- Email: [email protected]
MEMS capacitive flow sensor for natural gas pipelines
Accepted Manuscript Title: MEMS capacitive flow sensor for natural gas pipelines Author: Son D. Nguyen Igor Paprotny Paul K. Wright Richard M. White PII: DOI: Reference:
S0924-4247(14)00447-6 http://dx.doi.org/doi:10.1016/j.sna.2014.10.013 SNA 8930
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
15-4-2014 17-9-2014 7-10-2014
Please cite this article as: Son D. Nguyen, Igor Paprotny, Paul K. Wright, Richard M. White, MEMS capacitive flow sensor for natural gas pipelines, Sensors & Actuators: A. Physical (2014), http://dx.doi.org/10.1016/j.sna.2014.10.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
MEMS capacitive flow sensor for natural gas pipelines
a University
of California, Berkeley, USA of Illinois, Chicago, USA
cr
b University
ip t
Son D. Nguyena , Igor Paprotnyb , Paul K. Wrighta , Richard M. Whitea
us
Abstract
M
an
This paper presents the design, fabrication, and experimental results of an in-plane MEMS capacitive flow sensor that uses the displacement of a micro-fabricated paddle caused by dynamic gas pressure for measuring the velocity of the flow of surrounding gas. The fabrication process is simple; the prototype is fabricated on 100-μm device Silicon-On-Insulator wafers using only three photo-lithographic mask layers. The device area is 5.5 mm by 5.5 mm. A comb-drive capacitance is used as the transducer for the flow sensor. Measurements show that the output capacitance C is a quadratic function of the gas velocity v, C = k1 v 2 + k2 v + Cp , where k1 = -8.5 f F m s 2 , k2 = 73.6 f F m s and Cp = 16 pF . The advantage of using a capacitive sensing mechanism is that it is virtually insensitive to changes in ambient temperature. Experimental results show that the output capacitance changed only slightly, about 0.21% - 0.34%, when the temperature changed from 23 C - 43 C. Simplicity of fabrication, combined with insensitivity to variations in ambient temperature makes this sensor ideal for widespread deployment to monitor the flow in natural gas pipelines.
Ac ce pt e
d
Keywords: MEMS, flow sensors, drag force, capacitive transducer
1. Introduction
in ambient temperature and consequently not well suited for outdoor applications. Moreover, out-ofCost is currently the prohibitive factor preventing plane lateral structure [9, 10, 11] are difficult to fabwidespread deployment of natural gas flow sensors, ricate and have a potentially high cost. Out-of-plane and thus the development of inexpensive solutions MEMS-based flow sensors have been reported by Anis important. A variety of conventional flow sensors dre et al. [14] using capacitive readout mechanisms. exist, but few of these are in micro-scaled. In addiHowever, the sensors are quite low sensitivity and tion to reduced size, microfabrication offers a great temperature dependence due to the metal layer on reduction in the coast of the final devices. The overbended beam electrodes. whelming majority of existing MEMS flow sensors Optical transducers have also been used [15] but a are designed using the principles of asymmetric thermajor disadvantage of this approach is their high cost mal conduction [1, 2, 3, 4, 5, 6]. However, placement and large size. Dijkstra et al. [16] or later N. Izadi of a heated filament in a potentially combustible gas [17] and Dagamseh [18] reported a flow sensors using mixture is not possible due to intrinsic safety requirelateral hairs and capacitive transducers. Again, the ments. For non-thermal flow sensors, drag force acting on lateral structures complicate the fabrication process. mechanical structures has been exploited [7, 8, 9, In this work, an in-plane MEMS flow sensor using 10, 11, 12, 13]. The sensing of these flow sensors dynamic pressure acting on a paddle and comb-drive is detected using piezoresistors. However, the use of readout mechanisms is presented. The fabrication is piezoresitive transducer is highly sensitive to changes simple, resulting in overall low cost of the device, and Preprint submitted to Sensors and Actuators Physics A
September 16, 2014
Page 1 of 11
B
0
T zb F0
2. Design
Gas
Fd
M0
zp
L1 l
L
Cantilevers
Paddle
2.1. Device description
zf
Lf
Capacitors
cr
z
P
ip t
the capacitive pickup mechanism offers superior temperature characteristics compared with piezoresistive devices. This paper is an expansion of a previously published paper in IEEE MEMS 2014 [19].
us
Figure 2: Free-body diagram of the flow sensor cross-section.
Electrical pad
The fabricated flow sensor is designed to measure the gas velocity up to 18 m s at a nominal gas pressure up to 400 psi. Note that our design is such that the target measurement range, i.e., the maximum gas velocity and/or the maximum gas pressure, can be easily increased by using a thicker SOI device layer without changing the design layout.
an
cantilevers
Paddle
M
Gas flow
2.2. Modeling
Variable capacitors
g
Ac ce pt e
d
(a)
!
Figure 1: Simple schematic drawing of MEMS capacitive flow sensor, fabricated using Silicon-On-Insulator (SOI) wafers. The drawing is not to scale. The paddle is perpendicular to the gas flow direction
݀
݂
!
!
ߜ
! 0
fixed electrodes
(b)
zf Fig. 1 is a schematic drawing of the MEMS capaci- z p tive flow sensor, fabricated using Silicon-On-Insulator (SOI) wafers. A paddle supported by two cantilevers will deflect out-of-plane under a dynamic pressure δ d generated by the surrounding gas flow. Variable overlap area movable electrodes f comb-drive capacitors with movable electrodes attached to the paddle will change capacitance as the structure is deflected. By measuring the change of the capacitance, the velocity of the gas flow can be calcu- Figure 3: (a) 3D schematic of the comb-drive capacitor fingers lated. To increase the sensitivity of the device while (b) Cross-section of capacitor fingers on side view. keeping the critical layout dimension large, (e.g., the The gas flow regimes are fully turbulent for typical capacitance gaps and finger widths are larger than 10 transmission lines at high pressure [20]. In turbulent μm), three rows of digital capacitors are used.
