sensors Article
Fabrication of All-SiC Fiber-Optic Pressure Sensors for High-Temperature Applications Yonggang Jiang 1,2 , Jian Li 1 , Zhiwen Zhou 1 , Xinggang Jiang 1 and Deyuan Zhang 1, * 1 2
*
School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China; [email protected] (Y.J.); [email protected] (J.L.); [email protected] (Z.Z.); [email protected] (X.J.) International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, China Correspondence: [email protected]; Tel.: +86-10-8231-7707
Academic Editor: Vittorio M. N. Passaro Received: 26 July 2016; Accepted: 5 October 2016; Published: 17 October 2016
Abstract: Single-crystal silicon carbide (SiC)-based pressure sensors can be used in harsh environments, as they exhibit stable mechanical and electrical properties at elevated temperatures. A fiber-optic pressure sensor with an all-SiC sensor head was fabricated and is herein proposed. SiC sensor diaphragms were fabricated via an ultrasonic vibration mill-grinding (UVMG) method, which resulted in a small grinding force and low surface roughness. The sensor head was formed by hermetically bonding two layers of SiC using a nickel diffusion bonding method. The pressure sensor illustrated a good linearity in the range of 0.1–0.9 MPa, with a resolution of 0.27% F.S. (full scale) at room temperature. Keywords: pressure sensor; silicon carbide; ultrasonic assisted machining; bonding; harsh environment
1. Introduction High-temperature pressure sensors are of great significance for the monitoring of the dynamic pressures of well logging instruments, chemical reaction kettles, and even combustion chambers. Conventional piezoresistive pressure sensors using silicon diaphragms and piezoelectric pressure sensors are usually unable to withstand temperatures higher than 700 ◦ C [1,2]. Silicon carbide (SiC) is considered a promising material for the fabrication of high-temperature pressure sensors, owing to its mechanical robustness and chemical inertness at elevated temperatures. In addition, the electrical characteristics of SiC feature a wide bandgap, a high-breakdown electric field, and a low leakage current, making it a better candidate than silicon for high-temperature electronic applications [3,4]. High-temperature pressure sensors using SiC-based piezoresistive and capacitive detection mechanisms are available in the 350–600 ◦ C temperature range [5–9]. The silicon substrate and electrical wirings limit their sensing performances at elevated temperatures. Moreover, most of them are sensitive to temperature variations and electromagnetic interference (EMI). Compared with piezoresistive and capacitive sensors, fiber-optic pressure sensors offer high temperature capability, EMI immunity, and corrosion and oxidation resistance [10,11]. Diaphragm-based extrinsic Fabry-Perot interferometer (EFPI) pressure sensors are widely reported for healthcare, automotive, and aerospace applications, in which the diaphragm materials used include polymers [12,13], metals [14,15], silica [16,17], silicon [18,19], and other ceramics [20]. The working temperature of an EFPI pressure sensor is limited by the material of its diaphragm. Pulliam et al. [21] proposed an all-SiC EFPI pressure sensor with a deposited SiC diaphragm and a single-crystal SiC substrate. The SiC pressure sensor shows a nonlinear response that is attributed to the internal stress left during the deposition process. Several technological challenges remain for the development of all-SiC pressure sensors, such as micromachining for SiC diaphragms and hermetic bonding for sensor cavities. In this paper, we propose an EFPI pressure sensor using a single-crystal SiC diaphragm, which is advantageous for Sensors 2016, 16, 1660; doi:10.3390/s16101660
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Several technological challenges remain for the development of all-SiC pressure sensors, such as micromachining for SiC diaphragms and hermetic bonding for sensor cavities. In this paper, we propose an EFPI pressure sensor using a single-crystal SiC diaphragm, which is advantageous for its low internal stress and high mechanical reliability. Both the thin SiC diaphragm and the substrate are Sensors 2016, 16, 1660 9 fabricated by an ultrasonic vibration mill-grinding method. The sensor cavity is formed2 ofby a nickel diffusion bonding technique. its low internal stress and high mechanical reliability. Both the thin SiC diaphragm and the substrate are and fabricated by an ultrasonic vibration mill-grinding method. The sensor cavity is formed by a nickel 2. Design Calculations diffusion bonding technique.
The EFPI pressure sensor comprises of a SiC diaphragm, a SiC substrate, and an optical fiber as 2. Design and Calculations shown in Figure 1. As the diaphragm and substrate of the sensor head are both single-crystal SiC EFPI pressure sensor comprises of a the SiC signal diaphragm, a SiC substrate, andby an aoptical fiber as change with a sameThe coefficient of thermal expansion, fluctuation induced temperature shown in Figure 1. As the diaphragm and substrate of the sensor head are both single-crystal SiC with can be minimized. The cavity length of the SiC pressure sensor varies with the pressure difference a same coefficient of thermal expansion, the signal fluctuation induced by a temperature change can be betweenminimized. the innerThe cavity and the outer environment, which can be measured using a scheme shown cavity length of the SiC pressure sensor varies with the pressure difference between in Figure Thecavity lightand injected from a white-light is reflected from theinfiber endface and the1.inner the outer environment, whichsource can be measured usingpartly a scheme shown Figure 1. Thesurface light injected white-light source reflected partly from fiber endface andthrough the inner the same the inner of thefrom SiCadiaphragm. The istwo reflections thenthe propagate back surface of theinterference SiC diaphragm. The twoThe reflections thenlights propagate back through the same fiber and fiber and generate fringes. reflected propagate through a fiber circulator and generate interference fringes. The reflected lights propagate through a fiber circulator and are detected are detected by a mini-spectrometer. The Fabry-Perot (F-P) cavity length can be determined by by a mini-spectrometer. The Fabry-Perot (F-P) cavity length can be determined by demodulating the demodulating the generated interference fringes [22]. generated interference fringes [22].
Figure 1. Schematic illustration of the silicon carbide (SiC) sensor head and the measurement scheme.
Figure 1. Schematic illustration of the silicon carbide (SiC) sensor head and the measurement scheme. The SiC diaphragm is bonded with a SiC substrate as shown in Figure 2. Under uniformly
Thedistributed SiC diaphragm is bonded with a SiC substrate as shown in Figure 2. Under uniformly applied pressure, the center deflection of a circular sensor diaphragm is given by the distributed applied pressure, the center deflection of a circular sensor diaphragm is given by the following equation: 3 1 − µ2 p following equation: 4 y0 =
16Eh3
×r ,
(1)
where y0 is the deflection, p is the pressure applied 2on the diaphragm, µ is the Poisson’s ratio, E is 3(1 μ )p 4 Young’s modulus, and h and r are the thickness y0 and the3 effective r radius of the diaphragm, respectively. Eh as the, ratio of the deflection to the pressure Therefore, the pressure sensitivity (S), which is16 defined difference, can be calculated using Equation (2):
(1)
where y0 is the deflection, p is the pressure 3 1 − on µ2 the4 diaphragm, μ is the Poisson’s ratio, E is y0 applied S = = ×r . (2) Young’s modulus, and h and r are the pthickness 16Eh3 and the effective radius of the diaphragm, respectively. Therefore, the pressure sensitivity (S), which is defined as the ratio of the deflection to the pressure difference, can be calculated using Equation (2):
S
y0 3(1 2 ) 4 r p 16 Eh3 .
(2)
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Figure 2. 2. Deflection ofof rigidly clamped flat diaphragm under uniform pressure. Figure 2. Deflection of aa rigidly clamped flat diaphragm under uniform pressure. Figure Deflection a rigidly clamped flat diaphragm under uniform pressure.
