W3P.014
ACCELEROMETER USING MOSFET WITH MOVABLE GATE ELECTRODE:
ELECTROPLATING THICK NICKEL PROOF MASS ON FLEXIBLE PARYLENE BEAM FOR
ENHANCING SENSITIVITY
S. Aoyagi1, M. Suzuki1, J. Kogure1, T. Kong1, R. Taguchi1, T. Takahashi1, S. Yokoyama2, and H. Tokunaga3
1
Kansai University, Osaka, Japan
2
Hiroshima University, Hiroshima, Japan
3
M. T. C. Corp., Kanagawa, Japan
ABSTRACT
This paper proposes an accelerometer based on
metal-oxide-semiconductor field effect transistor
(MOSFET), the gate electrode of which is movable
because it is suspended by a flexible Parylene beam.
When acceleration is applied, the gate is moved due to
inertia force, making the change in drain current. It is
notable that any external amplifying circuitry is not
required, since the device itself acts as an electrical
transistor besides a mechanical sensing structure. The
fabrication starts from a conventional MOSFET, which is
preferable in terms of CMOS compatibility and
fabrication cost. For increasing the sensitivity, in the
present paper, a thick nickel (Ni) proof mass was
successfully electroplated on a flexible Parylene beam. It
was proven that the proposed MOSFET-type
accelerometer surely detects gravitational acceleration.
KEYWORDS
In our previous report, MOSFET was fabricated on a
SOI wafer having a thick silicon active layer (100 µm),
which was partially thinned for forming a beam and
leaving a thick proof mass [5]. The box oxide layer of it
was etched away by buffered HF, releasing the silicon
structure composed of beam and mass. This structure acts
as a floating gate electrode. When an inertia force is
applied to this electrode, it is displaced and the gate
capacitance is changed, causing the change in drain
current. By detecting the drain current electrically, the
input acceleration can be measured. However, our
developed sensor had following problems: 1) the beam
thickness is difficult to control in time etching, 2)
Young’s modulus of silicon (E=130 GPa) is rather large
to achieve a flexible beam for high sensitivity, and 3) SOI
wafer is expensive.
In the present paper, we propose two contrivances to
clear the problems: one is employing flexible Parylene
(E=3.2 GPa) as beam material, another is electroplating
Accelerometer, MOSFET, Electroplating, Parylene
Floating gate electrode
(Parylene/Al stack)
INTRODUCTION
Capacitive sensors are widely used as accelerometer
[1]. This type of sensors has two electrodes: one is fixed
and other is movable, and detects the displacement of the
movable electrode from changes in electrode charge. To
increase its sensitivity, the gap between two electrodes
should be filled with gaseous materials such as air,
vacuum, or any other gases. However, the capacitive
accelerometer has a problem that its sensitivity deceases
with its downsizing because the charge between two
electrodes is proportional to the area of electrodes.
Therefore, it requires an amplifier circuit when its size is
small. To overcome this circumstance, MOSFET sensor
is one of solutions because it can amplify a signal by itself.
Drain current of the MOSFET is changed due to the
charge accumulating effect of gate oxide. Therefore, if the
gate capacitance is changeable by making the gate
structure suspended, the MOSFET itself can become a
capacitive sensor.
Some kinds of FET-type capacitive sensors have
already been reported [2-5]. A vertical FET sensor was
fabricated by adding a floating gate structure on a
source/drain channel using film deposition: however, the
thickness of gate was limited [2, 3]. A lateral FET sensor
was also reported: however, its fabrication is somewhat
complicated [4]. On the other hand, our MOSFET sensor
is simply fabricated using a standard MOSFET. Therefore,
its fabrication is preferable in terms of CMOS
compatibility and fabrication cost.
978-1-4577-0156-6/11/$26.00 ©2011 IEEE
Drain
electrode
Merits: Any external amplifier
is not required, since it acts as
electrical transistor besides
mechanical sensing structure.
