MOEMS accelerometers bring together the advantages of both optical measurement and MEMS technique. It has higher resolution than traditional accelerometers and can be widely implemented in more application fields. Packaging is an important step for MOEMS accelerometers in their fabrication process. It can maintain the high parallelism of the upper surface of the proof mass and the grating, so that it helps to improve the temperature stability of accelerometers. In addition, it can reduce the effect of the external temperature on the sensitive structures, thereby reducing the changes of the zero drift and scale factor by temperature. Since the accelerometers measures the acceleration which involves the stress and strain of the springs, the thermal stress introduced during the packaging process will have significant side impacts on the device performance and life, etc. In this paper, we establish a finite element method (FEM) model of the MOEMS accelerometer which contains package and sensitive structure based on grating interferometry cavity. The FEM model considers the thermal coupling of sensitive structure and adhesive, adhesive and package substrate. Based on it, the influence of the thermal stress of the material of the adhesive and the substrate are studied. The results show that a good match between the coefficient of the thermal expansion (CTE) of the substrate and sensitive structure material and a reduced elastic modulus as well as the increase of thickness of the adhesive can effectively diminish the thermal stress. Besides, well designed packaging can help to reduce the zero drift and scale factor drift to minimum.
Cross-sensitivity is a crucial parameter since it detrimentally affect the performance of an accelerometer, especially for a high resolution accelerometer. In this paper, a suite of analytical and finite-elements-method (FEM) models for characterizing the mechanism and features of the cross-sensitivity of a single-axis MOEMS accelerometer composed of a diffraction grating and a micromachined mechanical sensing chip are presented, which have not been systematically investigated yet. The mechanism and phenomena of the cross-sensitivity of this type MOEMS accelerometer based on diffraction grating differ quite a lot from the traditional ones owing to the identical sensing principle. By analyzing the models, some ameliorations and the modified design are put forward to suppress the cross-sensitivity. The modified design, achieved by double sides etching on a specific double-substrate-layer silicon-on-insulator (SOI) wafer, is validated to have a far smaller cross-sensitivity compared with the design previously reported in the literature. Moreover, this design can suppress the cross-sensitivity dramatically without compromising the acceleration sensitivity and resolution.
There is a temperature drift of an accelerometer attributed to the temperature variation, which would adversely influence
the output performance. In this paper, a quantitative analysis of the temperature effect and the temperature compensation
of a MOEMS accelerometer, which is composed of a grating interferometric cavity and a micromachined sensing chip,
are proposed. A finite-element-method (FEM) approach is applied in this work to simulate the deformation of the
sensing chip of the MOEMS accelerometer at different temperature from -20°C to 70°C. The deformation results in the
variation of the distance between the grating and the sensing chip of the MOEMS accelerometer, modulating the output
intensities finally. A static temperature model is set up to describe the temperature characteristics of the accelerometer
through the simulation results and the temperature compensation is put forward based on the temperature model, which
can improve the output performance of the accelerometer. This model is permitted to estimate the temperature effect of
this type accelerometer, which contains a micromachined sensing chip. Comparison of the output intensities with and
without temperature compensation indicates that the temperature compensation can improve the stability of the output
intensities of the MOEMS accelerometer based on a grating interferometric cavity.
The ultrahigh static displacement-acceleration sensitivity of a mechanical sensing chip is essential primarily for an ultrasensitive
accelerometer. In this paper, an optimal design to implement to a single-axis MOEMS accelerometer consisting
of a grating interferometry cavity and a micromachined sensing chip is presented. The micromachined sensing chip is
composed of a proof mass along with its mechanical cantilever suspension and substrate. The dimensional parameters of
the sensing chip, including the length, width, thickness and position of the cantilevers are evaluated and optimized both
analytically and by finite-element-method (FEM) simulation to yield an unprecedented acceleration-displacement
sensitivity. Compared with one of the most sensitive single-axis MOEMS accelerometers reported in the literature, the
optimal mechanical design can yield a profound sensitivity improvement with an equal footprint area, specifically, 200%
improvement in displacement-acceleration sensitivity with moderate resonant frequency and dynamic range. The
modified design was microfabricated, packaged with the grating interferometry cavity and tested. The experimental
results demonstrate that the MOEMS accelerometer with modified design can achieve the acceleration-displacement
sensitivity of about 150μm/g and acceleration sensitivity of greater than 1500V/g, which validates the effectiveness of
the optimal design.
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