Miniaturized load sensor using quartz crystal resonator constructed through microfabrication and bonding
© Murozaki et al.; licensee Springer 2014
Received: 18 February 2014
Accepted: 12 May 2014
Published: 23 July 2014
Highly sensitive, wide-measurement-range compact load sensors are desirable for various applications, including measurement of biosignals, manipulation and stiffness measurement of cells, and so on. Conventional load sensors are highly sensitive but have relatively small measurement ranges. A load sensor using an AT-cut quartz crystal resonator (QCR) has superior characteristics such as, high accuracy, improved strength under compressive stress, long-term stability, and compact size. However, a retention mechanism is required to firmly support the QCR because the QCR is easily broken by stretching and bending motions. Conventional machining processes are not suitable for further miniaturization of the sensor. Even if the retention mechanism were miniaturized, the assembly process is complicated.
In this paper, we propose a novel design and fabrication method for a load sensor using the QCR. Using microfabrication and bonding, the assembly process was simplified. We demonstrate the feasibility of a miniaturized QCR load sensor whose volume is 24.6 mm3 (width is 4 mm, height is 5.6 mm, depth is 1.1 mm). The experimental results showed that the nonlinearity and hysteresis were 0.94% F.S. and 1.68% F.S., respectively. Additionally, sensitivity of the sensor was 1458 Hz/N. We improved the sensitivity and stability of the sensor; the fluctuation was 0.04 mN over a period of 1 min. Moreover, the effects of the temperature change were evaluated. The temperature and the sensor output were linear within the range of 20°C–50°C.
KeywordsQuartz crystal resonator Load sensor Microfabrication
Highly sensitive, wide-measurement-range, compact load sensors are desirable in many fields, such as the medical field and biological fields. Using a high-performance load sensor, we can realize health management by simultaneous detection of multiple biosignals, such as distribution of body (high load), pulse/blood pressure, and body motion by breath (small loads) on a bed/chair. Moreover, it could be possible to handle cells without damage and to measure the stiffness of the cells using the load sensor for force feedback in manipulation. Load sensors have employed different methods, such as the use of strain gauges [,], piezoelectric vibration [,], and capacitance changes []. Conventional load sensors can be highly sensitive; however, in most cases, their measurement ranges are relatively narrow.
To realize a high-performance load sensor with both high sensitivity and a wide measurement range, we focused on the quartz crystal resonator (QCR). We developed a load sensor using the QCR (QCR load sensor). Quartz crystal is a piezoelectric element. The QCR generates a periodic signal with high stability based on vibration. The resonant frequency of a QCR changes with high linearity depending on the external force [,]. In addition, quartz crystals have high strength under compressive stresses []. Therefore, a high-resolution and wide-measurement-range load sensor can be realized using the QCR.
However, the QCR is fragile and easily broken by stretching and bending []. Therefore, a retention mechanism is required to affix the QCR to avoid detrimental horizontal forces [-]. Moreover, the retention mechanism plays an important role in efficiently transmitting external loads to the QCR. Fabrication of the retention mechanism and assembly process are complicated; miniaturization of the retention mechanism is quite difficult. Additionally, the sensor output is not linear owing to a gap between the retention mechanism and the QCR, and therefore, preload must be applied to the QCR to obtain linear output. In our previous work, a screw mechanism was used to apply preload to the retention mechanism [,]. This increased the number of sensor parts and further complicated assembly. In addition, to achieve further miniaturization and improvements in the sensor sensitivity, conventional machining processes are not feasible owing to the small size of the sensor elements.
In this paper, we aim to improve the QCR load sensor with regard to miniaturization and sensitivity to increase its number of effective applications. We propose a novel design and fabrication method of the load sensor using microfabrication and bonding. In this study, the sensor was successfully miniaturized and its sensitivity was improved.
Conceptual design of QCR load sensor
Here, load sensitivity (S l ) is constant and depends on the QCR material properties (cross sectional area, temperature, cut direction, etc.). Therefore, to increase the sensor sensitivity, it is necessary to improve the load transfer efficiency. The load transfer efficiency is improved if the rigidity of the leaf spring in the vertical direction is low. However, it is difficult to miniaturize the leaf spring further using conventional machining processes. Therefore, we used microfabrication to miniaturize the leaf spring [,].
An AT-cut quartz crystal has superior temperature stability at room temperature [,]. When AC voltage is applied between two metal electrodes on both sides of the AT-cut QCR, thickness-shear vibration is generated along the quartz crystal’s electrical axis (x-axis).
Retention mechanism design
k l must be less than k q , to increase the load transfer efficiency (η), which is from Eq. (5). From Eq. (8), it is evident that high load transfer efficiencies can be realized by decreasing h. However, b must be sufficiently large to firmly support the QCR. Therefore, the ideal leaf spring has a large aspect ratio.
Here, we determined the dimensions of the QCR and the retention parts via calculation using Eqs. (5)–(8) such that the load transfer efficiency (η) is 0.950: w = 2.0 mm, H q = 3.5 mm, t = 0.1 mm, W = 4.0 mm, H s = 5.0 mm, b = 1 mm, h = 0.69 mm, and L = 1.6 mm.
Fabrication of the QCR
Electrode patterns were formed to both faces of the AT-cut quartz crystal plate (thickness of 100 μm) using a photolithography technique lift-off process []. Chromium (Cr) and gold (Au) were deposited using sputtering equipment (E-200S, Canon ANELVA).
Sheet resist (50X077, Nichigo-Morton Co.) was laminated to the quartz crystal.
Sheet resist was patterned by photolithography.
QCRs was formed into the pattern of sheet resist using sandblasting (Elfo-Blaster, Elfotec CO.).
Fabrication of the Si structure
Photoresist (OFPR800 15CP,Tokyo Ohka Kogyo CO.,LTD) was patterned on the Si wafer (thickness of 500 μm).
Silicon wafer was hollowed to a depth of approximately 50 μm using deep reactive-ion etching (DRIE) (Multiplex-ASE-L, Sumitomo Precision Products Co.).
Photoresist (SU-8 3025, MicroChem Corp.) was patterned on the Si wafer.
Silicon wafer was formed into the pattern of SU-8 by DRIE, and SU-8 was removed.
Si structure was bonded from both sides of the QCR. Epoxy adhesive was used to bond the sensor parts, and wiring was attached using conductive silver paste.
A quartz crystal plate can be easily formed into any arbitrary shape using sandblasting. In addition, a Si wafer can be formed into minute structures having a high aspect ratio and high accuracy using DRIE.
Results and discussion
Stability of sensor output
In this paper, we proposed a novel design and fabrication method for a QCR load sensor. Using microfabrication and bonding, the assembly process was simplified, and we achieved miniaturization of the QCR load sensor. The proposed retention mechanism has improved load transfer efficiency via the Si structure formed using DRIE. We successfully fabricated a high-sensitivity load sensor, and significant miniaturization of the sensor was realized using microfabrication and bonding techniques. The parts all have a flat structure and the fabrication process is based on photolithography, and therefore, is suitable for mass production.
In the future, we will work toward further improvements regarding the sensitivity and miniaturization of the sensor. A key issue will be the methods used to bond the quartz crystal plate and Si parts together.
Quartz crystal resonator
Deep reactive ion etching
This work was partially supported by Center of Innovation Program and A-STEP, JST.
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