SOI MICROMACHINED 5-AXIS MOTION SENSOR USING RESONANT
ELECTROSTATIC DRIVE AND NON-RESONANT CAPACITIVE DETECTION MODEYoshiyuki Watanabe1, Toshiaki Mitsui1, Takashi Mineta2, Yoshiyuki Matsu3 and Kazuhiro Okada3
Yamagata Research Institute of Technology, MEMS Laboratory, 2-2-1 Matsuei, Yamagata 990-2473, Japan
phone: +81-23-647-3125, fax: +81-23-647-3129, e-mail: wat@mmp.yamagata-rit.go.jp
2
Faculty of Science and Technology,Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan
3Wacoh corporation, Tsuzuki Bldg.4F, 4-244-1 Sakuragi-cho, Saitama 331-0852, Japan1ABSTRACTSENSOR STRUCTURE

This paper presents a 5-axis motion sensor which candetect 3-axis acceleration and 2-axis angular rate
capacitively. This sensor is fabricated by SOI
bulk-micromachining, and is vacuum sealed by anodic
bonding and activation of the non-evaporated getters. By
applying Z-axis reference vibration of a proof mass with
an electrostatic force at resonant frequency, 5-axis
motions can be detected capacitively at non-resonant
detection mode. Measured sensitivity of X-, Y- andZ-axis acceleration were approximately 1.9, 1.1 and 4.7
V/G, respectively. X- and Y-axis angular rate sensitivity
were approximately 6.7 and 12 mV/(deg/s). The sensor
chip size is 5.0×5.0×1.7mm3 .

The sensor structure is shown in Fig.1. The sensor
consists of three layers which are a glass, an SOI(SiliconOn Insulator) and a glass. The inner cavity is vacuum
sealed by anodic bonding and activation of thenon-evaporated getters in order to prevent the squeezed
film damping effect. A silicon proof mass, located in thecenter of the sensor, is suspended by a cross shapesilicon beam which is formed in an active silicon layer of
the SOI wafer. Each glass layer has five electrodes on thesurface of inside. The silicon pillars around the mass, arefixed on Glass-2 for interconnection of the electrodes onboth glass wafers to the electrodes through the glass
holes. The cross shape center electrodes (T0 and B0)
and surrounding electrodes (T1-T4 and B1-B4), areused for the electrostatic driving and the capacitive
detection, respectively.

Keywords: 5-axis motion, 3-axis acceleration, 2-axis
angular rate, resonant drive, non-resonant detection：Connected to electrodes on Glass-1：Connected to electrodes on Glass-2：Connected to Si frame (common GND)
T0 (for drive)
T1
T2
Electrodes throughINTRODUCTIONMEMS type accelerometers and angular rate sensors(gyroscopes) have come into use in wide fields, such as
automobile chassis control, auxiliary function of GPS(Global Positioning System)navigation, motion controlof the robot and I/O devices for amusement field. Inquite recent years, multi-axis motion sensing have beenrequired in many wide fields. Multi-axis motion sensing
can be performed, of course, using multiple uni-axis
sensors. However, its multi-axis sensing module has
disadvantages such as large size and high production
cost. Therefore the multi-axis motion sensor which candetect multi-axis acceleration and multi-axis angular ratewith only one chip, has serviceable advantages. We havealready reported a 5-axis motion sensor with Si-Si direct
bonded structure using silicon bulk-micromachining, andwe have demonstrated the principles of detection of3-axis acceleration and 2-axis angular rate [1, 2]. Thissensor have some disadvantages for practical using. Firstone was difficult to seal the sensor chip in vacuum
because of its open structure, and the second wasdifficult to fabricate the sensor with high accuracy. In
order to improve these problems, we have developed a
new 5-axis motion sensor with SOI structure fabricatedby SOI bulk-micromachining using deep RIE andvacuum anodic bonding processes.

the glass holes

Glass-1 (500µmt)

T4T3Si frame
Si proof mass

SOI (700µmt)
NEG
pocketSi-pillars

B2
B1ZX

Glass-2 (500µmt)

B4Y

B3

Fig.1

B0 (for drive)

