Measuring the deflection of a ...

Measuring the deflection of a micromachined cantilever-in-cantilever device using a piezoresistive sensor, - ...

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Proceedings of the 1999 IEEE Canadian Conferenceon Electrical and ComputerEngineering
Shaw ConferenceCenter, Edmonton, Alberta, Canada May 9-12 1999
Measuring the Deflection of a Micromachined Cantilever-In-Cantilever
Device using a Piezoresistive Sensor
Yuan
Ma,
Alexander M. Robinson, Ron
P.
W. Lawson, Bing Shen*,Derek Strembicke**,Walter
Allegretto***
Electrical and Computer Engineering Department
University of Alberta, Edmonton, Alberta, Canada T6G 2G7
*
GE Medical Systems,Milwaukee, WI, USA
53201
**
Optical ETC, Inc., Huntsville, AL, USA 35801
***
Department of Mathematical Sciences
University
of
Alberta,
Edmonton, Alberta, Canada T6G 2G1
Abstract
In this paper, we describe the use of polysilicon
piezoresistors to detect the deflection and resonance of a
magnetically actuated CMOS micromachined cantilever
device. We have introduced the Cantilever-In-Cantilever
(CIC) device in previous papers [8,9].
A
SEM picture of
a series of CIC devices is shown in
The CIC
structure involves nested cantilevers to enhance the
angular deflection of the inner cantilever. Possible
applications of the CIC include deflectable mirrors and
mirror
arrays, humidity sensor [IO], gas pressure sensor
[1
I], thin film deposition monitor
[
121, mass sensor and
magnetic field sensor. The deflection of the CIC
mirror
and the resonance shift
of
the CIC structures used as
humidity
or pressure sensors
are
measured
by the
piezoresistance
of
the polysilicon film deposited close to
the clamped ends in the outer
arms
of the structure.
The paper describes the detecting scheme for using
polysilicon, the strain analysis
of
the CIC structure and
the experiment results
of
applying polysilicon to
measure the deflection and resonance of
We have designed and tested a piezoresistor
for
detecting deflection of micromachined Cantilever-
In-Cantilever devices making use of the polycrystalline
siliconflm
of
the Mite1
1.5
pm
CMOS
IC
fabrication
process. Both static deflection and resonance
measurements have been investigated. The change
of
piezoresistance is about
0.06%
under static deflection,
which agrees with the
ANSYS
simulation results.
Under dynamic excitation, the piezoresistance AC
signal varies linearly with angular deflection
of
the
cantilever, although the variation depends
on
the
resonance mode. The resonant frequencies of the
modes
using
can
readily
be
determined
the
piezoresistor.
1. Introduction
the CIC
structures.
The piezoresistance effect in silicon and germanium
was discovered in 1954 [1,2]. Piezoresistivity is a
material property where the bulk resistivity is influenced
by the mechanical strains applied to the material.
Because of the large piezoresistivity in polycrystalline
silicon (polysilicon) and its favorable mechanical
behavior,piezoresistive silicon sensors have been widely
used as pressure, acceleration and flow sensors [3-71.
For
MicroElectroMechanical Systems (MEMS)
sensors and devices, polysilicon, available through the
standard CMOS IC fabrication process, can be used as a
piezoresistor. It can detect a structure’s strain, deflection,
motion, resonance frequencies
or
other strain-related
characteristics when the polysilicon is placed at a
suitable position in the structure.
2.
Piezoresistance
of
CMOS
Polysilicon Film
The piezoresistance change of polysilicon is a
function of the gauge factor
[13]
and the applied strain:
where G is the gauge factor and
<
is the applied strain.
The gauge factor of the polysilicon film has been proved
to be a function of the grain size and the doping
concentration [14].
A
typical value of 25 has been
obtained in reference [I41 from experiment results
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the gauge factor of the gate polysilicon fabricated with
the Mite1 1SpmCMOS process.
The commercial finite element package ANSYS is
used here to analyze the strain contour of the CIC
structure to find out the best position to place the
polysilicon film. The bigger the applied strain of the
polysilicon, the better the sensitivity
of
detection.
wide and consist of a sandwich of field oxide, metal,
CVD
oxide and nitride.
The devices are released isotropically with XeF2 or
anisotropically with EDP.
For the strain analysis of the triple CIC structure, only
half of the device is modeled with ANSYS in order
to
save the simulation time and memory usage. The
ANSYS simulation result of the strain component
parallel to the
arms
of a half triple CIC structure under a
static deflection into the paper is shown in
The
external magnetic field is 0.14
T
and the current flowing
inside the CIC is 10
mA.
The Lorentz forces are applied
to the corresponding segments at the ends of the
cantilevers in the
form
of uniform pressures over the
metal traces.
