micromachines
Article
A CMOS MEMS Humidity Sensor Enhanced by
a Capacitive Coupling Structure
Jian-Qiu Huang *, Baoye Li and Wenhao Chen
Key Laboratory of MEMS of the Ministry of Education, Southeast University, Sipailou 2, Nanjing 210096, China;
220143654@seu.edu.cn (B.L.); 230149404@seu.edu.cn (W.C.)
* Correspondence: hjq@seu.edu.cn; Tel.: +86-25-8379-2632 (ext. 8825); Fax: +86-25-8379-2939
Academic Editors: Ching-Liang Dai and Nam-Trung Nguyen
Received: 14 December 2015; Accepted: 18 April 2016; Published: 26 April 2016
Abstract: A capacitive coupling structure is developed to improve the performances of a capacitive
complementary metal oxide semiconductor (CMOS) microelectromechanical system (MEMS)
humidity sensor. The humidity sensor was fabricated by a post-CMOS process. Silver nanowires
were dispersed onto the top of a conventional interdigitated capacitive structure to form a coupling
electrode. Unlike a conventional structure, a thinner sensitive layer was employed to increase the
coupling capacitance which dominated the sensitive capacitance of the humidity sensor. Not only
static properties but also dynamic properties were found to be better with the aid of coupling
capacitance. At 25 ˝ C, the sensitive capacitance was 11.3 pF, the sensitivity of the sensor was
measured to be 32.8 fF/%RH and the hysteresis was measured to be 1.0 %RH. Both a low temperature
coefficient and a fast response (10 s)/recovery time (17 s) were obtained.
Keywords: CMOS MEMS; capacitive humidity sensor; nanowires; coupling electrode
1. Introduction
The growing applications of humidity sensors in various areas such as agriculture, industry and
household have promoted research in the design and fabrication technologies of humidity sensors [1–4].
There are many kinds of humidity sensors including electrolytic humidity sensors [5], resistive
humidity sensors [6,7], optical humidity sensors [8,9], capacitive humidity sensors [10,11] and so on.
Among them, capacitive humidity sensors are widely used because of advantages of low power
consumption, good performances and long-term stability [4,12].
Capacitive humidity sensors are normally based on the humidity-dependent dielectric constant
of a sensitive layer. Electrode geometry design has been studied in depth to optimize the performances
of capacitive humidity sensors or other chemical sensors [13–16]. Generally, there are two kinds of
capacitive humidity sensors: One is based on an interdigitated structure and the other is based on
a parallel plate structure [17]. Interdigitated structures are widely used because of their simple
fabrication processes [18–22]. In chemical sensors, interdigitated structures are usually applied
because one side of these structures can be open to the ambient conditions. However, the sensitive
capacitance and the sensitivity of an interdigitated capacitive structure are usually small [20–22].
Although finer geometry of the electrodes means higher sensitivity, small finger sizes are limited
by the lithography process. As a result, much effort has been made to improve the structure of
capacitive interdigitated sensors. In 2010, Lazarus et al. introduced a multi-stacked metal structure to
enhance the sensitivity of an interdigitated humidity sensor [23]. However, the fabrication process
was complex. In 2012, Kim et al. developed an interdigitated structure with increased height [24].
The sensitivity of the humidity sensor was enhanced but a thick sensitive layer was employed. It is
well known that the sensitivity increases with the thickness of a sensitive layer while the hysteresis
and the response/recovery time rise as well [13,25]. In our previous work, a poly-silicon heater was
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used to reduce both the hysteresis and the response/recovery time of a capacitive humidity sensor [26].
However, greater power consumption was required. To solve these problems, in this paper a capacitive
coupling method was applied to improve the performances of an interdigitated capacitive humidity
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sensor. As aMicromachines
result, both
static and dynamic properties were improved.
sensor [26]. However, greater power consumption was required. To solve these problems, in this
2. Structurepaper
and aOperating
Principle
capacitive coupling
method was applied to improve the performances of an interdigitated
capacitive humidity sensor. As a result, both static and dynamic properties were improved.
