JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 2 ...jchae2/Publications_files/3axis... · JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 2, ... mechanical noise

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 2, APRIL 2005 235

A Monolithic Three-Axis Micro-g MicromachinedSilicon Capacitive Accelerometer

Junseok Chae, Member, IEEE, Haluk Kulah, Member, IEEE, and Khalil Najafi, Fellow, IEEE

Abstract—A monolithic three-axis micro-g resolution siliconcapacitive accelerometer system utilizing a combined surfaceand bulk micromachining technology is demonstrated. Theaccelerometer system consists of three individual single-axisaccelerometers fabricated in a single substrate using a commonfabrication process. All three devices have 475- m-thick siliconproof-mass, large area polysilicon sense/drive electrodes, andsmall sensing gap ( 1 5 m) formed by a sacrificial oxidelayer. The fabricated accelerometer is 7 9 mm2 in size, has100 Hz bandwidth, 5 pF/g measured sensitivity and calculatedsub- g Hz mechanical noise floor for all three axes. Thetotal measured noise floor of the hybrid accelerometer assembledwith a CMOS interface circuit is 1.60 g Hz ( 1.5 kHz) and1.08 g Hz ( 600 Hz) for in-plane and out-of-plane devices,respectively. [1150]

Index Terms—Inertial sensors, micro-g, micromachinedaccelerometer, sigma-delta, switched-capacitor, three-axisaccelerometer.

I. INTRODUCTION

MEMS (microelectromechanical systems) have attractedmuch attention since miniaturized mechanical struc-

tures were developed by utilizing semiconductor fabricationtechnology. Sensors for measuring pressure, acceleration, anddevices such as the inkjet printer head have been commercial-ized since 1970s [1]–[3]. Among them, the micromachinedpressure sensor and accelerometer have been one of the mostsuccessful commercialized MEMS products for automotiveapplications [4]. In addition to low-medium performance ac-celerometers, demand for high-performance devices aimingat inertial navigation/guidance, unmanned aerial vehicles(UAV’s), seismometry, space gravity instruments, and manyconsumer applications such as computer peripherals, headsetfor virtual reality, has constantly increased due to the potentialfeatures of low-cost, small-size, and low-power dissipation[5]–[7].

Manuscript received September 2, 2003; revised August 30, 2004. This workwas supported by DARPA under Contract F30602-98-2-0231 and made useof the WIMS Engineering Research Center’s Shared Facilities supported bythe National Science Foundation under Award EEC-0096866. Subject EditorG. Stemme.

J. Chae is with the Department of Electrical Engineering and Computer Sci-ence and Center for Wireless Integrated Microsystems, University of Michigan,Ann Arbor, MI 48109-2122 USA (e-mail: [email protected]).

H. Kulah was with the Department of Electrical Engineering and ComputerScience, University of Michigan, Ann Arbor, MI 48109-2122 USA. He is nowwith the Department of Electrical and Electronics Engineering, Middle EastTechnical University (METU), Ankara, Turkey

K. Najafi is with the Department of Electrical Engineering and ComputerScience, University of Michigan, Ann Arbor, MI 48109-2122 USA.

Digital Object Identifier 10.1109/JMEMS.2004.839347

The requirements for these high-performance applicationsinclude low noise, high sensitivity, small drift, excellent sta-bility, small off-axis sensitivity, low-temperature sensitivity,and high resolution [8], [9]. In addition, many of these ap-plications require measurements along more than one axis.Of the requirements mentioned above, accuracy (stability)and resolution are the most challenging. The former is verydependent on the overall design and structure of the device, aswell as on the packaging and mounting and the materials used.The latter, depends heavily on the design and specific featuresof the sensing element and its readout electronics.

Since the first micromachined accelerometer was introducedin the late 1970s [1], several accelerometers have been de-veloped to achieve micro-g resolution. However, most of thereported high-performance capacitive accelerometers are onlysensitive to a single-axis [10]–[13]. For some applications suchas inertial navigation, a precision three-axis accelerometersystem is highly desired. In order to build such a system,typically individual devices are hybrid mounted on the faces ofa cube. This introduces misalignment of individual sensors, in-creases the cost, occupies large area, and requires complicatedpackaging [14], [15].