Sensor structure

PRINCIPLES
The mass movement by 5-axis motion is shown in
Fig.2(a). The proof mass suspended by a cross shapesilicon beam, is vibrated constantly along Z-axis by anelectrostatic force. Z-axis acceleration shifts the mass
along Z-axis, and X- and Y-axis acceleration tilts themass. When angular rate around X- and Y-axis isapplied, Coriolis' force makes tilting vibrationsynchronizing with Z-axis reference vibration, along
Y-and X-axis, respectively.
Block diagram of detection of 5-axis motions is shownin Fig.2(b). The proof mass is driven electrostaticallywith two center electrodes of T0 and B0 of which phases

silicon (act-Si, 20µmt) layers. The glass wafers for low
temperature anodic bonding (Asahi Technoglass Corp.,
SW-Z , 500µmt)are used. Fabrication process sequenceof the motion sensor is shown in Fig.3.
5µm

Process-1
Z-axis reference vibrationFzZX

Beam

Beam

Proof mass

BeamSi pillars

Si frame

max－2mVzY y

F= Fy = may＋2mVzY xmazFz

Cx+ :T1+T2
Cx－:T3+T4 T0
Cy+ :B1+B4Cy－:B2+B3 B0

Vx+
Vx－
Vy+
Vy－

Fig.2

Glass-2

Stoppers(1µm in height)
Au/Cr electrodes

NEG getters

Glass-1
Process-3

Driving voltage:
Amplitude: 5V(sine wave)
Off set : 5V
Frequency: 3.42kHz

Drive circuit

Fig.3

Synchronous
demodulator

Phase shifter+180deg

C-V converter
Cx+Cx－
Cy+Cy－BOXact-Si
Beam
Photoresist mask
NEG pocket

5µm* F： Inertial force act on the mass by 5-axis motions* ai (i=x,y,z)： i-axis acceleration* Y j (j=x,y)： j-axis angular rate
* m： Weight of a proof mass
* Vz： Velocity of the mass on reference vibration(a)Mass movement by 5-axis motions
SENSOR

Pt/Cr

Si proof mass

Process-2
Fx(1µm in height)

sub-Si
Au/CrYFx
FyStoppers

Contact pads

[Vx+]－[Vx－][Vy+]－[Vy－][Vx+]+[Vx－]

LPF

Ωx

LPF

ΩyLPF

Ax

LPF

AyLPF

Az(b)Block diagram of detection
Principle of 5-axis motion sensing

are shifted 180deg each other. Four capacitances of Cx+(:T1+T2), Cx- (:T3+T4), Cy+ (:B1+B4) and Cy(:B2+B3) are converted to Vx+, Vx-, Vy+ and Vy-,
respectively, with a C-V converter. Through an amplifier
and a low pass filter(LPF), 3-axis acceleration of Ax,
Ay and Az can be detected by [(Vx+)-(Vx-)],[(Vy+)-(Vy-)] and [(Vx+)+(Vx-)], respectively.
The 2-axis angular rates of Ix and Iy can be detectedby a synchronous demodulation, an amplification and alow pass filtration of [(Vy+)-(Vy-)] and[(Vx+)-(Vx-)], respectively. Since the angular rate
signals are synchronizing with Z-axis driving frequencyof 3.42kHz, the angular rate can be separated from low
frequency accelerations below several ten Hz with thesynchronous demodulator.

FABRICATIONThis sensor is fabricated by bulk-micromachining of aSOI wafer which consists of a silicon substrate (sub-Si,675µmt), a buried oxide (BOX, 2µmt) and an active

Fabrication process sequence of the sensor[process-1] The SOI wafer is etched from both sideswith 20wt.%TMAH aq. in order to form the masses,
beams, silicon pillars, and the top side gap. The sub-Si
and the act-Si layers are electrically interconnected via
the BOX layer with Au/Cr contact pads. The Pt/Cr metal
layer on the top side of the mass is formed for protection
from sticking during anodic bonding. The Pt/Cr metal
layer on the bottom side is formed for protection fromion attack after Deep RIE.
[process-2] After anodic bonding of the SOI wafer andthe lower glass wafer (Glass-2), sub-Si is etched throughby Deep RIE, consequently the proof masses, siliconpillars and the NEG pockets for insetting getters areformed.
[process-3] The NEG getters (Non evaporated getters,SAES Getters Japan) are inset in the NEG pockets. Anupper glass wafer (Glass-1) is anodically bonded to an
SOI wafer at 300℃, 30mTorr with applying voltage of-1kV to Glass-1 for vacuum sealing. Finally, after
shallow dicing of top side of Glass-1 in order to isolate
each electrode, the wafers were separated to each chip.
The act-Si and the sub-Si layers were interconnectedwith Au/Cr contact pads as showon in Fig.4 (a). Theconfiguration after deep etched silicon is shown in Fig.4(b). The etched profile was extremely perpendicular.
The vacuum sealed configuration is shown in Fig.4 (c).
The SOI and the Glass-1 wafers were bonded uniformly
in entire φ2 inch wafers. After an activation of NEG
getters at 450 ℃ in an inert gas, the wafers wereseparated to sensor chips of 5.0×5.0×1.7mm3 in size
(Fig.4(d)).

