- Photonics Research
- Vol. 9, Issue 4, 521 (2021)
Abstract
1. INTRODUCTION
Pressure sensing can provide rich interaction force information related to the object and is of significant importance in artificial intelligence input devices [1], electronic skin [2,3], organ system medical pressure monitoring [4], gas-pressure monitoring [5], and industry applications [6]. Piezo-resistivity-based pressure sensors making use of physic-electronic transduction are the most common sensor type because of their easy signal readout capability. Many micronanostructures, such as ultrathin gold nanowires [7] and urchin-like hollow carbon spheres [8], were proposed to enhance transduction efficiency. However, electronical sensors still suffer electromagnetic interference, a narrow working temperature range, a small pressure measurement range, and large temperature cross-sensitivity, which make them unusable in a harsh environment. For example, pressure sensing at a high-temperature compressor can play an important role in aeroengine investigation and its active control [9–11]. The compressor exit temperatures are on the order of , which prevents the use of traditional electronical sensors without a complex cooling method. In addition, temperature interference is a nerve-wracking obstruction for sensor performance, since the large and unstable temperature cross-sensitive characteristics of the sensor will deteriorate the measurement even if the sensor survives at high temperatures.
In recent years, fiber-optic pressure sensors fabricated by employing the microelectromechanical systems (MEMS) technology [12–16] have attracted a great deal of attention because of the feasibility of mass production and high consistency. Anodic bonding technology is the usual way to form a sealed vacuum Fabry–Perot (F–P) microcavity between glass and silicon. The pressure sensitivity can be easily adjusted by designing the thickness of elastic silicon diaphragm and the microcavity diameter. Due to the microcavity diameter of MEMS, fiber-optic pressure sensors are not limited to the diameter of the optical fiber [17]; a pressure sensitivity can be easily reached. However, in a wide temperature range, temperature cross-sensitivity is inevitably a severe problem. The temperature cross-sensitivity mainly arises for two reasons. One is the residual gas trapped in the microcavity. Although the anodic bonding process is carried out under a high vacuum condition, some gas will be produced and trapped in the microcavity because of the chemical reaction essence of the anodic bonding process. The residual gas will shrink or expand with temperature change and produce an undesirable force on the inside face of the pressure-sensing diaphragm. The other reason is the thermal stress at the interface between silicon and glass. The thermal stress caused by the thermal expansion mismatch between different materials will cause nonlinear variation of temperature cross-sensitivity. The two factors together lead to worse temperature-related stability and therefore deteriorate the measurement precision. The problem can be relaxed with Au/Au thermal-compression bonding to some degree by reducing gas production and relieving thermal expansion mismatch with an Au film as a buffer layer [14].
Using real-time temperature measurement to compensate for the pressure-temperature cross-error is an alternative method. For example, a hybrid fiber-optic Fabry–Perot interferometer (FPI) fabricated by double-sided anodic bonding of a through-hole-array-structured glass wafer and two silicon wafers for simultaneous pressure and temperature measurement was proposed [15], or temperature information was obtained by fluorescent material glued near the anodic bonding structure [16]. Nevertheless, the degree of temperature compensation effectiveness will depend largely on the temperature stability of the sensor. The compensation may fail in high-precision measurement when the thermal stress has not been eliminated completely. Therefore, reducing temperature cross response is an essential issue to be solved in order to improve pressure measurement accuracy under a wide dynamic temperature range.
