Piezoresistive MEMS Accelerometer Research Articles

Theoretical and numerical investigation of a new 3-axis SU-8 MEMS piezoresistive accelerometer

This paper investigates the static and dynamic performances of a new enhanced design of 3-axis MEMS polymer piezoresistive accelerometer. Its key geometric parameters are optimized using a developed numerical finite element model (FEM). Using FEM analysis, the evaluated induced stress fields show a clear improvement compared to two similar silicon-based designs found in literature. Under 100 g z-axis accelerations, the proposed design proof-mass deflection is 21.78 μm and the maximum induced stress in the beams is 13.2 MPa. With the incorporation of SU-8/Carbon Black (CB) piezoresistive layers in locations of maximum induced stress, the piezoresistive analysis is performed to deduce the variations in resistance values. Using FEM dynamic analysis, the Eigenfrequency is 1.08 kHz and the quality factor is 5.55. In addition, a complete analytical model is developed and we show that values from derived expressions are in good agreement with results obtained from FEM simulations. The details of a low-cost and simple fabrication process-flow is provided for the proposed design.

Fabrication and Performance of a Bulk 4h-Sic Mems Piezoresistive Accelerometer

Fabrication and Performance of a Bulk 4h-Sic Mems Piezoresistive Accelerometer

An Experimental and Analytical Study of Hybrid Accelerometers

Hybrid accelerometers comprise sensor elements from different technologies, capable of combining two output signals into a highly linear response. Typically, a piezoaccelerometer would be combined with a MEMS accelerometer to extend an otherwise AC coupled response down to DC. The complementary filter function is an effective and distortion-free method for combining a DC capacitive sensing element that is critically damped with an AC piezosensing element; however, this method does not work well with lightly damped DC sensing elements. A detailed examination was undertaken of the issues that arise when combining AC and DC sensing elements where the damping factor of the DC sensor element is < 1, as is the case with many MEMS piezoresistive accelerometers. Results include earlier work with a prototype hybrid accelerometer, as well as a later analytical model to investigate in detail the complex interaction between system parameters that lead to distortion, nonlinearity and changes in impulse response when lightly damped DC sensing elements are combined. The analysis concludes with a post-processing method investigated for the removal of spurious transformation coefficients that currently preclude the use of lightly damped DC sensors.

English

English

Out-of-plane dual flexure MEMS piezoresistive accelerometer with low cross axis sensitivity

Accelerometers are one of the widely explored microsensors with many transduction mechanisms. One of the primary concerns with piezoresistive microaccelerometers is their cross axis sensitivity. The main reason for cross axis response is the mechanical offset between the flexures and the center of mass of the proofmass. Cross axis sensitivity is generally minimized electronically using bridge configurations of piezoresistors or by depositing high density materials like gold over the proofmass to balance the mechanical structure. Here a novel non planar dual flexure geometry has been proposed which make the effective beam center of mass to align with that of the proof mass and thereby achieve low cross axis sensitivity by the mechanical design itself. The hybrid integration of electronic compensation with the mechanical compensation realizes a high performance accelerometer with ultralow cross axis sensitivity. Four piezoresistors placed on one flexure are configured such that the output voltage will be maximum for prime axis input while the bridge is balanced for the inputs along remaining two axes. The analytical model of the proposed structure, and the finite element analysis results are discussed. The device exhibits a very low cross axis sensitivity of 0.006% in Y direction and 0.005% in X direction while maintaining high prime axis sensitivity of 2.28 mV/g. The device also shows very low non linearity (1%) over a range of 100 g acceleration and a resonant frequency of 1.7 kHz.

Miniature accelerometers for sensing middle ear motion

Acceleration sensors are prevalent in handheld electronics, navigation systems, and physical activity monitors as well as for vibration measurement. There are many measurement applications, such as sensing human middle ear motion, that require miniature, low-noise sensors to detect low-amplitude vibration. Both capacitive and piezoresistive MEMS accelerometers have been used for vibration sensing of the middle ear. Recent capacitive designs are large (base area of 2.5 mm x 6.2 mm) and have a limited bandwidth of 6.44 kHz. Low-noise piezoresistive devices power requirements are high for hearing instruments, greater than 1 mW. Commercial piezoelectric accelerometers are a viable means for vibration detection; while small, they are still large and lacking proper electronic connections for middle ear sensing applications. The aforementioned problems motivate the design of a small, passive, broadband accelerometer with low input referred noise (IRN). In this work, we design a micromachined, piezoelectric bimorph cantilever accelerometer for broadband, low-amplitude vibration sensing. An analytic model is developed to minimize the IRN. The analytic expression allows for rapid design of many system parameters that are verified through finite element analysis.