Au/Cr contact metal

Si pillars

Si proof mass

NEG pocket(a)Interconnection of wafers (b)Si deep RIE configuration
WireThrough holepeaks of tilting vibration around X-and Y-axis into apeak completely. Although it is disadvantageous for highsensitive detection, the non-resonant detection mode hasan advantage of low mechanical coupling. It is alsofavorable that extreme high precise fabrication process is
not necessary. In addition, the peak height decreases asincreasing inner pressure generally [5]. However, thesepeak profiles have not changed for a year, therefore thesensor is considered to be sealed without leakage.

3-axis acceleration characteristics
Sensor chip(c)Vacuum sealing

Fig.4

Package(d)Developed motion sensor

Photographs of the 5-axis motion sensor

EXPERIMENTAL RESULTS
Vibration characteristics of the resonator
This sensor is designed for resonant drive andnon-resonant detection for suppression of the mechanicalcoupling [3,4]. The large amplitude of reference
vibration is required for high sensitive detection of theangular rate. The vibration characteristics of theresonator were investigated as follows. The resonator isdriven with a T3 electrode and the amplitude of thevibration is detected with a T1 electrode. Applied
amplitude voltage was varied to 5, 3, and 1V. The offset
voltage is fixed to 5V. Driving frequency was swept up
from 1kHz to 5kHz. As shown in Fig.5, three resonantpeaks were observed. A first peak at 3.42kHz
Z-axis simple
Harmonic vibration

10.6
0.4
0.2
-1

-0.8

-0.6

-0.4

0.2

0.4

0.6

0.8

1

0.6

0.8

1

0.6

0.8

1

-0.6
-0.8
-1

Acceleration (G)(a) X-axis acceleration
10.8

Ay/non-drive
Ay/DRIVEOutput voltage (V)200-0.2 -0.2 0-0.4

10Vpp
6Vpp2Vpp
T1:detection

15T3:drive
Vpp: 2, 6, 10V
Vos: 5V constant

100.8

Ax/non-drive
Ax/DRIVE

X- and Y-axis
tilting vibration

25

CV converted output(mVrms)

The static acceleration characteristics in the cases ofelectrostatic drive and non-drive conditions wereinvestigated by rotating along each axis undergravitational 1G. The electrostatic reference vibration
was carried out by applying 5Vpp+5Vos to T0 and B0electrodes. The driving frequency was approximately
3.42kH which was Z-axis resonant frequency. Measured
acceleration characteristics are shown in Fig.6. Eachoutput voltage increase in proportion to applied
acceleration. Each axis acceleration sensitivities underelectrostatic drive tends to be several % higher than that
under non-drive. It seems that the averaged capacitanceunder electrostatic drive becomes lager than that under
non-drive because the capacitance gap is changing
according to amplitude of the vibration. The X-axis

Output voltage (V)One chip

0.6
0.4
0.2-1-0.8

-0.6

-0.4

0-0.2 -0.2 0

0.2

0.4

-0.4
-0.6
-0.8
-1

5

Acceleration (G)(b) Y-axis acceleration

005

3.2

3.4

3.6

3.8

4

4.2

4.4

Vibration characterisitics of the resonatorcorresponded to the resonance of Z-axis simple
harmonic vibration, and the others of 4.05kHz and4.22kHz corresponded to the resonance of the two tilting
vibration around X-and Y-axis. This split into two peaks
is considered to be caused by imbalance of the four
silicon beams. It is quite difficult to merge the resonant