Although a few integrated single-chip three-axis capacitivemicroaccelerometers have been reported [16]–[18], due to smallmass, low sensitivity, or low-performance readout circuit, theiroutput noise floors are at best in the 0.04–1 range.Our group has reported in-plane (x- and y-axis) and out-of-plane(z-axis) capacitive silicon accelerometers with micro-g resolu-tion [19], [20]. Both implement a combined surface and bulkmicromachining technology [21] and utilize an almost identicalfabrication process. Thus, two in-plane and one out-of-planeaccelerometers can be integrated onto a single substrate. Thispaper presents a fully integrated three-axis accelerometer with ahybrid low-noise CMOS readout circuit providing micro-g mea-surement resolution for all three axes. In the following sections,first the structure of the three-axis accelerometer is described.Next, estimated performance with interface electronics, and ac-celerometer fabrication are discussed. Finally, measurement re-sults of the accelerometer-CMOS readout circuit system such assensitivity, noise floor, temperature, and drift characteristics arepresented.

II. SENSOR DESIGN

A. Accelerometer Structure

Fig. 1 shows the structure of the three-axis accelerometer.The three-axis chip is a monolithic integration of three indi-vidual single-axis accelerometers. This reduces the size of the

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236 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 2, APRIL 2005

Fig. 1. Three-axis single-chip micro-g accelerometer.

entire system. The lithographically defined alignment of thethree single-axis devices minimizes cross-axis sensitivity dueto the misalignment of individual devices. The three-axis ac-celerometer is mechanically connected together by polysiliconconnectors, which electrically isolate the three accelerometersto ensure cross-talk free operation. Unlike surface microma-chined devices, the three-axis chip utilizes a combined surfaceand bulk micromachining technology so that it has large struc-tural mass (full wafer thick, 475 ), and large area electrodeswith small sensing gap , which produces high-sen-sitivity low-noise accelerometers. The proof-mass is defined byanisotropic wet etching at the end of fabrication. Polysiliconelectrodes span the entire proof-mass and are anchored to asupporting rim.

Fig. 2 shows individual device structures. The Out-of-planeaccelerometer has its proof-mass separated by a narrow air gapfrom a fixed conductive plate made of polysilicon located at thetop and the bottom of the proof-mass, establishing a parallelplate capacitor. The polysilicon plate, while thin, is made verystiff by vertical stiffeners to ensure stable closed-loop operation[22]. In order to reduce damping to achieve sub-mechanical noise, the polysilicon plate is perforated to allowthe gas between the plate and the proof-mass to escapeeasily through the holes. According to our device structure,the damping needs to be to satisfy0.7 mechanical noise density. Detail derivationsand equations regarding the polysilicon plate perforation toachieve very low mechanical noise are presented in [23]. Thein-plane accelerometer has its proof-mass supported usinghigh aspect-ratio polysilicon springs, which are formed byrefilling deep-etched trenches. Polysilicon trench refilling isalso used to form vertical sense/drive electrodes, which areattached to the fixed support rim and span the entire width ofthe proof-mass. Anisotropic wet etching is performed at theend of the fabrication in order to release the proof-mass as wellas to remove un-needed silicon around the outside perimeter ofthe sense/drive electrodes as illustrated. The detailed individualdevice structures are explained in [24], [25]. The size of thethree-axis chip is . All three devices have largeproof-mass ( 2 milli-gram), large sense area ,and small sensing gap , resulting in high sensitivity

and sub- mechanical noise floor. The design specifica-tions of the three-axis accelerometer system are summarized inTable I. As shown, both in-plane and out-of-plane devices havehigh sensitivity and sub- mechanical noise.

III. FABRICATION

The three-axis accelerometer utilizes a combined surface andbulk micromachining technology, which has been well charac-terized to build individual devices [23]. The fabrication processis shown in Fig. 3. It is a seven-mask double-sided process.