2
Page 2 of 11
ip t
conditions, with a large Reynolds number, the drag capacitor finger width, d is the overlap length of the force on the paddle can be expressed as capacitor finger, δ is the gap between capacitor fingers and their frames, and f is the width of capacitor 1 2 Cd ρAp v , (1) frames; y1 and y2 are the distance of the drag forces Fd 2 on the movable capacitor fingers and their frames to where C is the local drag coefficient calculated em- the free end of the paddle, given by d
pirically, Ap is the area of the paddle, ρ is the density of the gas and v is the mean velocity of the gas. For a wide range of Reynolds number, the drag coefficient of a flat plate is in the range Cd = 1.1 - 1.3 [21]. In laminar flow conditions, with a small Reynolds number, the drag force is proportional to the velocity [7]. Fig. 2 shows the free-body diagram of the crosssection of the flow sensor with force distribution and deflection. The displacement due to drag force at the ends of the beams (cantilevers), the paddle, and the capacitor f ingers are respectively denoted as zb , zp , and zf . Details of comb-drive capacitors are shown in Fig. 3. By ignoring the drag force on the beams, since the beam area is about 0.25% of the paddle area, the total drag force can be replaced by a concentrated force Fd at a distance L1 from the free end of the cantilevers. Since it is assumed that the paddle, capacitor fingers and capacitor frames have the same drag coefficients, L1 can be expressed as
17 δ 6
3 d 2
f
cr
y1
(7)
36 36 52 δ d f. (8) 10 20 40 The slope and the deflection at the free end of the cantilevers due to the drag force Fd are given by
us
y2
Fd l 2 1 2EI
an
θb
2
L1 l
(9)
Fd l 3 3L1 1 (10) 3EI 2l where l, E, and I are the length, the Young s modulus, and the area moment of inertia of the cantilevers, respectively. Since the paddle and the capacitor fingers almost do not bend, the deflection slope of them is equal to the slope of deflection at the free end of the cantilevers. The deflection at the free end of the movable capacitors is given by
L1
Ac ce pt e
d
M
zb
L Ap 2 A
L
y1
Af A
L
y2
Acf , A
(2)
zf
where L is the paddle length; Ap , Af , and Acf are the areas of the paddle, movable capacitor fingers, and movable capacitor frames, respectively, calculated from Ap
Af
Acf
N wf d
Wf
A
WL
Ap
δ
3 f 4δ 2
2d
Af
Acf ,
f
zb
θb L
Lf
(11)
or 2Fd l3 η Ewt3
zf
(12)
with
(3)
η (4)
1
3 L1 2 l
3 1 2
2
L1 L l
5δ
3d l
2f
(13)
where w and t are the width and thickness of the cantilevers. There are two sources of capacitance in the device: the variable capacitance due to the comb-drive capacitor which is used to measure the flow, and the parasitic capacitance due to the four anchor locations to the substrate. Each anchor is separated from the silicon substrate by an insulating silicon dioxide layer,
(5) (6)
where W and L are the paddle width and length, N is the number of movable capacitor fingers, wf is the 3
Page 3 of 11
us
cr
ip t
where Ae is total area of four anchors (approximately 0.9 mm2 ), d is the relative permittivity of the oxide and tdo 10μm is the thickness of the buried oxide layer. The areas of the beam anchors and capacitor frame anchors are made as small as possible to reduce Cp . As illustrated in Fig. 3-b, the overlap areas of capacitors are different for each row. If it is assumed that the number of capacitor fingers on each row is equal (the actual number of capacitor fingers are 92, 92 and 98 for the first, second and third row, respectively), an analytical expression for the adjusted coefficient D is Figure 4: Deflection of the paddle and the capacitor fingers, calculated by Finite Element Method (FEM). The movable electrodes are replaced by a rectangular plate with equivalent surface areas for the purpose of the FEM analysis.
31 4
an
D
M
Displacement at free-end capacitor fingers P
d
Ac ce pt e C0 1
zf 1 t
D
Cp ,
(14)
where t is the device thickness, C0 is the initial capacitance (no deflection), Cp is the parasitic capacitance, and D is the adjusted coefficient due to the different overlap area of capacitor fingers for different rows (Fig. 3). The capacitance without gas flow C0 is given by C0
2N r 0
td , g
d 0
Ae , tdo
2f
.
(17)
100
Closed-form FEM calculation
80 60 40 20
0
0
5
10 15 Gas velocity [m/s]
20
Figure 5: Calculation (closed-form and FEM) of deflection at the free end of a capacitor finger vs. gas flow velocity at 400 psi pressure and 25 C (gas density = 0.668 kg m3 at 25 C and 1 atm (14.7 psi), drag-force coefficient Cd = 1.28. Total area A = 9.387 mm2 ). The drag force is 10.9 mN and 43.7 mN for a velocities of 10 m s and 20 m s, respectively.
(15)
where N is the total number of capacitor finger pairs, r and 0 are the relative permittivities of gas and vacuum, respectively; g is the gap between capacitor fingers. The parasitic capacitance is Cp
3d l
2.3. Design optimization
placing an equivalent capacitance in parallel with the variable capacitance. To simplify the calculations, the fringe effect is ignored, considering the capacitance is only between the two overlapping plates, i.e., the variable capacitance between capacitor fingers and the frames is ignored. Consequently, the capacitance for a deflection zf is C
2 L1 l 4δ η
The MEMS flow sensor is intended for application within natural gas pipelines which operate at very high gas pressure, 400 psi for nominal pressure and 1000 psi for maximum pressure. The typical gas velocity in natural gas pipelines is about 8 m s. As gas pressures increase, the gas density will increase and
(16) 4
Page 4 of 11
Table 1: Dimensions of the MEMS flow sensors
cr
ip t
Layout dimension 5.5 mm x 5.5 mm 2 mm x 4 mm 2 mm 4 mm 100 μm 1 mm 25, 50, 75 or 95 μm 10 or 15 μm 10 or 15 μm 10 μm 280 μm 100 μm 282
us
Symbol A0 A L W t l wb wf g δ d f N
an
Description chip area paddle area paddle length paddle width device thickness cantilever length cantilever width capacitor finger width capacitor finger gap gap between fingers and frames capacitor overlap length capacitor frame width number of capacitor fingers
or
30
M
d
η
Ac ce pt e
Capacitance [pF]
2 l3 Cd ρmax Avmax (19) 4 Et 25 For a nominal device thickness of 100 μm, and other parameters as shown in Table 1, the widths of 20 the cantilevers must be larger than 85 μm for a nominal pressure of 400 psi and vmax = 18 m s. However, 15 if the beam width is too large, the device sensitivity, i.e., the variable capacitance versus gas velocity, will 10 be low. In this design, the beam width of 95 μm (10% larger than the design value) is chosen for gas pressure up to the nominal pressure of 400 psi and 5 0 5 10 15 20 maximum gas velocity of 18 m s . A variety of beam Velocity [m/s] widths have been designed, e.g., 25 μm, 50 μm, and 75 μm, for lower maximum gas pressures of 100 psi, Figure 6: Calculated capacitance vs. gas flow velocity at nom- 200 psi and 300 psi, respectively. Moreover, withinal pressure of 400 psi and 25 C. out changing the layout design, the working ranges can be easily adjusted by choosing different wafer device layer thickness. For instance, for the same beam consequently the drag force Fd (Eq. 1) will increase. width of 75 μm, the device can measure vmax = 18 The variable capacitance in Eq. 14 is only valid when m s at a maximum pressure of 1000 psi when fabrithe capacitor finger displacement zf is less than the cated for 130-μm device layer thickness of SOI wafers. device thickness t. Given maximum operating conVarious capacitor finger gaps and finger widths (10 ditions, i.e., ρ = ρmax and v = vmax , the capacitor μm - 15 μm) have been designed to test the deep dry finger displacement zf must satisfy the condition: etching aspect ratio. The dimensions of the MEMS flow sensor are given in Table 1. (18) zf t Fig. 4 shows the deflection of the capacitor fin-
w
5
Page 5 of 11
gers and movable electrodes calculated using the Finite Element Method (FEM). In the FEM model, the drag force is converted to an equivalent pressure on the paddle and capacitor fingers. The FEM calculation results are shown in Fig. 5. The closed-form deflection calculated from (Eq. 12) are also plotted for comparison. There is good agreement between the analytical and FEM results. The FEM modal analysis shows that the out-of-plane natural frequency (in the z -direction in Fig. 4) is 2.7 kHz and the in-plane natural frequency (in the x -direction Fig. 4) is 16 kHz. Based on the deflection of the paddle, the variable capacitance versus the velocity of gas flow can be calculated using Eq. (14), assuming the drag coefficient of the rectangular paddle is about 1.28 (Fig. 6). As can be seen in Eqs. (1) and (14 that the device will respond with quadratic order of the gas velocity.
an
us
cr
ip t
layers. The process is similar to the previous fabrication of the MEMS electrostatic energy harvesters [22] without the electret deposition step. First, 200nm Cr/Au is evaporated using high-vacuum e-beam evaporation, and patterned to form electrical bond pads (Fig. 7-a). Next, a Deep-Reactive-Ion-Etch (DRIE) process is applied to both sides of the SOI wafers to define the cantilevers, digital capacitors, and paddle (Figs. 7-b and 7-c). A thick UV-baked layer of photoresist SPR-220 is used as a masking during the DRIE steps. The beam stiffness in the lateral direction is large enough to avoid the stiction effect between capacitor fingers during wet etching. Then, the movable parts are released by wet etching of the buried oxide using buffered hydrofluoric acid (Fig. 7-d). Fig. 8 shows an optical micrograph (center) of the device after fabrication. Scanning-Electron Microscope (SEM) images of specific components of the sensor are displayed in the insets of Fig. 8. The capacitor fingers and their frame are shown in Figs. 8-a and 8-b. Due to the intentional over-expose of thick photoresist SPR-220, the capacitor width is 1.8 μm smaller than the layout.