The pressure sensitivity variation with the dimensions the sensor diaphragm illustrated The pressure sensitivity variation with the dimensions ofof the sensor diaphragm is is illustrated inin Figure 3, which compares the analytical results obtained from Equation (2) and the finite element Figure 3, which compares the analytical results obtained from Equation (2) and the finite element both the calculation and the FEM analysis, the Young’s the Poisson’s method (FEM). both the calculation and the FEM analysis, the Young’s modulus and the Poisson’s method (FEM). InIn modulus and toto450 450 GPa and 0.142, respectively. ratioofof6H–SiC 6H–SiCare aresetsetto 450 GPa and 0.142, respectively.AsAsshown shownininFigure Figure3, 3,the thepressure pressure ratio GPa and 0.142, respectively. sensitivityincreases increasesbybyenlarging enlargingthe thediaphragm diaphragmradius radiusand anddecreasing decreasingthe thediaphragm diaphragmthickness. thickness. sensitivity decreasing diaphragm thickness. miniaturization Consideringsensor sensorminiaturization miniaturizationobjectives objectivesand andactual actualfabrication fabricationcapabilities, capabilities,the theradius radiusofofthe the Considering objectives capabilities, diaphragm is set to be 1.5 mm. The displacement resolution of the fiber-optic detection system being diaphragm is set to be 1.5 mm. The displacement resolution of the fiber-optic detection system being is set to be mm. The displacement resolution of the fiber-optic detection system approximately 2020nm, diaphragm should bebethinner than 5050µm ininin order toto achieve approximately nm, thesensor sensor diaphragm should thinner than order achievea a being approximately 20the nm, the sensor diaphragm should be thinner than 50µm µm order to resolutionwith witha aafull fullrange rangeofof of0–1 0–1MPa. MPa.The Thedimensions dimensionsof thediaphragm diaphragmcan canbebefurther further adetection detectionresolution resolution with full range 0–1 MPa. The dimensions ofofthe the diaphragm can miniaturized with the improvement in micro-grinding techniques. In this case, the thickness of miniaturizedwith withthe theimprovement improvementin in micro-grinding micro-grinding techniques. In ofthe the Inthis thiscase, case,the thethickness thickness of SiC diaphragm should decrease along with the radius SiC diaphragm should decrease along with the radius thesensor sensordiaphragm diaphragmto sustainthe the the SiC diaphragm should decrease along with the radiusofofthe the sensor diaphragm totosustain sustain pressure sensitivity. pressure sensitivity. sensitivity.
Figure 3. The pressure sensitivity varies with the dimensions of the sensor diaphragm. The dependency Figure 3. 3.The sensitivity varies with the of the sensor diaphragm. The Thepressure pressure sensitivity variesand with thedimensions dimensions is Figure evaluated here by theoretically calculation by FEM, respectively.of the sensor diaphragm. The dependency is is evaluated here byby theoretically calculation and byby FEM, respectively. dependency evaluated here theoretically calculation and FEM, respectively.
The frequency response of the SiC diaphragm isisofisofgreat importance when thethe dynamic range of The frequency response ofof the SiC diaphragm importance when range The frequency response the SiC diaphragm ofgreat great importance when thedynamic dynamic range the pressure sensor is considered. The diaphragm is defined as a free circular plate clamped ofofthe sensor is isconsidered. The is isdefined asvibrating circular plate thepressure pressure sensor considered. Thediaphragm diaphragm defined asa afree freevibrating vibrating circular plate at the edges. Its natural frequency f is expressed by Equation (3) [23]: n clamped atat the edges. ItsIts natural frequency fn fis expressed byby Equation (3)(3) [23]: clamped the edges. natural frequency n is expressed Equation [23]: αh E 1/21/2 1/2 fn = h . h E E fn fn 4πr2 3ρ (1 −µ2 ) 2 2
4π4rπr 33(1 ) ) (1. . 2 2
(3) (3)(3)
Here, ρ is the density of the SiC diaphragm, and α is a constant related to the vibrating modes of the diaphragm and is taken asthe 10.21 for its fundamental frequency. For a thickness of 50 µm is is the density ofof the SiC diaphragm, and αα is is aresonance constant related toto the vibrating modes ofof the Here, the density SiC diaphragm, and a constant related the vibrating modes the Here, and radius of 1.5 mm, the fundamental resonance frequency of the SiC diaphragm is calculated to diaphragm and asas10.21 a athickness diaphragm andis istaken taken 10.21forforitsitsfundamental fundamentalresonance resonancefrequency. frequency.For For thicknessofof5050µm µm be 124.5 kHz, which satisfies the requirements for most of theofhigh-temperature applications. and radius ofof 1.5 mm, the fundamental resonance frequency the SiC diaphragm is is calculated toto bebe and radius 1.5 mm, the fundamental resonance frequency of the SiC diaphragm calculated
124.5 kHz, which satisfies the requirements forfor most ofof the high-temperature applications. 124.5 kHz, which satisfies the requirements most the high-temperature applications.
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3. Fabrication
As single-crystal SiC is a hard and brittle material, the micromachining of the SiC substrate 3. Fabrication remains a great technological challenge, especially for a thin diaphragm with a low roughness. The ultrasonic vibration machining method has been proposed as an effectiveofcutting toremains reduce As single-crystal SiC is a hard and brittle material, the micromachining the SiCprocess substrate cutting force and improve surface quality.for Ultrasonic vibrationwith mill-grinding (UVMG) can attain a a great technological challenge, especially a thin diaphragm a low roughness. The ultrasonic small grinding force and a highhas machining quality, as which plays ancutting important role in andcutting brittle vibration machining method been proposed an effective process tohard reduce material [23]. The material removal process both brittle broken and force andprocessing improve surface quality. Ultrasonic vibrationhas mill-grinding (UVMG) canplastic attainremoval a small characteristics in the ultrasonic vibration mill-grinding processing ofrole SiC.inGrinding ultrasonic grinding force and a high machining quality, which plays an important hard and and brittle material parameters bothThe influence surfaceprocess finish has in UVMG. Thebroken grinding decreases the spindle processing [23]. materialthe removal both brittle andforce plastic removalas characteristics speed increases and the feed rate decreases [24]. As shown in Table with a spindle speed of in the ultrasonic vibration mill-grinding processing of SiC. Grinding and1,ultrasonic parameters both 45,000 rpm,the thesurface surfacefinish roughness decreases applyingforce an ultrasonic amplitude of 0.5 µm. In influence in UVMG. Theby grinding decreasesvibration as the spindle speed increases addition, a lower surface [24]. roughness can inbeTable obtained a smaller feed rpm, rate.the A surface sensor and the feed rate decreases As shown 1, withwith a spindle speedaxial of 45,000 diaphragm with a thickness of 43 µm is illustrated in Figure 4a. The inset inµm. Figure 4a shows aa lower small roughness decreases by applying an ultrasonic vibration amplitude of 0.5 In addition, surface roughness lay in the can center of the diaphragm, which can be removed by modifying the computer be obtained with a smaller axial feed rate. A sensor diaphragm with a thickness numerical (CNC) programs. Figure profileaof a machined surface by of 43 µm iscontrol illustrated in Figure 4a. The inset4b in shows Figure the 4a shows small surface lay in themeasured center of the adiaphragm, surface profiler (D-600, KLA-Tencor, CA,the USA), and thenumerical roughnesscontrol (Ra) is (CNC) as low programs. as 19 nm. which can be removed byMilpitas, modifying computer Figure 4b shows the profile of a machined surface measured by a surface profiler (D-600, KLA-Tencor, TableCA, 1. Variations of the sensor diaphragm feed rate and amplitude of ultrasonic Milpitas, USA), and roughness (Ra)roughness is as lowwith as 19the nm. vibration, in the case that the spindle rotation speed is 45,000 rpm. Table 1. Variations of sensor diaphragm roughness with the feed rate and amplitude of ultrasonic Wheel Spindle Speed Axial Feed Vibration Roughness vibration, in the case that the spindle rotation speed is 45,000 rpm. No.