The process starts with a shallow 3.5- -deep p++ borondiffusion on both sides of a double-side polished p-typesilicon wafer, defining the thickness of suspension beams andthe area of the proof-mass and supporting rim. Then, 70-deep trenches are made by using Deep Reactive Ion Etching(DRIE). The trenches are then refilled completely with a combi-nation of low-pressure chemical-vapor deposition (LPCVD) sil-icon oxide (sacrificial layer), silicon nitride, and doped polysil-icon. The polysilicon trench refilling is used to form verticalsense/drive electrodes and high aspect-ratio springs to supportthe proof-mass for in-plane devices. In addition, the polysil-icon in the refilled trenches also forms vertical stiffeners for theout-of-plane device. After polysilicon deposition, annealing isfollowed to alleviate any compressive stress in the polysilicon[26].

Next, the polysilicon and nitride films are etched using RIEand another LPCVD silicon oxide (capping oxide) is deposited.The oxide is patterned to form contact openings to the bulk sil-icon for the subsequent anisotropic wet etch in Ethylene-Di-amine Pyrocatechol (EDP). Then, contact metal is electroplated.To minimize the etch time in the anisotropic wet etching andhelp undercut the electrodes for in-plane devices by the etchant,some of the single-crystal silicon is etched by DRIE. After theDRIE, anisotropic wet etching is followed not only to release theproof-mass and the supporting rim but also to etch the unnec-essary silicon around the sense/drive electrodes for in-plane de-vices. This step is important to achieve high sensitivity in-planedevices. Details are explained in [24]. Finally, the sacrificialoxide layer is removed by etching in hydrofluoric acid (HF).

Fig. 4 shows a fabricated three-axis single-chip accelerom-eter. Two in-plane (x- and y-axis) and one out-of-plane (z-axis)devices are mechanically connected by polysilicon connectors.These connectors provide electrical isolation between indi-vidual devices to ensure cross-talk free operation as well asphotographically-defined alignment accuracy. The connectorsare not essential for the three-axis accelerometer and the ac-celerometer could operate without them. The connectors areadded to monitor individual single-axis devices for testing pur-pose as well as to enable die separation of individual devices.

IV. TEST RESULTS

A. Sensor System Performance

The capacitance changes from the accelerometer are read outby using a switched-capacitor front-end, which is known to beimmune to input parasitic capacitance [27]. This enables hybridassembly of the three-axis accelerometer with the readout cir-cuitry. In order to estimate the overall noise of the accelerometer


CHAE et al.: A MONOLITHIC THREE-AXIS MICRO-g MICROMACHINED SILICON CAPACITIVE ACCELEROMETER 237

Fig. 2. Individual device structures. (a) Out-of-plane accelerometer [25], [31]; (b) in-plane accelerometer [24].

TABLE ITHREE-AXIS SINGLE-CHIP ACCELEROMETER DESIGN SPECIFICATIONS

system, the readout circuit noise needs to be taken into account.In this paper, noise represents the overall system noise unlessit is stated as mechanical or electronic noise. The electronicnoise of the readout circuit was measured asat 1 MHz [20]. Assuming 0.2 V/pF circuit sensitivity, the esti-mated Input Referred Noise Density (IRND) can be calculatedas

where is mechanical noise,is electronic noise, and

is the product of sensitivities of the accelerometer and readoutcircuit. From design specifications of the accelerometers, theestimated IRND of the out-of-plane and in-plane accelerom-eters with readout electronics is expected to be 1.53and 0.91 , respectively.



Fig. 3. Fabrication process. (a) Boron doping; (b) DRIE trench; (c) oxide,nitride, poly deposition; (d) pattern oxide, nitride, poly; (e) electroplate metal;(f) anisotropic etching; (g) HF release.