M
3. Fabrication 3.1. Fabrication process
d
Ac ce pt e
Si substrate
a) Evaporation of Cr/Au for electrical connections
Si substrate
3.2. Dummy structures to protect cantilevers in DRIE
b) A deep-reactive-ion-etch (DRIE) Si device layer to define capacitor fingers, paddle, and cantilevers.
To protect the sidewall of the cantilevers during DRIE, dummy structures are added along the cantilevers. Two kinds of protective structures have been designed as shown in Fig. 9: dummy structures attached to the beams (9-a), and dummy structures not attached to the beams (Fig. 9-b). As shown in Figs. 9-c and -d, both designs can protect the sidewall of the cantilevers during DRIE with nearly vertical profiles. To avoid unexpected motions of the paddle in the lateral direction, mechanical end-stops were designed as shown in the inset of the Fig. 8-d. The gap between the paddle and the end-stops was designed smaller than the gap between capacitor fingers to avoid the capacitor fingers touching in the lateral inplane direction.
c) A DRIE Si substrate to create deflection space for the capacitor fingers and cantilevers. d) An HF (wet etch) releasing
Cr/Au
Si structure
Oxide
Photoresist
Figure 7: Fabrication process of MEMS flow sensor on Siliconon-Insulator (SOI) wafers
Fig. 7 shows the fabrication process of the sensors using SOI wafers. The fabrication process is simple and requires only three photo-lithographic mask 6
Page 6 of 11
(b)
11.8um
ip t
8.2um
cr
(a)
us
76.0
200 Pm
24.1
(d)
an
(c)
10.4um
M
Figure 8: Micrograph of the MEMS-capacitive flow sensor after fabrication. Inset figures: SEM images of (a) the capacitor fingers, (b) a corner of the sensor, (c) cantilever and its DRIE protections, (d) mechanical end-stops of the paddle in the lateral in-plane direction.
where v is mean air velocity, d is the air pipe diameter, and ν is the kinematic viscosity ( ν 1.568 10 5 4.1. Experimental set-up m2 /s at room temperature and atmospheric presThe sensors were characterized using an air flow in sure). The Reynolds number is larger than 4000 when a 4-inch diameter duct. The Reynolds number is v is larger than 0.62 m/s, indicating mainly turbulent (20) R vd ν,
Ac ce pt e
d
4. Experimental results and discussion
25 um
A
25.2 um
(c)
70.5 um
69.6 um
200 um 200 um
A’
A
(a)
A’
(d)
(b)
Figure 9: Variety of dummy structures to protect the cantilevers in DRIE.
7
Page 7 of 11
air flow in the duct. The experimental setup for the flow measurement is shown in Fig. 10. An air fan with adjustable speed via a variable transformer was used to generate the required air flow. The air flow velocity was measured using a digital anemometer (TPI 575C1) with vane probe, allowing measuring for an air flow up to 25 m s. For practical applications, a high-resolution capacitance-to-digital readout chip AD7747 (Analog Devices, Wilmington, MA, a resolution of 20 aF , accuracy of 10 f F , maximum capacitance of 17 pF ) could be used to measure the output capacitance [23]. For a purpose of characterization, a precision LCR meter E4980A was used to measure the capacitance between two electrodes of the flow sensor. The parallel circuit mode of capacitance (Cp-D) was used to measure small capacitance of the flow sensor. To minimize the stray capacitance of the test fixture of the LCR meter, the open/short correction was done before measurement. Using list sweep function, each data point of experiments was measured by averaging of 30 samples. The MEMS flow sensors were bonded on PCB (printed circuit boards) which were made a hole so that the air flow can go through. The PCB were attached on a probe to insert the device into the air duct. The other side of the duct, an air straightener was inserted to reduce the vortex. The average air velocity was measured by the anemometer with vane probe and was removed while testing the flow sensors to avoid generating vortexes.
16.5 increasing flow decreasing flow
16.22
15
16.2 16.18
2 C = -3.4e-3 v + 1.28e-3 v + 16.22
14.5 16.16 0
14
2
1
3
4
2
C = -8.5e-3 v + 73.6e-3 v + 16 0
us
13.5
ip t
15.5
cr
Capacitance [pF]
16
5
10 Velocity [m/s]
15
20
M
an
Figure 11: Measurement of variable capacitance vs. air velocity at the atmospheric pressure (14.7 psi) and the temperature of 25 C. Beam width of 25 μm. Capacitor finger gaps and finger widths are 10 μm. : experiments with increasing air flow. : experiments with decreasing air flow. Solid lines: the best fitting curves for up flow.
Data acquisition
Fan
Anemometer
15 14 25 P m analysis 25 P m experiments
13
50 P m analysis 50 P m experiements
12
Air duct
Variable transformer
16
Capacitance [pF]
Ac ce pt e
d
4.2. Device response
0
5
10 Velocity [m/s]
15
20
Figure 12: Measurements of capacitance vs. gas velocity in the air at the atmospheric pressure (14.7 psi) and the temperature of 25 C for different beam widths of 25 μm, and 50 μm. Capacitor finger gaps and finger widths are 10 μm.
Precision LCR meter
MEMS flow sensor
Fig. 11 shows the measured output capacitance of the flow sensor versus the airflow velocity for the smallest beam width - 25 μm and finger gaps and
Figure 10: Experimental set up for evaluating the MEMS flow sensor using an air flow. Insets: the flow sensor attached to PCB and the vane probe of the anemometer.
8
Page 8 of 11
finger widths of 10 μm. The air flow was increased from 0 to 19.2 m s and then decreased from 19.2 to 0 m s to measure the hysteresis. Each experimental point was measured by averaging of 30 samples. The experimental curves were repeated three times to obtain the error bar. The largest different capacitance of up flow and down flow is 0.4% at a velocity of 14.3 m s. The best fit to data is the quadratic function with the first order and the second order of the velocity, C = k1 v 2 + k2 v + Cp where k1 and k2 are the coefficients of the sensitivity. However, repsonse is slight different for low velocity regime (0 - 4 m s) and high velocity regime (4 - 19.2 m s). For low velocity regime, the coefficients k1 = -3.4 f F m s 2 and k2 = 1.28 f F m s . For high velocity regime, the coefficients k1 = -8.5 f F m s 2 and k2 = 73.6 fF m s . Fig. 12 shows the measured capacitance versus the gas velocity in the air for different beam widths of 25 μm and 50 μm. The capacitor finger gaps and finger widths are 10 μm. The closed-from calculation (from Eq. 14) were also plotted for comparison. The under-cut of capacitor finger gaps (11.8 μm instead of 10 μm) was included in the calculation. Although the experimental results differ from the closed-form calcalation, they follow the trend of the analytical curves. The discrepancy of analysis and experiments can be explained as follows. First, the analytical model does not include the capacitance fringe effects and the sidewall profile of capacitor finger is not vertical. Second, the measurement of velocity includes 2% error from the anemometer. Third, the parasitic capacitance Cp experiments is larger than because the model does not include the stray capacitance of wire connections. (C0 = 11.84 pF and Cp = 3.11 pF compared to Cp = 4.38 pF in experiments)
10.3
ip t
Flow sensor in parallel
10.2
10.15
cr
Capacitance [pF]
10.25
0
us
10.1
5
10 Velocity [m/s]
15
20
an
Figure 13: Response of the flow sensor when it is placed parallel to the air flow. Beam width of 50 μm. Finger gap of 10 μm and finger width of 15 μm.