Output voltage (V)Fig.5

4Az/non-drive
Az/DRIVEFrequency(kHz)32
10-1-0.8

-0.6

-0.4

-0.2

-1 00.2

0.4-2-3-4-5

Acceleration (G)Fig.6(c) Z-axis acceleration
3-axis acceleration characteristics

sensitivity is higher than that of Y-axis, because theinitial gap used for the X-axis detection is narrower thanthat of Y-axis. Cross axis sensitivity was less than 5%
and non-linearity was less than 2%F.S. Measured 3-axis
acceleration characteristics are summarized in Table 1.
Table 1 3-axis acceleration characteristics
Sensitivity (V/G)
non-drive
DRIVE
Ax
1.86
1.93
Ay
0.95
1.06
Az
3.51
4.67
Non-linearity＜2％f.s.
Cross-axis sensitivity＜5％2-axis angular rate characteristics
The angular rate characteristics were measured bymaking a half rotation of a turn table with a reference
gyro sensor setting nearby. The electrostatic resonant
drive was carried out under the same condition indicatedin a previous section. Measured angular rate signals of
the developed sensor and the reference gyro sensor are
shown in Fig.7(a). The signals of the developed motion

sensor follow that of the reference gyro sensor entirelywith low noise. As shown in Fig.7(b)and Fig.7(c),each axis output voltage increase in proportion to applied
angular rate. Measured 2-axis angular rate characteristics
are summarized in Table 2. In spite of the non-resonant
detection mode, stable detection of over 6mV/[deg/s]
can be obtained. The X-axis sensitivity is lower than thatof Y-axis because of the similar reason to acceleration
detection. The same as acceleration detection, 2-axis
sensitivities can be equalized by optimization of theinitial gap of top side and bottom side. Cross axis
sensitivity was less than 5% and non-linearity was lessthan 3%F.S.
Table 2 2-axis angular rate characteristics
Sensitivity (mV/[deg/s])
6.67
Ωx12.1
ΩyNon-linearity＜3％f.s.
Cross-axis sensitivity＜5％CONCLUSIONS

Reference gyro
Motion sensor

We have developed a new 5-axis motion sensor
fabricated by SOI bulk-micromachining using deep RIE
and vacuum anodic bonding processes. By using
electrostatic resonant drive at 3.42kHz and non-resonantcapacitive detection mode, we have achieved stabledetection of 3-axis acceleration and 2-axis angular rate.
Measured sensitivity of the 3-axis acceleration and the2-axis angular rate are over 1V/G and over 6mV/[deg/s],
respectively.

1.0V/div(Ref. gyro)

0.5V/div(Motion sensor)

1s/div(a) Measured angular rate signal
10.8

REFERENCES

Output voltage (V)

0.6
0.4
0.2
0-60-40-20-0.2

02040604060-0.4-0.6-0.8-1Angular rate (deg/s)(b) X-axis angular rate
10.8

Output voltage (V)

0.6
0.4
0.2
0-60-40-20-0.2

020-0.4
-0.6
-0.8
-1

Angular rate (deg/s)

Fig.7(c) Y-axis angular rate
2-axis angular rate characteristics[1] N.Taniguchi, H.Itano, K.Okada ： “ The 5-Axis
Motion Sensor”Technical Digest of the 16th Sensor
Symposium, pp.41-44, 1998.
[2] Y.Watanabe, T.Mitsui, T.Mineta, S.Kobayashi,N.Taniguchi, K.Okada：“Five-Axis Capacitive Motion
Sensor with Electrostatic Drive and Capacitive Detection
Fabricated by Silicon Bulk Micromachining”, Sensorsand Actuators A97-98, pp.109-115, April 2002.
[3]K.Maenaka, T.Fujita and M.Maeda ： “ Design
Problems on Vibratory Micro-Gyroscopes ” , T.IEE
Japan, Vol.118-E, No.7/8, pp.377-383, 1998.
[4] K.Ikeda and K.Maenaka：“Sensor Technologies
Based on Mechanical Resonance and Vibration”, T.IEE
Japan, Vol.120-E, No.3, pp.87-92, 2000.
[5] T.Mineta, S.Kobayashi, Y.Watanabe, S.Kanauchi,I.Nakagawa, E.Suganuma and M.Esashi：“Three-AxisCapacitive Accelerometer with Uniform Axial
Sensitivities ” Journal of Micromechanics andMicroengineering, pp.431-435, Vol.6, 1996.