Micromachined 5-axis Motion Sensor
with Electrostatic Drive and Capacitive Detection
Nobumitsu Taniguchi*, Kazuhiro Okada*
Yoshiyuki Watanabe**, Toshiaki Mitsui**, Takashi Mineta**, Seiya Kobayashi*** WACOH Corporation (http://www.wacoh.co.jp)
Tsuzuki Bldg. 4F, 4-244-1 Sakuragi-cho, Omiya-shi, Saitama Japan
TEL +81-48-641-9995, FAX +81-48-641-9996 e-mail:taniguchi@wacoh.co.jp**Yamagata Research Institute of Technology
2-2-1 Matsuei, Yamagata, 990-2473, JAPANAbstract
We have succeeded to measure 5-axis motion independently with one unit of sensor chip, which is made by siliconbulk-micromachining technology. The 5-axis motion is composed of 3-axis acceleration and 2-axis angular rate. This sensor has a seismicmass, which is attached on a thin silicon beam and vibrated along Z-axis. By acceleration, Newton’s force Fx, Fy and Fz act on the mass
(F = mA , m: mass, A: acceleration ). And by angular rate around X and Y-axis, the Coriolis force Fcy, and Fcx also act on the mass (Fc =2mΩVz, Ω: angular rate, Vz: velocity of the mass along the Z-axis). Through detecting the position of the mass, we measure 5-axis
motion. To vibrate the mass, we adopt the electrostatic force, and to measure the position of the mass, we used a change of electrostatic
capacitance.
Keywords: angular rate, acceleration, silicon micromachining, 2-axis, 3-axis, 5-axis

1 INTRODUCTION
To detect the movement of an object, the way of measuring themulti-axis acceleration and multi-axis angular rate is practicallyused in the various fields. Actually, numerous accelerometers
and angular rate sensors are used in this purpose, for example,
air bag system, chassis control system, video camcorder andcontroller for computer game. And the number of the sensor
and the application fields will increase in prospect.
Although the movement is originally vector quantity, most ofsensors are so-called uni-axial sensor, which detect only one
axial direction. So we have to use a few accelerometers and afew angular rate sensors. Therefore multi-axis motion sensor iseagerly required to minimize the size and cost of the system.
The 3-axis accelerometer [1,2,3,4] and the 2-axis angular rate
sensor [5] are developed. Through these experiences wealready presented 5-axis motion sensor to confirm the basic
theory of this sensor system [6]. Basic structure of the sensor isestablished at the thesis [6]. At the former sensor, we usedelectromagnetic coil to vibrate the mass. So the size of thesensor was not small enough, and fabrication was difficult.
This paper describes new type of 5-axis motion sensor made bysilicon bulk-micromachining technique. This time we adopt theelectrostatic force to vibrate the mass. Consequently, we havesucceeded to minimize the size and fabrication cost of thesensor.

Fig.1: The 5-axis motion sensor

2 STRUCTURE
As shown in Fig.2, this 5-axis motion sensor consists ofglass1-silicon1-silicon2-glass2 layer. In the silicon1 layer, 4beams (length: 2.2mm width: 0.4mm thickness: 0.04mm) and4 sensing plates are formed and connected each other at thecenter of the layer, and the other ends of the beams are attached
to a frame which is formed surrounded the silicon1layer.

The seismic mass is formed in silicon2 layer, and is suspended
by 4 beams at the center of the mass.
At the galass1 layer, 7.5μm gap and sensing electrodes (X+,X-, Y+, Y-, Sz) are formed, and at the glass2 layer, 7.5μm gapand driving electrodes (D1-D4, Dz) are also formed on thesurface of the glass.
ZY

X‑Y+ SzX+

X

Glass1

Y‑(t:0.5mm)

ElectrodeSensing plate
Beam

Silicon1(t:0.2mm)

Center pillar

Seismic mass

Silicon2

3.1 2-axis Angular rate
By the supplement of sign wave voltage to the electrodes(D1-D4), the mass vibrates along the Z-axis.
Under the conditions, when the mass is rotated around theX-axis, the Coriolis force is generated along the Y-axis
direction, as fig.4(a). And when the mass is rotated around theY-axis, the Coriolis force is generated along the X-axis
direction, as fig.4(b)X and Y-axis Coriolis force are described in equations (1) and(2) where Fcx and Fcy are Coriolis force, m is weight of themass, Ωy and Ωx are input angular rate to be measured, andVz is velocity of Z-axis vibration.
Fcx = 2 m Ωy Vz(1)
Fcy = 2 m Ωx Vz(2)
As shown in fig.5, the signal of X or Y-axis direction andreference signal, which is synchronized Vz, are input to thesynchronous detector. And though amplifier and filter, theangular rate can be measured.

