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Sensors and Actuators A 133 (2007) 272–277

A high-performance MEMS capacitive strain sensing system

Wen H. Ko ∗, Darrin J. Young, Jun Guo, Michael Suster, Hung-I Kuo, N. ChaimanonartElectrical Engineering and Computer Science Department, Case Western Reserve University, 10900 Euclid Ave, Cleveland, OH 44106, USA

Received 8 July 2005; received in revised form 10 January 2006; accepted 7 March 2006Available online 25 July 2006

bstract

This paper describes the design of a functional strain sensing module with large dynamic range (80 dB), DC to 10 kHz response, high resolution,nd mini size for industrial applications, such as the rolling-element bearings research. The design of the MEMS capacitive strain sensor employsechanical amplifications of package design and buckle beams as well as the linear differential comb capacitor. The sensor is interfaced with a low

oise charge amplifier, mixer, and filter circuits to provide an analog output that demonstrated a resolution of 0.09 microstrains with a maximum

ange of ±1000 microstrains. The sensor and the electronic circuits, including a temperature sensor, can be integrated on a chip, and packaged assmall functional unit. Additional electronics were integrated with the interface circuit on the chip that provide A/D conversion, radio frequencyower supply, and digital signal telemetry to a near-by control unit. Preliminary test results are compared with the design simulation.

2006 Elsevier B.V. All rights reserved.

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eywords: Strain sensors; Capacitive sensor; Mechanical gain in strain sensoelemetry system

. Introduction

High-performance strain sensing functional modules consist-ng of sensors and interface electronics are highly desirable fordvanced industrial applications, such as point-stress and torqueensing for ball-bearings, rotating shafts and blades, etc. Perfor-ance requirements with a resolution of 0.1 microstrains overwide bandwidth and a dynamic range of 80 dB are demanded

or these applications. Commercial metal foils and piezoresistivelement strain sensors are inadequate for the high-performancepplications [1,2]. Capacitive strain sensors have many advan-ages such as high sensitivity, low temperature dependence, lowoise, large dynamic range, and potential monolithic integrationith CMOS circuits [3].In this paper, a novel capacitive MEMS strain sensor is

atched to a low noise CMOS integrated interface sensing elec-ronics to achieve the performance of resolution, dynamic range,nd signal bandwidth desired, as a module with analog or digital

utputs. Additional electronics were designed and integratedith the sensing electronic to power the strain sensing module

nd to provide a digital data telemetry link to the external RF

∗ Corresponding author. Tel.: +1 216 368 4081; fax: +1 216 368 6039.E-mail address: whk@cwru.edu (W.H. Ko).

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924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2006.06.015

unctional strain sensing module; RF powered passive sensor module; Sensor

owering and control unit. This second module can then be useds a passive sensor module for strain measurement on rotatinghaft or other fast moving elements. The major design consid-ration of the sensor and the measured module performancere presented and agree closely with the design and simulatedoals.

. Sensor design

Strain sensors are widely used. Capacitive strain sensors havedvantages in temperature drift, sensitivity, noise, and dynamicange, as compared to metal foil and piezoresistive sensors.esearches were reported to measure residual stresses utilizingechanical amplification scheme [4] with a strain resolution of

0 microstrains. A novel high-performance capacitive strain sen-or employing mechanical amplifications of packaging designnd buckle beams suspension; and the comb structure for dif-erential capacitive linear output is designed. These mechanicalmplifications improve the sensor sensitivity and reduce sensi-ivity requirement and power dissipation on the interface circuits.ig. 1 shows a simplified schematic of a capacitive MEMS strain

ensor with buckle beams.

The device consists of three amplifying buckle beams, withomb fingers positioned at the structural center. The beams giveechanical amplification to the applied strain, and the sensor

W.H. Ko et al. / Sensors and Actuators A 133 (2007) 272–277 273

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/k)

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ecsutial capacitance output signal with mechanical gain twice thatof a single buckle beam set.

The device sensitivity and other performance were studied.They can be optimized by carefully selecting parameters such as

Fig. 1. Capacitive strain sensor.

ives linear differential capacitive output as a measure of thepplied strain, �x.

A folded packaging frame structure also is used in theackage of the capacitive sensor that provides a mechanical

onnection to the interface electronics. Furthermore, it gives andditional mechanical amplification, reduces the bonding atten-ation, and achieves a high transverse strain rejection ratio asill be explained later.Fig. 2 shows the principle of the buckle beam amplification

rchitecture. An applied strain causes a small lateral displace-ent, �x. With a small buckle angle, α, the center deflection of

he beam, �w, can be larger than �x thus resulting in a mechan-cal gain.