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In this paper, an all-silicon dual-cavity fiber-optic pressure sensor is proposed and demonstrated. A silicon substrate and a silicon diaphragm with etched cylindrical cavity are bonded together by silicon/silicon direct bonding to form a sealed vacuum F–P cavity for pressure sensing. The vacuum cavity and the silicon substrate act as two microcavities in series connection. We theoretically analyze the effect of thermal stress and residual gas pressure on pressure measurement, which indicates that the low-temperature cross-sensitivity can fundamentally improve the accuracy of pressure measurement over a wide temperature range. The all-silicon structure fabricated with direct bonding solves the problem of interface thermal mismatch, gas building up in the sealed microcavity, and additional reflective coating. The experiment results show that the pressure sensitivity is of under air pressure ranging from 20 to 280 kPa. The pressure-temperature cross-sensitivity of the proposed pressure sensor is as low as , which verifies that the thermal stress and residual gas issues have been overcome. To our best knowledge, this sensor presents the lowest pressure-temperature cross-sensitivity among the optical fiber pressure sensors with the capability of surviving high temperatures up to 700°C. Moreover, the silicon microcavity can be used for simultaneous temperature sensing, which is also a useful input and can be used for the further temperature compensation process. The optical fiber pressure sensor is promising for a wide range of applications, especially for pressure detection under a wide temperature range.
2. SENSOR CONFIGURATION AND OPERATION PRINCIPLE
Figure 1.(a) Schematic diagram of the all-silicon-based dual-cavity fiber-optic pressure sensor structure. (All the components are high-temperature resistant materials.) (b) Interference model of the dual-cavity structure with three reflective mirrors; (c) simulation of reflected spectra
A. Analysis of Sensing Characteristics of the Vacuum Cavity
Figure 2.All-silicon sensing chip’s mechanical deformation when external pressure and residual pressure are applied to it. Schematic diagram of the length and thickness of each part of the sensing chip.
Figure 3.Simulation results of the nonlinear response characters of the sensor model based on anodic bonding. (a) OPD response of
With our proposed all-silicon structure, there is almost no thermal stress at the bonding interface. Thus, Eqs. (10) and (11) will be simplified to constant values as and ,
B. Temperature Characteristics of the Silicon Cavity
can be used for temperature sensing because silicon owns a high thermo-optic coefficient and heat conductivity coefficient. Similarly, the relationship between OPD of and the change of pressure and temperature can be written as
The pressure cross-sensitivity of can be expressed as
The temperature sensitivity can be expressed as
According to Eqs. (12) and (17), the variation of OPDs corresponding to and can be expressed as a matrix to realize pressure and temperature simultaneous measurement,
3. SENSOR FABRICATION
The all-silicon sensing chip is fabricated by employing MEMS technology. A silicon-on-insulator (SOI) wafer with a device layer of thickness and a double-sided polished silicon wafer with thickness are chosen as the materials. The size and the orientation of the two wafers are both 100 mm and .
Figure 4.Fabrication processes of the proposed FPI sensing chip. (a) Spin the photoresist on the surface of the device layer of the SOI wafer; (b) photolithograph with the pre-prepared mask; (c) etch the cavity array by dry etching; (d) remove the photoresist; (e) prebond the SOI wafer with the silicon wafer; (f) anneal the prebonded wafer; (g) remove the handle layer and buried oxide layer by dry etching; (h) roughen the surface of the device layer by ultraviolet laser; (i) dice the bonded wafer into independent sensing chips; (j) assemble the sensing chip with silica capillary and gold-coated SMF.
Figure 5.Pictures of different parts of the fiber-optic pressure sensor structure. (a) Complete sensor after the MEMS process and package; (b) sectional view of a sensing chip and the inset is the detailed section view under a microscope; and (c) top view of the whole wafer before being roughened.
4. RESULTS AND DISCUSSION
A. Sensor Response to the Pressure Variation
Figure 6.Experimental configuration for investigation of the pressure characteristic of the sensor.
Figure 7.Example of the demodulation process from the reflection spectra. (a) Recorded interference spectra under 20°C and 100 kPa; (b) OPD results after taking fast Fourier transform of the reflection spectra.
According to the thin plate or small deflection theory [19], the maximum deformation of silicon diaphragm should be less than 20% of its thickness to ensure that the deformation changes linearly with external pressure. Thus, the maximum measurement pressure is about 385 kPa. The experiment was carried out when the temperature of the thermostat was stabled at , 0°C, 20°C, 40°C, and 60°C. At each temperature, the pressure was controlled to increase from 20 to 280 kPa, with a step of 20 kPa.
