A micro Fabry–Perot sensor for nano-lateral displacement sensing with enhanced sensitivity and pressure resistance1

Sensors and Actuators A 114 (2004) 163–170

A micro Fabry–Perot sensor for nano-lateral displacement sensing with enhanced sensitivity and pressure resistance夽,1 Chun-Jun Lin, Fan-Gang Tseng∗ BioMicroSystems and NanoFluidics Laboratory, Department of Engineering and System Science, National Tsing Hua University, #101, Sec. 2, Kuang Fu Road, Hsinchu, Taiwan 300, ROC Received 3 July 2003; received in revised form 14 December 2003; accepted 14 January 2004

Abstract This paper proposes a novel sensitivity/signal enhancement method for a nano-displacement sensor fabricated by polymer MEMS technology and operated by Fabry–Perot interferometry. The surface roughness of the floating element fabricated by UV lithography on SU-8 photoresist is better than 5 nm (Ra value) on 35 ␮m × 35 ␮m area and can be served as reflection mirror. Silicon oil with refractive index 1.406 is filled into the sensor cavity as an index matching medium for signal and sensitivity enhancement as well as a buffer material for pressure resistance and vibration reduction. With silicon oil filling, the sensor sensitivity can be improved by 1.85 times and signal intensity is increased by 16.4 dB, respectively. The minimum detectable displacement and shear stress have been demonstrated to be 10 nm and 0.33 Pa, respectively. The sensitivity of displacement and shear stress sensing are of 0.1249 nm/nm (wavelength shift/floating element displacement) and 6.825 nm/Pa (wavelength shift/shear stress), respectively. Besides, the signal spectrum shifts have been tested within 1 nm under static pressure from 1 to 6 atm or acoustic vibration from 1 Hz to 5 kHz, because of the incompressibility and damping effect of the silicon oil. However, the temperature dependency and hysteresis of the sensor due to the thermal effect need to be improved or compensated for practical applications in the future. © 2004 Published by Elsevier B.V. Keywords: Fiber-optic; Fabry–Perot; Nano-displacement; Shear stress; Floating element; Polymer MEMS

1. Introduction There have been many applications of fiber optic Fabry–Perot sensors such as strain [3], displacement [4], vibration [5], pressure [6], chemical gas and humidity [7] sensing or health monitoring [8], because of their small size, high accuracy, resistance to harsh environment and immunity from electromagnetic interference (EMI). However, few researches have reported on shear stress measurement, as a result, a novel miniaturized fiber optic sensor based on the principle of Fabry–Perot interferometer (FPI) and

夽 This paper is written based on TRANSDUCERS’03 contribution with abstract number PR196, paper number 2E78.P and entitled “A Novel Micro Fabry–Perot Sensor Utilizing Refractive Index Matched Medium For High Sensitive Shear Stress Sensing”. 1 This paper was published in error in a previous issue of Sens. Actuators. A 113 (2004) 12–19, and now appears as part of this Special Issue. ∗ Corresponding author. Tel.: +886-3-5715131x4270; fax: +886-3-5720724. E-mail address: [email protected] (F.-G. Tseng).

0924-4247/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.sna.2004.01.066

polymer MEMS fabrication technology has been proposed [1,2] recently not only on shear stress measurement but also on nano-displacement sensing. The proposed micro Fabry–Perot sensor, as a conceptual schematic shown in Fig. 1, is a floating-element type device consisting a flexible silicon rubber membrane (MRTV1, American Safety Inc., USA) with the functions of supporting the floating element and protecting the inner sensing components from obstructing by particles in the outside environments. The incident light from the optical fiber to the floating element is reflected and interfered with the first reflection light from the fiber cleaved surface, thus produces a signal sensitive to the distance change between the fiber surface and floating element. This paper reports the second-generation of this sensor featured cantilevered clamp structures for fiber easy alignment and silicon oil (refractive index 1.406) filling into the sensor cavity for sensitivity/signal enhancement, pressure resistance and acoustic vibration reduction. Thermal cycle test has also been carried out to fully understand the temperature dependency of this sensor.

