3-Axis Fully-Integrated Capacitive Tactile Sensor with Flip-Bonded CMOS on LTCC Interposer

Abstract

This paper reports a 3-axis fully integrated differential capacitive tactile sensor surface-mountable on a bus line. The sensor integrates a flip-bonded complementary metal-oxide semiconductor (CMOS) with capacitive sensing circuits on a low temperature cofired ceramic (LTCC) interposer with Au through vias by Au-Au thermo-compression bonding. The CMOS circuit and bonding pads on the sensor backside were electrically connected through Au bumps and the LTCC interposer, and the differential capacitive gap was formed by an Au sealing frame. A diaphragm for sensing 3-axis force was formed in the CMOS substrate. The dimensions of the completed sensor are 2.5 mm in width, 2.5 mm in length, and 0.66 mm in thickness. The fabricated sensor output coded 3-axis capacitive sensing data according to applied 3-axis force by three-dimensional (3D)-printed pins. The measured sensitivity was as high as over 34 Count/mN for normal force and 14 to 15 Count/mN for shear force with small noise, which corresponds to less than 1 mN. The hysteresis and the average cross-sensitivity were also found to be less than 2% full scale and 11%, respectively.

Full text

sensors
Article
3-Axis Fully-Integrated Capacitive Tactile Sensor with
Flip-Bonded CMOS on LTCC Interposer †
Sho Asano 1, *, Masanori Muroyama 2ID , Takahiro Nakayama 3, Yoshiyuki Hata 4,
Yutaka Nonomura 4ID and Shuji Tanaka 1
1Department of Robotics, Graduate School of Engineering, Tohoku University, 6-6-01 Aramaki Aza Aoba,
Aoba-ku, Sendai, Miyagi 980-8579, Japan; tanaka@mems.mech.tohoku.ac.jp
2Micro System Integration Center, Tohoku University, 519-1176 Aramaki Aza Aoba, Aoba-ku, Sendai,
Miyagi 980-0845, Japan; muroyama@mems.mech.tohoku.ac.jp
3
Partner Robot Division, Toyota Motor Corporation, 543 Kirigahora, Nishi-hirose-cho, Toyota, Aichi 470-0309,
Japan; takahiro_nakayama_aa@mail.toyota.co.jp
4System & Electronics Engineering Department III, Toyota Central R&D Labs., Inc., 41-1 Yokomichi,
Nagakute, Aichi 480-1192, Japan; yhata@mosk.tytlabs.co.jp (Y.H.); nonomura@meijo-u.ac.jp (Y.N.)
*Correspondence: asano@mems.mech.tohoku.ac.jp; Tel.: +81-22-795-6936
† This paper is an extended version of our published paper “3-axis fully-integrated surface-mountable
differential capacitive tactile sensor by CMOS flip-bonding”. In Proceedings of the 29th IEEE International
Conference on Micro Electro Mechanical Systems (MEMS 2016), Shanghai, China, 24–28 January 2016.
Received: 25 September 2017; Accepted: 24 October 2017; Published: 25 October 2017
Abstract:
This paper reports a 3-axis fully integrated differential capacitive tactile sensor
surface-mountable on a bus line. The sensor integrates a flip-bonded complementary metal-oxide
semiconductor (CMOS) with capacitive sensing circuits on a low temperature cofired ceramic (LTCC)
interposer with Au through vias by Au-Au thermo-compression bonding. The CMOS circuit and
bonding pads on the sensor backside were electrically connected through Au bumps and the LTCC
interposer, and the differential capacitive gap was formed by an Au sealing frame. A diaphragm
for sensing 3-axis force was formed in the CMOS substrate. The dimensions of the completed
sensor are 2.5 mm in width, 2.5 mm in length, and 0.66 mm in thickness. The fabricated sensor
output coded 3-axis capacitive sensing data according to applied 3-axis force by three-dimensional
(3D)-printed pins. The measured sensitivity was as high as over 34 Count/mN for normal force
and
14 to 15 Count/mN
for shear force with small noise, which corresponds to less than 1 mN.
The hysteresis and the average cross-sensitivity were also found to be less than 2% full scale and
11%, respectively.
Keywords:
3-axis tactile sensor; MEMS-CMOS integration; capacitive sensor; Au-Au thermo-compression
bonding; low temperature cofired ceramic
1. Introduction
Service robots such as nursing care robots, home assistant robots, and entertainment robots are
now being developed all over the world. In contrast with industrial robots, they operate in human
environments like homes and hospitals without fences and other safety measures. Tactile information
about physical contact allows for dexterous manipulation and safe operation in direct human-robot
interaction applications. For these purposes, whole-body tactile sensing is highly desired. 3-axis force
(normal and shear force) sensing is also important to detect the direction of the contact force and slip
of grasping objects [1,2].
As for whole-body tactile sensing, some papers have reported distributed tactile sensors on a large
area of a humanoid robot [
3
–
5
]. In order to reduce the number of wires, serial bus connection [
3
,
4
],
Sensors 2017,17, 2451; doi:10.3390/s17112451 www.mdpi.com/journal/sensors