Mesh Size
4
No.
5 6 7 8 9
(rpm)
600# Wheel Mesh Size
600# 4 5 6 7 8 9
Spindle 45,000 Speed (rpm)
45,000
600# 600#600# 600#600# 600# 600#600# 600#600#
45,000 45,000 45,000 45,000 45,000 45,000 45,000 45,000 45,000 45,000
(a)
Rate (μm/s)
Axial 0.16 Feed Rate (µm/s)
0.32
0.16 0.64 0.32 0.64 0.16 0.16 0.32 0.32 0.64 0.64
Amplitude (μm) Vibration 0 Amplitude (µm)
0
0 00 0 0.5 0.5 0.5 0.5 0.5 0.5
Ra (nm)
31 Roughness Ra (nm) 76
31 192 76 19219 19 2525 3131
(b)
Figure Figure 4. 4. A A sensor sensor diaphragm diaphragm fabricated fabricated by by the the UVMG UVMG method method with with aa thickness thickness of of 43 43 µm. µm. (a) (a) SEM SEM images, (b) measured surface roughness with a trace length of 400 µm. images; (b) measured surface roughness with a trace length of 400 µm.
The sensor cavity with a radius of 1.5 mm was fabricated on a SiC substrate using the UVMG The sensor cavity with a radius of 1.5 mm was fabricated on a SiC substrate using the UVMG method. A 150-µm-diameter through hole was formed in the SiC substrate using a picosecond pulsed method. A 150-µm-diameter through hole was formed in the SiC substrate using a picosecond laser machining technique to fit the optical fiber. In order to achieve a sealed reference cavity, it was pulsed laser machining technique to fit the optical fiber. In order to achieve a sealed reference cavity, necessary to make a hermetic bonding of the SiC layers and mount the optical fiber with a it was necessary to make a hermetic bonding of the SiC layers and mount the optical fiber with high-temperature ceramic adhesive. The nickel diffusion bonding method [25] was used to bond the a high-temperature ceramic adhesive. The nickel diffusion bonding method [25] was used to bond the SiC diaphragm and the substrate together. Both of the SiC layers were sputtered with a 500-nm-thick SiC diaphragm and the substrate together. Both of the SiC layers were sputtered with a 500-nm-thick
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nickel film, and then bonded high temperature 900 °C C with with aa pressure pressure of 42 42 MPa MPa for for 30 30 min. min. nickel film, and then bonded atataahigh temperature ofof900 °C with a pressure ofof42 MPa for 30 min. A cross-sectional view of the bonded interface is shown in Figure 5. cross-sectionalview viewofofthe thebonded bondedinterface interfaceisisshown shownininFigure Figure5.5. A cross-sectional
Figure5.5. 5.Bonding Bonding interface ofthe the SiC layers using the nickel diffusion methodwith withaaatemperature temperatureofof of Figure Figure Bondinginterface interfaceofof theSiC SiClayers layersusing usingthe thenickel nickeldiffusion diffusionmethod method with temperature 900°C and pressureofof of42 42MPa. MPa. ◦°C 900 900 Cand anda aapressure pressure 42 MPa.
Inorder order to verifythe the sealingperformance performance ofthe theNi Nidiffusion diffusionbonding, bonding,aaSiC SiCdiaphragm diaphragmand andaa InIn ordertotoverify verify thesealing sealing performanceofof the Ni diffusion bonding, a SiC diaphragm and SiC substrate were fabricated by UVMG and bonded to form a sensor cavity, in which no through SiC substrate were fabricated byby UVMG and bonded toto form a asensor a SiC substrate were fabricated UVMG and bonded form sensorcavity, cavity,ininwhich whichno nothrough through hole is formed. The leakage rate variation in the ambient temperature was measured using ahelium helium hole is formed. The leakage rate variation in the ambient temperature was measured using a hole is formed. The leakage rate variation in the ambient temperature was measured using a helium detector. As shown in Figure 6, the leakage rate of the sensor cavity increases abruptly at 127 °C and detector. ininFigure ◦ Cand detector.As Asshown shown Figure6,6,the theleakage leakagerate rateofofthe thesensor sensorcavity cavityincreases increasesabruptly abruptlyatat127 127°C and −8 atm·cc/s reaches 1.1 × 10 at 540 °C. This suggests that the bonding conditions should be improved −8 atm·cc/s at 540 °C. This suggests that the bonding conditions should be improved reaches 1.1 × 10 − 8 ◦ reaches 1.1 × 10 atm·cc/s at 540 C. This suggests that the bonding conditions should be improved toachieve achieve a lowleakage leakage to makethe the sensorfunctional functional at highertemperature temperature conditions. toto achieveaalow low leakagetotomake make thesensor sensor functionalatathigher higher temperatureconditions. conditions.
Figure 6. Leakage Leakage rate of the sensor cavity measured by helium detector variation with the Figure6.6. rate the sensor cavity measured byhelium helium detector variation withthe theambient ambient Figure Leakage rate ofofthe sensor cavity measured by detector variation with ambient temperature. temperature. temperature.
SiCpressure pressuresensor sensor head square shape with length 6mm, mm, and thickness TheSiC sensorhead headisisisofof of square shape with aside side length 6 mm, and a thickness The square shape with aaside length ofof6of and aathickness ofof approximately 660660 µm. Considering the coefficient thermal expansion, the sensor head of approximately µm. Considering the coefficient thermal expansion, thesensor sensorhead headisisthen then approximately 660 µm. Considering the coefficient ofofof thermal expansion, the assembled into a molybdenum package fitting with M14 threads as shown in Figure 7. Finally, assembled into a molybdenum package fitting with M14 threads as shown in Figure 7. Finally, a single assembled into a molybdenum package fitting with M14 threads as shown in Figure 7. Finally, aa singlemode modeoptical optical fiberisisassembled assembled intothe the sensor head using microdisplacement displacement worktable mode optical fiber isfiber assembled into theinto sensor head using a micro displacement worktableworktable and sealed single sensor head using aamicro and sealed with a high-temperature ceramic adhesive to achieve the prototype device. with a high-temperature ceramic adhesive to achieve the prototype device. and sealed with a high-temperature ceramic adhesive to achieve the prototype device.
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Figure Figure7.7.The Thefiber-optic fiber-opticpressure pressuresensor sensorwith withaafull fullSiC SiCsensor sensorhead. head.