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Fig. 1. (a) Top view of the inner side of the proposed Fabry–Perot sensor before final packaging process and (b) A-A cross-section of conceptual design of the sensor after packaging process.

2. Sensor design and operation principle The detail design and operation principle of the second-generation sensor device will be introduced in this section. To improve the sensitivity of the sensor and overcome the disturbance from pressure and acoustic vibration on the sensing results, a new material is proposed to demonstrate these purposes. 2.1. Sensor design concept Fig. 1(a) shows the top view of the inner side of the conceptual design of this second-generation fiber optic Fabry–Perot sensor while Fig. 1(b) shows the A–A cross-section of Fig. 1(a) of the sensor. The sensor design, based on the floating element concept, is close to that of the previous generation [1,2], except two important modifications to enhance sensor performance. The first one is the adding of clamp structure for easy alignment of optical fibers. Since the distance between the cleaved fiber end and floating element affects the sensitivity and signal intensity of the sensor greatly, accurate definition of this distance during the fiber packaging is very important. As the fiber arranged closer to the floating element, the sensitivity becomes larger, however, the dynamic range will become smaller (Section 3 details the calculation). To have the dynamic range of 2 ␮m (250 nm/0.1249 nm/nm), the initial distance has been set to 50 ␮m, while the corresponding sensitivity is around 0.03 nm/nm from the calculation. The distance of 50 ␮m is precisely defined by the clamp structure during fiber assembly. On the other hand, the micro Fabry–Perot sensor is designed for sensing lateral displacement instead of vertical one. However, if there is no mechanism resisting vertical

movement as in the first generation, the floating element, sitting on a flexible membrane, may have large vertical displacement while exposed to a pressurized environment. Besides, the resistance mechanism of the vertical movement should not restrict the lateral movement otherwise it will reduce sensor sensitivity. As a result, liquid is designed filling inside the sensing cavity for pressure resistance by its incompressibility while not effect the lateral movement of the floating movement. With this design, not only pressure effect can be greatly reduced but the acoustic vibration can be damped down by the viscosity of the liquid. Among many liquids, silicon oil has been selected for the following reasons: (1) silicon oil does not react with the sensor structure and is chemically and physically stable during operation; (2) silicon oil with refractive index 1.406 close to that of fiber core with 1.48, can be employed as an index matching material to reduce diffraction effect for light transmitting from the fiber core to the sensor cavity, thus the diverging angle of the light emitted from the fiber cleaved surface will be reduced to enhance signal intensity; (3) silicon oil is very transparent to the applied wavelength, providing a high efficient light-wave-guide. As a result, silicon oil is employed as a filling liquid throughout this study. It is possible that the floating element will tilt due to the bending of the flexible membrane under the large gradient of shear stress or pressure and the Fabry–Perot gap spacing will be changed. However, this micro fiber-optic Fabry–Perot sensor is designed for applying in the environment with low gradient of shear stress or pressure such as on the surface of submarines or the wing of airplanes. In addition, the smaller the area of the membrane, the bending of the membrane can be minimized effectively. 2.2. Operation principle of the sensor The operational principle of the sensor is shown in Fig. 1(b). When flowing through the sensor surface, the fluid exerts a shear stress force on the MRTV1 membrane, thus induces a lateral displacement of the floating element. Sensing the displacement is based on the principle of Fabry–Perot interferometer. More detail schematic description is shown as Fig. 2. As the incident light (I) passing

Resonant cavity d , n2

Fiber core n1 I RI*I

T

R1 R2

Rj

Rj Partially Refractive

Fig. 2. Schematic description of the resonant cavity within the fiber optic Fabry–Perot interferometer.

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through the cleaved fiber end, partial light is reflected by the cleaved surface as R1 and the transmitted light is reflected back to the fiber by the floating element as R2 . Actually there will be more than two reflections between the two mirrors, as illustrated in the figure. However, only the first two reflections, R1 and R2 , with highest intensity are considered in the following calculation. The final interfered spectrum with the intensity of RI I, interfered between R1 and R2 , can be expressed as RI =

2Rj (1 − cos φ) 1 + R2j

− 2Rj cos φ

,

φ=

4πn2 d λ2

N = 0, 1, 2, . . .