Sensors 2017,17, 2451 2 of 14
or matrix scanning [
5
] are used. Flexible printed circuit boards (PCBs) also allow for conformable
implementation on curved surfaces of a robot. However, the discrete components on PCBs are relatively
large in size, which causes low spatial resolution. Recently, 3-axis tactile sensors have been developed
based on microelectromechanical systems (MEMS) technology [
6
,
7
]. In [
8
], a piezoresisitive tactile
sensor with surface-doped and sidewall-doped beams embedded in polydimethylsiloxane (PDMS)
was presented. This sensor demonstrated measurement of 3-axis components of applied pressure
independently. In [
9
], the slip detection of a grasping object was demonstrated by microcantilevers with
a thin film strain gauge embedded in PDMS. In these studies, the tactile sensor and the readout circuit
are connected one-by-one, therefore the number of wires will increase for connecting many sensors.
Flexible tactile sensor arrays based on strain gauges [
10
] and multiple capacitors [
11
] demonstrated high
spatial resolution like human fingertips, but the wiring problem was not completely solved. Another
problem is long wire connection between the tactile sensor and the readout circuit, which increases
noise and parasitic capacitance. The monolithic integration of MEMS sensor and complementary
metal-oxide semiconductor (CMOS) circuit (MEMS-CMOS integration) enables the reduction of noise
and parasitic capacitance, as well as the miniaturization of the device. CMOS-integrated tactile sensor
arrays with a spatial resolution of 1 mm or higher have been reported [
12
,
13
]. Also, a capacitive tactile
sensor implemented by CMOS standard process and in-house post-CMOS process has been proposed [
14
].
However, detection of shear force is not examined by these CMOS-integrated tactile sensors.
We have developed a tactile sensor, which can be surface-mounted on a flexible and stretchable bus
line by integrating a MEMS capacitive sensor and a CMOS circuit [
15
]. The integrated tactile sensor can
detect contact force by a diaphragm with a boss formed in the CMOS substrate. Thermo-compression
bonding by planarized Au bumps and an Au sealing frame is used to integrate the CMOS on an low
temperature cofired ceramic (LTCC) interposer, which provides interconnections from the CMOS
circuit to the bus lines. The sensor has demonstrated the following functions: (1) data reduction by
human-inspired threshold and adaptation operations; and (2) the adjustment of sensitivity and data
rate after implementation. However, this integrated tactile sensor can detect normal force only because
of limited functionalities.
The objective of this study is to develop a CMOS-integrated 3-axis tactile sensor surface-mountable
on a bus line. Our final target is a networked event-driven tactile sensor system with numerous
distributed 3-axis integrated tactile sensors on a flexible and stretchable bus line, as shown in Figure 1.
In this study, we focus on the 3-axis integrated tactile sensor. In order to detect 3-axis force, we have
used a new CMOS substrate with four channels of differential capacitive sensing circuits. Also, we have
improved the sensor structure from the previous study to enhance sensitivity to not only normal force but
also shear force [
16
]. This paper is organized as follows. In Section 2, we show the structure and working
principle of the sensor and the results of finite element method (FEM) simulation. In Section 3, the fabrication
process of the sensor is presented. In Section 4, an experimental setup for evaluation and the measurement
results of normal force and shear force are described. Finally, Section 5concludes this paper.
Sensors 2017, 17, 2451 2 of 14
conformable implementation on curved surfaces of a robot. However, the discrete components on
PCBs are relatively large in size, which causes low spatial resolution. Recently, 3-axis tactile sensors
have been developed based on microelectromechanical systems (MEMS) technology [6,7]. In [8], a
piezoresisitive tactile sensor with surface-doped and sidewall-doped beams embedded in
polydimethylsiloxane (PDMS) was presented. This sensor demonstrated measurement of 3-axis
components of applied pressure independently. In [9], the slip detection of a grasping object was
demonstrated by microcantilevers with a thin film strain gauge embedded in PDMS. In these
studies, the tactile sensor and the readout circuit are connected one-by-one, therefore the number of
wires will increase for connecting many sensors. Flexible tactile sensor arrays based on strain
gauges [10] and multiple capacitors [11] demonstrated high spatial resolution like human fingertips,
but the wiring problem was not completely solved. Another problem is long wire connection
between the tactile sensor and the readout circuit, which increases noise and parasitic capacitance.
The monolithic integration of MEMS sensor and complementary metal-oxide semiconductor
(CMOS) circuit (MEMS-CMOS integration) enables the reduction of noise and parasitic capacitance,
as well as the miniaturization of the device. CMOS-integrated tactile sensor arrays with a spatial
resolution of 1 mm or higher have been reported [12,13]. Also, a capacitive tactile sensor
implemented by CMOS standard process and in-house post-CMOS process has been proposed [14].
However, detection of shear force is not examined by these CMOS-integrated tactile sensors.
We have developed a tactile sensor, which can be surface-mounted on a flexible and stretchable
bus line by integrating a MEMS capacitive sensor and a CMOS circuit [15]. The integrated tactile
sensor can detect contact force by a diaphragm with a boss formed in the CMOS substrate.
Thermo-compression bonding by planarized Au bumps and an Au sealing frame is used to integrate
the CMOS on an low temperature cofired ceramic (LTCC) interposer, which provides
interconnections from the CMOS circuit to the bus lines. The sensor has demonstrated the following
functions: (1) data reduction by human-inspired threshold and adaptation operations; and (2) the
adjustment of sensitivity and data rate after implementation. However, this integrated tactile sensor
can detect normal force only because of limited functionalities.
The objective of this study is to develop a CMOS-integrated 3-axis tactile sensor
surface-mountable on a bus line. Our final target is a networked event-driven tactile sensor system
with numerous distributed 3-axis integrated tactile sensors on a flexible and stretchable bus line, as
shown in Figure 1. In this study, we focus on the 3-axis integrated tactile sensor. In order to detect
3-axis force, we have used a new CMOS substrate with four channels of differential capacitive
sensing circuits. Also, we have improved the sensor structure from the previous study to enhance
sensitivity to not only normal force but also shear force [16]. This paper is organized as follows. In
Section 2, we show the structure and working principle of the sensor and the results of finite element
method (FEM) simulation. In Section 3, the fabrication process of the sensor is presented. In Section
4, an experimental setup for evaluation and the measurement results of normal force and shear force
are described. Finally, Section 5 concludes this paper.
(a) (b)
Figure 1. Conceptual figure of the 3-axis integrated tactile sensors on a flexible and stretchable bus
line: (a) System diagram; (b) Physical implementation.
Figure 1.
Conceptual figure of the 3-axis integrated tactile sensors on a flexible and stretchable bus line:
(a) System diagram; (b) Physical implementation.

Sensors 2017,17, 2451 3 of 14
2. Design
2.1. Sensor Structure
Figure 2shows the structure of the 3-axis integrated tactile sensor. The dimensions of the sensor are
designed to be 2.5 mm in width, 2.5 mm in length, and 0.66 mm in thickness. The sensor has a flipped
CMOS with a sensing diaphragm above an LTCC interposer. The CMOS circuit is used for differential
capacitive sensing and serial bus communication. Interconnections from the CMOS circuit are provided
through Au bumps and Au vias of the LTCC interposer, which makes the fabrication process simpler
as compared to using through silicon vias (TSVs) [
17
,
18
] or through silicon grooves [
19
]. The LTCC
interposer is suitable for wafer-level packaging due to three-dimensional (3D) internal wiring as well
as its small mismatch of coefficient of thermal expansion compared to Si [
20
]. The capacitive sensor
structure is sealed with an Au sealing frame. Although we have formed a diaphragm with tapered boss
by anisotropic wet etching in the previous study [
15
], the side surface of the diaphragm and the boss
is vertical in this study to enhance sensitivity to normal force and shear force. The capacitive gap of
4.5
µ
m, which is smaller than that of the previous sensor (10
µ
m), also enables the enhancement of
sensitivity and the prevention of large diaphragm deflection. As described later, differential capacitive
sensing like a seesaw for X-axis and Y-axis allows shear force detection with high sensitivity while
one capacitance changes and another is fixed for Z-axis.
Sensors 2017, 17, 2451 3 of 14
2. Design
2.1. Sensor Structure
Figure 2 shows the structure of the 3-axis integrated tactile sensor. The dimensions of the sensor
are designed to be 2.5 mm in width, 2.5 mm in length, and 0.66 mm in thickness. The sensor has a
flipped CMOS with a sensing diaphragm above an LTCC interposer. The CMOS circuit is used for
differential capacitive sensing and serial bus communication. Interconnections from the CMOS
circuit are provided through Au bumps and Au vias of the LTCC interposer, which makes the
fabrication process simpler as compared to using through silicon vias (TSVs) [17,18] or through
silicon grooves [19]. The LTCC interposer is suitable for wafer-level packaging due to
three-dimensional (3D) internal wiring as well as its small mismatch of coefficient of thermal
expansion compared to Si [20]. The capacitive sensor structure is sealed with an Au sealing frame.
Although we have formed a diaphragm with tapered boss by anisotropic wet etching in the previous
study [15], the side surface of the diaphragm and the boss is vertical in this study to enhance
sensitivity to normal force and shear force. The capacitive gap of 4.5 µm, which is smaller than that
of the previous sensor (10 µm), also enables the enhancement of sensitivity and the prevention of
large diaphragm deflection. As described later, differential capacitive sensing like a seesaw for
X-axis and Y-axis allows shear force detection with high sensitivity while one capacitance changes
and another is fixed for Z-axis.
Figure 2. Structure of the 3-axis integrated tactile sensor.
2.2. Working Principle
Figure 3a shows the electrode layout of the 3-axis integrated tactile sensor. The sensor has 5
sensor capacitors C
X−
, C
X+
, C
Y−
, C
Y+
, and C
Z+
under the diaphragm, and a fixed capacitor C
Zref
, which is
apart from the other electrodes. When normal force F
Z
is applied to the boss of the sensor,
capacitances except for C
Zref
increase as shown in Figure 3b. In the case that shear force is applied,
capacitances correspond to the force direction change. For example, if X-axis shear force F
X
is applied
to the sensor in positive direction as shown in Figure 3c, C
X+
increases while C
X−
decreases. The other
capacitances do not change in this case.
Figure 2. Structure of the 3-axis integrated tactile sensor.
2.2. Working Principle
Figure 3a shows the electrode layout of the 3-axis integrated tactile sensor. The sensor has 5 sensor
capacitors C
X−
,C
X+
,C
Y−
,C
Y+
, and C
Z+
under the diaphragm, and a fixed capacitor C
Zref
, which is
apart from the other electrodes. When normal force F
Z
is applied to the boss of the sensor, capacitances
except for C
Zref
increase as shown in Figure 3b. In the case that shear force is applied, capacitances
correspond to the force direction change. For example, if X-axis shear force F
X
is applied to the sensor