X
Fig. 2 Triple Cantilever-In-Cantilever plan view and
arrangement of metal layers
The distribution of the strain
on
the bottom surface of
the structure in the direction parallel
to
the arms (Adx) is
shown in different grayscale in
The maximum
strain (absolute value) occurs at the clamped end
of
the
outer arm, and has a value of about 0.27~10".The length
of the high strain region is about one third the length of
the
arm.
The negative sign in the figure indicates the
compression of the region. It is the best location to place
the piezoresistors for static deflections.
The polysilicon films are placed in the outer
arms,
about one third the length of the arm, and aligned
parallel to the arms as shown in
The thickness of
the
arm
is approximately 3.8 pm when the field oxide is
present, and the polysilicon layer is located 1.5 pm
below the neutral plane. Without the field oxide layer, as
Fig. 1 SEM picture of a group of CIC devices
3.
Strain Analysis
of
the CIC structures
A
plan view of a triple CIC structure is shown in
The actuation of the device is produced by Lorentz
forces arising from the interaction between an external
magnetic field
(B
)
and the current (I,) flowing in the
cantilever beams [8].
The central cantilever is 240 pm
x
260 pm and has an
isolated metal layer which can be used as a mirror for
optical applications of the device as well as detect the
angular deflection. The cantilever arms are about 15 pm
-
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 some of our devices have been fabricated, the thickness
is
3.1
ym
and the polysilicon is
1.8
pm below the neutral
plane.
simulation results of the applied strain. According to
(1)
and the simulation result in Fig.
3,
the piezoresistance
change of the polysilicon at
10
mA
actuation current can
be approximated by:
AR
R
C.Ac
= 25~0.27~10-~
=6.85~10-'
(2)
-=
The experimental result will
be
smaller than this due
to the extra polysilicon used for the connection close to
the left end of the
arms.
Fig.
5
illustrates the resistance change of the
piezoresistor versus the actuation current after
eliminating the thermal effects [9]. The experimental
data are least-squares
fitted
by straight lines.
The room temperature resistance of the piezoresistor
is 69
w2.
From Fig. 5, we can
see
that the resistance
change of the polysilicon within the current range from
0
to
40
mA
is linear. However, it is only about 0.06% at 10
mA,
which agrees with the prediction calculated by (2).
Fig. 3 The
ANSYS
result of strain component parallel to
the
arms
of a half triple CIC structure
L
I
0.2
0.1
top
view
E
L
'
maal
B
I
0
.o
si& view
-0.1
Fig.
4
The placement
of
polysilicon inside the CIC outer
arms
so
-0.2
10
20
40
I
5
Resistance variation
of
piezoresistor with thermal
effects eliminated
4. Static Deflection Detection Analysis
In
our
experiment, the static deflection of the CIC was
measured with the polysilicon piezoresistor.
A
DC
current is supplied to the aluminum leads to actuate the
device. Another DC current
from
a constant current
source (10=1.58 PA) is applied to the piezoresistor.
A
DMM
is used to measure the voltage across the
piezoresistor and hence its resistance.
The change of piezoresistance of the polysilicon in
the CIC structure under
5.
Dynamic Detection Analysis
We can consider the applied strain to the polysilicon
as two components:
cL
and
cT,
the strain component
parallel and perpendicular to the current flow. In the case
of the CIC vibrating at its resonant frequency, the strain
component
cL
is modulated by the vibration, while the
static deflection can be
calculated by
the gauge factor and
the
ANSYS
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 resonances, nevertheless the piezoresistance
measurement is still sensitive for determining the
resonant frequencies. As well, detecting an electrical
signal is significantly more convenient than observing a
laser beam on a curved screen. Hence the piezoresistor is
a useful component of the CIC device used as a sensor,
when the resonant frequency is the significant parameter
to
be measured.
change of strain component
ACT
is equal to zero.
Assuming small deflections of the CIC structure, the
induced strain is proportional to the vibration amplitude
A.cosWt:
Ct
=
CL.0
+
Ay,
=
CL.0
+
C.
A
*COS
Ut
(3)
A
is the amplitude of the angular deflection of the
device. The factor c depends on the actual structure and
the exact location of the piezoresistors. Substituting
A<L
into
(l),
the resistance change is then:
AR
R
-=G.A<,=G.c.A.cos
(4)
Ot
which means the change of the vibration amplitude can
be detected by the change of the piezoresistance. When
the polysilicon piezoresistors are driven with a DC
current
I,,,
the amplitude of the change in voltage AV
across the piezoresistor is:
AR
R
AV
=
AR
.
lo=
-.
R.
Io=
c.G.
A.