N = 0, 1, 2, . . .

When the constructive condition stands, their relationship between the distance of fiber surface to floating element and wave length as well medium indices can be represented by the following equation: d=

λ2 1 (2N + 1) 4 n2

λ1 n2 = λ2 n1

and

(2)

After combining the above equations, we can obtain d=

n1 1 (2N + 1) 2 λ1 , 4 n2

N = 0, 1, 2, . . .

3. Fabrication process and result The fabrication process and fabrication result of the sensor will be elaborated in this section. Polymer MEMS technology is employed to map out the process flow to demonstrate the advantages of low temperature and low cost. 3.1. Process flow

and a destructive one as φ = 2Nπ,

To have high sensitivity and large dynamic range, the cyclic recording is desired for this sensor.

(1)

where I is the intensity of incident light and RI is the coefficient of interfered light intensity. The interfered spectrum is highly related to the wavelength of incident light (λ1 ) as well as the transmitting light in the medium (λ2 ), refractive index of the medium (n2 ) as well as the fiber core (n1 ), and the cavity distance (d) between the floating element and the cleaved fiber end. The spectrum is a constructive interference as φ = (2N + 1)π,

(3)

The fabrication process of the multi-purposes fiber optic Fabry–Perot sensor illustrated in Fig. 3 begins by molding 30 ␮m thick MRTV1 membrane on the Teflon coated silicon substrate (Fig. 3(a)). Because of the hydrophobic property of the Teflon film, the MRTV1 membrane with the fabricated sensor structures can be easily released from the silicon substrate after the end of the process. Oxygen plasma (75 W/10 Pa/10 sccm/15 s) is employed as hydrophilic surface treatment (Fig. 3(b)) to enhance the coating ability of SU-8 resist before SU-8 lithography process. Two lithography steps are then performed on SU-8 resist layers, each layer with thickness of 200 ␮m, defining the floating element, fiber grooves and glue channels, respectively (Fig. 3(c), (d)). After developing of SU-8 (Fig. 3(e)) and dicing of MRTV1 membrane, the sensor device is directly detached from the substrate (Fig. 3(f)). Before deposition of 1000 Å gold reflection film on the front side (toward the fiber) of the floating element by sputtering process (Fig. 3(g)), the sensor device, tilts out-off plane at 90◦ with the front side of floating element facing to the sputter target, is put onto the sample holder of the sputter. Then a fiber, deposited with 30 Å gold film on the cleaved end

Refractive indices for the fiber core and silicon oil are 1.48, and 1.406, respectively. The shift of the interfered spectrum is mainly due to the variation of cavity distance. As a result, the displacement of the floating element can be obtained by measuring the spectrum shift of the interfered signal. The orientation of the spectrum shift also determines the flow direction, thus realizes directional sensing. The sensitivity (S) of the sensor can also be improved by selecting a medium (silicon oil in current research) with larger index (n2 ) than that of air, as shown in the following equation: S=

n2

λ1 4 = 2

d n1 (2N + 1)

(4)

Since the signal has a cyclic character, the dynamic range D of the sensor will be inversely proportional to the sensitivity within a signal cycle: D=

λwhole spectrum S

165

(5)

Fig. 3. The fabrication process of the micro Fabry–Perot sensor.