B. Electrostatic Measurement

The three-axis chip has been tested electrostatically. Electro-static measurement shows pull-in voltages of 1.9 and 2.5 V forin-plane, and 3.7 V for out-of-plane devices as shown in Fig. 5.This indicates spring constants of 17.1, 19.5, 12.0 N/m, for X, Y,Z axis accelerometers, respectively. Note that the out-of-planedevice has higher pull-in voltage than the in-plane devices sincewe performed the electrostatic test on a flat stage where theout-of-plane device experiences the 1 g gravitational bias. Inorder to extrapolate the spring constant of the out-of-plane de-vice from its pull-in voltage, we took this 1 g bias effect intoaccount. The out-of-plane device has a lower spring constantthan expected. It is because the boron doping level of the partic-ular batch of wafers was lower than expected and resulted in asmaller thickness for the support beams, which gives a smallerspring constant. The spring constants of the in-plane devices areclose to the estimated values.

C. Dividing Head (Precision Turn Table) Measurement

Fig. 6 shows measured differential capacitance vs. input ac-celeration by using a dividing head (i.e., precision turn table)and a HP-4284A precision LCR meter. Note that in-plane de-vices have smaller offset (0.09 pF) than the out-of-plane de-vice (0.2 pF). The offset for the out-of-plane device (

at zero acceleration) is due to gap variation on topand bottom of the wafer. However, the offset for in-plane de-vices ( at zero acceleration) is more immuneto fabrication variation. In the range of 0.3 g, the three-axischip provides sensitivity of 8.0, 7.9, 4.9 pF/g for X, Y, Z axes, re-

Fig. 4. Photograph of the integrated single-chip three-axis accelerometer,measuring 7 � 9 mm .

Fig. 5. Electrostatic measurements show pull-in voltages of 1.9, 2.5, 3.7 V forX, Y, Z-axis accelerometers.

spectively, with a small offset and good linearity. The measuredsensitivities of in-plane and out-of-plane devices are 15% and70% higher than estimated, respectively. This is because fab-ricated accelerometers have lower spring constants and largersensing gaps, very sensitive to the sensitivity (proportional to

, than expected.

D. Hybrid Module With Switched Capacitor Readout Circuit

Capacitance changes produced by the accelerometer are readout using a switched capacitor circuit, which can operateeither in open- or closed-loop. The circuit includes chopper sta-bilization and correlated double sampling to cancel 1/f noise,amplifier offset and compensate for finite amplifier gain. It op-erates using a 1 MHz clock and can resolve better than 10 aF( 10 Hz) with dynamic range of 120 dB for 1 Hz BW, whiledissipating less than 12 mW from 5 V supply [27]. Fig. 7 shows ahybrid three-axis accelerometer with two CMOS readoutcircuits in a dual inline package (DIP). Each readout circuit con-tains two readout circuits and external reference capac-itors are used to establish a full-bridge scheme. The size of the



Fig. 6. Measured capacitance change vs. input acceleration shows sensitivitiesof 8.0, 7.9, 4.9 pF/g for X, Y, Z axes.

Fig. 7. Hybrid accelerometer module in a DIP package.

entire package is . The mounting of the deviceon a Printed Circuit Board (PCB) is critical for the three-axis ac-celerometer because the device has double sided features suchthat it needs to be suspended from the PCB. A nonconductiveepoxy has been used to mount the device. The epoxy is curedat 100 for 2 h after the device is mounted. Although thismethod is not an optimal approach to reduce sensitivity due toCoefficient of Thermal Expansion (CTE) mismatch, it is the bestapproach available due to its simplicity. An improved packagingand mounting scheme will be necessary to realize devices withimproved temperature performance.