4.4.Characterization Characterizationand andDiscussions Discussions Characterization and Discussions The mechanical frequency The mechanical frequency response the sensor sensor diaphragm diaphragm was measured using laser response of of the was measured using aa laser Doppler Polytec, Waldbronn, Germany). As in 8,8,the Dopplermicro microsystem systemanalyzer analyzer(MSA-500, (MSA-500, Polytec, Waldbronn, Germany). As shown in Figure 8, micro system analyzer (MSA-500, Polytec, Waldbronn, Germany). Asshown shown inFigure Figure the fundamental resonance frequency isis approximately 78 kHz. the fundamental resonance frequency is approximately kHz.The Theresonance resonancefrequency frequencyof of the the fundamental resonance frequency approximately 7878 kHz. The resonance frequency of diaphragm as the pressure measurement for many high-temperature chambers diaphragm is is sufficiently sufficiently high, high-temperature chambers is sufficientlyhigh, high,as asthe thepressure pressuremeasurement measurementfor formany many high-temperature chambers isin in the 0–10 kHz dynamic range. thethe 0–10 kHz dynamic range. is in 0–10 kHz dynamic range.
Figure 8. Frequency response of the SiC diaphragm with a thickness of 43 µm. Figure 8. Frequency response of the SiC diaphragm with a thickness of 43 µm. Figure 8. Frequency response of the SiC diaphragm with a thickness of 43 µm.
Thesensor sensor wasconnected connected toan an adjustablepressure pressure chamberfor for calibration.AAcontinuous continuous white The The sensorwas was connectedto to anadjustable adjustable pressurechamber chamber forcalibration. calibration. A continuouswhite white light source, a circulator, and a spectrum analyzer were used for cavity length detection using the the light light source, source, aa circulator, circulator, and and aa spectrum spectrum analyzer analyzer were were used used for for cavity cavity length length detection detection using using the scheme shown in Figure 1. A standard interference spectrum of the EFPI pressure sensor is illustrated scheme schemeshown shownin inFigure Figure1.1.A Astandard standardinterference interferencespectrum spectrumof ofthe theEFPI EFPIpressure pressuresensor sensorisisillustrated illustrated in Figure 9, which shows obvious interferences, with an F-P cavity length of approximately 35 µm. in inFigure Figure9,9,which whichshows showsobvious obviousinterferences, interferences,with withan anF-P F-Pcavity cavitylength lengthof ofapproximately approximately35 35 µm. µm. As shown in Figure 10, the cavity length decreases linearly with the increase of the applied pressure As Asshown shownin inFigure Figure10, 10,the thecavity cavitylength lengthdecreases decreaseslinearly linearlywith withthe theincrease increaseof ofthe theapplied appliedpressure pressure from 0.1 0.1 MPa to to 0.9 MPa. MPa. The fluctuation of the cavity length is approximately 0.27% F.S. (full scale) as from from 0.1MPa MPa to0.9 0.9 MPa.The Thefluctuation fluctuationof ofthe thecavity cavitylength lengthisisapproximately approximately0.27% 0.27%F.S. F.S.(full (fullscale) scale) shown in Figure 11, while the pressure is held at 0.2 MPa for 400 min at room temperature. as asshown shownin inFigure Figure11, 11,while whilethe thepressure pressureisisheld heldat at0.2 0.2MPa MPafor for400 400min minat atroom roomtemperature. temperature. A conventional conventional single-modeSiO SiO2 opticalfiber fiber wasused used for this this prototype device. device. Inorder order to A A conventional single-mode single-mode SiO22 optical optical fiber was was used for for this prototype prototype device. In In order to to explorethe thetemperature temperaturelimit limitof ofthe thepressure pressuresensor, sensor,aasapphire-based sapphire-basedoptical opticalfiber fibershould shouldbe beutilized utilized explore explore the temperature limit of the pressure sensor, a sapphire-based optical fiber should be utilized toreplace replace theSiO SiO2 fiber.As As a result,the the characterizationof of thisprototype prototype devicewas was onlyconducted conducted to to replacethe the SiO22fiber. fiber. Asaaresult, result, thecharacterization characterization ofthis this prototypedevice device wasonly only conducted at room temperature. The fiber-optic SiC pressure sensor should be calibrated at high temperature at at room room temperature. temperature. The The fiber-optic fiber-optic SiC SiC pressure pressure sensor sensor should should be be calibrated calibrated at at high high temperature temperature conditions in future works. conditions conditionsin infuture futureworks. works.
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Figure 9. A standard interference spectrum of the EFPI pressure sensor with a cavity length of approximately 35 µm. Figure 9. A standard interference spectrum of the EFPI pressure sensor with a cavity length of Figure9.9. AAstandard standardinterference interferencespectrum spectrumofofthe theEFPI EFPIpressure pressuresensor sensorwith witha acavity cavitylength lengthofof Figure approximately 35 µm. approximately 35 µm. approximately 35 µm.
Figure 10. Cavity length as a function of pressures measured at room temperature. Figure 10. Cavity length as a function of pressures measured at room temperature. Figure 10. Cavity length as a function of pressures measured at room temperature. Figure 10. Cavity length as a function of pressures measured at room temperature.
Figure 11. The fluctuation of the cavity length while sustaining the pressure at 0.2 MPa for 400 min at Figure 11. The fluctuation of the cavity length while sustaining the pressure at 0.2 MPa for 400 min at room temperature. room temperature. Figure 11. The fluctuation of the cavity length while sustaining the pressure at 0.2 MPa for 400 min at Figure 11. The fluctuation of the cavity length while sustaining the pressure at 0.2 MPa for 400 min at room temperature. room temperature.
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5. Conclusions An all-SiC structure diaphragm-based EFPI pressure was developed. The SiC diaphragm was fabricated with a thickness of 43 µm and a surface roughness of 19 nm using ultrasonic vibration mill-grinding. The sensor head was formed using a nickel diffusion bonding technique. The pressure sensor shows a good linearity in the range of 0.1–0.9 MPa, with a resolution of 0.27% F.S. at room temperature. As the all-SiC pressure sensor needs to work at temperatures over 1000 ◦ C, future studies must work to improve the bonding process and investigate the high temperature characteristics of the pressure sensor. Acknowledgments: This work was funded by the National Key Basic Research Program of China (No. 2015CB059900). The frequency response measurement using a Micro system analyzer was performed with the assistance of Jinling Yang in the Institute of Semiconductors, CAS. The authors also acknowledge the helpful advice on sensor characterization from Jingming Song, Beihang University. Author Contributions: Yonggang Jiang conceived and designed the experiments; Zhiwen Zhou, Jian Li, and Xinggang Jiang fabricated the sensor and performed the characterization experiments; Yonggang Jiang and Deyuan Zhang analyzed the data; Yonggang Jiang, Jian Li, and Deyuan Zhang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Abeysinghe, D.C.; Dasgupta, S.; Boyd, J.T.; Jackson, H.E. A novel MEMS pressure sensor fabricated on an optical fiber. IEEE Photonics Technol. Lett. 2001, 13, 993–995. [CrossRef] Yin, J.; Liu, T.; Jiang, J.; Liu, K.; Wang, S.; Qin, Z.; Zou, S. Batch-Producible Fiber-Optic Fabry-Perot Sensor for Simultaneous Pressure and Temperature Sensing. IEEE Photonics Technol. Lett. 2014, 26, 2070–2073. Parkes, W.; Djakov, V.; Barton, J.S.; Watson, S.; MacPherson, W.N.; Stevenson, J.T.M.; Dunare, C.C. Design and fabrication of dielectric diaphragm pressure sensors for applications to shock wave measurement in air. J. Micromech. Microeng. 