(t:0.2mm)ｚｚElectrode

Glass2ＶｚＶｚΩｙΩｘ(t:0.5mm)ｘ
Ｆ ｃｙD3

D4

D2 Dz

D1(a) X-axis rotation

Fig.2: Structure of the sensor

ｘｙ(b) Y-axis rotation

Fig.4: Principal of 2-axis angular rate sensor
AMPX

3 PRINCIPLEｙＦ ｃｘ

LPFAx

Vacuum chamber
Y

When a force acts on the mass, which is suspended by beams,
the position of the mass is changed along the direction of theforce. By the force of X or Y-axis direction, the position of themass changed as shown in fig.3(a), and by the force of Z-axis
direction, the mass is displaced as shown in fig.3(b). At thesame time, the distance between the 4 sensing plates and 4
electrodes is changed. So measuring the electrostatic
capacitance between the 4 sensing plates and 4 electrodes, theforce works on the mass can be measured.

AｙC/V ConverterZ

AｚSynchronous
detectorreference signal
Drive circuit
oscillator
Drive signal
Phase/Gaincontroller

Fx,Fy(a) X(Y)-axis force

Fz(b) Z-axis force

Fig. 3: Displacement of the seismic mass

Fig.5: Block diagram

ΩxΩy3.2 3-axis acceleration(3)

4 FABRICATION
The fabrication process sequence is shown in Fig.6.
(a) A silicon wafer (Silicon1) was anisotropically wet etchedthrough with TMAH solution from both sides to form thebeams, sensing plates and center pillar.
(b) A silicon wafer (Silicon2) was fusion bonded to the silicon1
at 1100℃ in nitrogen atmosphere.
(c) After thermal oxidation, the mass was released from theframe by anisotropic wet etching.
(d) Two glass wafers that have the gap of 7.5μm and Pt/Cr
fixed electrodes were anodically bonded to the both sides of theSi-Si structure. Finally, grooves for chip-break lines were diced,
and the wafers were separated to sensor chips.
The chip size of the sensor is 8.4mm×8.0mm×1.4mm.

The frequency characteristics of this sensor are as shown inFig.7. The resonant frequency of X- and Y-axis are 2030Hz and2040Hz shown as Fig.7(b). And the resonant frequency ofZ-axis is 1880Hz as Fig.7(a). Because the structure of thesensor is symmetric with respect to the X- and Y-axis, theresonant frequency of these two axis are very close.
This time we vibrate the mass along Z-axis at 1875Hz, which isclose to resonant frequency of Z-axis, using the way ofapplying 4Vpp sine wave with 4V offset voltage to theelectrodes D1-D4.

1. 0
1880Hz

0. 8
0. 6
0. 4
0. 201920

1820
Frequency ( Hz)(a) Z-dri ve

Dri ve：Dz (2Vpp+8Vos) ，Sense：Sz

1. 02030Hz

0. 82040Hz

0. 6
0. 4
0. 201960
Frequency ( Hz)(b) Y-dri ve

2060

Dri ve：D1 (2Vpp+2Vos) ，Sense：D3

Fig.7: Frequency characteristics(a) Anisotropic wet etching
Silicon1

5.1 Frequency characteristics of each axis

Magni t ude ( a. u. )Fx (y,z) = m Ax (y,z)

But the mass is vibrating along the Z-axis. Therefore thevibration signal is overlapped in the Z-axis direction’sacceleration signal. And in the X- and Y-axis direction’sacceleration signal, the signal of Coriolis force is overlapped.
The acceleration to be measured is ranged from DC to 30Hz.
On the other hand, the frequency of these unexpected signals isalmost as same as vibration frequency. Therefore using the low
pass filter, as shown in Fig.5 acceleration can be separated.