Monolithic-integrated piezoresistive MEMS accelerometer pressure sensor with glass-silicon-glass sandwich structure

A monolithic composite MEMS sensor with sandwich structure is designed, simulated, fabricated and characterized. It consists of a rectangular diaphragm piezoresistive pressure sensor and a double-cantilever-mass piezoresistive accelerometer. The professional MEMS software, Intellisuite 8.5, is used to simulate the performances of the composite sensor. The composite sensor is fabricated on a (100)—silicon wafer by using MEMS bulk-micromachining and anodic bonding technology. One-step wet anisotropic etching process on the backside of the wafer can form the main backside shape of the composite sensor including the mass of accelerometer and the pressure sensing diaphragm. The glass-silicon-glass sandwich structure is formed with α-Si (amorphous silicon)-glass anodic bonding on the top surface of the wafer and Si-glass anodic bonding at the bottom. The fabricated composite sensor is measured, resulting in 33.0 μV/V kPa sensitivity for the 450 kPa-ranged pressure sensor, as well as, 11.2 μV/V g sensitivity for the 125 g-ranged accelerometer. The die size of the composite sensor chip is 2.5 mm × 2.5 mm × 1.4 mm. The measured results show that the composite sensor is appropriate for the application fields such as automobile, aerospace and environment monitoring.

SIMULATION STUDY OF CONVEX CORNER UNDERCUTTING IN KOH AND TMAH FOR A MEMS PIEZORESISTIVE ACCELEROMETER

Undercutting is a common problem in wet anisotropic etching. This problem in turn, influences the performance and sensitivity of MEMS devices. This paper investigates the use of corner compensation to prevent convex corner undercutting in a MEMS piezoresistive accelerometer. The Intellisuite CAD simulation software was used for designing the mask with corner compensation and for analysing wet anisotropic etching profiles in potassium hydroxide (KOH) and tetra-methyl-ammonium-hydroxide (TMAH) solutions at different concentrations and temperatures. Perfect 90 degrees corners on the proof mass was successfully etched using a corner compensation design at etching temperature of 63 °C for KOH and 67.7 °C for TMAH with 25 wt% and 10.3 wt% concentration levels, respectively. Etching in TMAH required lower concentration level, thus making the etching process safer. However, TMAH required longer time to etch perfect convex corners compared to KOH. Nevertheless, both KOH and TMAH etchants have been successfully used to etch perfect convex corners by using the designed corner compensation mask.

Design, Simulation and Fabrication of Triaxial MEMS High Shock Accelerometer.

On the basis of analyzing the disadvantage of other structural accelerometer, three-axis high g MEMS piezoresistive accelerometer was put forward in order to apply to the high-shock test field. The accelerometer's structure and working principle were discussed in details. The simulation results show that three-axis high shock MEMS accelerometer can bear high shock. After bearing high shock impact in high-shock shooting test, three-axis high shock MEMS accelerometer can obtain the intact metrical information of the penetration process and still guarantee the accurate precision of measurement in high shock load range, so we can not only analyze the law of stress wave spreading and the penetration rule of the penetration process of the body of the missile, but also furnish the testing technology of the burst point controlling. The accelerometer has far-ranging application in recording the typical data that projectile penetrating hard target and furnish both technology guarantees for penetration rule and defend engineering.

Design, Fabrication and Test of In-Plane MEMS Piezoresistive High-g Accelerometer

The new structure of in-plane high-g MEMS sensor for 10,000-150,000g has been designed in our paper, and it was packaged in dimensions of 17.5mm×12mm×4.5mm. In our experiments, the sensitivity reach to 0.335uv/g by the Hopkinson bar system and it also is in quick respose to the frequence. Meanwhile, the results show that all acceleration-time pulses are in very close agreement for acceleration amplitudes to about 150000g. It is a new way to monolithically integral triaxial piezoresistive high-g accelerometer in the future.