2007, 17, 1334–1342. [CrossRef] Pulliam, W.; Russler, P.; Mlcak, R.; Murphy, K.; Kozikowski, C. Micromachined, SiC fiber optic pressure sensors for high-temperature aerospace applications. Proc. SPIE 2000, 4202. [CrossRef] Peng, W.; Pickrell, G.R.; Wang, A.B. High-temperature fiber optic cubic-zirconia pressure sensor. Opt. Eng. 2005, 44, 124402. [CrossRef] Xu, J.C.; Pickrell, G.; Yu, B.; Han, M.; Zhu, Y.Z.; Wang, X.W.; Cooper, K.L.; Wang, A.B. Epoxy-free high temperature fiber optic pressure sensors for gas turbine engine applications. Proc. SPIE 2004, 5590. [CrossRef] Wang, Y.; Lin, B.; Wang, S.; Cao, X. Study on the system matching of ultrasonic vibration assisted grinding for hard and brittle materials processing. Int. J. Mach. Tools Manuf. 2014, 77, 66–73. [CrossRef] Zhang, D.; Zhao, Y.; Jiang, Y. SiC-SiC Bonding Methods for Ultra High-Temperature Pressure Sensors. Nanotechnol. Precis. Eng. 2014, 12, 258–262. © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
Fabrication of All-SiC Fiber-Optic Pressure Sensors for High-Temperature Applications Yonggang Jiang 1,2 , Jian Li 1 , Zhiwen Zhou 1 , Xinggang Jiang 1 and Deyuan Zhang 1, * 1 2
*
School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China; [email protected] (Y.J.); [email protected] (J.L.); [email protected] (Z.Z.); [email protected] (X.J.) International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, China Correspondence: [email protected]; Tel.: +86-10-8231-7707
Academic Editor: Vittorio M. N. Passaro Received: 26 July 2016; Accepted: 5 October 2016; Published: 17 October 2016
Abstract: Single-crystal silicon carbide (SiC)-based pressure sensors can be used in harsh environments, as they exhibit stable mechanical and electrical properties at elevated temperatures. A fiber-optic pressure sensor with an all-SiC sensor head was fabricated and is herein proposed. SiC sensor diaphragms were fabricated via an ultrasonic vibration mill-grinding (UVMG) method, which resulted in a small grinding force and low surface roughness. The sensor head was formed by hermetically bonding two layers of SiC using a nickel diffusion bonding method. The pressure sensor illustrated a good linearity in the range of 0.1–0.9 MPa, with a resolution of 0.27% F.S. (full scale) at room temperature. Keywords: pressure sensor; silicon carbide; ultrasonic assisted machining; bonding; harsh environment
1. Introduction High-temperature pressure sensors are of great significance for the monitoring of the dynamic pressures of well logging instruments, chemical reaction kettles, and even combustion chambers. Conventional piezoresistive pressure sensors using silicon diaphragms and piezoelectric pressure sensors are usually unable to withstand temperatures higher than 700 ◦ C [1,2]. Silicon carbide (SiC) is considered a promising material for the fabrication of high-temperature pressure sensors, owing to its mechanical robustness and chemical inertness at elevated temperatures. In addition, the electrical characteristics of SiC feature a wide bandgap, a high-breakdown electric field, and a low leakage current, making it a better candidate than silicon for high-temperature electronic applications [3,4]. High-temperature pressure sensors using SiC-based piezoresistive and capacitive detection mechanisms are available in the 350–600 ◦ C temperature range [5–9]. The silicon substrate and electrical wirings limit their sensing performances at elevated temperatures. Moreover, most of them are sensitive to temperature variations and electromagnetic interference (EMI). Compared with piezoresistive and capacitive sensors, fiber-optic pressure sensors offer high temperature capability, EMI immunity, and corrosion and oxidation resistance [10,11]. Diaphragm-based extrinsic Fabry-Perot interferometer (EFPI) pressure sensors are widely reported for healthcare, automotive, and aerospace applications, in which the diaphragm materials used include polymers [12,13], metals [14,15], silica [16,17], silicon [18,19], and other ceramics [20]. The working temperature of an EFPI pressure sensor is limited by the material of its diaphragm. Pulliam et al. [21] proposed an all-SiC EFPI pressure sensor with a deposited SiC diaphragm and a single-crystal SiC substrate. The SiC pressure sensor shows a nonlinear response that is attributed to the internal stress left during the deposition process. Several technological challenges remain for the development of all-SiC pressure sensors, such as micromachining for SiC diaphragms and hermetic bonding for sensor cavities. In this paper, we propose an EFPI pressure sensor using a single-crystal SiC diaphragm, which is advantageous for Sensors 2016, 16, 1660; doi:10.3390/s16101660
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Several technological challenges remain for the development of all-SiC pressure sensors, such as micromachining for SiC diaphragms and hermetic bonding for sensor cavities. In this paper, we propose an EFPI pressure sensor using a single-crystal SiC diaphragm, which is advantageous for its low internal stress and high mechanical reliability. Both the thin SiC diaphragm and the substrate are Sensors 2016, 16, 1660 9 fabricated by an ultrasonic vibration mill-grinding method. The sensor cavity is formed2 ofby a nickel diffusion bonding technique. its low internal stress and high mechanical reliability. Both the thin SiC diaphragm and the substrate are and fabricated by an ultrasonic vibration mill-grinding method. The sensor cavity is formed by a nickel 2. Design Calculations diffusion bonding technique.
The EFPI pressure sensor comprises of a SiC diaphragm, a SiC substrate, and an optical fiber as 2. Design and Calculations shown in Figure 1. As the diaphragm and substrate of the sensor head are both single-crystal SiC EFPI pressure sensor comprises of a the SiC signal diaphragm, a SiC substrate, andby an aoptical fiber as change with a sameThe coefficient of thermal expansion, fluctuation induced temperature shown in Figure 1. As the diaphragm and substrate of the sensor head are both single-crystal SiC with can be minimized. The cavity length of the SiC pressure sensor varies with the pressure difference a same coefficient of thermal expansion, the signal fluctuation induced by a temperature change can be betweenminimized. the innerThe cavity and the outer environment, which can be measured using a scheme shown cavity length of the SiC pressure sensor varies with the pressure difference between in Figure Thecavity lightand injected from a white-light is reflected from theinfiber endface and the1.inner the outer environment, whichsource can be measured usingpartly a scheme shown Figure 1. Thesurface light injected white-light source reflected partly from fiber endface andthrough the inner the same the inner of thefrom SiCadiaphragm. The istwo reflections thenthe propagate back surface of theinterference SiC diaphragm. The twoThe reflections thenlights propagate back through the same fiber and fiber and generate fringes. reflected propagate through a fiber circulator and generate interference fringes. The reflected lights propagate through a fiber circulator and are detected are detected by a mini-spectrometer. The Fabry-Perot (F-P) cavity length can be determined by by a mini-spectrometer. The Fabry-Perot (F-P) cavity length can be determined by demodulating the demodulating the generated interference fringes [22]. generated interference fringes [22].
Figure 1. Schematic illustration of the silicon carbide (SiC) sensor head and the measurement scheme.
