Accelerometers are among the most mature sensor technologies with a broad range of applications in multiple fields and industries. They represent the most widely used microelectromechanical system ...

Acceleration Sensors: Sensing Mechanisms, Emerging Fabrication Strategies, Materials, and Applications

The presence of residual stress is inevitable and major constraint for MEMS devices as they induce deformation, fracture, fatigue and micro structural changes in the structure. This paper presents the influence of residual stresses (up to − 2100 MPa) on the performance of AlN based piezoelectric MEMS accelerometer structure. The MEMS structure consists of an AlN coated silicon proof mass (800 µm × 800 µm × 300 µm) suspended by four beams (400 µm × 200 µm × 10 µm). Under the influence of residual stress (− 2100 MPa), the first three modal frequencies (13.54 kHz, 20.95 kHz and 20.95 kHz) of the accelerometer structure are reduced to 5.19 kHz, 10.17 kHz and 10.17 kHz respectively. The location of proof mass deflection and maximum von-Mises stress also get altered. Hence the structure sensitivity is also modified from 3.64 × 10−2/g (at no residual stress) to 2.54 × 10−4/g (at − 2100 MPa). The damping factor of the structure also gets modified from 3.96 to 10.38 due to the residual stress. Thus, the accelerometer frequency response (3 dB bandwidth) is found to be diminished from 1.81 to 0.26 kHz due to the presence of residual stress.

Influence of residual stress on performance of AlN thin film based piezoelectric MEMS accelerometer structure

This paper discusses the design and fabrication of MEMS differential capacitive accelerometer (z-axis sensitive) structure. The accelerometer structure consists of one each movable and reference capacitors in the single accelerometer die fabricated using highly conductive (p-type, resistivity: 0.001 Ω cm) SOI substrate. Resonant frequencies of the designed movable and reference capacitive structures were found to be 9.6 kHz and 150 kHz respectively. Corresponding rest capacitance (at 0 g) of both the capacitors was 2.21 pF. The movable and reference structures showed a deflection of 0.14 µm and 0.6 nm respectively at 50 g applied acceleration. Corresponding changes in capacitances of the movable and reference capacitors were 82.3 fF and < 0.33 fF respectively. The designed accelerometer showed a scale factor sensitivity of the movable capacitor was of ~ 1.65 fF/g. The device demonstrated a dynamic range of in − 17 g to 42 g with a full-scale non-linearity of ~ 3%. Corresponding measured scale factor sensitivity in the centrifuge test was found to be ~ 47 mV/g with an acceleration resolution of ~ 17 mg. The device exhibited cross-axis sensitivity of ~ 2% in the full-scale range. Measured 3 dB bandwidth (380 Hz) of the device matches reasonably with the simulated value (~ 400 Hz).

Design and fabrication of SOI technology based MEMS differential capacitive accelerometer structure

Piezoelectric accelerometers present higher sensitivities and better signal-to-noise ratios in comparison with piezoresistive and capacitive accelerometers. A micro-electromechanical system (MEMS) piezoelectric accelerometer was designed and manufactured with four L-shaped beams covered by Lead Zirconate Titanate (PZT) thin film with $1~\mu \text{m}$ in thickness which was deposited using sol-gel methods. The micro devices, whose size and the optimized PZT distribution were determined by theoretical and simulation analysis, were fabricated on the silicon substrate using PZT films working in the ${d}_{31}$ mode. Different from the conventional vertical crossbeam accelerometers, the designed structure devotes to the larger area piezoelectric film laid on L-shaped supporting beams with higher energy transferring efficiency for sensing. Then the chips of accelerometer were fabricated, and the proposed sensor was packaged on the printed circuit board (PCB). In addition, the microsphere was integrated in the center of the beams. Finally, the properties of the accelerometer were measured by vibration measurement system. The measuring results show that the initial sensitivity of the accelerometer is 28.14mV/g at 500Hz. The proposed piezoelectric accelerometer has high sensitivity and low resonance frequency, and the frequency response on each beam is consistent, providing a new structure for high-performance accelerometer.