R .I~.cos
O
t
(5)
Thus the vibration of the CIC structure can be
determined by the voltage change across the polysilicon
piezoresistor. The results of such measurements are
shown in Fig.
6,
for a triple CIC structure.
Fig.
6
shows two resonances of the structure, at 7.0
kHz and
10.8
kHz,
as indicated by both the piezoresistor
voltage and also by the reflection of a laser beam off the
central mirror
[8].
The AC current flowing in the
aluminum leads was
3
mA
(RMS)
and the external
magnetic field was
0.14
T. There is no significant
difference in the resonance frequencies measured by the
two methods.
The relative response measured by the two methods,
however, shows different values, depending on which
resonant mode is excited. Such behaviour is expected, as
the relationship between the strain at the base
of
the
outer arm and the angular deflection of the central
platform differs with the different resonance modes of
the CIC, as discussed in
[
81.
For a specific device and resonance mode, however,
the utility of the piezoresistor signal is dependent on the
linearity between the signal and the deflection of the
center platform. We have investigated this linearity by
varying the actuating AC current through the structure
and measuring both the piezoresistive signal and the
platform deflection. The results are plotted in Fig.
7,
which shows a decidedly linear behaviour.
Although the relationship between the piezoresistor
voltage signal and the deflection is different at different
Fig.
6
Comparisonof the piezoresistance detection
method with the laser deflection measurement of a triple
CIC structure
:
2_i
_i/
b
-
>
t
-
1.00
0.0
10.0
20.0
30.0
40.0
theta (degrees)
Fig.
7
Comparisonof the piezoresistor signal and the
deflection
of
the center platform for the first resonance
mode
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sacrificial layer etch-stop technique",
Tech.
Dig.
Th
lnt.
Con$ Solid-state Sensors and Actuators (Transducers
6. Conclusions
'93),
1993, pp. 632-635.
We have investigated the use of a piezoresistor in
micromachined CIC devices using the polysilicon layer
from a standard CMOS IC fabrication process. Static
deflection and dynamic resonance have been
investigated. The piezoresistance signal is proportional
to the angular deflection of the cantilever. Our results
show that the polysilicon layer is an effective component
for measuring the structural deflection and the resonant
frequencies.
[6] R. Legtenberg,
S.
Bouwstra,
J.
H.
J.
Fluitman,
"Resonating microbridge
mass
flow sensor with low-
temperature glass-bonded cap wafer",
Sensors and
Actuators A,
vol. 25-27, 1991, pp. 723-727.
[7]
S.
Bouwstra, P. Kemna, R. Legtenberg, "Thermally
excited resonating membrane
mass
flow sensor",
Sensors
and Actuators
A,
vol.
20,
1989, pp. 213-223.
[8] B. Shen, W. Allegretto, Y. Ma, B. Yu, M. Hu and A.
M. Robinson, Cantilever Micromachined Structures in
CMOS Technology with Magnetic Actuation,
Sensors
and Materials,
Vol. 9, No. 6, 1997, pp. 347-362.
[9]
Y.
Ma, A. M. Robinson, R. P. W. Lawson, W.
Allegretto, and T. Zhou, "Static and dynamic
characterization of magnetically actuated CMOS-
micromachined
cantilever-in-cantilever
devices",
Can. J.
Acknowledgements
We gratefully acknowledge the services of the
Canadian Microelectronics Corporation and Mite1 in the
fabrication of the devices used in this work. We also
thank the Alberta Microelectronic Corp. for device
bonding assistance. We thank Martin Spacek for the help
with the data acquisition unit.
Phys,
76, 1998, pp. 747-758.
[lo]
D.
Strembicke,
F.
E. Vermeulen and A. M.
Robinson, "Humidity measurements using Resonating
CMOS microcantilever structures",
IEEE Can. Con.
Electrical and Computer Engineering,
Edmonton, to be
published in May, 1999.
[
111 K. Brown, W. Allegretto,
F. E.
Vermeulen, R.
P.
W.
Lawson, et al., "Cantilever-in-Cantilever Pressure
Sensors Fabricated in CMOS Technology",
IEEE Can.
Con. Electrical and Computer Engineering,
Edmonton,
to be published in May, 1999.
[12]
M. Spacek,
K.
Brown, Y. Ma,
A.
M. Robinson, R.
P.
W. Lawson, and W. Allegretto, "CMOS Cantilever
Microstructures as Thin Film Deposition Monitors",
IEEE Can. Con5 Electrical and Computer Engineering,
Edmonton, to be published in May, 1999.
[13] J. Y. W. Seto, "Piezoresistive properties of
polycrystalline silicon",
J.
Appl. Phy., vol. 47, 1976, pp.
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,
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