For very small �x, the angle, α, can be considered constant.he mechanical gain is constant. The sensor output is linear withtrain input, as given in Eq. (1).

mech = �w

�x∼= 1

2ctgα (1)

However, as the �x increases, the angle α change with theoad can no more be omitted, which results in the non-linear

Amech = �w

�x∼= 2(tanα/

tan2α[(tan2(kL/2)/4k)(−sin(2kL) + 2kL) +

Amech = �w

�x∼= 2(tanα

tan2α[(tanh2(kL/2)/4k)(sinh(2kL) − 2kL) +

roperty of the mechanical gain, as approximated,

∗mech = 1

2 tanα

[1 − 1

2 tan2α

�x

L

]. (2)

Fig. 2. Principle of a buckle beam amplification.

Fig. 3. A buckle beam strain sensor with differential output.

The complete analytical solution of the mechanical gain ofhe buckle beams is given by Eq. (3), one for tension and thether for compression.

n(kL/2) − h

2kL) + 2kL/4k) − (tan(kL/2)/2k)(cos(2kL) − 1)]

tanh(kL/2) − h

h(2kL) + 2kL/4k) − (tanh(kL/2)/2k)(cosh(2kL) − 1)]

(3)

A FEA simulation was also made. Fig. 3 shows the com-arison of analytical and FEA stimulated results. FEA givesame result as Eq. (3), Eq. (2) gives an error about 2% at 1000icrostrain, tension. The parameters of the buckle beam used

re: Lg = 1000 �m, Lb = 300 �m and the buckle angle = 5.7◦. Theechanical gain for a buckle beam is 5. For the sensor with two

uckle beams as a set, the gain is 10. These results agree wellith the experimental data observed on fabricated strain sensors.The linear differential capacitive output can be obtained by

mploying sensing comb fingers as shown in Fig. 4. By designingomb drive fingers at the center beam as shown, for a compres-ive strain, the top, Cx+, beam and bottom Cx− beam will movep and the center beam will move down, resulting in a differen-

Fig. 4. A buckle beam strain sensor with differential output.

274 W.H. Ko et al. / Sensors and Actuators A 133 (2007) 272–277

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Fig. 6. Fabrication process flow chart.

Fig. 7. SEM picture of a buckle beam strain sensor with two sets of buckledbeams.

Fig. 5. Sensor on a packaged frame.

uckling angle, fingers numbers, thickness and gap size, usinghe developed FEA simulation program. Optimized sensor has auckle angle of 5.7◦, a structural thickness of 20 �m, a minimumir gap size of 3.6 �m, and a lateral gauge length of 1 mm, a totalumber of 260 sensing fingers [5].

The sensor also employs novel packaging design with aolded frame as shown in Fig. 5. The sensor and the foldedrame can be simplified as two springs. By carefully selectinghe geometrical parameters, the stiffness of the two springs cane designed to be weak as compared to bonding area strength,hus achieving an additional mechanical gain, Ap = Lp/Lg, and

inimizing the strain attenuation loss caused by the bondingdhesive interface [6].

A high transverse strain rejection ratio (the sensitivity ration X direction over that in Y direction) is also achieved due tohe structural high transverse strength. The microstructure alsomploys a strain isolation scheme to minimize the interferencetrain introduced by the electronics carrier and wire connections.

. Fabrication and test result

A one-mask fabrication process, as shown in Fig. 6, wasesigned to verify the design and simulation results. The fabri-ation process started with a SOI wafer with device/oxide layerf 20/2 �m. The wafer was first patterned for a DRIE process,hen diced, cleaned, and followed by a timed HF release process.

200 ´̊A Aluminum layer was then sputtered on the device toeduce the sensor series resistance and resistive thermal noise.he prototype fabrication achieved a yield close to 100% due

o the simplicity of the design and fabrication process. An SEMicture of the sensor is shown in Fig. 7 with parameters given inection 2.

The sensor was bonded on a stainless steel beam by epoxy,hen measured using a three-point testing fixture developed for

evice testing [6]. The mechanical gain was determined bynalyzing the captured optical images. Fig. 8 shows a typicalharacteristic curve of the sensor capacitance change versus thepplied compressive strain. An overall differential mechanical Fig. 8. Measured capacitance change as a function of the applied strain.