High Sensitivity MEMS Accelerometer Using PZT-Based Four L-Shaped Beam Structure

A mathematical model of a sensor is vital to deeply comprehend its working principle and implement its optimal design. However, mathematical models of piezo-resistive eight-beam three-axis accelerometers have rarely been reported. Furthermore, those works are largely focused on the analysis of sensing acceleration in the normal direction, rather than in three directions. Therefore, a complete mathematical model of a piezo-resistive eight-beam three-axis accelerometer is developed in this paper. The validity of the mathematical model is proved by a Finite Element Method (FEM) simulation. Furthermore, the accelerometer is fabricated and tested. The prime sensitivities of X, Y and Z axes are 0.209 mV/g, 0.212 mV/g and 1.247 mV/g at 160 Hz, respectively, which is in accord with the values obtained by the model. The reason why the prime sensitivity SZZ is bigger than SXX and SYY is explained. Besides, it is also demonstrated why the cross-sensitivities SXZ and SYZ exceed SZX and SZY. Compared with the FEM model, the developed model could be helpful in evaluating the performance of three-axis accelerometers in an accurate and rapid way.

A Mathematical Model of a Piezo-Resistive Eight-Beam Three-Axis Accelerometer with Simulation and Experimental Validation.

This paper presents the effect of vacuum packaging on the band-width of push–pull type capacitive accelerometer structure. The accelerometer structure (for ± 30 g application) consists of silicon proof mass (1000 μm × 1000 μm × 30 μm) suspended by four L-shaped beams over 1 μm deep cavity. The fixed electrodes (Au) are on Pyrex glass substrates which are anodically bonded to the Si substrate. The squeeze film damping factor (ς) arising due to the trapped air between the electrodes is found to be ~ 2600 at atmospheric pressure. Introduction of holes in the proof-mass reduces ς to 750. In such a highly damped environment, the operational bandwidth (3 dB) bandwidth is found to be only around 20 Hz which is not suitable for many of the navigation applications. With vacuum packaging (vacuum level 100–760 Torr), there is not much noticeable improvement in the bandwidth due to high level of ς (> 150). In the 1–100 Torr range, the bandwidth is found to be improving linearly (from 20 Hz to 2 kHz) with the improving vacuum level. Further improvement of vacuum level (< 1 Torr) leads to the under-damped (ς < 1) condition and the proof mass start oscillating which also undesirable. Thus, vacuum packaging in the 1–10 Torr range (having ς: 2–15) is found to be suitable for achieving optimum operation bandwidth for the current accelerometer sensor.

Effect of vacuum packaging on bandwidth of push–pull type capacitive accelerometer structure

Deep boron diffusion often induces residual stress in bulk micromachined MEMS structures, which may affect the MEMS devices operation. In this study, we studied the modal patterns of MEMS vibratory gyroscope under the residual stress (100 – 1000 MPa). Modal patterns and modal frequencies of the gyro are found to be dependent on the residual stress values. Without any residual stress, the modal frequencies drive and sense modeswere found to be 20.06 kHz and 20.36 kHz respectively. In presence of 450 MPa residual stress, the modal frequencies of the drive and sense modes were changed to 42.75 kHz and 43.07 kHz respectively.

Effect of residual stress on modal patterns of MEMS vibratory gyroscope

The paper reports on process design for fabrication of a navigational grade ±30 g MEMS capacitive push-pull accelerometer based on SOI (Silicon-On-Insulator) technology using Pyrex Glass-Silicon-Pyrex Glass multi-stack. The accelerometer structure is fabricated by DWP (Dissolve Wafer Process) technique. The complete fabrication process and released structure results after two wafers process (SOI and Pyrex-Glass) step are discussed in this paper. After the release of accelerometer structure, another patterned glass wafer would be anodically bonded on top. The scheme of three wafer assembly of push-pull accelerometer is also proposed.

Abha Panchal

Papers

Effect of vacuum packaging on bandwidth of push–pull type capacitive accelerometer structure

Design and fabrication of SOI technology based MEMS differential capacitive accelerometer structure

Effect of residual stress on modal patterns of MEMS vibratory gyroscope

Process Design for Fabrication of Multi-stack MEMS Capacitive Push-Pull Accelerometer Based on SOI Technology