Ac ce pt e
d
M
However, the capacitance was changed about 0.3% at a velocity of 16 m s. The vortex of the air after go through the first edge of the printed circuit board (PCB) and the tilt of PCB due to set-up may be this causing. With cylindrical pumps on the surface of the paddle and difference of suspension design, the device in parallel flow could measure the pressure gradient as presented by Zhao et al. [23]. 4.4. Temperature effect
It is important to note that all flow sensors are sensitive to the variations in the density of the gas, which in turn is a function of its temperature. It is thus undesirable if the transduction mechanism is also sensitive to changes in the ambient temperature. The benefit of the capacitive (compared to e.g., piezoresistive transduction) is its insensitivity to temperature fluctuations. To verify this, the flow sensor was tested at dif4.3. Device in parallel flow ferent air flow temperatures. A heater was placed in The response of the flow sensor when it is placed the duct to heat the air. The output capacitance was parallel to the air flow is shown in Fig. 13. Ideally, measured at constant velocity when increasing the the output capacitance would not change in this sit- temperature. The air temperature was adjusted from uation because the gas flow hits the paddle edge and 23 C to 43 C for low velocity and to 37 C for high the beams are very stiff in this direction (36 times velocity. The experimental results are shown in Fig. stiffer than the stiffness of perpendicular direction). 14. The output capacitance changed very slightly 9
Page 9 of 11
9.7
Lab, University of California, Berkeley. The author would like to thank Christine E. Gregg and Sean Wihera for valuable suggestions.
air velocity = 5.6 m/s
References
ip t
air velocity = 11.8 m/s 9.5
[1] C. G. Schabmueller, Flow sensors, Artech House, 2004, Ch. 9, pp. 213 – 248.
cr
9.4 air velocity = 14.8 m/s
9.3
[2] Y.-H. Wang, C.-P. Chen, C.-M. Chang, C.-P. Lin, C.-H. Lin, L.-M. Fu, C.-Y. Lee, Mems-based gas flow sensors, Microfluidics and Nanofluidics 6 (3) (2009) 333–346.
9.2 25
30
35
40
45
us
Capacitance [pF]
9.6
0
Figure 14: Response of the flow sensor under changing temperatures.
[3] N. Nguyen, Micromachined flow sensors - a review, Flow Measurement and Instrumentation 8 (1) (1997) 7 – 16.
an
Temperature ( C)
[4] S. Wu, Q. Lin, Y. Yuen, Y. Tai, Mems flow sensors for nano-fluidic applications, in: Micro Electro Mechanical Systems, 2000. MEMS 2000. The Thirteenth Annual International Conference on, 2000, pp. 745–750.
5. Conclusion
d
M
when changing the temperature. In particular, the capacitance increased 0.21%, 0.34% and 0.25% for the velocity of 5.6 m s, 11.8 m s and 14.8 m s, respectively.
Ac ce pt e
In-plane MEMS capacitive flow sensors have been designed, modeled, and fabricated using a bulk micromachining technology with only three photolithographic mask layers. Experimental results with an airflow showed that output capacitance of the device is a quadratic function of air velocity, C = k1 v 2 + k2 v + Cp , where k1 = -8.5 f F m s 2 , k2 = 73.6 f F m s and Cp = 16 pF. Experimental results show that the output capacitance changed only slightly, about 0.21% - 0.34%, when the temperature changed from 23 - 43 C. A shield to protect the device from particles in the gas stream, as well as an erosion and corrosion coating, will be considered in future work. 6. Acknowledgments This project is sponsored by the California Energy Commission, contract number 500-10-044. The devices were fabricated in Marvell Nano-fabrication
[5] J. Sun, D. Cui, L. Zhang, X. Chen, H. Cai, H. Li, Fabrication and characterization of a doubleheater based {MEMS} thermal flow sensor, Sensors and Actuators A: Physical 193 (0) (2013) 25 – 29. [6] M. Shikida, Y. Yamazaki, K. Yoshikawa, K. Sato, A {MEMS} flow sensor applied in a variable-air-volume unit in a building airconditioning system, Sensors and Actuators A: Physical 189 (0) (2013) 212 – 217. [7] V. Gass, B. H. Van Der Schoot, N.-F. de Rooij, Nanofluid handling by micro-flow-sensor based on drag force measurements, in: Micro Electro Mechanical Systems, 1993, MEMS ’93, Proceedings An Investigation of Micro Structures, Sensors, Actuators, Machines and Systems. IEEE., 1993, pp. 167–172. [8] L. Zhang, X. Ye, Z. Zhou, J. Yao, A micromachined single-crystal silicon flow sensor with a cantilever paddle, in: Micromechatronics and Human Science, 1997. Proceedings of the 1997 International Symposium on, 1997, pp. 225–229.
10
Page 10 of 11
M
an
us
cr
ip t
[9] Y. Ozaki, T. Ohyama, T. Yasuda, I. Shimoyama, [18] A. Dagamseh, Bio-inspired hair flow-sensor arAn air flow sensor modeled on wind receptor rays: from nature to mems, Ph.D. thesis, Unihairs of insects, in: Micro Electro Mechanical versity of Twente (2011). Systems, 2000. MEMS 2000. The Thirteenth Annual International Conference on, 2000, pp. 531– [19] S. Nguyen, I. Paprotny, P. Wright, R. White, Inplane capacitive mems flow sensor for low-cost 536. metering of flow velocity in natural gas pipelines, [10] Y. Su, A. G. R. Evans, A. Brunnschweiler, in: Micro Electro Mechanical Systems (MEMS), Micromachined silicon cantilever paddles with 2014 IEEE 27th International Conference on, piezoresistive readout for flow sensing, Journal 2014, pp. 971–974. of Micromechanics and Microengineering 6 (1) [20] P. M. Coelho, C. Pinho, Considerations about (1996) 69. equations for steady state flow in natural gas [11] J. Chen, Z. Fan, J. Zou, J. Engel, C. Liu, Twopipelines, Journal of the Brazilian Society of Medimensional micromachined flow sensor array for chanical Sciences and Engineering 29 (2007) 262 fluid mechanics studies, Journal of Aerospace – 273. Engineering 16 (2) (2003) 85–97. [21] M. Sadraey, Aircraft Performance Analysis, [12] Z. Fan, J. Chen, J. Zou, D. Bullen, C. Liu, VDM Verlag Dr. Muller, 2009. F. Delcomyn, Design and fabrication of artificial lateral line flow sensors, Journal of Microme- [22] S. D. Nguyen, E. Halvorsen, I. Paprotny, chanics and Microengineering 12 (5) (2002) 655. Bistable springs for wideband microelectromechanical energy harvesters, Applied Physics Let[13] L. Du, Z. Zhao, C. Pang, Z. Fang, Drag force ters 102 (2) (2013) –. micro solid state silicon plate wind velocity sensor, Sensors and Actuators A: Physical 151 (1) (2009) 35 – 41.