W.H. Ko et al. / Sensors and Actuators A 133 (2007) 272–277 275

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Fig. 9. Electronic st

ain of 19 and a sensitivity of 210 aF/microstrain with nonlinear-ty less than 0.5% FS were achieved, close to the FEA simulation218 aF/microstrain) with a discrepancy less than 5%.

The device has also demonstrated a transverse strain rejec-ion ratio of 100. A high resolution of 0.1 microstrain withinhe bandwidth from DC-10 kHz and a full range of ±1000

icrostrains can be achieved with a matching interface circuit.

. Interface electronics

The prototype electronic strain sensing architecture is pre-ented in Fig. 9. The MEMS sensors, modeled as differentialapacitors, are driven by a clock signal with amplitude of 3 Vnd are interfaced by a charge amplifier, which converts the sen-or capacitance change to an output voltage. A clock frequencyf 1 MHz is chosen to modulate the sensor information, and tohift away from the 1/f noise of the amplifier, a critical meanso achieve a high sensitivity. The charge amplifier output is then

ixed by the same clock signal and low-pass filtered to obtainhe desired strain information. A second-order low-pass filterith a cut-off frequency of 150 kHz is designed to achieve a

inear phase characteristic.

Fig. 10. Sensing electronics die photo.

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ensing architecture.

A low noise charge amplifier with an input referred noisepectral density of 5 nV/(Hz)1/2, is designed and fabricatedsing a 1.5 �m CMOS process and consumes 1.5 mA currentrom a 3 V supply. Fig. 10 shows the chip photo with theore interface electronics occupying an area of approximately.6 mm × 1.7 mm.

The MEMS sensor chip is first bonded to a stainless steelubstrate and then wire bonded to the sensing electronics to formhe prototype system and evaluated in the test set up. Fig. 11hows the measured output voltage versus applied input strain,ndicating that the system can achieve an input signal of ±1000

icrostrains, corresponding to an output voltage of ±380 mV,ith a linearity of 1.5% of full scale, and with a low noise levelf 375 nV/(Hz)1/2, equivalent to a resolution of 37.5 �V over0 kHz bandwidth, thus a 80 dB dynamic range [7].

Fig. 12 is the RF powering architecture, where an external RFower source is used to drive a tuned series resonator consistingf LB1B and CB1B [8]. The resistor, RB1B , represents the overalleries resistance associated with the resonator. The RF signal isoupled to a parallel resonator, consisting of L2, and C2, and aotal loop resistance, RB2B , tuned to the same frequency. The sig-al is then rectified by an integrated CMOS fullwave rectifier to

V and further regulated to achieve a stable 3 V DC supply with2 mA current driving capability to power the sensor module.

An integrated CMOS fullwave rectifier used to rectify theeceived RF signal and convert it into DC is shown in Fig. 13,

Fig. 11. Output voltage vs. input strain.

276 W.H. Ko et al. / Sensors and Actuators A 133 (2007) 272–277

Fig. 12. The RF powering el

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here VAC-Input represents the coupled RF signal across the par-llel resonator, which is rectified to produce a DC output voltage,DC-Output. The RF powering and signal telemetry unit was inte-rated onto the same chip of the interface electronics and is usedo power the capacitive strain sensor and the low noise interfacelectronics. A strain resolution of 0.1 microstrains over a 10 kHzandwidth and a dynamic range 80 dB were achieved. Which ishe same performance achieved using 3 V battery.

. Conclusion

The analysis, design, fabrication and evaluation of a high-erformance capacitive strain sensor with low noise interfacemplifier and RF powering electronics is presented. The workas carried out by a research team. The evaluated results on

ensor modules fabricated agree well with the FEA simula-ion. These modules will be packaged for needed applica-ions.

cknowledgment

This work is supported by U.S. Army Research Office (ARO)nder contract #DADD19-02-1-0198.

eferences

1] M.L. Nagy, C. Apanius, J.W. Siekkinen, A user friendly, high-sensitivitystrain gauge, Sensors 18 (June 2001) 20–27.

2] M. Hrovat, D. Belavic, Z. Samardzija, J. Holc, An investigation of thick-film

resistor, fired at different temperatures, for strain sensors, in: InternationalSpring Seminar on Electronics Technology, May 2001, pp. 32–36.

3] L.L. Chu, L. Que, M.-H. Li, Y.B. Gianchandani, Measurements of mate-rial properties using differential capacitive strain sensors, J. MEMS 11 (5)(October 2002), pp. 489–498.

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ectronics architecture.