5 RESULT

Magni t ude ( a. u. )By acceleration, Newton’s force Fx, Fy and Fz act on the massand change the position of the mass. By change of electrostatic
capacitance, the acceleration can be measured. [1,2,3,4] And
the equations is as (3).

BeamCenter piller(b) Si‑Si fusion bonding

SiO 2

5.2 3-axis acceleration

Silicon1
Silicon2(c) Anisotropic wet ctching
Silicon1
Silicon2(d) Anodic bonding
Glass1

Electrode

Silicon1
Silicon2

Seismic mass

Glass2

Fig.6: Fabrication process

Acceleration characteristics were measured using gravitation ofearth by rotating along each axis. During the measurement, themass is vibrated along Z-axis. Results of 3-axis acceleration test
are shown in Fig.8. The sensitivities of Ax, Ay, Az acceleration
were 3.23V/G, 3.12V/G and 3.40V/G, which were equivalent to
5.9fF/G, 5.7fF/G and 23fF/G, respectively. The cross-axis
sensitivity was approximately 5%. These results were similar to
the non-driven measurement results.

4.00

4.00
Ax
AyAz

3.00

5.3 2-axis angular rate

2.00

2.00

1.00

1.00

Measured characteristics of angular rates around X and Y-axis
are shown in Fig.9. The sensitivity of Ωx and Ωy were
6.8mV/[deg/s] and 7.0mV/[deg/s], which were equivalent to
2.7aF/[deg/s], 2.8aF/[deg/s], respectively. The cross-axis
sensitivity of Ωx against Ωy were less than 3%. The
cross-axis sensitivity of Ωz against Ωx and Ωy was almost
0%.

3.00

Output (V)Output (V)

AxAy
Az

0.00
-1-0.500.5

1-1.00

0.00
-1

-0.5

0

0.5

-1.00

-2.00

-2.00

-3.00

-3.00
-4.00

-4.00

Acceleration (G)Acceleration (G)(A) X-Axis Acceleration

1(B) Y-Axis Acceleration

6 CONCLUSION
4.00
Ax
AyAz

We have succeeded to develop the 5-axis motion sensor, which
can be measured 3-axis acceleration and 2-axis angular rate
independently with one unit of sensor chip.
Measured sensitivities of accelerations are approximately
23fF/G in Az, and approximately 6fF/G in Ax and Ay. The
cross-axis sensitivity was approximately 5%. The sensitivities
of angular rates are 3aF/[deg/s] in Ωx and Ωy at anelectrostatic resonant drive of 1875Hz. The cross-axis
sensitivity was less than 3%. The chip size of developed sensor
is 8.4mm×8.0mm×1.4mm.

3.00

1.00
0.00
-1-0.500.5

1-1.00
-2.00
-3.00
-4.00

Acceleration (G)(C) Z-Axis Acceleration

Fig. 8: Acceleration characteristics

7 References
600

Output (mV)

Ωx

400

Ωy200
0-80-60-40-20-200020

406080-400-600Angular rate (deg/sec)(a) Angular rate around X-axis (Ωx)

600
Ωx

Output(mV)

Output (V)

2.00

400

Ωy200
0-80-60-40-20-200

0204060

80

-400
-600

Angular rate (deg/sec)(b) Angular rate around Y-axis (Ωy)

Fig .9: Angular rate characteristics[1] K.Okada “Tri-axial piezoresistive accelerometer”
Tech.Dig. of The 11th Sensor Symposium, 1992,
pp245-248[2] T.Mineta et al. “Three-axis capacitive accelero-meter withuniform axial sensitivity” Transducer’95, pp554-557[3] H.Takao, et al. “Three dimensional vector accelerometerusing SOI structure for high temperature”, Proceedings ofTransducers ’95, pp.683-686[4] O. Torayashiki et al. “Capacitiv type 3-axis accelero-meter”
Tech. Dig. of The 14th SensorSymposium, 1996, pp19-22[5] H.Tamura, et al. “Vibratory Gyro-Sensor Using a FlexuralSame-Form Double Mode Disk Resonator”, Proceedingsof 18th Symposium on Ultrasonic Electronics, ’97,
pp21-22[6] N.Taniguchi et al. “The 5-axis Motion Sensor” Tech. Dig.
of The 16th Sensor Symposium, 1998, pp41-44