Figure 1. Schematic illustration of the silicon carbide (SiC) sensor head and the measurement scheme. The SiC diaphragm is bonded with a SiC substrate as shown in Figure 2. Under uniformly
Thedistributed SiC diaphragm is bonded with a SiC substrate as shown in Figure 2. Under uniformly applied pressure, the center deflection of a circular sensor diaphragm is given by the distributed applied pressure, the center deflection of a circular sensor diaphragm is given by the following equation: 3 1 − µ2 p following equation: 4 y0 =
16Eh3
×r ,
(1)
where y0 is the deflection, p is the pressure applied 2on the diaphragm, µ is the Poisson’s ratio, E is 3(1 μ )p 4 Young’s modulus, and h and r are the thickness y0 and the3 effective r radius of the diaphragm, respectively. Eh as the, ratio of the deflection to the pressure Therefore, the pressure sensitivity (S), which is16 defined difference, can be calculated using Equation (2):
(1)
where y0 is the deflection, p is the pressure 3 1 − on µ2 the4 diaphragm, μ is the Poisson’s ratio, E is y0 applied S = = ×r . (2) Young’s modulus, and h and r are the pthickness 16Eh3 and the effective radius of the diaphragm, respectively. Therefore, the pressure sensitivity (S), which is defined as the ratio of the deflection to the pressure difference, can be calculated using Equation (2):
S
y0 3(1 2 ) 4 r p 16 Eh3 .
(2)
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Figure 2. 2. Deflection ofof rigidly clamped flat diaphragm under uniform pressure. Figure 2. Deflection of aa rigidly clamped flat diaphragm under uniform pressure. Figure Deflection a rigidly clamped flat diaphragm under uniform pressure.
The pressure sensitivity variation with the dimensions the sensor diaphragm illustrated The pressure sensitivity variation with the dimensions ofof the sensor diaphragm is is illustrated inin Figure 3, which compares the analytical results obtained from Equation (2) and the finite element Figure 3, which compares the analytical results obtained from Equation (2) and the finite element both the calculation and the FEM analysis, the Young’s the Poisson’s method (FEM). both the calculation and the FEM analysis, the Young’s modulus and the Poisson’s method (FEM). InIn modulus and toto450 450 GPa and 0.142, respectively. ratioofof6H–SiC 6H–SiCare aresetsetto 450 GPa and 0.142, respectively.AsAsshown shownininFigure Figure3, 3,the thepressure pressure ratio GPa and 0.142, respectively. sensitivityincreases increasesbybyenlarging enlargingthe thediaphragm diaphragmradius radiusand anddecreasing decreasingthe thediaphragm diaphragmthickness. thickness. sensitivity decreasing diaphragm thickness. miniaturization Consideringsensor sensorminiaturization miniaturizationobjectives objectivesand andactual actualfabrication fabricationcapabilities, capabilities,the theradius radiusofofthe the Considering objectives capabilities, diaphragm is set to be 1.5 mm. The displacement resolution of the fiber-optic detection system being diaphragm is set to be 1.5 mm. The displacement resolution of the fiber-optic detection system being is set to be mm. The displacement resolution of the fiber-optic detection system approximately 2020nm, diaphragm should bebethinner than 5050µm ininin order toto achieve approximately nm, thesensor sensor diaphragm should thinner than order achievea a being approximately 20the nm, the sensor diaphragm should be thinner than 50µm µm order to resolutionwith witha aafull fullrange rangeofof of0–1 0–1MPa. MPa.The Thedimensions dimensionsof thediaphragm diaphragmcan canbebefurther further adetection detectionresolution resolution with full range 0–1 MPa. The dimensions ofofthe the diaphragm can miniaturized with the improvement in micro-grinding techniques. In this case, the thickness of miniaturizedwith withthe theimprovement improvementin in micro-grinding micro-grinding techniques. In ofthe the Inthis thiscase, case,the thethickness thickness of SiC diaphragm should decrease along with the radius SiC diaphragm should decrease along with the radius thesensor sensordiaphragm diaphragmto sustainthe the the SiC diaphragm should decrease along with the radiusofofthe the sensor diaphragm totosustain sustain pressure sensitivity. pressure sensitivity. sensitivity.
Figure 3. The pressure sensitivity varies with the dimensions of the sensor diaphragm. The dependency Figure 3. 3.The sensitivity varies with the of the sensor diaphragm. The Thepressure pressure sensitivity variesand with thedimensions dimensions is Figure evaluated here by theoretically calculation by FEM, respectively.of the sensor diaphragm. The dependency is is evaluated here byby theoretically calculation and byby FEM, respectively. dependency evaluated here theoretically calculation and FEM, respectively.
The frequency response of the SiC diaphragm isisofisofgreat importance when thethe dynamic range of The frequency response ofof the SiC diaphragm importance when range The frequency response the SiC diaphragm ofgreat great importance when thedynamic dynamic range the pressure sensor is considered. The diaphragm is defined as a free circular plate clamped ofofthe sensor is isconsidered. The is isdefined asvibrating circular plate thepressure pressure sensor considered. Thediaphragm diaphragm defined asa afree freevibrating vibrating circular plate at the edges. Its natural frequency f is expressed by Equation (3) [23]: n clamped atat the edges. ItsIts natural frequency fn fis expressed byby Equation (3)(3) [23]: clamped the edges. natural frequency n is expressed Equation [23]: αh E 1/21/2 1/2 fn = h . h E E fn fn 4πr2 3ρ (1 −µ2 ) 2 2
4π4rπr 33(1 ) ) (1. . 2 2
(3) (3)(3)
Here, ρ is the density of the SiC diaphragm, and α is a constant related to the vibrating modes of the diaphragm and is taken asthe 10.21 for its fundamental frequency. For a thickness of 50 µm is is the density ofof the SiC diaphragm, and αα is is aresonance constant related toto the vibrating modes ofof the Here, the density SiC diaphragm, and a constant related the vibrating modes the Here, and radius of 1.5 mm, the fundamental resonance frequency of the SiC diaphragm is calculated to diaphragm and asas10.21 a athickness diaphragm andis istaken taken 10.21forforitsitsfundamental fundamentalresonance resonancefrequency. frequency.For For thicknessofof5050µm µm be 124.5 kHz, which satisfies the requirements for most of theofhigh-temperature applications. and radius ofof 1.5 mm, the fundamental resonance frequency the SiC diaphragm is is calculated toto bebe and radius 1.5 mm, the fundamental resonance frequency of the SiC diaphragm calculated
124.5 kHz, which satisfies the requirements forfor most ofof the high-temperature applications. 124.5 kHz, which satisfies the requirements most the high-temperature applications.