The humidity sensor with a conventional structure and an improved structure are shown in
Figure 1. A2. coupling
electrode
employed in the improved structure to enhance the sensitive
Structure and
OperatingisPrinciple
capacitance as well
as the sensitivity.
coupling
electrode
is improved
made ofstructure
silver nanowire
networks
The humidity
sensor with aThe
conventional
structure
and an
are shown in
so that a grid
structure
is obtained
to allow
the penetration
of water
molecules.
Although
conductive
Figure
1. A coupling
electrode
is employed
in the improved
structure
to enhance
the sensitive
capacitance
as well
as the
The coupling
electrode
is made of
nanowire
nano-particles
have been
used
in sensitivity.
an interdigitated
resistive
structure
tosilver
enhance
the networks
performance of
so that a grid structure is obtained to allow the penetration of water molecules. Although
a gas sensor [27], it is not practical for an interdigitated capacitive structure. In an interdigitated
conductive nano-particles have been used in an interdigitated resistive structure to enhance the
resistive structure,
conductive
nano-particles
were employed
as dispersed
to reduce
performance
of a gas sensor
[27], it is not practical
for an interdigitated
capacitive electrodes
structure. In an
electrical losses
in
the
sensitive
material.
However,
in
this
paper
the
top
electrode
should
be
continuous
interdigitated resistive structure, conductive nano-particles were employed as dispersed electrodes
reduce
electricalcapacitance
losses in the sensitive
material.
However,
in this paper
the top electrode
should
to maximizetothe
coupling
between
the two
interdigitated
electrodes.
So that
nanowires
be continuous to maximize the coupling capacitance between the two interdigitated electrodes. So
instead of nano-particles are used to form a continuous coupling electrode.
that nanowires instead of nano-particles are used to form a continuous coupling electrode.
capacitive
humidity sensor:
conventional
structure,
Figure 1.of
Sketch
of an interdigitated
Figure 1. Sketch
an interdigitated
capacitive
humidity
sensor:(a) (a)
conventional
structure;
(b) capacitive coupling structure.
(b) capacitive coupling structure.
Unlike a conventional interdigitated structure, a thinner sensitive layer is preferred to increase
the coupling capacitance
and the
sensitivity aofthinner
the sensor.
Considering
of to
theincrease
Unlike both
a conventional
interdigitated
structure,
sensitive
layertheislimitation
preferred
leakage current a 0.1–0.2 μm thick sensitive layer can be used which is 10–20 times thinner than that
both the coupling capacitance and the sensitivity of the sensor. Considering the limitation of the
in a conventional interdigitated structure. However, in the improved structure, an additional
leakage current
a 0.1–0.2
thick sensitive
can
be used
which
is 10–20 oftimes
thinnerlayer
than that in
nanowire
layer µm
is deposited
on the top,layer
so it is
porous
and the
total thickness
the nanowire
and interdigitated
the sensitive layerstructure.
is still thinner.
Therefore,
and structure,
the response/recovery
time are
a conventional
However,
inthe
thehysteresis
improved
an additional
nanowire
reduced.on
Actually,
the capacitive
couplingand
structure
is a combination
interdigitated
layer is deposited
the top,
so it is porous
the total
thicknessofofanthe
nanowirestructure
layer and the
and a parallel plate structure. In a conventional parallel plate structure, the connection between the
sensitive layer is still thinner. Therefore, the hysteresis and the response/recovery time are reduced.
top electrode and its corresponding pad should be carefully designed to avoid step coverage
Actually, theproblems
capacitive
coupling
of an
interdigitated
structure
and a parallel
over
different structure
materials. isAsa combination
to the capacitive
coupling
structure the
top electrode
(nanowire
networks in thisparallel
paper) isplate
not connected
outside
it is just a between
floating electrode
is
plate structure.
In a conventional
structure,
the and
connection
the topwhich
electrode
and
more robust
a conventional
parallel
plate structure.
its corresponding
padthan
should
be carefully
designed
to avoid step coverage problems over different
Figure 2 shows the equivalent circuit of the capacitive coupling structure. Where Cf is the
materials. As to the capacitive coupling structure the top electrode (nanowire networks in this paper)
capacitance between adjacent fingers, Rc is the parasitic resistance of the coupling electrode between
is not connected
outside
isthe
just
a floating
electrode
which
is more
thanelectrode.
a conventional
adjacent
fingers and
and Cit
c is
coupling
capacitance
between
each finger
androbust
the coupling
parasitic capacitance between the interdigitated electrode and the substrate are not included in
parallel plateThe
structure.
model because
it can be eliminated
connecting
the substrate
to the
ground [28].