Sensors 2017,17, 2451 4 of 14
in positive direction as shown in Figure 3c, C
X+
increases while C
X−
decreases. The other capacitances
do not change in this case.
The capacitance change is detected by the CMOS circuit. Figure 4shows the optical micrograph
of the CMOS substrate and the schematic diagram of the readout circuit. The CMOS circuit has
a capacitance-frequency (CF) converter for each capacitor, and the oscillation frequency fis written as
f=4.74 ×104
Csens +497, (1)
where C
sens
is the sensor capacitance and the units of fand C
sens
are MHz and fF, respectively.
The 3-axis output values NX,NY, and NZare given by
NX=fX−·P
fclock
−fX+·P
fclock , (2)
NY=fY−·P
fclock
−fY+·P
fclock , (3)
NZ=fZref·P
fclock
−fZ+·P
fclock , (4)
where the subscripts of fcorrespond to the capacitor (e.g., f
Zref
is the oscillation frequency of C
Zref
),
and Pand f
clock
are the count period and the clock frequency, respectively. In this study, Pand f
clock
are set to be 2
13
clock cycles and 1.56 MHz, respectively. When F
Z
is applied, N
Z
increases, because
f
Zref
is fixed and f
Z+
decreases. In contrast, N
X
and N
Y
do not change because the decreases of
f
X−
and f
X+
as well as f
Y−
and f
Y+
are the same in principle due to the symmetry of the electrode
layout.
If FXis
applied to the positive direction, N
X
increases because f
X−
increases and f
X+
decreases,
while NYand NZdo not change because the other oscillation frequencies do not change.
Sensors 2017, 17, 2451 4 of 14
The capacitance change is detected by the CMOS circuit. Figure 4 shows the optical micrograph
of the CMOS substrate and the schematic diagram of the readout circuit. The CMOS circuit has a
capacitance-frequency (CF) converter for each capacitor, and the oscillation frequency f is written as
f = 4.74 × 104
Csens + 497 , (1)
where C
sens
is the sensor capacitance and the units of f and C
sens
are MHz and fF, respectively. The
3-axis output values N
X
, N
Y
, and N
Z
are given by
NX = fX · P
fclock
 fX+ · P
fclock
, (2)
NY = fY · P
fclock
fY+ · P
fclock
, (3)
NZ = fZref · P
fclock
 fZ+ · P
fclock
, (4)
where the subscripts of f correspond to the capacitor (e.g., f
Zref
is the oscillation frequency of C
Zref
),
and P and f
clock
are the count period and the clock frequency, respectively. In this study, P and f
clock
are set to be 2
13
clock cycles and 1.56 MHz, respectively. When F
Z
is applied, N
Z
increases, because
f
Zref
is fixed and f
Z+
decreases. In contrast, N
X
and N
Y
do not change because the decreases of f
X−
and
f
X+
as well as f
Y−
and f
Y+
are the same in principle due to the symmetry of the electrode layout. If F
X
is
applied to the positive direction, N
X
increases because f
X−
increases and f
X+
decreases, while N
Y
and
N
Z
do not change because the other oscillation frequencies do not change.
(b)
(a) (c)
Figure 3. (a) Electrode layout; Working principles for (b) normal force F
Z
and (c) X-axis shear force
F
X
.
Figure 3.
(
a
) Electrode layout; Working principles for (
b
) normal force F
Z
and (
c
)X-axis shear force F
X
.

