of all possible six axis motion components with a single bloc based on SMSD approach. The angular velocity detection method is based on the Coriolis Effect which requires a certain translation applied to a suspended mass attached to a piezoelectric membrane. Two methods have been examined: first one is based on the Eigenfrequencies of the membrane, and second one is based on the inverse piezoelectric effect to actuate the membrane. So, electrodes deposited on the piezoelectric membrane are employed for sensing and actuating mechanisms based on direct/reverse piezoelectric effects. As a primary investigation, a modal analysis of the IMU macro model consisting of a piezoelectric Buzzer is presented. FEM simulations and measurement values show a good agreement. Applied stress on the membrane submitted to a linear acceleration is evaluated and improved designs, allowing 3 to 4 times higher stress values, are investigated.

Optimization of Induced Voltage From CMOS-Compatible MEMS Electrodynamic Microphone With Coaxial Planar Inductances

This paper presents an improved design of the electrodynamic CMOS-MEMS microphone. The studied microphone is based on two integrated concentric planar inductors, a fixed outer one and an inner one attached on a flexible vibrating diaphragm. First, the magnetic field, generated by the outer inductor, was evaluated. Unlike conventional electrodynamic microphones based on Faradays law, the induced voltage was found to be proportional to the product of the diaphragm displacement and its velocity. Even though the induced output voltage is in the range of a few microvolts, and its frequency is doubled compared to that of the incident acoustic wave. This result pushed us to adjust and improve the basic principle of this kind of microphone by trying to generate a vertical downward shift of the microphone diaphragm while maintaining the COMS compatibility. The Lorentz force appears to be a possible alternative to the wafer bonding, as it will be automatically generated when we bias the inner inductor by a dc current. First, this paper shows that the induced voltage will be linear for a vertical fluctuation magnitude lower than 10 μm with an induced voltage in the range of several microvolts. Using the developed vertical offset technique, we show that induced voltages can be increased by a factor of 10 and the dynamic response of the electrodynamic microsensor will be appropriate for a use as a microphone.

Conductive behavior modeling of dual-axis CMOS MEMS convective accelerometers using 3D FEM and spherical model

This paper presents heat conduction modeling of dual axis micromachined convective accelerometers. Results from FEM simulation are used to develop an analytical model of heat conduction main parameters. Two variables are used in FEM simulations: heater temperature and cavity depth. The latter parameter has a large impact on the overall conductive behavior of thermal accelerometers since it fixes the volume where the heat bubble can expand. Simulation results are used in a derived spherical model to develop an analytical expression of outer isotherm equivalent radius. The hot bubble radius and form are closely related to sensor geometry parameters and temperature. Two distinct equivalent radius modeling are studied and are used to express both heater heat transfer coefficient and common mode. These physically-based derived expressions govern the overall sensor conductive behavior. It is also shown that these derived expressions are still valid for different sensor design geometries.

Mechanical-thermal noise in CMOS micromachined inductive microphone

In this paper, we present mechanical-thermal noise characterization and calculation for the new state-of-the art monolithically integrated CMOS micromachined inductive microphone. This new acoustic sensor has a suspended moving membrane attached to the substrate with 4 L-shaped arms. This membrane has 1.4times1.4 mm2 surface, 22 mug mass and its natural frequency was found to be around 200 kHz. This sensor was analyzed for mechanical-thermal noise by modeling the suspended membrane with its mass-spring oscillator diagram and adding a force generator alongside each damper. The system damping factor was found to be 4times10-2 N.s/m, which gives a fluctuating force spectral density of 2.57times10-11 N/radicHz. This noise pressure spectral density corresponds to an A-weighted sound level of about 38 dB(A) SPL. Consequently, this microphone is well suited for recording low-level signals in quiet environments.

Thikness dependence investigation of the mutual inductance link in concentric planar transformers

In this paper, we present a new analytic approach for the calculation of the mutual inductance between two concentric planar conductors. From this study, we aim to prove that coil thickness has a minor impact on mutual inductance value. This study will help to determine a simplified adapted expression of the induced voltage output at our electrodynamic microphone ends. This micromachined new sensor exploits the concentric transformer principle which relies on the use of two planar inductors: an outer fixed one and an inner suspended one. If the outer is biased, a mutual inductance link will be generated. This link strongly depends on inductors coil design geometry and the induced output voltage. The present work proves that for long conductors (large number of turns), coil thicknesses above 6μm will have no significant influence on mutual inductance values. This has been proven by an increase of only 0.01% if coil thickness reaches as high as 20 μm. Moreover, for thicknesses under 2μm, mutual inductance value is constant arround 7.2μH. This allows the use of much simpler expressions, with no thickness dependence, to evaluate mutual inductance values.

Behavior analysis of a 3-axis detection push-pull piezoresistive MEMS accelerometer

This paper reports a new concept of a 3-axis accelerometer design along with proposed fabrication steps and a circuit readout configuration. In order to achieve a better performance in terms of induced output, the study focuses on the optimization of the structure dimensions and the optimal piezoresistors location. The proposed model will be analyzed using Finite Element Modeling (FEM) to fully investigate the stress field in the structure based on the seismic mass movement and dimensions of attachment arms. Using Finite Element Analysis results, the push-pull operation mode has been confirmed. Maximum stress magnitude with an appropriate profile has been studied in terms of various key geometric parameters. Under an acceleration of 100g, symmetric stress magnitude profile with an approximate value of 3MPa, has been detected. This paper also discusses the structure of the built-in network of the piezoresistive Wheatstone bridge circuit and its position. We also show a flip-chip process flow for the ongoing fabrication of a polymer based accelerometer prototype.