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by e-beam evaporation, is aligned to the floating element and fixed in the fiber groove by UV curable epoxy (#363, LOCTITE, Fig. 3(h)). The cavity distance between the cleaved fiber end and the floating element is designed and controlled to be 50 ␮m by the clamp structure. Finally, silicon oil with refractive index 1.406 is filled into the cavity as a refractive index matching medium for sensitivity enhancement, followed by a Pyrex glass attaching onto the device and sealed with UV curable epoxy (Fig. 3(i), (j)). 3.2. The fabricated sensor devices Fig. 4(a) illustrates the SEM picture of the secondgeneration micro Fabry–Perot sensor with dimensions of 5 mm, 2 mm and 400 ␮m in length, width and height, respectively. The floating element with dimensions of 200 mm × 200 mm × 400 ␮m stands on the inner side of the released MRTV1 membrane with dimensions of 1.5 mm, 1.5 mm and 30 ␮m in length, width and thickness. Fig. 4(b) shows the close-up of the floating element, clamp structure and fiber. The clamp structure is designed for fiber positioning and alignment, the initial distance between the cleaved fiber-end and floating element is designed to be 50 ␮m in this sensor. Because the patterns of the clamp structure and the upper part (the 2nd SU-8 layer and where the fiber aligns to) of the floating element are designed in the same mask level (Mask #2), the optical fiber is supported by the first SU-8 layer as shown in Fig. 1(a) and (b) and can be easily aligned to the floating element by the clamp structure during each fiber packaging process. The inner widths of the U-groove and the end of the clamp are 135 and 110 ␮m, respectively, compared to the diameter of the optical fiber with125 ␮m. As the optical fiber inserting into the clamp structure, it will be held at the end of the clamp. Fig. 4(c) shows the packaged sensor devices. The surface roughness of the floating element was also characterized to be less than 5 nm (Ra value) on the 35 ␮m× 35 ␮m area by atomic force microscopy (AFM) scanning as shown in Fig. 5(a) and (b) and good enough for optical applications.

4. Experiment setup Testing experiments, including displacement and shear stress calibrations, pressure, acoustic vibration and thermal cycle testing, of this micro Fabry–Perot sensor have been successfully carried out by employing a fiber optic detection system illustrated in Fig. 6. The system consists a broadband ELED light source (␭: 1550 nm, 3 dB-half spectrum-width: 75 nm, PD-LD, USA) driven by a current controller (LDC210, Profile Optische Systeme, Germany), a three ports optical circulator (M-CN-15-T-C-2-FC/FC, FOCI, Taiwan), an optical spectrum analyzer (MS9710C, Anritsu, Japan) and single mode fibers (FS-SC-7324, 3MTM ,

Fig. 4. (a) SEM picture of the fabricated second-generation sensor before packaging, (b) close-up of the clamp structure, fiber and floating element and (c) the packaged devices.

USA). Besides, the displacement calibration experiment was implemented by a tilt and translation stage driven by LVPZT amplifier position controller (E-662.SR, Physik Instrumente (PI), Germany) with a displacement resolution of 2.5 nm.

5. Testing results and discussions Spectrum readouts are directly obtained from the optical spectrum analyzer (OSA) as shown in Fig. 7(a)–(c). The spectrum profile of the input broadband ELED light source shown in Fig. 7(a) has the center wavelength at 1550 nm

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167

Fig. 5. (a) Surface morphology of the floating element by AFM scanning on a 35 ␮m × 35 ␮m area and (b) cross-section and roughness analysis of the floating-element surface.

ELED,1550nm

Optical Circulator

SMF Current Controller

SMF

1 2 3

Input

Output

Optical Spectrum Analyzer (OSA)

Sensor device Fig. 6. Fiber optic detection system for the sensor testing.

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Wavelength shift (nm)

16 14

y = 0.1249x + 1.2308

Cavity filled with Silicon oil Cavity filled with Air

12 10 8 6 4

y = 0.0639x + 0.2615

2 0 0

10

20

30

40

50

60

70

Wavelength shift (nm)

16 14 12

90

100

110

y = 6.825x + 1.275

Cavity filled with Silicon oil Cavity filled with Air

10 8 6 y = 3.7071x + 0.6214

4 2 0 0.0

(b)

80

Floating element displacement (nm)

(a)

0.5

1.0

1.5

2.0

Shear Stress (Pa)

Fig. 8. (a) The relationship between the floating element’s displacement and wavelength shift of the sensor cavity filled with silicon oil or air and (b) wavelength shift versus shear stress induced on the MRTV1 membrane with sensor cavity filled with silicon oil or air.