Ac ce pt e
d
[23] Z. Zhao, M. Shin, J. M. Gallman, R. D. White, A microfabricated shear sensor array on a chip with pressure gradient calibration, Sensors and [14] N. Andre, B. Rue, G. Scheen, D. Flandre, L. A. Actuators A: Physical 205 (0) (2014) 133 – 142. Francis, J.-P. Raskin, Out-of-plane mems-based mechanical airflow sensor co-integrated in {SOI} {CMOS} technology, Sensors and Actuators A: Physical 206 (0) (2014) 67 – 74. [15] R. Philip-Chandy, P. J. Scully, R. Morgan, The design, development and performance characteristics of a fibre optic drag-force flow sensor, Measurement Science and Technology 11 (3) (2000) N31. [16] M. Dijkstra, J. J. van Baar, R. J. Wiegerink, T. S. J. Lammerink, J. H. de Boer, G. J. M. Krijnen, Artificial sensory hairs based on the flow sensitive receptor hairs of crickets, Journal of Micromechanics and Microengineering 15 (7) (2005) S132. [17] N. Izadi, Bio-inspired mems aquatic flow sensor arrays, Ph.D. thesis, University of Twente (2011). 11
Page 11 of 11
S0924-4247(14)00447-6 http://dx.doi.org/doi:10.1016/j.sna.2014.10.013 SNA 8930
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
15-4-2014 17-9-2014 7-10-2014
Please cite this article as: Son D. Nguyen, Igor Paprotny, Paul K. Wright, Richard M. White, MEMS capacitive flow sensor for natural gas pipelines, Sensors & Actuators: A. Physical (2014), http://dx.doi.org/10.1016/j.sna.2014.10.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
MEMS capacitive flow sensor for natural gas pipelines
a University
of California, Berkeley, USA of Illinois, Chicago, USA
cr
b University
ip t
Son D. Nguyena , Igor Paprotnyb , Paul K. Wrighta , Richard M. Whitea
us
Abstract
M
an
This paper presents the design, fabrication, and experimental results of an in-plane MEMS capacitive flow sensor that uses the displacement of a micro-fabricated paddle caused by dynamic gas pressure for measuring the velocity of the flow of surrounding gas. The fabrication process is simple; the prototype is fabricated on 100-μm device Silicon-On-Insulator wafers using only three photo-lithographic mask layers. The device area is 5.5 mm by 5.5 mm. A comb-drive capacitance is used as the transducer for the flow sensor. Measurements show that the output capacitance C is a quadratic function of the gas velocity v, C = k1 v 2 + k2 v + Cp , where k1 = -8.5 f F m s 2 , k2 = 73.6 f F m s and Cp = 16 pF . The advantage of using a capacitive sensing mechanism is that it is virtually insensitive to changes in ambient temperature. Experimental results show that the output capacitance changed only slightly, about 0.21% - 0.34%, when the temperature changed from 23 C - 43 C. Simplicity of fabrication, combined with insensitivity to variations in ambient temperature makes this sensor ideal for widespread deployment to monitor the flow in natural gas pipelines.
Ac ce pt e
d
Keywords: MEMS, flow sensors, drag force, capacitive transducer
1. Introduction
in ambient temperature and consequently not well suited for outdoor applications. Moreover, out-ofCost is currently the prohibitive factor preventing plane lateral structure [9, 10, 11] are difficult to fabwidespread deployment of natural gas flow sensors, ricate and have a potentially high cost. Out-of-plane and thus the development of inexpensive solutions MEMS-based flow sensors have been reported by Anis important. A variety of conventional flow sensors dre et al. [14] using capacitive readout mechanisms. exist, but few of these are in micro-scaled. In addiHowever, the sensors are quite low sensitivity and tion to reduced size, microfabrication offers a great temperature dependence due to the metal layer on reduction in the coast of the final devices. The overbended beam electrodes. whelming majority of existing MEMS flow sensors Optical transducers have also been used [15] but a are designed using the principles of asymmetric thermajor disadvantage of this approach is their high cost mal conduction [1, 2, 3, 4, 5, 6]. However, placement and large size. Dijkstra et al. [16] or later N. Izadi of a heated filament in a potentially combustible gas [17] and Dagamseh [18] reported a flow sensors using mixture is not possible due to intrinsic safety requirelateral hairs and capacitive transducers. Again, the ments. For non-thermal flow sensors, drag force acting on lateral structures complicate the fabrication process. mechanical structures has been exploited [7, 8, 9, In this work, an in-plane MEMS flow sensor using 10, 11, 12, 13]. The sensing of these flow sensors dynamic pressure acting on a paddle and comb-drive is detected using piezoresistors. However, the use of readout mechanisms is presented. The fabrication is piezoresitive transducer is highly sensitive to changes simple, resulting in overall low cost of the device, and Preprint submitted to Sensors and Actuators Physics A
September 16, 2014
Page 1 of 11
B
0
T zb F0
2. Design
Gas
Fd
M0
zp
L1 l
L
Cantilevers
Paddle
2.1. Device description
zf
Lf
Capacitors
cr
z
P
ip t
the capacitive pickup mechanism offers superior temperature characteristics compared with piezoresistive devices. This paper is an expansion of a previously published paper in IEEE MEMS 2014 [19].
us
Figure 2: Free-body diagram of the flow sensor cross-section.
Electrical pad
The fabricated flow sensor is designed to measure the gas velocity up to 18 m s at a nominal gas pressure up to 400 psi. Note that our design is such that the target measurement range, i.e., the maximum gas velocity and/or the maximum gas pressure, can be easily increased by using a thicker SOI device layer without changing the design layout.
an
cantilevers
Paddle
M
Gas flow
2.2. Modeling
Variable capacitors
g
Ac ce pt e
d
(a)
!
Figure 1: Simple schematic drawing of MEMS capacitive flow sensor, fabricated using Silicon-On-Insulator (SOI) wafers. The drawing is not to scale. The paddle is perpendicular to the gas flow direction
݀
݂
!
!
ߜ
! 0
fixed electrodes
(b)
zf Fig. 1 is a schematic drawing of the MEMS capaci- z p tive flow sensor, fabricated using Silicon-On-Insulator (SOI) wafers. A paddle supported by two cantilevers will deflect out-of-plane under a dynamic pressure δ d generated by the surrounding gas flow. Variable overlap area movable electrodes f comb-drive capacitors with movable electrodes attached to the paddle will change capacitance as the structure is deflected. By measuring the change of the capacitance, the velocity of the gas flow can be calcu- Figure 3: (a) 3D schematic of the comb-drive capacitor fingers lated. To increase the sensitivity of the device while (b) Cross-section of capacitor fingers on side view. keeping the critical layout dimension large, (e.g., the The gas flow regimes are fully turbulent for typical capacitance gaps and finger widths are larger than 10 transmission lines at high pressure [20]. In turbulent μm), three rows of digital capacitors are used.
2
Page 2 of 11
ip t
conditions, with a large Reynolds number, the drag capacitor finger width, d is the overlap length of the force on the paddle can be expressed as capacitor finger, δ is the gap between capacitor fingers and their frames, and f is the width of capacitor 1 2 Cd ρAp v , (1) frames; y1 and y2 are the distance of the drag forces Fd 2 on the movable capacitor fingers and their frames to where C is the local drag coefficient calculated em- the free end of the paddle, given by d
pirically, Ap is the area of the paddle, ρ is the density of the gas and v is the mean velocity of the gas. For a wide range of Reynolds number, the drag coefficient of a flat plate is in the range Cd = 1.1 - 1.3 [21]. In laminar flow conditions, with a small Reynolds number, the drag force is proportional to the velocity [7]. Fig. 2 shows the free-body diagram of the crosssection of the flow sensor with force distribution and deflection. The displacement due to drag force at the ends of the beams (cantilevers), the paddle, and the capacitor f ingers are respectively denoted as zb , zp , and zf . Details of comb-drive capacitors are shown in Fig. 3. By ignoring the drag force on the beams, since the beam area is about 0.25% of the paddle area, the total drag force can be replaced by a concentrated force Fd at a distance L1 from the free end of the cantilevers. Since it is assumed that the paddle, capacitor fingers and capacitor frames have the same drag coefficients, L1 can be expressed as
17 δ 6
3 d 2
f
cr
y1
(7)
36 36 52 δ d f. (8) 10 20 40 The slope and the deflection at the free end of the cantilevers due to the drag force Fd are given by
us
y2
Fd l 2 1 2EI
an
θb
2
L1 l
(9)
Fd l 3 3L1 1 (10) 3EI 2l where l, E, and I are the length, the Young s modulus, and the area moment of inertia of the cantilevers, respectively. Since the paddle and the capacitor fingers almost do not bend, the deflection slope of them is equal to the slope of deflection at the free end of the cantilevers. The deflection at the free end of the movable capacitors is given by
L1
Ac ce pt e
d
M
zb
L Ap 2 A
L
y1
Af A
L
y2
Acf , A
(2)
zf
where L is the paddle length; Ap , Af , and Acf are the areas of the paddle, movable capacitor fingers, and movable capacitor frames, respectively, calculated from Ap
Af
Acf
N wf d
Wf
A
WL
Ap
δ
3 f 4δ 2
2d
Af
Acf ,
f
zb
θb L
Lf
(11)
or 2Fd l3 η Ewt3
zf
(12)
with
(3)
η (4)
1
3 L1 2 l
3 1 2
2
L1 L l
5δ
3d l
2f
(13)
where w and t are the width and thickness of the cantilevers. There are two sources of capacitance in the device: the variable capacitance due to the comb-drive capacitor which is used to measure the flow, and the parasitic capacitance due to the four anchor locations to the substrate. Each anchor is separated from the silicon substrate by an insulating silicon dioxide layer,
(5) (6)
where W and L are the paddle width and length, N is the number of movable capacitor fingers, wf is the 3
Page 3 of 11
us
cr
ip t
where Ae is total area of four anchors (approximately 0.9 mm2 ), d is the relative permittivity of the oxide and tdo 10μm is the thickness of the buried oxide layer. The areas of the beam anchors and capacitor frame anchors are made as small as possible to reduce Cp . As illustrated in Fig. 3-b, the overlap areas of capacitors are different for each row. If it is assumed that the number of capacitor fingers on each row is equal (the actual number of capacitor fingers are 92, 92 and 98 for the first, second and third row, respectively), an analytical expression for the adjusted coefficient D is Figure 4: Deflection of the paddle and the capacitor fingers, calculated by Finite Element Method (FEM). The movable electrodes are replaced by a rectangular plate with equivalent surface areas for the purpose of the FEM analysis.
