Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 120 (2015) 857 – 860
EUROSENSORS 2015
All-Quartz High Accuracy MEMS Pressure Sensor Based on Double-Ended Tuning Fork Resonator J. Wang, C. Zhao, G. H. Zhao, X.F. Jin, S.M. Zhang, J.B. Zou Beijing Research Institute of Telemetry, No. 1 Nandahongmen Road, Fengtai District,Beijing 100076, P. R. CHINA
Abstract This paper reports an all-quartz high accuracy MEMS absolute pressure sensor based on high Q double-ended tuning fork (DETF) resonator, which is suitable for application in automotive, petroleum, meteorological, aerospace and spacecraft field. etc. The main parts of the MEMS pressure sensor, including the DETF resonator, diaphragm and back cavity structure, are all fabricated by quartz crystal using MEMS process. These pieces are bonded together as ‘sandwich’ structure to form the absolute pressure sensing unit using glass paste under low temperature and vacuum condition. This process could effectively eliminate the thermal stress effect and form the reference vacuum cavity. The experimental results show that the quality factor of the DETF resonator is up to 61000, the sensitivity is up to 7.35Hz/kPa in the operating range 0~250kPa, the pressure error is only 0.021%FS over the temperature range -40~+60. ©2015 2015The TheAuthors. Authors. Published by Elsevier Ltd. © Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015. Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
Keywords: All-quartz MEMS, double-ended tuning fork resonator, absolute pressure sensor
1. Introduction. Compared with piezoresistive or capacitive pressure sensors, resonant pressure sensors are attractive for high accuracy, high stability, low power consumption and inherently digital-type output1. High accuracy resonant pressure sensors commonly employ resonators such as resonant beams or tuning forks as resonant strain gauges to sense pressure-induced stresses in a diaphragm2. R.J. Cheng.et al has reported a resonant pressure sensor by combination of DETF quartz resonator and silicon diaphragm3. However, the mismatch of between quartz crystal and silicon could affect the accuracy and stability of the pressure sensor. In this paper, we report an all-quartz * Corresponding author. Tel.: +86-10-68750293; fax: +86-10-88520335. E-mail address:
[email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
doi:10.1016/j.proeng.2015.08.732
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MEMS absolute pressure sensor based on quartz crystal DETF resonator, quartz crystal diaphragm and quartz crystal back cavity structure. The pressure sensor which we report here is believed to be higher accuracy and stability than that using quartz resonator and silicon diaphragm, because the three main parts are all fabricated by quartz crystal with same orientation through MEMS process. 2. Device design. The pressure sensing element consists of three parts, namely quartz crystal DETF resonator, quartz crystal diaphragm and quartz crystal back cavity structure, as schematically shown in Fig.1(a). The diaphragm includes a thin portion which deforms in response to pressure and a pair of bosses for fixing the quartz crystal DEFT resonator. The bosses, together with DETF resonator are used for sensing the strain of the diaphragm because of the applied pressure. The sensitivity of the sensor is relative with the thickness of bosses, which enlarging the strain of the diaphragm. The quartz crystal DETF resonator is located between the diaphragm and the back cavity. The back cavity structure is used for avoiding the DETF resonator contacting with the bottom. The three parts of sensing element are bonded together using glass frit bonding technology to form a vacuum cavity for sensing pressure.
Fig.1. Schematic drawing of (a) pressure sensing unit; (b) DETF resonator with vibration attenuation; (c) electrodes on the DETF resonator.
Figure.1(b) shows the schematic drawing of the quartz crystal DETF resonator with vibration attenuation structure and the electrode distribution on the DETF resonator. The DETF length and width are in the y and x direction of the Z-cut quartz crystal wafer, respectively. Electrodes are arranged as shown in figure.1(c), so that the electric field is mainly from the field in the x direction to obtain the maximum electrical mechanical coupling, according to the finite element analysis. The vibration attenuation structure and stress equilibration joint reduce the transference of vibration energy from the DETF to the diaphragm, and guarantee high Q value of the DETF4. The fundamental resonant frequency f is dependent upon the force F aligned along the length L, can be expressed as5
f f 0 1 SF Where,
f 0 a0 S as
t2 L2
L2 Ebt 3
E
(1) (2) (3)
Where E is Young’s modulus, is mass density, L, t and b are length, width and thickness of the DETF resonator, respectively. f0 is the fundamental resonant frequency, and constant a0,aS are 1.026 and 0.294, respectively.