Sensors 2017,17, 2451 5 of 14
Sensors 2017, 17, 2451 5 of 14
(a) (b)
(c) (d)
Figure 4. (a) Optical micrograph of the complementary metal-oxide semiconductor (CMOS)
substrate; (b) Schematic diagram of the capacitive readout circuit; Time responses of (c) the voltage
of the sensor capacitor V
C
and (d) the oscillation output voltage V
out
.
2.3. FEM Simulation
FEM simulation software (Femtet, Murata Software Co., Ltd., Tokyo, Japan) was used to
estimate the diaphragm deflection and the capacitance change when force is applied to the 3-axis
integrated tactile sensor. The 3-axis integrated tactile sensor is designed to distribute on the bus
lines, and the contact force is divided to some sensors. Thus, the maximum applied force was set to 1
N on the assumption of normal manipulation operations. The target minimum detectable force was
decided as 1 mN for achieving a dynamic range of 1000:1. The measurement range can be changed
by the dimensions of the sensor (e.g., the diaphragm thickness, the size of the boss, and the thickness
of the Au bumps, and the sealing frame). The dimension of the diaphragm was determined to be 1.3
mm square, in which no analog circuits is located, to avoid piezoelectric effect to the analog circuits.
The thickness of the diaphragm was determined to be 50 µm, as with the previous study [15]. The
dimension of the boss is 0.4 mm square. In this simulation as well as the measurement described in
Section 4, normal force was applied to the top surface of the boss, and shear force was applied to one
of the side surfaces from the top to 200 µm below the top of the boss.
Figure 5a,b show the deflection of the diaphragm by normal force F
Z
and X-axis shear force F
X
of 1 N applied to the positive direction, respectively. The maximum deflections of 3.21 µm for F
Z
and
1.03 µm for F
X
are smaller than the initial capacitive gap of 4.5 µm. The Z-axis stiffness was
calculated as 3.1 × 10
5
N/m. The maximum principal stresses for F
Z
and F
X
of 1 N were estimated as
365 MPa and 542 MPa, respectively. They are smaller than the reported fracture stress of Si
specimens microfabricated by deep reactive ion etching (DRIE) [21]. The eigenfrequencies of the
parallel motion of the diaphragm and the rotation motion of the boss were estimated to be 223 kHz
and 399 kHz, respectively. These values are much higher than the frequency of the motion of robots
and the detectable stimuli frequency of human tactile receptors. Figure 5c,d show the change of
3-axis output values calculated by the simulated 3-axis capacitances as a function of F
Z
and F
X
,
respectively. The nonlinearity of the change of N
Z
is caused by the relationship of the oscillation
frequency and the capacitance, as described in Equation (1). The sensitivity was estimated as 103
Count/mN for F
Z
from 0 N to 0.5 N and 22.6 Count/mN for F
X
from −1 N to 1 N, which is high
Figure 4.
(
a
) Optical micrograph of the complementary metal-oxide semiconductor (CMOS) substrate;
(
b
) Schematic diagram of the capacitive readout circuit; Time responses of (
c
) the voltage of the sensor
capacitor VCand (d) the oscillation output voltage Vout.
2.3. FEM Simulation
FEM simulation software (Femtet, Murata Software Co., Ltd., Tokyo, Japan) was used to
estimate the diaphragm deflection and the capacitance change when force is applied to the 3-axis
integrated tactile sensor. The 3-axis integrated tactile sensor is designed to distribute on the bus lines,
and the contact force is divided to some sensors. Thus, the maximum applied force was set to 1 N
on the assumption of normal manipulation operations. The target minimum detectable force was
decided as 1 mN for achieving a dynamic range of 1000:1. The measurement range can be changed
by the dimensions of the sensor (e.g., the diaphragm thickness, the size of the boss, and the thickness
of the Au bumps, and the sealing frame). The dimension of the diaphragm was determined to be
1.3 mm square, in which no analog circuits is located, to avoid piezoelectric effect to the analog
circuits. The thickness of the diaphragm was determined to be 50
µ
m, as with the previous study [
15
].
The dimension of the boss is 0.4 mm square. In this simulation as well as the measurement described
in Section 4, normal force was applied to the top surface of the boss, and shear force was applied to
one of the side surfaces from the top to 200 µm below the top of the boss.
Figure 5a,b show the deflection of the diaphragm by normal force F
Z
and X-axis shear
force FX
of 1 N applied to the positive direction, respectively. The maximum deflections of 3.21
µ
m
for FZand 1.03 µm
for F
X
are smaller than the initial capacitive gap of 4.5
µ
m. The Z-axis stiffness was
calculated as
3.1 ×105N/m
. The maximum principal stresses for F
Z
and F
X
of 1 N were estimated as
365 MPa and 542 MPa, respectively. They are smaller than the reported fracture stress of Si specimens
microfabricated by deep reactive ion etching (DRIE) [
21
]. The eigenfrequencies of the parallel
motion of the diaphragm and the rotation motion of the boss were estimated to be 223 kHz and
399 kHz, respectively. These values are much higher than the frequency of the motion of robots and
the detectable stimuli frequency of human tactile receptors. Figure 5c,d show the change of 3-axis
output values calculated by the simulated 3-axis capacitances as a function of F
Z
and F
X
, respectively.
The nonlinearity of the change of N
Z
is caused by the relationship of the oscillation frequency and

Sensors 2017,17, 2451 6 of 14
the capacitance, as described in Equation (1). The sensitivity was estimated as 103 Count/mN for F
Z
from 0 N to 0.5 N and 22.6 Count/mN for F
X
from
−
1 N to 1 N, which is high enough to satisfy
the minimum force sensitivity of 1 mN. The capacitance at unload condition was calculated as 643 fF
for CX−,CX+,CY−, and CY+, 368 fF for CZref, and 332 fF for CZ+.
Sensors 2017, 17, 2451 6 of 14
enough to satisfy the minimum force sensitivity of 1 mN. The capacitance at unload condition was
calculated as 643 fF for C
X−
, C
X+
, C
Y−
, and C
Y+
, 368 fF for C
Zref
, and 332 fF for C
Z+
.
(a) (b)
(c) (d)
Figure 5. Finite element method (FEM) simulation results: Diaphragm deflection by (a) normal force
F
Z
of 1 N and (b) X-axis shear force F
X
of 1 N applied to the positive direction; Calculated change of
3-axis output values by (c) F
Z
and (d) F
X
.
3. Fabrication
Figure 6 illustrates the fabrication process of the integrated tactile sensor. A multi-project wafer
(MPW) fabricated by 0.18 µm CMOS process of Taiwan Semiconductor Manufacturing Co., Ltd.
(TSMC) (Hsinchu, Taiwan) was used for the tactile sensor. The MPW has a laser ablation area, as
shown in Figure 4a. Figure 7 shows the surface roughness of the MPW. The peak-to-valley
roughness of the laser ablation area is over 20 µm, which is not suitable for the subsequent process.
In order to decrease the surface roughness, a 20-µm-thick SiO
2
layer was deposited by plasma
enhanced chemical vapor deposition (Figure 6b) and the surface was planarized by chemical
mechanical polishing. The CMOS substrate was back-ground to make the surface flat and to reduce
the thickness to 300 µm (Figure 6c) [22]. After this planarization process, the surface roughness
became small enough for the following processes.
Figure 5.
Finite element method (FEM) simulation results: Diaphragm deflection by (
a
) normal force F
Z
of 1 N and (
b
)X-axis shear force F
X
of 1 N applied to the positive direction; Calculated change of 3-axis
output values by (c)FZand (d)FX.
3. Fabrication
Figure 6illustrates the fabrication process of the integrated tactile sensor. A multi-project wafer
(MPW) fabricated by 0.18
µ
m CMOS process of Taiwan Semiconductor Manufacturing Co., Ltd. (TSMC)
(Hsinchu, Taiwan) was used for the tactile sensor. The MPW has a laser ablation area, as shown in
Figure 4a. Figure 7shows the surface roughness of the MPW. The peak-to-valley roughness of
the laser ablation area is over 20
µ
m, which is not suitable for the subsequent process. In order to
decrease the surface roughness, a 20-
µ
m-thick SiO
2
layer was deposited by plasma enhanced chemical
vapor deposition (Figure 6b) and the surface was planarized by chemical mechanical polishing.
The CMOS substrate was back-ground to make the surface flat and to reduce the thickness to 300
µ
m
(Figure 6c) [
22
]. After this planarization process, the surface roughness became small enough for
the following processes.