With aC is the
Figure this
2 shows
the equivalent
circuit ofbythe
capacitive
coupling
structure.
Where
f
thinner sensitive layer, Cc is usually much larger than Cf so that the sensitive capacitance is
capacitance dominated
between by
adjacent
fingers,
R
is
the
parasitic
resistance
of
the
coupling
electrode
between
c
the coupling capacitance.
adjacent fingers and Cc is the coupling capacitance between each finger and the coupling electrode.
The parasitic capacitance between the interdigitated electrode and the substrate are not included in
this model because it can be eliminated by connecting the substrate to the ground [28]. With a thinner
sensitive layer, Cc is usually much larger than Cf so that the sensitive capacitance is dominated by the
coupling capacitance.
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Figure2.2. Equivalent
circuit
of an interdigitated
capacitive humidity
sensor
with a nanowire
Figure
Equivalent
circuit
of an interdigitated
capacitive
humidity
sensorcoupling
with aelectrode.
nanowire
coupling electrode.
3. Fabrication Process
Figure
Equivalentsensor
circuit ofwith
an interdigitated
capacitive
humidity
sensorwas
with afabricated
nanowire coupling
electrode.
The 2.humidity
a capacitive
coupling
structure
by a post
CMOS
3. Fabrication
Process
process. As shown in Figure 3a, standard 3-μm 1-poly-1-metal CMOS process was used to construct
3. Fabrication
The
humidityProcess
sensor
with
a capacitive
coupling
fabricated
a post
CMOS
the interdigitated
structure
as described
in our
previousstructure
work [12].was
First,
a 0.045-μmbythick
silicon
process.
As
in Figure
3a,
standard
3-µm
1-poly-1-metal
CMOS
process
was
construct
dioxide
(SiO
2) layer
was thermal
on a coupling
(100)
p-type
silicon
(Si) wafer.
Then
thick
Theshown
humidity
sensor
with
agrown
capacitive
structure
was
fabricated
bya 0.45-μm
aused
postto
CMOS
poly-silicon
(poly-Si)
layeraswas
deposited
by
low pressure
chemical
vaporwas
process. As shown
in Figure
3a,
standard
CMOS
used to (LPCVD)
construct
the interdigitated
structure
described
in3-μm
oura1-poly-1-metal
previous
work
[12].process
First,
adeposition
0.045-µm
thick silicon
process.
The poly-Si
layer was
patternedintoour
form
an integrated
heater
(theaheater
wasthick
not used
in
the (SiO
interdigitated
structure
as described
previous
work [12].
First,
0.045-μm
silicon
dioxide
2 ) layer was thermal grown on a (100) p-type silicon (Si) wafer. Then a 0.45-µm thick
this
paper).
Next,
an
LPCVD
insulating
layer
of
SiO
2p-type
with
a
thickness
of
0.55
μm
was
deposited
and
dioxide
(SiO
2) layer
was
thermal
grown
on
a
(100)
silicon
(Si)
wafer.
Then
a
0.45-μm
thick
poly-silicon (poly-Si) layer was deposited by a low pressure chemical vapor deposition (LPCVD)
etched
to form
contactlayer
holes.was
Afterwards,
aluminumchemical
layer was
useddeposition
to fabricate(LPCVD)
a pair of
poly-silicon
(poly-Si)
depositeda sputtered
by a low pressure
vapor
process. The poly-Si layer was patterned to form an integrated heater (the heater was not used in
interdigitated
electrodes.
thickness to
of form
the electrodes
was
1.2 μm.
Finally,was
a 1-μm
thick
process. The poly-Si
layer The
was patterned
an integrated
heater
(the heater
not used
in
this paper). Next, an LPCVD insulating layer of SiO2 with a thickness of 0.55 µm was deposited and
passivation
deposited
and viaslayer
wereofopened
in the
pad regions.
this paper). layer
Next, was
an LPCVD
insulating
SiO2 with
a thickness
of 0.55 μm was deposited and
etched
to form
contact
holes.