In-plane and out-of-plane accelerometers combined withreadout circuit provide system gain of 0.49 and 0.96 V/g,respectively. We do not know the source of this discrepancy.The open-loop output noise of the hybrid module and a 50reference resistor are measured with a HP 3561 dynamicsignal analyzer, and shown in Fig. 8. This figure shows thatthe resistor has 32 noise density which matcheswell with estimated thermal noise of the resistor (note that themeasurement bandwidth is 11.72 Hz in these measurementsand is included to account for the two output channels ofthe readout circuit). The hybrid module shows higher noisedensity for both in-plane and out-of-plane devices at low fre-quency. The in-plane device can resolve 13.8 -rms at 100 Hzand 5.5 -rms above 1.5 kHz, which correspond to IRNDof 4.0 at 100 Hz and 1.60 above 1.5 kHz,

Fig. 8. Measured output noise floor of the hybrid module shows 1.60�g=pHz

(>1.5 kHz), 1.08 �g=pHz (>600 Hz) for in-plane and out-of-plane devices,

respectively. (a) In-plane accelerometer; (b) out-of-plane accelerometer.

respectively. On the other hand, the out-of-plane device shows9.34 -rms at 100 Hz and 3.7 -rms above 600 Hz, whichindicate IRND of 2.7 at 100 Hz and 1.08above 600 Hz, respectively. Table II summarizes the measuredspecifications of the three-axis accelerometer, the interfacecircuit, and the hybrid module.

E. Temperature and Drift Characteristics

The drift of the accelerometer is determined primarily bythe package and assembly. Therefore, to determine the truestability of the device, the packaging approach needs to be im-proved. As mentioned in the previous section, the packagingscheme of the current system is not optimum. Nevertheless,the temperature and drift characteristics of this system havebeen measured in an environmental chamber (ESPEC-SU240).



TABLE IIMEASURED THREE-AXIS ACCELEROMETER SYSTEM SPECIFICATIONS

Fig. 9. Temperature and drift characteristics show TCO of 500 ppm= C upto 80 C and drift of �400 ppm for 1 h.

Fig. 9 shows rest capacitances that were recorded by using aLCR meter every 1 min for 1 h at 25 , 50 , and 80 .The temperature coefficient of offset (TCO) is obtained tobe 70 up to 50 and 500 up to 80

. This is mainlycaused by the CTE mismatch of the device and its packaging.CTE of the materials used for the device itself such as sil-icon, polysilicon, silicon nitride are 2.4–2.9 , whilePCB and nonconductive epoxy have CTE of 20–25and 40–60 , respectively [28]–[30]. This indicatesthat packaging and assembly techniques are major limitingfactors. Thus, improved packaging and assembly techniquesare expected to significantly reduce TCO. The drift of theaccelerometer is also measured from room temperature up to80 . The measurement data shows 400 ppm for 1 h at50 Standard deviation of .

V. CONCLUSION

A single-chip three-axis silicon capacitive accelerometerwith a switched-capacitor frond-end modulator readoutcircuitry is demonstrated with micro-g resolution. The ac-celerometer is a monolithic integration of three individualsingle-axis accelerometers. Utilizing a combined surface andbulk micromachining technology, full-wafer thick proof-mass,large sense area, small sensing gap have beenachieved. The fabricated accelerometer chip isin size, has 5 pF/g measured sensitivity and sub-mechanical noise floor for all three axes. The total measurednoise of the accelerometer hybrid assembled with CMOSinterface circuit is 1.60 ( 1.5 kHz), 1.08( 600 Hz) for in-plane and out-of-plane devices, respectively.The accelerometer has TCO of 70 up to 50 and500 up to 80 . Drift of the accelerometer is mea-sured to be 400 ppm for 1 h.

ACKNOWLEDGMENT

The authors thank Dr. N. Yazdi and Dr. A. Salian for theircontributions to the above work, R. Gordenker and B. Casey fordevice bonding and testing stages, and the staff at WIMS, TheUniversity of Michigan.

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[28] A. V. Chavan, “An Integrated High Resolution Barometric PressureSensing System,” Ph.D. dissertation, EECS at The University ofMichigan, Ann Arbor, MI, 2000.

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Junseok Chae (M’98) received the B.S. degree inmetallurgical engineering from the Korea University,Seoul, Korea, in 1998 and the M.S. and Ph.D. degreesin electrical engineering and computer science fromthe University of Michigan, Ann Arbor, in 2000 and2003, respectively.