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3. Fabrication
As single-crystal SiC is a hard and brittle material, the micromachining of the SiC substrate 3. Fabrication remains a great technological challenge, especially for a thin diaphragm with a low roughness. The ultrasonic vibration machining method has been proposed as an effectiveofcutting toremains reduce As single-crystal SiC is a hard and brittle material, the micromachining the SiCprocess substrate cutting force and improve surface quality.for Ultrasonic vibrationwith mill-grinding (UVMG) can attain a a great technological challenge, especially a thin diaphragm a low roughness. The ultrasonic small grinding force and a highhas machining quality, as which plays ancutting important role in andcutting brittle vibration machining method been proposed an effective process tohard reduce material [23]. The material removal process both brittle broken and force andprocessing improve surface quality. Ultrasonic vibrationhas mill-grinding (UVMG) canplastic attainremoval a small characteristics in the ultrasonic vibration mill-grinding processing ofrole SiC.inGrinding ultrasonic grinding force and a high machining quality, which plays an important hard and and brittle material parameters bothThe influence surfaceprocess finish has in UVMG. Thebroken grinding decreases the spindle processing [23]. materialthe removal both brittle andforce plastic removalas characteristics speed increases and the feed rate decreases [24]. As shown in Table with a spindle speed of in the ultrasonic vibration mill-grinding processing of SiC. Grinding and1,ultrasonic parameters both 45,000 rpm,the thesurface surfacefinish roughness decreases applyingforce an ultrasonic amplitude of 0.5 µm. In influence in UVMG. Theby grinding decreasesvibration as the spindle speed increases addition, a lower surface [24]. roughness can inbeTable obtained a smaller feed rpm, rate.the A surface sensor and the feed rate decreases As shown 1, withwith a spindle speedaxial of 45,000 diaphragm with a thickness of 43 µm is illustrated in Figure 4a. The inset inµm. Figure 4a shows aa lower small roughness decreases by applying an ultrasonic vibration amplitude of 0.5 In addition, surface roughness lay in the can center of the diaphragm, which can be removed by modifying the computer be obtained with a smaller axial feed rate. A sensor diaphragm with a thickness numerical (CNC) programs. Figure profileaof a machined surface by of 43 µm iscontrol illustrated in Figure 4a. The inset4b in shows Figure the 4a shows small surface lay in themeasured center of the adiaphragm, surface profiler (D-600, KLA-Tencor, CA,the USA), and thenumerical roughnesscontrol (Ra) is (CNC) as low programs. as 19 nm. which can be removed byMilpitas, modifying computer Figure 4b shows the profile of a machined surface measured by a surface profiler (D-600, KLA-Tencor, TableCA, 1. Variations of the sensor diaphragm feed rate and amplitude of ultrasonic Milpitas, USA), and roughness (Ra)roughness is as lowwith as 19the nm. vibration, in the case that the spindle rotation speed is 45,000 rpm. Table 1. Variations of sensor diaphragm roughness with the feed rate and amplitude of ultrasonic Wheel Spindle Speed Axial Feed Vibration Roughness vibration, in the case that the spindle rotation speed is 45,000 rpm. No.
Mesh Size
4
No.
5 6 7 8 9
(rpm)
600# Wheel Mesh Size
600# 4 5 6 7 8 9
Spindle 45,000 Speed (rpm)
45,000
600# 600#600# 600#600# 600# 600#600# 600#600#
45,000 45,000 45,000 45,000 45,000 45,000 45,000 45,000 45,000 45,000
(a)
Rate (μm/s)
Axial 0.16 Feed Rate (µm/s)
0.32
0.16 0.64 0.32 0.64 0.16 0.16 0.32 0.32 0.64 0.64
Amplitude (μm) Vibration 0 Amplitude (µm)
0
0 00 0 0.5 0.5 0.5 0.5 0.5 0.5
Ra (nm)
31 Roughness Ra (nm) 76
31 192 76 19219 19 2525 3131
(b)
Figure Figure 4. 4. A A sensor sensor diaphragm diaphragm fabricated fabricated by by the the UVMG UVMG method method with with aa thickness thickness of of 43 43 µm. µm. (a) (a) SEM SEM images, (b) measured surface roughness with a trace length of 400 µm. images; (b) measured surface roughness with a trace length of 400 µm.
The sensor cavity with a radius of 1.5 mm was fabricated on a SiC substrate using the UVMG The sensor cavity with a radius of 1.5 mm was fabricated on a SiC substrate using the UVMG method. A 150-µm-diameter through hole was formed in the SiC substrate using a picosecond pulsed method. A 150-µm-diameter through hole was formed in the SiC substrate using a picosecond laser machining technique to fit the optical fiber. In order to achieve a sealed reference cavity, it was pulsed laser machining technique to fit the optical fiber. In order to achieve a sealed reference cavity, necessary to make a hermetic bonding of the SiC layers and mount the optical fiber with a it was necessary to make a hermetic bonding of the SiC layers and mount the optical fiber with high-temperature ceramic adhesive. The nickel diffusion bonding method [25] was used to bond the a high-temperature ceramic adhesive. The nickel diffusion bonding method [25] was used to bond the SiC diaphragm and the substrate together. Both of the SiC layers were sputtered with a 500-nm-thick SiC diaphragm and the substrate together. Both of the SiC layers were sputtered with a 500-nm-thick
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nickel film, and then bonded high temperature 900 °C C with with aa pressure pressure of 42 42 MPa MPa for for 30 30 min. min. nickel film, and then bonded atataahigh temperature ofof900 °C with a pressure ofof42 MPa for 30 min. A cross-sectional view of the bonded interface is shown in Figure 5. cross-sectionalview viewofofthe thebonded bondedinterface interfaceisisshown shownininFigure Figure5.5. A cross-sectional
Figure5.5. 5.Bonding Bonding interface ofthe the SiC layers using the nickel diffusion methodwith withaaatemperature temperatureofof of Figure Figure Bondinginterface interfaceofof theSiC SiClayers layersusing usingthe thenickel nickeldiffusion diffusionmethod method with temperature 900°C and pressureofof of42 42MPa. MPa. ◦°C 900 900 Cand anda aapressure pressure 42 MPa.
Inorder order to verifythe the sealingperformance performance ofthe theNi Nidiffusion diffusionbonding, bonding,aaSiC SiCdiaphragm diaphragmand andaa InIn ordertotoverify verify thesealing sealing performanceofof the Ni diffusion bonding, a SiC diaphragm and SiC substrate were fabricated by UVMG and bonded to form a sensor cavity, in which no through SiC substrate were fabricated byby UVMG and bonded toto form a asensor a SiC substrate were fabricated UVMG and bonded form sensorcavity, cavity,ininwhich whichno nothrough through hole is formed. The leakage rate variation in the ambient temperature was measured using ahelium helium hole is formed. The leakage rate variation in the ambient temperature was measured using a hole is formed. The leakage rate variation in the ambient temperature was measured using a helium detector. As shown in Figure 6, the leakage rate of the sensor cavity increases abruptly at 127 °C and detector. ininFigure ◦ Cand detector.As Asshown shown Figure6,6,the theleakage leakagerate rateofofthe thesensor sensorcavity cavityincreases increasesabruptly abruptlyatat127 127°C and −8 atm·cc/s reaches 1.1 × 10 at 540 °C. This suggests that the bonding conditions should be improved −8 atm·cc/s at 540 °C. This suggests that the bonding conditions should be improved reaches 1.1 × 10 − 8 ◦ reaches 1.1 × 10 atm·cc/s at 540 C. This suggests that the bonding conditions should be improved toachieve achieve a lowleakage leakage to makethe the sensorfunctional functional at highertemperature temperature conditions. toto achieveaalow low leakagetotomake make thesensor sensor functionalatathigher higher temperatureconditions. conditions.
Figure 6. Leakage Leakage rate of the sensor cavity measured by helium detector variation with the Figure6.6. rate the sensor cavity measured byhelium helium detector variation withthe theambient ambient Figure Leakage rate ofofthe sensor cavity measured by detector variation with ambient temperature. temperature. temperature.
SiCpressure pressuresensor sensor head square shape with length 6mm, mm, and thickness TheSiC sensorhead headisisisofof of square shape with aside side length 6 mm, and a thickness The square shape with aaside length ofof6of and aathickness ofof approximately 660660 µm. Considering the coefficient thermal expansion, the sensor head of approximately µm. Considering the coefficient thermal expansion, thesensor sensorhead headisisthen then approximately 660 µm. Considering the coefficient ofofof thermal expansion, the assembled into a molybdenum package fitting with M14 threads as shown in Figure 7. Finally, assembled into a molybdenum package fitting with M14 threads as shown in Figure 7. Finally, a single assembled into a molybdenum package fitting with M14 threads as shown in Figure 7. Finally, aa singlemode modeoptical optical fiberisisassembled assembled intothe the sensor head using microdisplacement displacement worktable mode optical fiber isfiber assembled into theinto sensor head using a micro displacement worktableworktable and sealed single sensor head using aamicro and sealed with a high-temperature ceramic adhesive to achieve the prototype device. with a high-temperature ceramic adhesive to achieve the prototype device. and sealed with a high-temperature ceramic adhesive to achieve the prototype device.