Fig. 7. Spectrum readouts from the OSA of (a) ELED light source, (b) interfered spectrum of the sensor and (c) spectrum shift due to floating element’s displacement.

and 3 dB-half spectrum width about 75 nm. The interfered spectrum from the Fabry–Perot sensor shown in Fig. 7(b) demonstrates the constructive and destructive interference at the certain cavity distance of the sensor. As the cavity

distance varies due to the floating element’s displacement, spectrum will shift as shown in Fig. 7(c). The displacement calibration was carried out by measuring the wavelength shift of spectrum valley caused by the variation of the floating element’s displacement. A liner relationship with the maximum deviation of 22% from the linearity at the floating element’s displacement of 40 nm was obtained and shown in Fig. 8(a), and two sensors with silicon oil filling and no silicon oil (air) filling were tested and had the sensitivity of 0.1249 and 0.0639, respectively. The sensitivity was improved by 1.95 times compared to the calculation result of 1.98 times from Eq. (4). A 1.5% deviation illustrated a good matching for the estimation to the real sensor signals. The minimum detectable displacement for this sensor has also been determined to be 10 nm. Shear stress calibration has also been carried out in a rectangle flow channel of 0.5 mm depth, 10 mm width and 15 cm length, respectively. The corresponding shear stress versus spectrum valley shift is shown in Fig. 8(b). Sensor cavities filled with either air or silicon oil has very different sensitivity. Due to index matching by filling silicon oil inside sensor cavity, the sensitivity increases by 1.85 times than that of the air filling case from Fig. 8(b), which is also close to the calculation result of 1.98 times improvement estimated by Eq. (4). The minimum detectable shear stress was 0.33 Pa, determined by the noise level in the system. Besides the improvement of sensor sensitivity, filling

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169

Wavelength shift (nm)

1.2 1 0.8 0.6 0.4 0.2 0 1

2

3

(a)

4

5

6

Pressure (atm)

Dip Wavelength(nm)

1553 1552 1551 1550 1549 1548 1

10

(b)

100 Frequency (Hz)

1000

10000

Fig. 9. Signal intensity comparison for the sensor cavity filled with (a) pure DI water and (b) silicon oil.

Dip_Wavelength(nm)

Fig. 10. (a) Static pressure and (b) acoustic vibration testing of the sensor.

1595 1590 1585 1580 1575 1570 1565 1560 1555

Low->High High->Low 25

30

35

40

45

50

55

60

65

70

75

80

Temperature (ºC)

silicon oil also effectively enhances the signal intensity by 16.4 dB, as shown in Fig. 9(b), compared to Fig. 9(a) obtained by filling DI water. This is because the refractive index of silicon oil is close to that of the optical fiber core (silica, index 1.48), thus reducing the light dispersion in the medium. In addition to index matching medium, silicon oil was also employed to withstand the outside pressure and acoustic vibration by its incompressibility and damping property. The silicon oil-filled sensor has been tested to withstand the static pressure up to 6 atm and acoustic vibration with frequency from 1 Hz to 5 kHz. There was only 1 nm spectrum shift in maximum under aforementioned conditions, as shown in Fig. 10(a) and (b), respectively. To apply this sensor to real environment, thermal characteristic is another important consideration. In the experiment of temperature variation, the sensor was left on a hotplate with temperature stabilization at each testing point for 5 min before measurement, and the result is shown in Fig. 11. The sensor shows an average temperature dependency of 0.33 nm/◦ C, and also a hysteresis. This thermal characteristic may result from the thermal expansion coefficient mis-

Fig. 11. Hyeteresis phenomenon of the sensor through the thermal cycle testing.

match between SU-8 resist and fiber core, and friction effect among the assembly of sensor body, UV glue, silicon oil, and optical fiber. Detailed on this phenomena is still under going and several ways, including dummy sensor cancellation and structural rearrangement of the sensor assembly, will be considered to compensate the temperature effect for the application in the environment of large temperature variation.