31 4
an
D
M
Displacement at free-end capacitor fingers P
d
Ac ce pt e C0 1
zf 1 t
D
Cp ,
(14)
where t is the device thickness, C0 is the initial capacitance (no deflection), Cp is the parasitic capacitance, and D is the adjusted coefficient due to the different overlap area of capacitor fingers for different rows (Fig. 3). The capacitance without gas flow C0 is given by C0
2N r 0
td , g
d 0
Ae , tdo
2f
.
(17)
100
Closed-form FEM calculation
80 60 40 20
0
0
5
10 15 Gas velocity [m/s]
20
Figure 5: Calculation (closed-form and FEM) of deflection at the free end of a capacitor finger vs. gas flow velocity at 400 psi pressure and 25 C (gas density = 0.668 kg m3 at 25 C and 1 atm (14.7 psi), drag-force coefficient Cd = 1.28. Total area A = 9.387 mm2 ). The drag force is 10.9 mN and 43.7 mN for a velocities of 10 m s and 20 m s, respectively.
(15)
where N is the total number of capacitor finger pairs, r and 0 are the relative permittivities of gas and vacuum, respectively; g is the gap between capacitor fingers. The parasitic capacitance is Cp
3d l
2.3. Design optimization
placing an equivalent capacitance in parallel with the variable capacitance. To simplify the calculations, the fringe effect is ignored, considering the capacitance is only between the two overlapping plates, i.e., the variable capacitance between capacitor fingers and the frames is ignored. Consequently, the capacitance for a deflection zf is C
2 L1 l 4δ η
The MEMS flow sensor is intended for application within natural gas pipelines which operate at very high gas pressure, 400 psi for nominal pressure and 1000 psi for maximum pressure. The typical gas velocity in natural gas pipelines is about 8 m s. As gas pressures increase, the gas density will increase and
(16) 4
Page 4 of 11
Table 1: Dimensions of the MEMS flow sensors
cr
ip t
Layout dimension 5.5 mm x 5.5 mm 2 mm x 4 mm 2 mm 4 mm 100 μm 1 mm 25, 50, 75 or 95 μm 10 or 15 μm 10 or 15 μm 10 μm 280 μm 100 μm 282
us
Symbol A0 A L W t l wb wf g δ d f N
an
Description chip area paddle area paddle length paddle width device thickness cantilever length cantilever width capacitor finger width capacitor finger gap gap between fingers and frames capacitor overlap length capacitor frame width number of capacitor fingers
or
30
M
d
η
Ac ce pt e
Capacitance [pF]
2 l3 Cd ρmax Avmax (19) 4 Et 25 For a nominal device thickness of 100 μm, and other parameters as shown in Table 1, the widths of 20 the cantilevers must be larger than 85 μm for a nominal pressure of 400 psi and vmax = 18 m s. However, 15 if the beam width is too large, the device sensitivity, i.e., the variable capacitance versus gas velocity, will 10 be low. In this design, the beam width of 95 μm (10% larger than the design value) is chosen for gas pressure up to the nominal pressure of 400 psi and 5 0 5 10 15 20 maximum gas velocity of 18 m s . A variety of beam Velocity [m/s] widths have been designed, e.g., 25 μm, 50 μm, and 75 μm, for lower maximum gas pressures of 100 psi, Figure 6: Calculated capacitance vs. gas flow velocity at nom- 200 psi and 300 psi, respectively. Moreover, withinal pressure of 400 psi and 25 C. out changing the layout design, the working ranges can be easily adjusted by choosing different wafer device layer thickness. For instance, for the same beam consequently the drag force Fd (Eq. 1) will increase. width of 75 μm, the device can measure vmax = 18 The variable capacitance in Eq. 14 is only valid when m s at a maximum pressure of 1000 psi when fabrithe capacitor finger displacement zf is less than the cated for 130-μm device layer thickness of SOI wafers. device thickness t. Given maximum operating conVarious capacitor finger gaps and finger widths (10 ditions, i.e., ρ = ρmax and v = vmax , the capacitor μm - 15 μm) have been designed to test the deep dry finger displacement zf must satisfy the condition: etching aspect ratio. The dimensions of the MEMS flow sensor are given in Table 1. (18) zf t Fig. 4 shows the deflection of the capacitor fin-
w
5
Page 5 of 11
gers and movable electrodes calculated using the Finite Element Method (FEM). In the FEM model, the drag force is converted to an equivalent pressure on the paddle and capacitor fingers. The FEM calculation results are shown in Fig. 5. The closed-form deflection calculated from (Eq. 12) are also plotted for comparison. There is good agreement between the analytical and FEM results. The FEM modal analysis shows that the out-of-plane natural frequency (in the z -direction in Fig. 4) is 2.7 kHz and the in-plane natural frequency (in the x -direction Fig. 4) is 16 kHz. Based on the deflection of the paddle, the variable capacitance versus the velocity of gas flow can be calculated using Eq. (14), assuming the drag coefficient of the rectangular paddle is about 1.28 (Fig. 6). As can be seen in Eqs. (1) and (14 that the device will respond with quadratic order of the gas velocity.
an
us
cr
ip t
layers. The process is similar to the previous fabrication of the MEMS electrostatic energy harvesters [22] without the electret deposition step. First, 200nm Cr/Au is evaporated using high-vacuum e-beam evaporation, and patterned to form electrical bond pads (Fig. 7-a). Next, a Deep-Reactive-Ion-Etch (DRIE) process is applied to both sides of the SOI wafers to define the cantilevers, digital capacitors, and paddle (Figs. 7-b and 7-c). A thick UV-baked layer of photoresist SPR-220 is used as a masking during the DRIE steps. The beam stiffness in the lateral direction is large enough to avoid the stiction effect between capacitor fingers during wet etching. Then, the movable parts are released by wet etching of the buried oxide using buffered hydrofluoric acid (Fig. 7-d). Fig. 8 shows an optical micrograph (center) of the device after fabrication. Scanning-Electron Microscope (SEM) images of specific components of the sensor are displayed in the insets of Fig. 8. The capacitor fingers and their frame are shown in Figs. 8-a and 8-b. Due to the intentional over-expose of thick photoresist SPR-220, the capacitor width is 1.8 μm smaller than the layout.