Figure 8.Demodulation results of OPDs’ response to pressure from 20 to 280 kPa at low temperatures. The demodulation results corresponding to (a) vacuum cavity
On the other hand, as Fig. 8(b) shows, the OPD of remains almost unchanged with pressure increasing to 280 kPa, which demonstrates that the silicon microcavity is insensitive to pressure.
B. Sensor Response to the High-Temperature Variation
Figure 9.Experimental configuration for the investigation of the high-temperature characters of the sensor.
Figure 10.Demodulation results of OPDs’ response to high temperature from 100°C to 700°C under atmosphere environment. The demodulation results corresponding to (a) vacuum cavity
From Fig. 10(a), it can be observed that OPD of the vacuum cavity shifts with temperature linearly with a low-temperature cross-sensitivity of 0.256 nm/°C and 0.261 nm/°C, corresponding to the heating and cooling process, respectively, which is a little smaller than the theoretical value. This is because Young’s modulus of silicon decreases with increasing temperature. Since the experiment was carried out over a large temperature range, the silicon diaphragm deformation increased under the same external pressure. On the other hand, the lower linearity and coincidence in Fig. 10(a) further verified the complication of temperature interference, such as the tiny difference of two silicon wafer materials and tiny gas existence.
In Fig. 10(b), the OPD response of the silicon cavity exhibits a quadratic relation with temperature, with no hysteresis. The fittings to the heating and cooling data give the first-order coefficients of 121.670 nm/°C and 124.185 nm/°C, respectively, and the second-order coefficients of and , respectively, which agree well with the theoretical value of 111.660 nm/°C and .
In summary, from the discussion above, the sensor shows the ultralow temperature cross-sensitivity and excellent stability during the high-temperature cycle experiment. Pressure and temperature can be simultaneously obtained by solving the following matrix equation: Comparison of the Proposed Fiber-Optic Pressure Sensors in Terms of Structure, Pressure Sensitivity, Temperature Cross-Sensitivity, and Pressure-Temperature Cross-SensitivityType Structure Pressure Sensitivity Temperature Pressure-Temperature MEMS The present work 33.034 nm/kPa 0.197 nm/°C 5.96 Pa/°C Silicon-glass-silicon double-sided anodic bonding [ 12.816 nm/kPa 3.365 nm/°C 263 Pa/°C Silicon-glass anodic bonding [ Silicon-glass thermal compression bonding [ 47.26 nm/kPa 3.4 nm/°C 71.9 Pa/°C All sapphire direct bonding [ All-silica SMF-MMF-silica diaphragm [ 24.8 nm/kPa 1.48 nm/°C 60 Pa/°C SMF-HC-PBF-HCF [ 1.336 nm/kPa 0.1 nm/°C 74 Pa/°C Fiber-tip air bubble FPI [ 24.44 nm/kPa 2.6 nm/°C 106 Pa/°C SMF with side-open F–P cavity [ 4.071 pm/kPa (wavelength shift) 0.83 pm/°C (wavelength shift) 204 Pa/°C SMF fabricated by femtosecond laser [ 0.56 nm/kPa 15.86 Pa/°C
5. CONCLUSION
In this paper, we theoretically analyzed the factors that influence the temperature cross-sensitivity of a microcavity pressure sensor. An all-silicon dual-cavity fiber-optic pressure sensor was successfully fabricated. The sensing chip comprises two silicon layers, which are bonded together by direct bonding. A sealed vacuum F–P microcavity was formed between the silicon substrate and the silicon diaphragm for pressure sensing. The silicon substrate acts as the second solid microcavity for temperature sensing. The sensor can eliminate the influence of thermal stress and residual pressure greatly. The experiment results showed that the ultralow pressure-temperature cross-sensitivity of was successfully obtained. In addition, the sensor can survive a temperature of up to 700°C. Therefore, the proposed fiber-optic pressure sensor provides an excellent candidate for pressure measurement in harsh and complicated environments.
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