6. Conclusions The performance enhancement of a nano-displacement sensor based on the principle of Fabry–Perot interferometry has been demonstrated in this paper. With a refractive index-matched medium, silicon oil, filling into the sensor cavity, the signal intensity of the sensor can be enhanced effectively by 16.4 dB and the sensor sensitivity has also been improved by 1.95 times. The sensitivity, minimum

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detectable displacement and shear stress of this sensor were characterized to be 6.825 nm/Pa (spectrum shift/shear stress), 10 nm and 0.33 Pa, respectively. The ability of pressure resistance and vibration reduction has also been carried out by the silicon oil filling method, and demonstrated only 1 nm signal shift for the sensor under 6 atm static pressure or vibration signal between 1 Hz and 5 kHz. With the silicon oil filling technique, signal perturbation problems can be overcome mostly and the sensor performance has been improved extensively. However, the temperature dependency and hysteresis phenomenon of the sensor need to be further overcome by signal processing or compensation techniques.

Acknowledgements This work was partially supported by SBIR program through POC Inc., Torrance, CA, USA. We also appreciated the partial financial support from National Science Council, Taiwan.

[5] N. Furstenau, M. Schmidt, Fiber-optic extrinsic Fabry–Perot interferometer vibration sensor with two-wavelength passive quadrature readout, Instrum. Meas.: IEEE Trans. 47 (1998) 143–147. [6] M.J. Gander, W.N. MacPherson, J.S. Barton, R.L. Reuben, J.D.C. Jones, R. Stevens, K.S. Chana, S.J. Anderson, T.V. Jones, Embedded micromachined fiber optic Fabry–Perot pressure sensors in aerodynamics applications, Technical Digest of IEEE Sensors Conference, Orlando, USA 2 (2002) 1707–1712. [7] I.R. Matlas, F.J. Arregui, R.O. Claus, K.L. Cooper, Molecularly self-assembled optical fiber sensors, Technical Digest of IEEE Sensors Conference, Orlando, USA 1 (2002) 198–202. [8] J.W. Borinski, C.D. Boyd, J.A. Dietz, J.C. Duke, M.R. Home, Fiber optic sensors for predictive health monitoring, in: Proceedings of the AUTOTESTCON, IEEE, 2001, pp. 250–262.

Biographies Chun-Jun Lin was born in 1974 in Taichung, Taiwan, ROC. He received his BS degree from the Department of Control Engineering in National Chiao Tung University in 1996 and MS degree from the Department of Engineering and System Science in National Tsing Hua University in 2000, respectively. He is now a PhD student in the Department of Engineering and System Science, National Tsing Hua University. His major research work is focused on the MEMS design and fabrication technology and fiber optic microsensors for biomedical sensing applications.

References [1] F.G. Tseng, C.J. Lin, A high sensitive Fabry–Perot shear stress sensor employing flexible membrane and double SU-8 structures, Technical Digest of IEEE Sensors Conference, Orlando, USA 2 (2002) 969– 972. [2] F.G. Tseng, C.J. Lin, Polymer MEMS based Fabry–Perot shear stress sensor, Sens. J., IEEE 3 (2003) 812–817. [3] M. Jiang, E. Gerhard, A simple strain sensor using a thin film as a low-finesse fiber-optic Fabry–Perot interferometer, Sens. Actuators A 88 (2001) 41–46. [4] T. Wang, S. Zheng, Z. Yang, A high precision displacement sensor using a low-finesse fiber-optic Fabry–Pérot interferometer, Sens. Actuators A 69 (1998) 134–138.

Fan-Gang Tseng was born in 1967 in Taichung, Taiwan, ROC. In 1998, he received his PhD degree in mechanical engineering from the University of California, Los Angeles (UCLA), USA, with an emphasis on MEMS technology. His PhD dissertation was on the design, fabrication and applications of a novel microdroplet injector system. This novel system is currently under technology transfer for commercialization. After 1 year staying with USC/Information Science Institute as a senior engineer working on a new microfabrication process, EFAB, he has been an assistant professor with Engineering and System Science Department of National Tsing-Hua University, Taiwan since August 1999, and advanced to associate professor on August 2002. His interests are in the fields of Bio-MEMS and Micro/nano-Fluidics Systems.