In this report:
For increasing sensitivity,
• Flexible polymer Parylene
beam is employed.
• Electroplating thick Ni
proof mass on polymer is
successfully realized.
Ni proof mass
A
A’
Source
electrode
(a) Top view
Acceleration
Gate
Source
Ni
Parylene mass
Al
Airgap
n+-Si
Channel
p-Si substrate
Drain
VG
VD
Al
SiO2
n+-Si
(b) A-A’ cross section
b
Concrete size of
Parylene beam
h: 4 µm
b: 100 µm
l : 1,000 µm
l
h
(c) Overview
Figure 1: Proposed accelerometer using MOSFET in which
gate electrode is movable due to input acceleration.
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Transducers’11, Beijing, China, June 5-9, 2011
thick Ni proof mass on Parylene surface. Although
adhesion between electroplated metal and polymer is
generally not strong, we realized it by searching
conditions.
FEATURES OF DEVELOPED SENSOR
Figure 1 shows a schematic of proposed
accelerometer, in which gate electrode made of Al is
movable to applied acceleration because it is suspended
OPTIMAZATION FOR ELECTROPLATING
OF NICKEL
Table 1: Parameters of propose MOSFET shown in Fig.1
100 µm
1,000 µm
p-Si (100), 1-10 Ω·cm
1.0 µm
1.0 µm
1.6 mg
4.0 µm
7.40
7.4
0.08
VG =25 V
VD =20 V
7.35
7.30
7.3
0.06
7.25
0.04
7.20
7.2
7.15
0.02
7.10
7.1
0
7.05
7.00
7.0
0.0
0.2
0.4
0.6
0.8
The fabrication of proof mass on the floating Parylene
beam in MOSFET sensor is the most important technique
in this study. It is difficult to fabricate a metallic proof
mass on a polymer beam by electroplating method,
because adhesion between a metal and polymer is
generally not strong. We tried to experimentally find a
seed metal most suitable for electroplating of Ni mass on
a Parylene film by carrying out adhesion test. In this test,
Deflection of floating gate
electrode (µm)
Saturated drain current (µA)
Gate length L
Gate width W
Substrate
Thickness of gate oxide
Initial airgap
Weight of Ni mass
Thickness of Parylene
by a flexible Parylene beam. Parameters of the MOSFET
are shown in Table 1. The Change in drain current with
respect to applied force was simulated using finite
element method (FEM) software (COMSOL,
Multiphysics ver. 3.5a) and SPICE software. The
simulation result is shown in Fig. 2. Looking at this figure,
the drain current is proportional to the applied
acceleration. Its sensitivity is 0.3 µA/g within the range
from 0 to 1 g, where g is the gravitational acceleration.
Ni plate
(anode)
Sample
(cathode)
Plating bath
1.0
DC power
supply
Applied acceleration ( g )
Figure 2: Applied acceleration versus saturated drain
current and deflection of gate electrode of proposed
MOSFET sensor (simulation result).
：p-Si
：Parylene
Materials
：poly- 4 - pyridine
：Sputtered Ni
Figure 4: Setup for electroplating of Ni.
Table 2: Parameters of Ni electroplating.
Nickel (II) sulfate hexahydrate
Nickel (II) chloride hexahydrate
Contents of
plating solution Boric acid
Pure water
Temperature of plating solution
Revolution of magnetic stirrer
Applied current (adopted value at device fabrication)
：Sputtered
：Electroless-plated Ni
：Photosensitive film resist
：Electroplated Ni
O2 plasma
(50 Wm 2min)
36 g
6.75 g
5.25 g
150 cm3
53 °C
80 rpm
15 mA
60
(b) Exposure to O2 plasma to
improve adhesion between Parylene
and seed layer for electroplating.
Mass height (µm)
(a) Deposition of Parylene film
(4 µm in thickness) on Si
Hot plate with
magnetic stirrer
Plating time: 1 h
50
40
30
20
10
0
0
5
10
15
Current (mA)
20
Figure 5: Current dependency of plating rate of Ni mass.