Sensors 2017,17, 2451 7 of 14
Sensors 2017, 17, 2451 7 of 14
(a) (e) (i)
(b) (f) (j)
(c) (g) (k)
(d) (h) (l)
Figure 6. Fabrication process of the 3-axis integrated tactile sensor: (a) Received CMOS substrate; (b)
Thick SiO
2
film deposition; (c) Planarization and back grinding; (d) Pad opening; (e) Au
electroplating; (f) Planarization of Au bumps and an Au sealing frame; (g) Formation of a ground
(GND) electrode and rewiring; (h) Formation of 3-axis capacitor electrodes and rewiring; (i) Au-Au
thermo-compression bonding; (j) Formation of a diaphragm; (k) Formation of bonding pads; (l)
Surface mounting on a glass substrate with interconnections by an anisotropic conductive film
(ACF).
The SiO
2
layer was patterned by dry etching for pad opening (Figure 6d). After depositing
Au/Pt/Ti seed layers by sputtering, Au of 9 µm thickness was electroplated using a sulfite-based
plating solution (Microfab Au 310, Electroplating Engineers of Japan Ltd., Kanagawa, Japan)
(Figure 6e). The roles of Au, Pt, and Ti layers are the electrode to electroplate Au, diffusion
prevention and adhesion, respectively. The electroplated Au was annealed at 350 °C for 30 min to
remove volatile components. After covering the surface of the substrate with a resist for protection,
the electroplated Au was planarized by fly-cutting process using a surface planer (DAS8920, DISCO
Corporation, Tokyo, Japan) with a diamond bit (Figure 6f) [23]. After removing the resist, the
thickness of the electroplated Au was confirmed to be approximately 4.5 µm. The Au/Pt/Ti seed
layers were then patterned for forming a ground (GND) electrode and rewiring (Figure 6g). The Au
layer was patterned by I
2
/KI solution and Pt/Ti layers were patterned by Ar ion milling. The 3-axis
capacitor electrodes were formed on an LTCC interposer (Via-Wafer, Nikko Company, Ishikawa,
Japan) by the same procedure as stated above (Figure 6h). Figure 8a,b show the optical micrographs
of the CMOS substrate and the LTCC interposer substrate just before bonding, respectively.
The surface of these substrates were activated by Ar ion milling. After alignment by an
mask/bonding aligner (MA/BA8, SÜSS MicroTec, Garching, Germany), the substrates were bonded
by a wafer bonder (SB6e, SÜSS MicroTec, Garching, Germany) at 300 °C for 30 min with a bonding
pressure of about 130 MPa (Figure 6i). A diaphragm was formed in the CMOS substrate by DRIE
(Figure 6j). The thickness of the diaphragm was approximately 50 µm. Bonding pads were formed
by Au electroplating and patterning the seed layers (Figure 6k). Figure 8c,d show the fabricated
sensor after dicing. The sensing diaphragm with the boss is formed on the front side to receive
contact force. The bonding pads formed on the backside are for providing the power (3.3 V for
analog circuits and input/output (I/O) and 1.8 V for digital circuits), GND, signals, and resets to the
CMOS circuit.
Figure 6.
Fabrication process of the 3-axis integrated tactile sensor: (
a
) Received CMOS substrate;
(
b
) Thick SiO
2
film deposition; (
c
) Planarization and back grinding; (
d
) Pad opening; (
e
) Au electroplating;
(
f
) Planarization of Au bumps and an Au sealing frame; (
g
) Formation of a ground (GND) electrode and
rewiring; (
h
) Formation of 3-axis capacitor electrodes and rewiring; (
i
) Au-Au thermo-compression
bonding; (
j
) Formation of a diaphragm; (
k
) Formation of bonding pads; (
l
) Surface mounting on a glass
substrate with interconnections by an anisotropic conductive film (ACF).
Sensors 2017, 17, 2451 8 of 14
Figure 7. Surface profile of the multi-project wafer (MPW) before and after planarization.
(a) (b)
(c) (d) (e)
Figure 8. Fabrication results: Optical micrographs of (a) CMOS substrate with Au bumps, an Au
sealing frame and a GND electrode and (b) low temperature cofired ceramic (LTCC) interposer
substrate with 3-axis sensing electrodes; (c) Front side and (d) back side views of a fabricated tactile
sensor; (e) Surface-mounted tactile sensor on a glass substrate with interconnections.
The fabricated sensor was surface-mounted on a glass substrate with Au/Ni/Au/Cr
interconnections by an anisotropic conductive film (ACF) [24] (Figure 6l). The ACF (MF-301,
Hitachi Chemical Co., Ltd., Tokyo, Japan) has Au-plated plastic particles with a diameter of 10 µm
and a density of approximately 600 pcs./mm
2
. The bonding pads and the interconnections are
electrically connected, keeping the isolation of the adjacent bonding pads. A flip-chip bonder
Figure 7. Surface profile of the multi-project wafer (MPW) before and after planarization.
The SiO
2
layer was patterned by dry etching for pad opening (Figure 6d). After depositing
Au/Pt/Ti seed layers by sputtering, Au of 9
µ
m thickness was electroplated using a sulfite-based
plating solution (Microfab Au 310, Electroplating Engineers of Japan Ltd., Kanagawa, Japan) (Figure 6e).

Sensors 2017,17, 2451 8 of 14
The roles of Au, Pt, and Ti layers are the electrode to electroplate Au, diffusion prevention and adhesion,
respectively. The electroplated Au was annealed at 350
◦
C for 30 min to remove volatile components.
After covering the surface of the substrate with a resist for protection, the electroplated Au was
planarized by fly-cutting process using a surface planer (DAS8920, DISCO Corporation, Tokyo, Japan)
with a diamond bit (Figure 6f) [
23
]. After removing the resist, the thickness of the electroplated Au was
confirmed to be approximately 4.5
µ
m. The Au/Pt/Ti seed layers were then patterned for forming
a ground (GND) electrode and rewiring (Figure 6g). The Au layer was patterned by I
2
/KI solution
and Pt/Ti layers were patterned by Ar ion milling. The 3-axis capacitor electrodes were formed on
an LTCC interposer (Via-Wafer, Nikko Company, Ishikawa, Japan) by the same procedure as stated
above (Figure 6h). Figure 8a,b show the optical micrographs of the CMOS substrate and the LTCC
interposer substrate just before bonding, respectively.
Sensors 2017, 17, 2451 8 of 14
Figure 7. Surface profile of the multi-project wafer (MPW) before and after planarization.
(a) (b)
(c) (d) (e)
Figure 8. Fabrication results: Optical micrographs of (a) CMOS substrate with Au bumps, an Au
sealing frame and a GND electrode and (b) low temperature cofired ceramic (LTCC) interposer
substrate with 3-axis sensing electrodes; (c) Front side and (d) back side views of a fabricated tactile
sensor; (e) Surface-mounted tactile sensor on a glass substrate with interconnections.
The fabricated sensor was surface-mounted on a glass substrate with Au/Ni/Au/Cr
interconnections by an anisotropic conductive film (ACF) [24] (Figure 6l). The ACF (MF-301,
Hitachi Chemical Co., Ltd., Tokyo, Japan) has Au-plated plastic particles with a diameter of 10 µm
and a density of approximately 600 pcs./mm
2
. The bonding pads and the interconnections are
electrically connected, keeping the isolation of the adjacent bonding pads. A flip-chip bonder
Figure 8.
Fabrication results: Optical micrographs of (
a
) CMOS substrate with Au bumps, an Au
sealing frame and a GND electrode and (
b
) low temperature cofired ceramic (LTCC) interposer
substrate with 3-axis sensing electrodes; (
c
) Front side and (
d
) back side views of a fabricated tactile
sensor; (e) Surface-mounted tactile sensor on a glass substrate with interconnections.
The surface of these substrates were activated by Ar ion milling. After alignment by
an mask/bonding aligner (MA/BA8, SÜSS MicroTec, Garching, Germany), the substrates were bonded
by a wafer bonder (SB6e, SÜSS MicroTec, Garching, Germany) at 300
◦
C for 30 min with a bonding
pressure of about 130 MPa (Figure 6i). A diaphragm was formed in the CMOS substrate by DRIE
(Figure 6j). The thickness of the diaphragm was approximately 50
µ
m. Bonding pads were formed by
Au electroplating and patterning the seed layers (Figure 6k). Figure 8c,d show the fabricated sensor
after dicing. The sensing diaphragm with the boss is formed on the front side to receive contact force.