M
3. Fabrication 3.1. Fabrication process
d
Ac ce pt e
Si substrate
a) Evaporation of Cr/Au for electrical connections
Si substrate
3.2. Dummy structures to protect cantilevers in DRIE
b) A deep-reactive-ion-etch (DRIE) Si device layer to define capacitor fingers, paddle, and cantilevers.
To protect the sidewall of the cantilevers during DRIE, dummy structures are added along the cantilevers. Two kinds of protective structures have been designed as shown in Fig. 9: dummy structures attached to the beams (9-a), and dummy structures not attached to the beams (Fig. 9-b). As shown in Figs. 9-c and -d, both designs can protect the sidewall of the cantilevers during DRIE with nearly vertical profiles. To avoid unexpected motions of the paddle in the lateral direction, mechanical end-stops were designed as shown in the inset of the Fig. 8-d. The gap between the paddle and the end-stops was designed smaller than the gap between capacitor fingers to avoid the capacitor fingers touching in the lateral inplane direction.
c) A DRIE Si substrate to create deflection space for the capacitor fingers and cantilevers. d) An HF (wet etch) releasing
Cr/Au
Si structure
Oxide
Photoresist
Figure 7: Fabrication process of MEMS flow sensor on Siliconon-Insulator (SOI) wafers
Fig. 7 shows the fabrication process of the sensors using SOI wafers. The fabrication process is simple and requires only three photo-lithographic mask 6
Page 6 of 11
(b)
11.8um
ip t
8.2um
cr
(a)
us
76.0
200 Pm
24.1
(d)
an
(c)
10.4um
M
Figure 8: Micrograph of the MEMS-capacitive flow sensor after fabrication. Inset figures: SEM images of (a) the capacitor fingers, (b) a corner of the sensor, (c) cantilever and its DRIE protections, (d) mechanical end-stops of the paddle in the lateral in-plane direction.
where v is mean air velocity, d is the air pipe diameter, and ν is the kinematic viscosity ( ν 1.568 10 5 4.1. Experimental set-up m2 /s at room temperature and atmospheric presThe sensors were characterized using an air flow in sure). The Reynolds number is larger than 4000 when a 4-inch diameter duct. The Reynolds number is v is larger than 0.62 m/s, indicating mainly turbulent (20) R vd ν,
Ac ce pt e
d
4. Experimental results and discussion
25 um
A
25.2 um
(c)
70.5 um
69.6 um
200 um 200 um
A’
A
(a)
A’
(d)
(b)
Figure 9: Variety of dummy structures to protect the cantilevers in DRIE.
7
Page 7 of 11
air flow in the duct. The experimental setup for the flow measurement is shown in Fig. 10. An air fan with adjustable speed via a variable transformer was used to generate the required air flow. The air flow velocity was measured using a digital anemometer (TPI 575C1) with vane probe, allowing measuring for an air flow up to 25 m s. For practical applications, a high-resolution capacitance-to-digital readout chip AD7747 (Analog Devices, Wilmington, MA, a resolution of 20 aF , accuracy of 10 f F , maximum capacitance of 17 pF ) could be used to measure the output capacitance [23]. For a purpose of characterization, a precision LCR meter E4980A was used to measure the capacitance between two electrodes of the flow sensor. The parallel circuit mode of capacitance (Cp-D) was used to measure small capacitance of the flow sensor. To minimize the stray capacitance of the test fixture of the LCR meter, the open/short correction was done before measurement. Using list sweep function, each data point of experiments was measured by averaging of 30 samples. The MEMS flow sensors were bonded on PCB (printed circuit boards) which were made a hole so that the air flow can go through. The PCB were attached on a probe to insert the device into the air duct. The other side of the duct, an air straightener was inserted to reduce the vortex. The average air velocity was measured by the anemometer with vane probe and was removed while testing the flow sensors to avoid generating vortexes.
16.5 increasing flow decreasing flow
16.22
15
16.2 16.18
2 C = -3.4e-3 v + 1.28e-3 v + 16.22
14.5 16.16 0
14
2
1
3
4
2
C = -8.5e-3 v + 73.6e-3 v + 16 0
us
13.5
ip t
15.5
cr
Capacitance [pF]
16
5
10 Velocity [m/s]
15
20
M
an
Figure 11: Measurement of variable capacitance vs. air velocity at the atmospheric pressure (14.7 psi) and the temperature of 25 C. Beam width of 25 μm. Capacitor finger gaps and finger widths are 10 μm. : experiments with increasing air flow. : experiments with decreasing air flow. Solid lines: the best fitting curves for up flow.
Data acquisition
Fan
Anemometer
15 14 25 P m analysis 25 P m experiments
13
50 P m analysis 50 P m experiements
12
Air duct
Variable transformer
16
Capacitance [pF]
Ac ce pt e
d
4.2. Device response
0
5
10 Velocity [m/s]
15
20
Figure 12: Measurements of capacitance vs. gas velocity in the air at the atmospheric pressure (14.7 psi) and the temperature of 25 C for different beam widths of 25 μm, and 50 μm. Capacitor finger gaps and finger widths are 10 μm.
Precision LCR meter
MEMS flow sensor
Fig. 11 shows the measured output capacitance of the flow sensor versus the airflow velocity for the smallest beam width - 25 μm and finger gaps and
Figure 10: Experimental set up for evaluating the MEMS flow sensor using an air flow. Insets: the flow sensor attached to PCB and the vane probe of the anemometer.
8
Page 8 of 11
finger widths of 10 μm. The air flow was increased from 0 to 19.2 m s and then decreased from 19.2 to 0 m s to measure the hysteresis. Each experimental point was measured by averaging of 30 samples. The experimental curves were repeated three times to obtain the error bar. The largest different capacitance of up flow and down flow is 0.4% at a velocity of 14.3 m s. The best fit to data is the quadratic function with the first order and the second order of the velocity, C = k1 v 2 + k2 v + Cp where k1 and k2 are the coefficients of the sensitivity. However, repsonse is slight different for low velocity regime (0 - 4 m s) and high velocity regime (4 - 19.2 m s). For low velocity regime, the coefficients k1 = -3.4 f F m s 2 and k2 = 1.28 f F m s . For high velocity regime, the coefficients k1 = -8.5 f F m s 2 and k2 = 73.6 fF m s . Fig. 12 shows the measured capacitance versus the gas velocity in the air for different beam widths of 25 μm and 50 μm. The capacitor finger gaps and finger widths are 10 μm. The closed-from calculation (from Eq. 14) were also plotted for comparison. The under-cut of capacitor finger gaps (11.8 μm instead of 10 μm) was included in the calculation. Although the experimental results differ from the closed-form calcalation, they follow the trend of the analytical curves. The discrepancy of analysis and experiments can be explained as follows. First, the analytical model does not include the capacitance fringe effects and the sidewall profile of capacitor finger is not vertical. Second, the measurement of velocity includes 2% error from the anemometer. Third, the parasitic capacitance Cp experiments is larger than because the model does not include the stray capacitance of wire connections. (C0 = 11.84 pF and Cp = 3.11 pF compared to Cp = 4.38 pF in experiments)
10.3
ip t
Flow sensor in parallel
10.2
10.15
cr
Capacitance [pF]
10.25
0
us
10.1
5
10 Velocity [m/s]
15
20
an
Figure 13: Response of the flow sensor when it is placed parallel to the air flow. Beam width of 50 μm. Finger gap of 10 μm and finger width of 15 μm.