J. Wang et al. / Procedia Engineering 120 (2015) 857 – 860
3. Fabrication Figure.2 shows the fabrication process of the DETF resonator using quartz crystal anisotropic chemical etching micromachining and three dimensions electrodes deposition technology based on shadow mask process. The thickness of the Z-cut quartz crystal wafer is 200μm. Cr/Au film was deposited at the both sides of the wafer by heat evaporation, and patterned with typical photolithography process as the mask for quartz crystal anisotropic wet etching. Then the electrode pattern was transferred to the photoresist layer by standard photolithography process and then electrodes shapes were formed by wet etching. At last, the side electrodes were deposited based on the shadow mask process. The main process flow of the diaphragm and back cavity structure is similar to the DETF resonator.
Fig.2. Fabrication process of the quartz crystal DETF resonator.
The three parts are bonded together as ‘sandwich’ structure to form the absolute pressure sensing unit using glass paste under low temperature and vacuum condition. In view of the special structure, The DETF is bonded to the diaphragm in the first step and to back cavity structure forming vacuum-cavity in the second step6. Fig. 3 illustrates the image of the pressure sensing unit. And the quality factor of the DETF resonator is up to 61000. The packaged prototype is shown in Fig.4. The pressure sensing unit was fixed on the backside of the print circuit board with pierce oscillator circuit.
Fig. 3. Image of the fabricated all-quartz pressure sensing unit.
Fig.4. The packaged pressure sensor.
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4. Results and discussions Pierce oscillator circuit was used to excite the DETF resonator and output the steady square signal. Universal frequency counter was used to measure the frequency output as a function of pressure over the temperature range 40 ~+60 , and the result is shown in Fig. 5(a). The sensitivity is up to 7.35Hz/kPa in the operating range 0~250kPa at room temperature. Fig. 5(b) shows that the pressure error band, including the combined effects of nonlinearity, hysteresis, repeatability and temperature errors over the compensated temperature range, is only 0.021%FS.
Fig. 5. (a) Frequency output of the pressure sensor versus applied pressure at different temperature; (b) Pressure error band with four order polynomial compensation over the pressure and temperature range.
5. Conclusion A novel all-quartz high accuracy MEMS absolute pressure sensor based on high Q double-ended tuning fork (DETF) resonator has been designed and fabricated by the MEMS fabrication process. The sensitivity of the pressure sensor is up to 7.35Hz/kPa in the operating range 0~250kPa at room temperature, and the pressure error band is only 0.021%F.S. over the temperature range -40~+60. This research results are useful for designing and fabricating many kinds of MEMS sensors based on quartz double-ended tuning fork (DETF) resonator. References [1] Errol P. Eernisse, Roger W. Ward, Robert B. Wiggins, Survey of Quartz Bulk Resonator Sensor Technologies, J. Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions, 1988, Vol. 35, Issue 3, pp 323-330. [2] O. Brand, H. Baltes, Micromachined resonant sensors––an overview, Sensors Update,1998, Vol. 4, No. 1, pp. 3-51. [3] R.J. Cheng, Y.L. Zhao, C. Li, B. Tian, Z.L. Yu, K.Y. Liu, Design and fabrication of a resonant pressure sensor by combination of DETF quartz resonator and silicon diaphragm, J. Microsyst Technol, March 2015, Vol. 21, Issue 3, pp 631-640. [4] Jian Wang, Cong Zhao, Shiming Zhang, Jiangbo Zou, Quartz crystal double-ended tuning fork resonator for high resolution force sensing, J. Key Engineering Materials Vols.609-610, 2014, pp 1181-1184. [5] Errol P.Eerniss, J.M.Paros, Practical considerations for miniature quartz resonator force transducers, 37th Annual Symposium on Frequency Control, Philadelphia, Pennsylvania, USA, 1983. [6] Cong Zhao, Jian Wang, Shiming Zhang, Jiangbo Zou, Glass frit as a hermetic joining material for bonding among three wafers with metallic film feed-through, J. Key Engineering Materials, 2014, Vol. 609-610, pp 489-494.