4] L. Lin, A.P. Pisano, R.T. Howe, A micro strain gauge with mechanical ampli-fier, J. MEMS 6 (4) (December 1997), pp. 313–321.

5] J. Guo, H. Kuo, D.J. Young, W.H. Ko, Buckled beam linear output capacitivestrain sensor, in: Solid-State Sensor, Actuator and Microsystems Workshop2004 at Hilton Head, June 2004, pp. 399–403.

6] H. Kuo, J. Guo, D.J. Young, W.H. Ko, Packaging of micro strain sensors,IEEE Sensors 2004, October 2004, pp. 546–549.

7] M. Suster, J. Guo, N. Chaimanonart, W. Ko, D. Young, Low-noise CMOSintegrated sensing electronics for capacitive MEMS strain sensors, in: IEEECustom Integrated Circuits Conference, October 2004, pp. 693–696.

8] N. Chaimanonart, W. Ko, D. Young, Remote RF powering system for MEMSstrain sensors, in: Third IEEE International Conference on Sensors, Vienna,October 2004, pp. 1522–1525.

iographies

en H. Ko received his BSEE from Xiamen University, China, in 1946, andis PhD degrees from Case Institute of Technology, Cleveland, Ohio, in 1959.e has been a faculty member of EE and BME Departments at Case Westerneserve University for 40 years. He is a professor Emeritus in EE of CWRUow.

arrin J. Young received his BS with honors, MS, and PhD degrees from theepartment of Electrical Engineering and Computer Sciences at University ofalifornia at Berkeley in 1991, 1993, and 1999, respectively. His doctoral dis-

ertation emphasizes on microelectromechanical devices design and fabricationechnologies for radio frequency analog signal processing. Between 1991 and993, he worked at Hewlett-Packard Laboratories in Palo Alto, CA, where heesigned a shared memory system for a DSP-based multiprocessor architecture.uring the summer of 1997, he worked at Rockwell Semiconductor Systems inewport Beach, CA, where he designed silicon bipolar RF analog circuits for

ellular telephony applications. Between 1997 and 1998, he was also at Lawrenceivermore National Laboratory, working on the design and fabrication of three-imensional RF MEMS coil inductors for wireless communications. Dr. Youngoined the Department of Electrical Engineering and Computer Science at Case

estern Reserve University in 1999, where he is currently an associate profes-or. His research interests include MEMS and nano-electro-mechanical devicesesign, fabrication, and integrated analog circuits design for sensing, commu-ication, biomedical implant, and general industrial applications.

un Guo received his BS degree from Tsinghua University, China, in 1993,nd MS degree from Case Western Reserve University (CWRU) in 2000. From000 to 2002, he joined a telecommunication company where he was involved inesigning MEMS wavelength optical switches and related high-voltage drivingircuitry. Since 2002, he has been working towards his PhD degree at CWRUdvised by Dr. Wen H. Ko. His current research activities are oriented towardshe design, fabrication, and testing of MEMS physical sensors and nano-electro-

echanical devices for a variety of application fields.

ichael Suster received his BS and MS degrees in electrical engineering fromase Western Reserve University in 2002 and 2004, respectively. He is currentlyursuing his PhD degree in electrical engineering at Case where he is a recipient

f the Case Prime Fellowship. His current research interests include analogntegrated circuit design for wireless MEMS sensors.

ung-I Kuo received the BS degree in electrical engineering from Tatung Insti-ute of Technology, Taipei, Taiwan, in 1992 and the MS and PhD degrees

Actua

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NEThailand, in 2001. Currently, he is now working toward his PhD degree at

W.H. Ko et al. / Sensors and

n electrical engineering and computer science from Case Western Reserveniversity, Cleveland, OH, in 1999 and 2002, respectively. His dissertationork focused on design, fabricate and testing MEMS devices based on 3C-

ilicon carbide on insulator substrates. Upon completion of the PhD program,e joined the Department of Electrical Engineering and Computer Science, Caseestern Reserve University, Cleveland, OH, as research associate. He has 10

ublications in journal/conference proceedings. His research interests includeicro/nanofabrication technologies and packaging for MEMS devices.

Cifa

tors A 133 (2007) 272–277 277

attapon Chaimanonart received his bachelor degree in engineering fromlectrical Engineering Department at Chulalongkorn University, Bangkok,

ase Western Reserve University in Cleveland, OH. Professor Darrin J. Youngs his research advisor. His research interests are integrated-circuit designor wireless communications and interface electronics for biomedical sensingpplication.