Ac ce pt e
d
M
However, the capacitance was changed about 0.3% at a velocity of 16 m s. The vortex of the air after go through the first edge of the printed circuit board (PCB) and the tilt of PCB due to set-up may be this causing. With cylindrical pumps on the surface of the paddle and difference of suspension design, the device in parallel flow could measure the pressure gradient as presented by Zhao et al. [23]. 4.4. Temperature effect
It is important to note that all flow sensors are sensitive to the variations in the density of the gas, which in turn is a function of its temperature. It is thus undesirable if the transduction mechanism is also sensitive to changes in the ambient temperature. The benefit of the capacitive (compared to e.g., piezoresistive transduction) is its insensitivity to temperature fluctuations. To verify this, the flow sensor was tested at dif4.3. Device in parallel flow ferent air flow temperatures. A heater was placed in The response of the flow sensor when it is placed the duct to heat the air. The output capacitance was parallel to the air flow is shown in Fig. 13. Ideally, measured at constant velocity when increasing the the output capacitance would not change in this sit- temperature. The air temperature was adjusted from uation because the gas flow hits the paddle edge and 23 C to 43 C for low velocity and to 37 C for high the beams are very stiff in this direction (36 times velocity. The experimental results are shown in Fig. stiffer than the stiffness of perpendicular direction). 14. The output capacitance changed very slightly 9
Page 9 of 11
9.7
Lab, University of California, Berkeley. The author would like to thank Christine E. Gregg and Sean Wihera for valuable suggestions.
air velocity = 5.6 m/s
References
ip t
air velocity = 11.8 m/s 9.5
[1] C. G. Schabmueller, Flow sensors, Artech House, 2004, Ch. 9, pp. 213 – 248.
cr
9.4 air velocity = 14.8 m/s
9.3
[2] Y.-H. Wang, C.-P. Chen, C.-M. Chang, C.-P. Lin, C.-H. Lin, L.-M. Fu, C.-Y. Lee, Mems-based gas flow sensors, Microfluidics and Nanofluidics 6 (3) (2009) 333–346.
9.2 25
30
35
40
45
us
Capacitance [pF]
9.6
0
Figure 14: Response of the flow sensor under changing temperatures.
[3] N. Nguyen, Micromachined flow sensors - a review, Flow Measurement and Instrumentation 8 (1) (1997) 7 – 16.
an
Temperature ( C)
[4] S. Wu, Q. Lin, Y. Yuen, Y. Tai, Mems flow sensors for nano-fluidic applications, in: Micro Electro Mechanical Systems, 2000. MEMS 2000. The Thirteenth Annual International Conference on, 2000, pp. 745–750.
5. Conclusion
d
M
when changing the temperature. In particular, the capacitance increased 0.21%, 0.34% and 0.25% for the velocity of 5.6 m s, 11.8 m s and 14.8 m s, respectively.
Ac ce pt e
In-plane MEMS capacitive flow sensors have been designed, modeled, and fabricated using a bulk micromachining technology with only three photolithographic mask layers. Experimental results with an airflow showed that output capacitance of the device is a quadratic function of air velocity, C = k1 v 2 + k2 v + Cp , where k1 = -8.5 f F m s 2 , k2 = 73.6 f F m s and Cp = 16 pF. Experimental results show that the output capacitance changed only slightly, about 0.21% - 0.34%, when the temperature changed from 23 - 43 C. A shield to protect the device from particles in the gas stream, as well as an erosion and corrosion coating, will be considered in future work. 6. Acknowledgments This project is sponsored by the California Energy Commission, contract number 500-10-044. The devices were fabricated in Marvell Nano-fabrication
[5] J. Sun, D. Cui, L. Zhang, X. Chen, H. Cai, H. Li, Fabrication and characterization of a doubleheater based {MEMS} thermal flow sensor, Sensors and Actuators A: Physical 193 (0) (2013) 25 – 29. [6] M. Shikida, Y. Yamazaki, K. Yoshikawa, K. Sato, A {MEMS} flow sensor applied in a variable-air-volume unit in a building airconditioning system, Sensors and Actuators A: Physical 189 (0) (2013) 212 – 217. [7] V. Gass, B. H. Van Der Schoot, N.-F. de Rooij, Nanofluid handling by micro-flow-sensor based on drag force measurements, in: Micro Electro Mechanical Systems, 1993, MEMS ’93, Proceedings An Investigation of Micro Structures, Sensors, Actuators, Machines and Systems. IEEE., 1993, pp. 167–172. [8] L. Zhang, X. Ye, Z. Zhou, J. Yao, A micromachined single-crystal silicon flow sensor with a cantilever paddle, in: Micromechatronics and Human Science, 1997. Proceedings of the 1997 International Symposium on, 1997, pp. 225–229.
10
Page 10 of 11
M
an
us
cr
ip t
[9] Y. Ozaki, T. Ohyama, T. Yasuda, I. Shimoyama, [18] A. Dagamseh, Bio-inspired hair flow-sensor arAn air flow sensor modeled on wind receptor rays: from nature to mems, Ph.D. thesis, Unihairs of insects, in: Micro Electro Mechanical versity of Twente (2011). Systems, 2000. MEMS 2000. The Thirteenth Annual International Conference on, 2000, pp. 531– [19] S. Nguyen, I. Paprotny, P. Wright, R. White, Inplane capacitive mems flow sensor for low-cost 536. metering of flow velocity in natural gas pipelines, [10] Y. Su, A. G. R. Evans, A. Brunnschweiler, in: Micro Electro Mechanical Systems (MEMS), Micromachined silicon cantilever paddles with 2014 IEEE 27th International Conference on, piezoresistive readout for flow sensing, Journal 2014, pp. 971–974. of Micromechanics and Microengineering 6 (1) [20] P. M. Coelho, C. Pinho, Considerations about (1996) 69. equations for steady state flow in natural gas [11] J. Chen, Z. Fan, J. Zou, J. Engel, C. Liu, Twopipelines, Journal of the Brazilian Society of Medimensional micromachined flow sensor array for chanical Sciences and Engineering 29 (2007) 262 fluid mechanics studies, Journal of Aerospace – 273. Engineering 16 (2) (2003) 85–97. [21] M. Sadraey, Aircraft Performance Analysis, [12] Z. Fan, J. Chen, J. Zou, D. Bullen, C. Liu, VDM Verlag Dr. Muller, 2009. F. Delcomyn, Design and fabrication of artificial lateral line flow sensors, Journal of Microme- [22] S. D. Nguyen, E. Halvorsen, I. Paprotny, chanics and Microengineering 12 (5) (2002) 655. Bistable springs for wideband microelectromechanical energy harvesters, Applied Physics Let[13] L. Du, Z. Zhao, C. Pang, Z. Fang, Drag force ters 102 (2) (2013) –. micro solid state silicon plate wind velocity sensor, Sensors and Actuators A: Physical 151 (1) (2009) 35 – 41.
Ac ce pt e
d
[23] Z. Zhao, M. Shin, J. M. Gallman, R. D. White, A microfabricated shear sensor array on a chip with pressure gradient calibration, Sensors and [14] N. Andre, B. Rue, G. Scheen, D. Flandre, L. A. Actuators A: Physical 205 (0) (2014) 133 – 142. Francis, J.-P. Raskin, Out-of-plane mems-based mechanical airflow sensor co-integrated in {SOI} {CMOS} technology, Sensors and Actuators A: Physical 206 (0) (2014) 67 – 74. [15] R. Philip-Chandy, P. J. Scully, R. Morgan, The design, development and performance characteristics of a fibre optic drag-force flow sensor, Measurement Science and Technology 11 (3) (2000) N31. [16] M. Dijkstra, J. J. van Baar, R. J. Wiegerink, T. S. J. Lammerink, J. H. de Boer, G. J. M. Krijnen, Artificial sensory hairs based on the flow sensitive receptor hairs of crickets, Journal of Micromechanics and Microengineering 15 (7) (2005) S132. [17] N. Izadi, Bio-inspired mems aquatic flow sensor arrays, Ph.D. thesis, University of Twente (2011). 11
Page 11 of 11