(d) Formation of mold (e) Electroplating of Ni (f) Removal of the
using photosensitive (approximately 50 µm mold using photofilm resist for electro in thickness).
sensitive film resist.
-plating.
Figure 3: Process flow of preparing three kinds of samples
having different seed layers for electroplating.
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Mass height (µm)
(c1) Spin coating of (c2) DC sputtering of (c3) DC sputtering of
diluted poly-4-pyridine, Ni (0.2 µm in FeNi (0.2 µm in
thickness).
and electroless plating thickness).
of Ni.
Applied current: 15 mA
60
50
40
30
20
10
0
0
50
100
Plating time (min)
Figure 6: Height of plated Ni mass versus plating time.
three kinds of seed metals were employed, which are
electroless plated Ni, sputtered Ni, and sputtered FeNi.
Figure 3 shows the process flow of preparing samples.
Parylene (4 µm in thickness) was deposited on Si wafer,
followed by preparing abovementioned metal seed layer.
A mold for electroplating was formed by patterning
photosensitive dry film (Hitachi Chemical, HM4056: 56
µm in thickness). The surface of seed layer was flushed
by diluted HCl acid (0.1 w%) for 1 min just before
electroplating Ni for avoiding surface oxidation. Then,
electroplating of Ni was carried out. Setup and condition
of electroplating are shown in Fig. 4 and Table 2,
respectively. Finally, the mold was removed by NaOH
solution (5 %).
Current and time dependency of the growth rate of
electroplating Ni is shown in Figs. 5 and 6, respectively.
As shown in these graphs, the rate of electroplating Ni
increases as the current and the time increase. The peeling
of Ni occurred at the mold removal process when the
applied current was 20 mA or more. Therefore, the
applied current was fixed to 15 mA.
Adhesion of the plated Ni mass to each sample was
verified by the tape test which is specified by Japanese
Industrial Standards committee (JIS Z1522). The
procedure and the result are shown in Figs. 7(a) and (b),
respectively. As the result, adhesion of sputtered Ni to
Parylene was the best. As shown in Fig. 7(b), all the
electroplated Ni masses actually did not peel from
Parylene when the seed layer is sputtered Ni and the
bottom area of Ni mass is over 0.08 mm2. In cases of small
Materials
：p-Si
+
：n -Si
：SiO2
：Ni
Tape (Adhesive force: 3 N/cm )
：Parylene
Fabrication of MOSFET
MEMS process
(a) Fabrication of trenches on
SOI wafer for device isolation.
(g) Formation of Parylene gate
structure..
(b) Formation of Si gate electrode by reactive ion etching.
(h) DC sputtering of Ni which
acts as seed layer for electroplating.
2
Ni mass
：Al
：Photosensitive film resist
1) Press a tape on Ni mass array for 10 s.
Peel off
(c) Formation of source and
drain region by HF etching of
berried oxide layer of SOI wafer.
2) Peel off the tape and count Ni masses sticking to the tape.
Probability of peeled mass (%)
(a) Procedure of tape test conforming to JIS-Z1522
60
Seed layer
: Sputtered Ni
: Sputtered FeNi
: Electroless plated Ni
50
40
( i ) Formation of mold using
photosensitive film resist for
electroplating.
30
(d) Ion implantation of
Antimony and annealing for
activation of source and drain.
20
10
0
0.02
0.03
0.04
0.08
Bottom area of Ni mass (mm2)
(b) Probability of peeled Ni proof mass
( j ) Electroplating of Ni
(formation of proof mass).
0.12
Figure 7: Probability of peeled Ni proof mass after tape test.
(e) Formation of contact hole by
HF etching.
(k) Removal of mold and
unnecessary seed layer.
245 µm
100 µm
(a) Mass size is 100 × 250 × t40 µm
100 µm
(b) Mass size is 100 × 250 × t 70 µm
100 µm
( f) Fabrication of source, drain
and gate electrodes.