Sensors 2017,17, 2451 9 of 14
The bonding pads formed on the backside are for providing the power (3.3 V for analog circuits and
input/output (I/O) and 1.8 V for digital circuits), GND, signals, and resets to the CMOS circuit.
The fabricated sensor was surface-mounted on a glass substrate with Au/Ni/Au/Cr
interconnections by an anisotropic conductive film (ACF) [
24
] (Figure 6l). The ACF (MF-301, Hitachi
Chemical Co., Ltd., Tokyo, Japan) has Au-plated plastic particles with a diameter of 10
µ
m and
a density of approximately 600 pcs./mm
2
. The bonding pads and the interconnections are electrically
connected, keeping the isolation of the adjacent bonding pads. A flip-chip bonder (MODEL-6000,
HiSOL, Inc., Tokyo, Japan) was used for surface mounting, and a bonding pressure of 3 N was applied
at 170
◦
C. Figure 8e shows a surface-mounted tactile sensor on a glass substrate with interconnections.
4. Experiments and Results
4.1. Experimental Method
Figure 9a shows experimental system diagram. The integrated tactile sensor was connected
to a field programmable gate array (FPGA) based relay node (EDA-004, HuMANDATA Ltd.,
Osaka, Japan) and DC-DC converters. After providing power and configuration data, the tactile sensor
recognizes the configuration data via clock and data recovery system [
25
,
26
], and starts transmitting
sensing data packets. The data packet includes the sensor ID, 3-axis capacitive sensing data, and a cyclic
redundancy check (CRC) error check code, as shown in Figure 9b. The clock frequency and data rate
of the data packet were 1.34 MHz and 81 samples/s, respectively. The packet data were decoded
by the relay node, and the decoded data are analyzed by a host software. The power consumption
of the fabricated tactile sensor was estimated as 5.8 mW (1.2 mW for 3.3 V power and 4.6 mW for
1.8 V power).
Sensors 2017, 17, 2451 9 of 14
(MODEL-6000, HiSOL, Inc., Tokyo, Japan) was used for surface mounting, and a bonding pressure
of 3 N was applied at 170 °C. Figure 8e shows a surface-mounted tactile sensor on a glass substrate
with interconnections.
4. Experiments and Results
4.1. Experimental Method
Figure 9a shows experimental system diagram. The integrated tactile sensor was connected to a
field programmable gate array (FPGA) based relay node (EDA-004, HuMANDATA Ltd., Osaka,
Japan) and DC-DC converters. After providing power and configuration data, the tactile sensor
recognizes the configuration data via clock and data recovery system [25,26], and starts transmitting
sensing data packets. The data packet includes the sensor ID, 3-axis capacitive sensing data, and a
cyclic redundancy check (CRC) error check code, as shown in Figure 9b. The clock frequency and
data rate of the data packet were 1.34 MHz and 81 samples/s, respectively. The packet data were
decoded by the relay node, and the decoded data are analyzed by a host software. The power
consumption of the fabricated tactile sensor was estimated as 5.8 mW (1.2 mW for 3.3 V power and
4.6 mW for 1.8 V power).
Figure 10 illustrates the experimental setup. Normal force was applied to the fabricated sensor
by pushing a 3D-printed plastic pin vertically using a linear stage via a reference force gauge
(ZTS-5N, IMADA Co., Ltd., Aichi, Japan). A 3D-printed pin with a concave shape (200 µm depth)
was pushed horizontally to the side surface of the boss by moving a positioning stage to apply
shear force.
(a) (b)
Figure 9. (a) Experimental system diagram; (b) Digital signal output waveform from the fabricated
3-axis integrated tactile sensor.
Figure 9.
(
a
) Experimental system diagram; (
b
) Digital signal output waveform from the fabricated
3-axis integrated tactile sensor.
Figure 10 illustrates the experimental setup. Normal force was applied to the fabricated sensor by
pushing a 3D-printed plastic pin vertically using a linear stage via a reference force gauge (ZTS-5N,
IMADA Co., Ltd., Aichi, Japan). A 3D-printed pin with a concave shape (200
µ
m depth) was pushed
horizontally to the side surface of the boss by moving a positioning stage to apply shear force.

Sensors 2017,17, 2451 10 of 14
Sensors 2017, 17, 2451 10 of 14
(a) (b)
Figure 10. Measurement setup for applying (a) normal force and (b) shear force.
4.2. Results
Figure 11 plots the change of 3-axis output values for the applied force. Each plot shows the
average of 200 output values. As shown in Figure 11a, the Z-axis output value N
Z
increased by
applying normal force F
Z
up to 1.3 N, while the change of N
X
and N
Y
are small. The sensitivity for F
Z
up to 0.5 N is calculated as 34.5 Count/mN. The standard deviations of N
Z
at unload condition was
3.5 Count. This corresponds to F
Z
of 0.10 mN, which is calculated by the sensitivity of 34.5
Count/mN. The hysteresis for F
Z
is less than 0.3% full scale (FS). The nonlinear change of N
Z
is due to
parasitic capacitance and the nonlinear relationship between the oscillation frequency and the
capacitance. Suppose the elastic deformation of the diaphragm, the applied normal force F
Z
is given
by
FZ = k∆d, (5)
where k and Δd are the stiffness and the deflection of the diaphragm, respectively. The Z-axis sensor
capacitance C
Z+
is written as
CZ+ = εSZ+
d  ∆d + Cp, (6)
where ε is permittivity of vacuum, S
Z+
is the area of the Z-axis sensor capacitor C
Z+
, d is the initial
gap of the capacitor electrodes, and C
p
is parasitic capacitance. Combining Equations (1) and (4)–(6)
gives the following equation:
NZ = fZref · P
fclock
 4.74 × 104
d
d  ΔdCZ+,0 + Cp + 497 · P
fclock
, (7)
where C
Z+,0
is expressed as C
Z+,0
= εS
Z+
/d. The Z-axis output value at unload condition N
Z0
is given by
NZ0 = fZref · P
fclock
 4.74 × 104
CZ+,0 + Cp + 497 · P
fclock. (8)
The change of the Z-axis output value ΔN
Z
(i.e., N
Z
− N
Z0
) is written as
ΔNZ = 4.74 × 104CZ+,0PFZ
fclock(Cp + 497)(CZ+,0 + Cp + 497) ·󰇧CZ+,0 + Cp + 497
Cp + 497 kd  FZ󰇨1
, (9)
where the units of f
clock
, C
Z+,0
, and C
p
are MHz, fF, and fF, respectively. Comparing Equation (9) and
the equation of the approximate curve in Figure 11a, k and C
p
are calculated as 2.9 × 10
5
N/m and
0.96 pF, respectively. The calculated stiffness is smaller than the simulated one of 3.1 × 10
5
N/m. The
Figure 10. Measurement setup for applying (a) normal force and (b) shear force.
4.2. Results
Figure 11 plots the change of 3-axis output values for the applied force. Each plot shows
the average of 200 output values. As shown in Figure 11a, the Z-axis output value N
Z
increased by
applying normal force F
Z
up to 1.3 N, while the change of N
X
and N
Y
are small. The sensitivity for F
Z
up to 0.5 N is calculated as
34.5 Count/mN
. The standard deviations of N
Z
at unload condition was
3.5 Count. This corresponds to F
Z
of 0.10 mN, which is calculated by the sensitivity of
34.5 Count/mN
.
The hysteresis for F
Z
is less than 0.3% full scale (FS). The nonlinear change of N
Z
is due to parasitic
capacitance and the nonlinear relationship between the oscillation frequency and the capacitance.
Suppose the elastic deformation of the diaphragm, the applied normal force FZis given by
FZ=k∆d, (5)
where kand
∆
dare the stiffness and the deflection of the diaphragm, respectively. The Z-axis sensor
capacitance CZ+ is written as
CZ+=εSZ+
d−∆d+Cp, (6)
where
ε
is permittivity of vacuum, S
Z+
is the area of the Z-axis sensor capacitor C
Z+
,dis the initial gap
of the capacitor electrodes, and C
p
is parasitic capacitance. Combining Equations (1) and (4)–(6) gives
the following equation:
NZ=fZref·P
fclock
−4.74 ×104
d
d−∆dCZ+,0 +Cp+497
·P
fclock , (7)
where C
Z+,0
is expressed as C
Z+,0
=
ε
S
Z+
/d. The Z-axis output value at unload condition N
Z0
is
given by
NZ0 =fZref·P
fclock
−4.74 ×104
CZ+,0 +Cp+497·P
fclock . (8)
The change of the Z-axis output value ∆NZ(i.e., NZ−NZ0) is written as
∆NZ=4.74 ×104CZ+,0PFZ
fclock(Cp+497)(CZ+,0 +Cp+497·CZ+,0 +Cp+497
Cp+497 kd −FZ−1
, (9)