Since then, he has been a Postdoctoral ResearchFellow. His areas of interests are MEMS sensors,mixed-signal interface electronics, MEMS pack-aging, and ultrafast pulse (femtosecond) laser formicro-/nanostructures. He holds a couple of U.S.

patents.Dr. Chae received the 1st place prize and the best paper award in DAC (De-

sign Automation Conference) student design contest in 2001 with the paperentitled “Two-dimensional position detection system with MEMS accelerom-eter for mouse application”. He had an invited talk at Microsoft, Inc., regarding“MEMS technology for consumer electronic applications.”

Haluk Kulah (M’02) received the B.Sc. and M.Sc.degrees in electrical engineering with high honorsfrom Middle East Technical University (METU),Ankara, Turkey, in 1996 and 1998, respectively.He received the Ph.D. degree in electrical engi-neering from the University of Michigan, AnnArbor, in 2003.

From 2003 to 2004, he was employed as aResearch Fellow at the Department of ElectricalEngineering and Computer Science, University ofMichigan. In August 2004, he has joined Electrical

and Electronics Engineering Department of METU as an Assistant Professor.His research interests include: micromachined sensors, mixed signal interfaceelectronics design for MEMS sensors, and MEMS-based energy scavenging.

Dr. Kulah was the winner of several prizes in Design Automation Conference(DAC) 2000, 2002, and 2002 Student Design Contests, which is sponsored by anumber of companies including CADENCE, Mentor Graphics, TI, IBM, Intel,and Compaq. His M.Sc. thesis received 1999 Thesis of the Year Award givenby Prof. M. N. Parlar Education and Research Foundation in METU.



Khalil Najafi (S’84–M’86–SM’97–F’00) was bornin 1958. He received the B.S., M.S., and the Ph.D.degree in 1980, 1981, and 1986, respectively, allin electrical engineering from the Department ofElectrical Engineering and Computer Science,University of Michigan, Ann Arbor. From 1986 to1988, he was employed as a Research Fellow, from1988 to 1990, as an Assistant Research Scientist,from 1990 to 1993, as an Assistant Professor, from1993 to 1998, as an Associate Professor, and sinceSeptember 1998, as a Professor and the Director of

the Solid-State Electronics Laboratory, Department of Electrical Engineeringand Computer Science, University of Michigan. His research interests include:micromachining technologies, solid-state micromachined sensors, actuators,and MEMS; analog integrated circuits; implantable biomedical microsystems;hermetic micropackaging; and low-power wireless sensing/actuating systems.

Dr. Najafi was awarded a National Science Foundation Young InvestigatorAward from 1992–1997, was the recipient of the Beatrice Winner Award forEditorial Excellence at the 1986 International Solid-State Circuits Conference,of the Paul Rappaport Award for coauthoring the Best Paper published in theIEEE TRANSACTIONS ON ELECTRON DEVICES, and of the Best Paper Award atISSCC 1999. In 2001, he received the Faculty recognition Award, and in 1994the University of Michigan’s “Henry Russel Award” for outstanding achieve-ment and scholarship, and was selected as the “Professor of the Year” in 1993.In 1998, he was named the Arthur F. Thurnau Professor for outstanding con-tributions to teaching and research, and received the College of Engineering’sResearch Excellence Award. He has been active in the field of solid-state sen-sors and actuators for more than eighteen years, and has been involved in severalconferences and workshops dealing with solid-state sensors and actuators, in-cluding the International Conference on Solid-State Sensors and Actuators, theHilton-Head Solid-State Sensors and Actuators Workshop, and the IEEE/ASMEMicro Electromechanical Systems (MEMS) Conference. He is the Editor forSolid-State Sensors for IEEE TRANSACTIONS ON ELECTRON DEVICES, Asso-ciate Editor for IEEE JOURNAL OF SOLID-STATE CIRCUITS, an Associate Editorfor the Journal of Micromechanics and Microengineering, Institute of PhysicsPublishing, and an editor for the Journal of Sensors and Materials.


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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 2 ...jchae2/Publications_files/3axis... · JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 2, ... mechanical noise