(c) Ni mass with maximum
height in this study, which is
fabricated using five stacked
layers of film resist. Its size
is 100 × 250 × t 245 µm.
( l ) Removal of poly-Si gate
using XeF2 gas..
Figure 9: Fabrication
accelerometer.
Figure 8: SEM images of electroplated Ni masses.
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process
of
MOSFET
type
Floating gate (Parylene/Al stack)
Anchor
Drain
Metallic board
Sample
Drain
Source
Fsinθ
Ni mass
500 µm
Source
Spacer
Fcosθ
Ni mass
θ
100 µm
F=mg
Anchor
Figure 10: SEM image of MOSFET with floating gate.
(a) Measurement setup
area lower than 0.04 mm2 mass, peel-off occurs: however,
it may not occur under usual usage, considering that the
tape test is the severest case.
Figure 8 shows scanning electron microscope (SEM)
images of electroplated Ni. As a result of employing
sputtered Ni as seed metal, a Ni mass with the thickness
over several tens µm was able to electroplate. Even if the
height of Ni mass is over that of the mold, it did not peel
off after removal of the mold, as shown in Fig. 8(b). The
maximal achieved thickness of Ni mass in this study was
245 µm, as shown in Fig 8(c). In this case, five stacked
layers of photosensitive dry films were used.
Drain current ID (µA)
14
Tilt angle θ
VG = 20 V
0°
10
30°
8
60°
4 µA
6
4
2
0
0
5
10
15
20
25
Drain voltage VD (V)
(b) Measurement setup
Saturated drain current (µA)
FABRICATION
OF
ACCELEROMETER
USING MOSFET AND EVALUATION OF ITS
CHARACTERISTICS
Figure 9 shows fabrication process of the
accelerometer using MOSFET. After fabrication of
conventional MOSFET, Parylene beams are patterned.
Then, Ni mass is electroplated using the sputtered seed
Ni layer and a mold made of photoresistive film. Finally,
Si gate of MOSFET is etched by XeF2 gas for releasing
the floating gate structure. The SEM images of
completed device are shown in Fig. 10.
For verifying basic potential of the fabricated
accelerometer, ID-VD characteristic of MOSFET was
measured by changing the inclination, as shown in Fig.
11(a). Let the tilt angle be θ, the effective force applied to
floating gate becomes mgcosθ, where m is mass, g
gravitational acceleration. The measurement results are
shown in Figs 11(b) and (c). Looking at these figures, ID is
surely decreased with increasing θ : therefore, it can
safely be said that the fabricated device is functional as an
accelerometer.
In our previous report using Si beam and mass [5], the
rate of ID change with respect to applied force was 0.2
mA/gf. In this report, the ID change between 0º and 60º is
4 µA (see Fig. 11(b)), the mass is 1.6 mg, so the rate is
(4×10-3)/cos60º/(1.6×10-3)=5 mA/gf, which is 25 times
larger than the previous one, showing the effectiveness of
Parylene beam and thick Ni mass.
12
10
8
6
60
Tilt angleθ (degree)
30
VG =20 V, VD =10 V
6.3
0
8.3
6.8
4
2
0
0.50
0.87
1.00
Cosθ
(c) Cosθ versus drain current ID
Figure 11: Tilt angle dependency of ID-VD characteristic of
fabricated MOSFET accelerometer. Basic potential for
detecting static gravitational acceleration was verified.
This work was supported in part by JSPS (Japan
Society for the Promotion of Science) KAKENHI
(22310083). This work was supported in part by the
Kansai University Special Research Fund, 2010.
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REFERENCES:
CONTACT
ACKNOWLEGEMENT
[1] G.
Kovacs,
“Micromachined
Transducers
Sourcebook”, McGraw-Hill, pp. 232-237, 1998.
* S. Aoyagi, tel: +81-6-6368-0823;
[email protected]
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