Design, fabrication and characterization of high performance SOI MEMS piezoresistive accelerometers

Piezoresistive accelerometers have served as the frontrunners in micromachined accelerometer technology and have undergone modifications with the primary focus on device complexity and novel processes to circumvent performance trade-offs. This work comprises the analysis, design and development of micromachined SOI MEMS-based piezoresistive accelerometers for inertial sensing applications using minimalistic component design and a custom fabrication process to realize robust devices capable of withstanding more than ~106 cycles of error-free operation. Extensive simulation studies have been carried out to validate and tune the design parameters of the analytical models used. The devices have been subjected to an exhaustive range of static and dynamic tests to characterize their response which has been sensitive and highly linear with low noise, an intrinsic quality of piezoresistive sensors coupled with precision design and fabrication.

A simple analytical design approach based on computer aided analysis of bulk micromachined piezoresistive MEMS accelerometer for concrete SHM applications

Structural Health Monitoring (SHM) using non destructive testing generally involves measurement of shift in natural frequency of the monitored structure. This paper presents the simulation using CoventorWare MEMS design tool and analysis of three bulk micromachined piezoresistive MEMS accelerometers namely device A, B and C that are specifically intended for SHM applications. The devices A and B have been designed for the same natural frequency (100Hz) but with different geometries. The device C has the maximum deflection sensitivity. The modal, piezoresistive and stress analyses show that beam length (L) must be less than the half side length (a) of the proof mass for achieving maximum voltage sensitivity. Thus Device-A has been selected for further analysis and the various performance factors for the Device-A have been obtained using simulation experiments and the results show that this device has excellent voltage sensitivity (3.56mV/g/V), appreciably smaller cross axes sensitivities (32.8μV/g/V), very low noise floor (4.53μg/Hz) and high resolution (12.72μg) compared with the already reported piezoresistive accelerometer designed for SHM applications and certain general purpose accelerometers available in the global market. The frequency analysis on two devices (Devices A and D) show that the resonant frequency of the sensor should be low for achieving maximum sensitivity and the damping factor (ξ) must be 0.7 for getting the maximum bandwidth over which the sensitivity remains constant (60Hz). Finally, a standard analytical design procedure for the design of piezoresistive MEMS accelerometers has been developed and presented based on the various observations and results of this study. Further, the design approach for high packing density has also evolved.

Design, fabrication and experiment of a MEMS piezoresistive high-g accelerometer

High-g accelerometers are widely used in explosion and shock measurement. This paper describes a MEMS piezoresistive high-g accelerometer whose range is more than 50000g. It is designed on the basis of silicon on insulator (SOI) solid piezoresistive chip. The chip has a structure where both ends of the beam are fixed. Through the stress analysis and mode analysis of the accelerometer, the detailed parameters of the structure are established. The experimental results obtained from the drop hammer shock machine test and live-fire test show good properties of the accelerometer such as good output characteristic, repeatability and fast response speed. Therefore, the accelerometer in this paper meets the requirement of explosion and shock measurement basically.

Effects of package on the performance of MEMS piezoresistive accelerometers

Package is one of the key technologies for MEMS accelerometers. This paper investigated the influence of package on performances of custom-designed MEMS piezoresistive accelerometer. Based on the designed package process, the effect of die adhesive and potting materials on the sensor sensitivity and residual stress were studied by theory analysis and experimental test. The results showed that: with more quantity of die adhesive when the thickness was fixed, the sensitivity of MEMS piezoresistive accelerometer was much smaller; in addition, with thicker die adhesive, the accelerometer had much bigger sensitivity and much smaller residual stress increment after the die attach process. Meanwhile, the sensitivity was proportional to the density of potting material, and it varied with the elastic modulus.

Design and Simulation for Tri-Axial Piezoresistive MEMS Accelerometer

This paper presents analytical models for tri-axial accelerometer based on one seismic mass and eight beams. The model makes it possible to better understand and to predict the behavior of the accelerometer. The accelerometer discussed in this paper was designed to measure high g-forces in every single axis. We use finite element mechanics analysis and design for optimization of the device structure. FEA software ANSYS was used to simulate the behavior of the accelerometer when it was subjected to static high-g forces. The analytical results of simulation and theoretical demonstrate that the accelerometer we designed is feasible.

