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A resonant microbeam accelerometer of a novel highly symmetric structure based on MEMS bulk-silicon technology is proposed and some numerical modeling results for this scheme are presented. The accelerometer consists of two proof masses, four supporting hinges, two anchors, and a vibrating triple beam, which is clamped at both ends to the two proof masses. LPCVD silicon rich nitride is chosen as the resonant triple beam material, and parameter optimization of the triple-beam structure has been performed. The triple beam is excited and sensed electromagnetically by film electrodes located on the upper surface of the beam. Both simulation and experimental results show that the novel structure increases the scale factor of the resonant accelerometer, and ameliorates other performance issues such as cross axis sensitivity of insensitive input acceleration, etc.

Resonant sensors have many advantages over the conventional type, such as high resolution, wide dynamic range, and quasi-digital nature of the output signal. One of the most commonly found silicon resonators is a microbeam clamped at both ends, which is sensitive to axial loads. This load can be made proportional to the physical parameters (acceleration, force, pressure etc.) via some kind of converting mechanism. To date, there have been several successful examples of resonant silicon accelerometers fabricated by both surface [

In order to increase the scale factor and decrease cross-sensitivity, this paper presents a resonant microbeam accelerometer of a novel highly symmetric structure, shown schematically in

The newly designed electromagnetically excited resonant micro accelerometer is shown in

To enhance the quality factor of the resonant micro accelerometer, a triple-beam resonator shown in

Parameter optimization mainly concerns two factors as follows:

1) Resonant frequency distinction between M1and M3

If the resonant frequency of M1 is too close to the resonant frequency of M3, when the triple-beam is working in the resonant M3 state, energy dissipation would be significant and the quality factor would be relatively low. Based on this assertion, the resonant frequency of M1 should be set much lower than the resonant frequency of M3, which can be achieved by adjusting the length of the decoupling region.

2) Coupling effect

The coupling effect criterion is that the vibrating amplitude difference of the central beam and the outer beam of M3 should be set as a minimum in that when the central beam has twice the width of the two outer beams, if the central beam has the same vibrating amplitude as that of outer beams, the sheer force and moment applied by three beams on decoupling region can be cancelled, which means there will be no energy dissipation.

When the inertial force caused by the proof mass under out-of-plane (Z-direction) acceleration bends the leverage structure, the axial force, which is several times larger than the inertial force, because of the action of the leverage structure on the resonator, will modify its intrinsic resonant frequency.

The main advantage of this design is that unlike conventional cantilever-based micro accelerometers, the cross-sensitivity issue can be successfully resolved by means of only one resonator. Because of symmetric placement of two proof masses at both ends of the resonator, when this device is under in-plane (X-direction and Y-direction) acceleration, there is nearly no frequency shift though there is resonator deformation.

Another advantage of this structure lies in its triple-beam resonator excitation mechanism. The configuration of separate drive circuit and pick off circuit shown in

Finite element modeling is an integral and essential aspect of the design and development process for resonant beam sensors. Commercial FEA software has been used for the numerical modelling to examine this design. The central beam dimensions are 800 μm × 80 μm × 3 μm, those of the supporting beam 600 μm × 100 μm × 20 μm and the proof mass dimensions are 2 mm × 1.2 mm × 200 μm. The material properties for the modeling are given as: for silicon, density, 2.33×10^{−15} kg/μm^{3}, Young’s Modulus, 1.65×10^{5} MPa, Poisson’s Coefficient, 0.22. for silicon nitride, density, 3.1×10^{−15} kg/μm^{3}, Young’s Modulus, 3.85×10^{5} MPa, Poisson’s Coefficient, 0.245 [

First, computer simulations are carried out on axial stress distribution.

Next, modal frequency analysis is carried out at different applied acceleration loads. The results are given in

Simulation results from this Table show that resonant frequencies M1–M3 change as applied loads vary, indicating vibrations of the two micro beams. Taking the shift of frequencies of each modal as output, one can get scale factors (denoted as

The starting materials are 3 inch p type <100> 300 μm silicon wafers, with the resistivity of 0.01 Ωcm. Standard bulk-silicon micromachining technology was used to manufacture this resonant micro accelerometer which combines ICP deep etching and KOH anisotropic wet etching technology. The basic fabrication steps are shown in

First, a thin silicon oxide (100 nm) and a thick low stress silicon rich SiN film (3 μm) are grown on both sides of the wafer by thermal oxidation and LPCVD methods, respectively. Using thick positive photoresist as the mask, SiN and SiO_{2} on the reverse side are removed by RIE etching selectively to define the proof mass, then, ICP deep etching is performed to etch silicon to the depth of 100 μm (a).

Second, by means of lift-off technology, gold film electrodes have been formed on the SiN film on the front side of the wafer (b). Then, using thick positive photoresist as the mask, SiN is removed by RIE etching selectively to define the SiN beam, supporting beam and the proof masses, and using the same mask, ICP deep etching is conducted to etch silicon to the depth of 40 μm (c) .

Finally, the wafer is immersed in hot KOH solution, and the same etch is performed from the front and back sides of the wafer. The proof mass and suspension system are formed by time-controlled etching in KOH (d) and the SiN beams are released by undercutting.

The frequency response function between

Static sensitivity measurements were performed in closed-loop operation by an off chip self-oscillating circuit. By turning the device in the gravitation field, several devices are measured, with sensitivities of one single chip ranging between 1,000 and 1,500 Hz/g, in agreement with the FEM predicated value, though slightly lower.

A silicon rich SiN resonant microbeam accelerometer of a novel highly symmetric structure is presented. The parameter optimization of the triple-beam resonator has been conducted in order to enhance the quality factor. The novel structure, which includes two symmetric proof masses at both ends of the resonator, can effectively eliminate cross-sensitivity. This excitation mechanism highly can avoid disturbing modes of the triple beam resonator, which further enhances the quality factor of the design. The major fabrication steps have been described in this paper and FEA simulation shows that the sensitivity of this design is more than 1,600 Hz/g over the ± 5g full scale. Preliminary measurements are in good agreement with simulated results. Further investigation into the acceleration sensing performance is being carried out.

This research was supported by the Chinese National Natural Science Foundation (No. 60674112).

Schematic view of an electromagnetically excited resonant micro accelerometer.

Schematic view of the triple-beam resonator.

Three major flexure modes of triple-beam resonator.

Distribution of

Simulated differential frequency output of a pair of identical sensors over the ±5g full scale.

Fabrication process flow for a resonant accelerometer.

SEM of the triple beam.

Measured gain and phase frequency response in air.

Simulation results of modal frequency analysis at different applied acceleration loads.

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M1 | 40773 | 42385 | 40774 | 40775 |

M2 | 65798 | 67250 | 65800 | 65799 |

M3 | 73722 | 75336 | 73722 | 73722 |