Sensors 2017,17, 2451 11 of 14
where the units of f
clock
,C
Z+,0
, and C
p
are MHz, fF, and fF, respectively. Comparing Equation (9)
and the equation of the approximate curve in Figure 11a, kand C
p
are calculated as 2.9
×
10
5
N/m
and 0.96 pF, respectively. The calculated stiffness is smaller than the simulated one of
3.1 ×105N/m
.
The possible reason is the dimension error (e.g., the thickness of the diaphragm was smaller than
the designed value). The parasitic capacitance is caused by the CMOS circuit (e.g., pads and I/O
buffers) and the interconnections for rewiring. The parasitic capacitance of the CMOS circuit is about
several hundred fF. Thicker isolation layer between the CMOS circuit and the interconnections for
rewiring enables the decreasing of parasitic capacitance. When large normal force over 1.3 N is applied,
NZbecomes stable. This is probably because the capacitor electrodes contact by the force.
Sensors 2017, 17, 2451 11 of 14
possible reason is the dimension error (e.g., the thickness of the diaphragm was smaller than the
designed value). The parasitic capacitance is caused by the CMOS circuit (e.g., pads and I/O
buffers) and the interconnections for rewiring. The parasitic capacitance of the CMOS circuit is
about several hundred fF. Thicker isolation layer between the CMOS circuit and the
interconnections for rewiring enables the decreasing of parasitic capacitance. When large normal
force over 1.3 N is applied, N
Z
becomes stable. This is probably because the capacitor electrodes
contact by the force.
(a)
(b) (c)
Figure 11. Change of output value of the 3-axis integrated tactile sensor by (a) normal force F
Z
; (b)
shear force F
X
and (c) shear force F
Y
.
The X-axis output value N
X
and the Y-axis output value N
Y
were linearly changed by the
applied shear force, as shown in Figure 11b,c. The sensitivity is calculated to be 15.1 Count/mN for
X-axis and 14.1 Count/mN for Y-axis. The standard deviations of N
X
and N
Y
at unload condition
were 12.4 Count and 7.6 Count, respectively. These correspond to F
X
of 0.82 mN and F
Y
of 0.54 mN.
The hysteresis is less than 1.7% FS for X-axis and 1.1% FS for Y-axis. The average cross-sensitivity is
calculated by the following procedures [27]: (1) the 3-axis output values are converted to measured
3-axis force by the approximation curves shown in Figure 11; and, (2) the average cross-axis
sensitivity is calculated as the average of the ratio of the measured cross-axis force to the measured
principal-axis force. When F
Z
is applied, the average cross-axis sensitivity is 8.2% for X-axis and
10.4% for Y-axis. In the case that F
X
or F
Y
is applied, the average cross-axis sensitivity is less than
10%. The calculation of the stiffness and the parasitic capacitance for X-axis and Y-axis is not possible
by the procedure as stated above because the Z-axis output value is determined by a fixed capacitor
(i.e., f
Zref
can be assumed as a constant frequency) and a variable capacitor, while the output values of
X-axis and Y-axis are calculated by two variable capacitors. Also, definition of the gap change Δd is
difficult for X-axis and Y-axis because the gap change is not uniform due to the tilt of the diaphragm
(Figure 3c), although the gap of the Z-axis sensor capacitor changes like a parallel plate type
capacitor (Figure 3b). As described in Equations (2)–(4), the output values for each axis is calculated
Figure 11.
Change of output value of the 3-axis integrated tactile sensor by (
a
) normal force F
Z
; (
b
) shear
force FXand (c) shear force FY.
The X-axis output value N
X
and the Y-axis output value N
Y
were linearly changed by the applied
shear force, as shown in Figure 11b,c. The sensitivity is calculated to be 15.1 Count/mN for X-axis and
14.1 Count/mN for Y-axis. The standard deviations of N
X
and N
Y
at unload condition were 12.4 Count
and 7.6 Count, respectively. These correspond to F
X
of 0.82 mN and F
Y
of 0.54 mN. The hysteresis
is less than 1.7% FS for X-axis and 1.1% FS for Y-axis. The average cross-sensitivity is calculated by
the following procedures [
27
]: (1) the 3-axis output values are converted to measured 3-axis force by
the approximation curves shown in Figure 11; and, (2) the average cross-axis sensitivity is calculated as
the average of the ratio of the measured cross-axis force to the measured principal-axis force. When F
Z
is applied, the average cross-axis sensitivity is 8.2% for X-axis and 10.4% for Y-axis. In the case
that F
X
or F
Y
is applied, the average cross-axis sensitivity is less than 10%. The calculation of the
stiffness and the parasitic capacitance for X-axis and Y-axis is not possible by the procedure as stated
above because the Z-axis output value is determined by a fixed capacitor (i.e., f
Zref
can be assumed