A miniature in-plane piezoresistive MEMS accelerometer for detection of slider off-track motion in hard disk drives

A miniature in-plane pizoresistive MEMS accelerometer was designed, fabricated and characterized for detection of slider off-track motion in hard disk drives. The structure of the accelerometer consists of a central supporting beam and two stress-magnifying sensing beams. Under geometric constraints imposed by the trailing side of a pico slider, the accelerometer design was optimized to achieve approximately pure axial deformation in the sensing beams and a maximum sensitivity with a specified natural frequency of 300 kHz. Fabricated on a silicon-on-insulator (SOI) wafer, the accelerometer with a half Wheatstone bridge was wirebonded to external pads and interfaced with an amplifier circuit on a printed circuit board (PCB). The noise level, sensitivity, nonlinearity were characterized with vibration testing on a shaker. The miniature accelerometer (1 × 0.3 × 0.3 mm3) with a weight of only 0.2 mg offers a much higher resonant frequency with a comparable sensitivity compared with those in previous work.

A robust and flexible optimization technique for efficient shrinking of MEMS accelerometers

We report on the automated determination of the minimal required area of a MEMS accelerometer conforming to given specifications. For a realistic nonlinear sensor model this process is only possible by the use of numerical optimization, which typically has the difficulty of finding the global minimum or is time consuming. A miniaturized sensor’s chip size reduces manufacturing cost and leads to more competitive package sizes and new, unforeseen applications. Size reduction is especially important for consumer applications like mobile phones and navigation devices, where an increasing demand for accelerometers is expected in the near future. With further miniaturization of a sensor it is increasingly important to find the optimal design in order to use chip area as efficiently as possible. To achieve a robust and flexible automated area reduction without loss of functionality we uniquely combine available genetic and gradient-based optimization algorithms. Furthermore, we reduce the model complexity, apply different scaling techniques and adapt optimization algorithm settings. The application to a capacitive and a piezoresistive MEMS accelerometer shows significant improvement of efficiency when compared with the use of currently available optimization algorithms.

Miniaturization limits of piezoresistive MEMS accelerometers

We present the miniaturization limits of axially loaded piezoresistive MEMS accelerometers. Therefore we identify limiting factors on the basis of FEM-verified analytical models. To ensure a broad discussion we compare two different axially loaded topologies: first a conventional topology, which can be manufactured already today, and second a future-oriented topology utilizing nanowires. To enable a realistic comparison of the different topologies we shrink the sensor while maintaining a specific performance (e.g. sensitivity and noise) considering design limitations such as fracture of silicon and buckling. To find the minimum total sensor area under certain constraints and therefore the optimal geometric and material parameters we apply optimization techniques to our analytical models. It will be seen that the piezoresistive transducer principle for MEMS accelerometers has a promising shrink potential with minimum total sensor dimensions as low as 150 × 150 × 10 μm3 achievable by use of currently available manufacturing processes.

Cross-axis sensitivity reduction of a silicon MEMS piezoresistive accelerometer

This paper presents realization of a MEMS piezoresistive single axis accelerometer using dual doped TMAH solution. The silicon micromachined structure consists of a heavy proofmass supported by four thin flexures and sandwiched between top and bottom glass plates. Boron diffused piezoresistors located near fixed points of the flexure are used for sensing the developed stress due to applied acceleration. Based on the initial results an improved design has also been considered to achieve reduced cross-axis sensitivity and nonlinearity. The fabricated sensor tested upto 13 g acceleration shows average sensitivity of 0.556 mV/g along normal to the proofmass plane. The measured cross-axis sensitivity was 3.272 μV/g for X-axis and 3.442 μV/g for Y-axis which is less than 1% of Z-axis sensitivity. Comparing two designs there was an improvement of 63% sensitivity along Y-axis for the design with flexures placed along the proofmass edges.

Response of piezoresistive MEMS accelerometers and pressure transducers to high gamma dose

Several piezoresistive microelectromechanical (MEMS) sensors are operated in a gamma ray environment to doses of 800 kGy. The pressure transducers and accelerometers are micromachined silicon-on-insulator (SOI) and bulk silicon devices, respectively. Both sensor types experienced similar performance degradation: a drift in offset voltage with a slight increase in sensitivity. We explain the drift in offset voltage for all sensors tested by correlating the change in resistance of the silicon piezoresistors to the formation of oxide and interface trapped hole charges. We demonstrate how these charges effectively reduce the volume for current flow through the piezoresistors due to the creation of a depletion region surrounding the periphery of the gage resistors. Differences in the magnitude of the output voltage drift of the two sensor types are determined to be related to the unique construction of each sensor.