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Figure Figure7.7.The Thefiber-optic fiber-opticpressure pressuresensor sensorwith withaafull fullSiC SiCsensor sensorhead. head.
4.4.Characterization Characterizationand andDiscussions Discussions Characterization and Discussions The mechanical frequency The mechanical frequency response the sensor sensor diaphragm diaphragm was measured using laser response of of the was measured using aa laser Doppler Polytec, Waldbronn, Germany). As in 8,8,the Dopplermicro microsystem systemanalyzer analyzer(MSA-500, (MSA-500, Polytec, Waldbronn, Germany). As shown in Figure 8, micro system analyzer (MSA-500, Polytec, Waldbronn, Germany). Asshown shown inFigure Figure the fundamental resonance frequency isis approximately 78 kHz. the fundamental resonance frequency is approximately kHz.The Theresonance resonancefrequency frequencyof of the the fundamental resonance frequency approximately 7878 kHz. The resonance frequency of diaphragm as the pressure measurement for many high-temperature chambers diaphragm is is sufficiently sufficiently high, high-temperature chambers is sufficientlyhigh, high,as asthe thepressure pressuremeasurement measurementfor formany many high-temperature chambers isin in the 0–10 kHz dynamic range. thethe 0–10 kHz dynamic range. is in 0–10 kHz dynamic range.
Figure 8. Frequency response of the SiC diaphragm with a thickness of 43 µm. Figure 8. Frequency response of the SiC diaphragm with a thickness of 43 µm. Figure 8. Frequency response of the SiC diaphragm with a thickness of 43 µm.
Thesensor sensor wasconnected connected toan an adjustablepressure pressure chamberfor for calibration.AAcontinuous continuous white The The sensorwas was connectedto to anadjustable adjustable pressurechamber chamber forcalibration. calibration. A continuouswhite white light source, a circulator, and a spectrum analyzer were used for cavity length detection using the the light light source, source, aa circulator, circulator, and and aa spectrum spectrum analyzer analyzer were were used used for for cavity cavity length length detection detection using using the scheme shown in Figure 1. A standard interference spectrum of the EFPI pressure sensor is illustrated scheme schemeshown shownin inFigure Figure1.1.A Astandard standardinterference interferencespectrum spectrumof ofthe theEFPI EFPIpressure pressuresensor sensorisisillustrated illustrated in Figure 9, which shows obvious interferences, with an F-P cavity length of approximately 35 µm. in inFigure Figure9,9,which whichshows showsobvious obviousinterferences, interferences,with withan anF-P F-Pcavity cavitylength lengthof ofapproximately approximately35 35 µm. µm. As shown in Figure 10, the cavity length decreases linearly with the increase of the applied pressure As Asshown shownin inFigure Figure10, 10,the thecavity cavitylength lengthdecreases decreaseslinearly linearlywith withthe theincrease increaseof ofthe theapplied appliedpressure pressure from 0.1 0.1 MPa to to 0.9 MPa. MPa. The fluctuation of the cavity length is approximately 0.27% F.S. (full scale) as from from 0.1MPa MPa to0.9 0.9 MPa.The Thefluctuation fluctuationof ofthe thecavity cavitylength lengthisisapproximately approximately0.27% 0.27%F.S. F.S.(full (fullscale) scale) shown in Figure 11, while the pressure is held at 0.2 MPa for 400 min at room temperature. as asshown shownin inFigure Figure11, 11,while whilethe thepressure pressureisisheld heldat at0.2 0.2MPa MPafor for400 400min minat atroom roomtemperature. temperature. A conventional conventional single-modeSiO SiO2 opticalfiber fiber wasused used for this this prototype device. device. Inorder order to A A conventional single-mode single-mode SiO22 optical optical fiber was was used for for this prototype prototype device. In In order to to explorethe thetemperature temperaturelimit limitof ofthe thepressure pressuresensor, sensor,aasapphire-based sapphire-basedoptical opticalfiber fibershould shouldbe beutilized utilized explore explore the temperature limit of the pressure sensor, a sapphire-based optical fiber should be utilized toreplace replace theSiO SiO2 fiber.As As a result,the the characterizationof of thisprototype prototype devicewas was onlyconducted conducted to to replacethe the SiO22fiber. fiber. Asaaresult, result, thecharacterization characterization ofthis this prototypedevice device wasonly only conducted at room temperature. The fiber-optic SiC pressure sensor should be calibrated at high temperature at at room room temperature. temperature. The The fiber-optic fiber-optic SiC SiC pressure pressure sensor sensor should should be be calibrated calibrated at at high high temperature temperature conditions in future works. conditions conditionsin infuture futureworks. works.
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Figure 9. A standard interference spectrum of the EFPI pressure sensor with a cavity length of approximately 35 µm. Figure 9. A standard interference spectrum of the EFPI pressure sensor with a cavity length of Figure9.9. AAstandard standardinterference interferencespectrum spectrumofofthe theEFPI EFPIpressure pressuresensor sensorwith witha acavity cavitylength lengthofof Figure approximately 35 µm. approximately 35 µm. approximately 35 µm.
Figure 10. Cavity length as a function of pressures measured at room temperature. Figure 10. Cavity length as a function of pressures measured at room temperature. Figure 10. Cavity length as a function of pressures measured at room temperature. Figure 10. Cavity length as a function of pressures measured at room temperature.
Figure 11. The fluctuation of the cavity length while sustaining the pressure at 0.2 MPa for 400 min at Figure 11. The fluctuation of the cavity length while sustaining the pressure at 0.2 MPa for 400 min at room temperature. room temperature. Figure 11. The fluctuation of the cavity length while sustaining the pressure at 0.2 MPa for 400 min at Figure 11. The fluctuation of the cavity length while sustaining the pressure at 0.2 MPa for 400 min at room temperature. room temperature.
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5. Conclusions An all-SiC structure diaphragm-based EFPI pressure was developed. The SiC diaphragm was fabricated with a thickness of 43 µm and a surface roughness of 19 nm using ultrasonic vibration mill-grinding. The sensor head was formed using a nickel diffusion bonding technique. The pressure sensor shows a good linearity in the range of 0.1–0.9 MPa, with a resolution of 0.27% F.S. at room temperature. As the all-SiC pressure sensor needs to work at temperatures over 1000 ◦ C, future studies must work to improve the bonding process and investigate the high temperature characteristics of the pressure sensor. Acknowledgments: This work was funded by the National Key Basic Research Program of China (No. 2015CB059900). The frequency response measurement using a Micro system analyzer was performed with the assistance of Jinling Yang in the Institute of Semiconductors, CAS. The authors also acknowledge the helpful advice on sensor characterization from Jingming Song, Beihang University. Author Contributions: Yonggang Jiang conceived and designed the experiments; Zhiwen Zhou, Jian Li, and Xinggang Jiang fabricated the sensor and performed the characterization experiments; Yonggang Jiang and Deyuan Zhang analyzed the data; Yonggang Jiang, Jian Li, and Deyuan Zhang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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