Sensors 2017,17, 2451 12 of 14
as a constant frequency) and a variable capacitor, while the output values of X-axis and Y-axis are
calculated by two variable capacitors. Also, definition of the gap change
∆
dis difficult for X-axis and
Y-axis because the gap change is not uniform due to the tilt of the diaphragm (Figure 3c), although the
gap of the Z-axis sensor capacitor changes like a parallel plate type capacitor (Figure 3b). As described
in Equations (2)–(4), the output values for each axis is calculated by the difference of the count value of
the two capacitors. If the count value of each capacitor can be obtained, estimation of the parasitic
capacitances for X-axis and Y-axis will be possible.
Figure 12 shows the response of the 3-axis integrated tactile sensor to temperature change.
The fabricated sensor was heated by a silicone rubber heater from room temperature (22
◦
C) to 60
◦
C.
The change of the output value corresponds to less than 7.5 mN for X-axis, 69 mN for Y-axis, and 15 mN
for Z-axis.
The performance of the fabricated sensor is summarized in Table 1. The performance well fits
the tactile sensing for a robot, which we are considering. In addition, the sensor characteristics
measured in this study were repeatable because of the almost fatigue-free characteristics of Si and
avoidance of the mechanical stress to the analog circuits, which is an important advantage compared
with elastomer-based tactile sensors.
Sensors 2017, 17, 2451 12 of 14
by the difference of the count value of the two capacitors. If the count value of each capacitor can be
obtained, estimation of the parasitic capacitances for X-axis and Y-axis will be possible.
Figure 12 shows the response of the 3-axis integrated tactile sensor to temperature change. The
fabricated sensor was heated by a silicone rubber heater from room temperature (22 °C) to 60 °C.
The change of the output value corresponds to less than 7.5 mN for X-axis, 69 mN for Y-axis, and 15
mN for Z-axis.
The performance of the fabricated sensor is summarized in Table 1. The performance well fits
the tactile sensing for a robot, which we are considering. In addition, the sensor characteristics
measured in this study were repeatable because of the almost fatigue-free characteristics of Si and
avoidance of the mechanical stress to the analog circuits, which is an important advantage compared
with elastomer-based tactile sensors.
Figure 12. Response of the 3-axis integrated tactile sensor as a function of temperature.
Table 1. The performance of the fabricated 3-axis integrated tactile sensor.
Force Sensitivity [Count/mN] Noise [Count] Hysteresis [%] Average Cross-Axis Sensitivity [%]
X-Axis Y-Axis Z-Axis
FX (−1 to 1 N) 15.1 12.4 ≤ 1.7 - 5.5 9.0
FY (−1 to 1 N) 14.1 7.6 ≤ 1.1 5.2 - 7.2
FZ (0 to 1.3 N 1) ≥ 34.5 3.5 ≤ 0.3 8.2 10.4 -
1 Z-axis output value NZ saturate when normal force of over 1.3 N is applied.
5. Conclusions
We have developed a 3-axis integrated tactile sensor surface-mountable on a bus line. In order
to simplify the fabrication process, a CMOS substrate with circuits for differential capacitive sensing
and serial bus communication was flip-bonded on a LTCC interposer substrate by Au-Au
thermo-compression bonding, and then the sensing diaphragm was formed from the backside of the
CMOS substrate. For the evaluation of the fabricated sensor, normal and shear force was applied
independently by 3D-printed pins and movable stages. The fabricated tactile sensor demonstrated
3-axis force detection with a high sensitivity of 15.1 Count/mN for X-axis shear force, 14.1 Count/mN
for Y-axis shear force, and over 34.5 Count/mN for Z-axis normal force. The noise at unload
condition corresponded to smaller than 1 mN. Also, the hysteresis and the average cross-sensitivity
were less than 2% FS and 11%, respectively. The 3-axis integrated tactile sensor developed in this
study is useful for the tactile sensation of robots working with human.
Acknowledgments: This study was performed in R & D Center of Excellence for Integrated Microsystems,
Tohoku University under the program “Formation of Innovation Center for Fusion of Advanced Technologies”
supported by Special Coordination Funds for Promoting Science and Technology. This work was supported by
VLSI Design and Education Center (VDEC), the University of Tokyo in collaboration with Synopsys, Inc.,
Cadence Design Systems, Inc. and Mentor Graphics, Inc. This work was partially supported by Grant-in-Aid for
JSPS Research Fellow (Grant number 16J01558). The authors would like to thank Hitachi Chemical Co., Ltd. for
Figure 12. Response of the 3-axis integrated tactile sensor as a function of temperature.
Table 1. The performance of the fabricated 3-axis integrated tactile sensor.
Force Sensitivity [Count/mN] Noise [Count] Hysteresis [%] Average Cross-Axis Sensitivity [%]
X-axis Y-axis Z-axis
FX(−1 to 1 N) 15.1 12.4 ≤1.7 - 5.5 9.0
FY(−1 to 1 N) 14.1 7.6 ≤1.1 5.2 - 7.2
FZ(0 to 1.3 N1)≥34.5 3.5 ≤0.3 8.2 10.4 -
1Z-axis output value NZsaturate when normal force of over 1.3 N is applied.
5. Conclusions
We have developed a 3-axis integrated tactile sensor surface-mountable on a bus line.
In order to simplify the fabrication process, a CMOS substrate with circuits for differential capacitive
sensing and serial bus communication was flip-bonded on a LTCC interposer substrate by Au-Au
thermo-compression bonding, and then the sensing diaphragm was formed from the backside of
the CMOS substrate. For the evaluation of the fabricated sensor, normal and shear force was applied
independently by 3D-printed pins and movable stages. The fabricated tactile sensor demonstrated
3-axis force detection with a high sensitivity of 15.1 Count/mN for X-axis shear force, 14.1 Count/mN
for Y-axis shear force, and over 34.5 Count/mN for Z-axis normal force. The noise at unload condition
corresponded to smaller than 1 mN. Also, the hysteresis and the average cross-sensitivity were less

Sensors 2017,17, 2451 13 of 14
than 2% FS and 11%, respectively. The 3-axis integrated tactile sensor developed in this study is useful
for the tactile sensation of robots working with human.
Acknowledgments:
This study was performed in R & D Center of Excellence for Integrated Microsystems, Tohoku
University under the program “Formation of Innovation Center for Fusion of Advanced Technologies” supported
by Special Coordination Funds for Promoting Science and Technology. This work was supported by VLSI Design
and Education Center (VDEC), the University of Tokyo in collaboration with Synopsys, Inc., Cadence Design
Systems, Inc. and Mentor Graphics, Inc. This work was partially supported by Grant-in-Aid for JSPS Research
Fellow (Grant number 16J01558). The authors would like to thank Hitachi Chemical Co., Ltd. for providing ACF.
The authors would also like to thank Travis Bartley who previously belonged to Tohoku University for developing
measurement system software.
Author Contributions:
Sho Asano performed the design, the fabrication, the evaluation, and the analysis
of the tactile sensor and wrote the paper. Masanori Muroyama designed and evaluated the CMOS circuit.
Masanori Muroyama, Takahiro Nakayama and Yutaka Nonomura designed the specification of the CMOS circuit.
Yoshiyuki Hata and Yutaka Nonomura provided helpful comments and suggestions on the design and the data
analysis. Masanori Muroyama and Shuji Tanaka planned and supervised the research.
Conflicts of Interest: The authors declare no conflict of interest.
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