Afterwards,
aluminumlayer
layer
was
used
to fabricate
a of
pair of
etched
to form
contact
holes.
Afterwards,aasputtered
sputtered aluminum
was
used
to fabricate
a pair
interdigitated
electrodes.
The
thickness
of
the
electrodes
was
1.2
µm.
Finally,
a
1-µm
thick
passivation
interdigitated electrodes. The thickness of the electrodes was 1.2 μm. Finally, a 1-μm thick
layerpassivation
was deposited
wereand
opened
in the
pad regions.
layer and
was vias
deposited
vias were
opened
in the pad regions.
Figure 3. Fabrication process of the humidity sensor: (a) interdigitated structure fabricated by a
complementary metal oxide semiconductor (CMOS) process, (b) structure after deposition of a
polyimide film (c) structure after deposition of a silver nanowire electrode.
Figure
3. Fabricationprocess
processof
of the
the humidity
humidity sensor:
interdigitated
structure
fabricated
by a by
Figure
3. Fabrication
sensor:(a)(a)
interdigitated
structure
fabricated
Following
themetal
CMOS
process,
a two-step
post process,
process
was
First,
a deposition
0.1-μm
complementary
metal
oxide
semiconductor
(CMOS)
(b)
structure
after
deposition
of athick
a complementary
oxide
semiconductor
(CMOS)
process;
(b)applied.
structure
after
of
polyimide
was
spun
onto
the
wafer
to
act
as
a
humidity
sensitive
material
(Figure
3b). A
polyimide
film
(c)
structure
after
deposition
of
a
silver
nanowire
electrode.
a polyimide film (c) structure after deposition of a silver nanowire electrode.
lithographic and dry etching process was used to remove the polyimide in the pad regions (which
was not
drawn the
in the
figure).
The polyimide
waspost
cured
under was
nitrogen
purge
as follows:
ramp
to
Following
CMOS
process,
a two-step
process
applied.
First,
a 0.1-μm
thick
Following
the
CMOS
process,
a
two-step
post
process
was
applied.
First,
a
0.1-µm
thick
polyimide
150
in
half
an
hour;
hold
at
150
°C
for
1
h;
ramp
to
250
in
half
an
hour
;
hold
at
250
°C
for
1
h;
ramp
polyimide was spun onto the wafer to act as a humidity sensitive material (Figure 3b). A
was spun
onto the
wafer
to act as
a humidity
sensitive
material
(Figure in
3b).
lithographic
and dry
lithographic
and
dry etching
process
was used
to remove
the polyimide
theA
pad
regions (which
wasprocess
not drawn
the figure).
The the
polyimide
was cured
nitrogen(which
purge aswas
follows:
ramp toin the
etching
wasinused
to remove
polyimide
in theunder
pad regions
not drawn
150The
in half
an hour; was
hold cured
at 150 °C
for 1nitrogen
h; ramp to
250 inashalf
an hour
; holdtoat150
250in
°Chalf
for 1anh;hour;
ramp hold
figure).
polyimide
under
purge
follows:
ramp
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˝ C for 1 h; ramp to 350 ˝ C in half an hour;
at
C for
1 h; an
ramp
to ;250
inat
half
at 250
to 150
350 ˝°C
in half
hour
hold
250an°Chour;
for 1hold
h; cool
to 20
°C during ~3 h. At last, silver nanowires
˝
˝
hold
at
250
C
for
1
h;
cool
to
20
C
during
~3
h.
At
last,
silver
nanowires
were dispersed
ontoInc.,
the
were dispersed onto the polyimide with a SonoPlot GIX Microplotter
instrument
(SonoPlot,
polyimide
SonoPlot
GIXthe
Microplotter
instrument
(SonoPlot,
Inc., Middleton,
WI, USA) toof
form
Middleton,with
WI, aUSA)
to form
coupling electrode
as shown
in Figure
3c. The concentration
the
the
coupling
electrode
as shown
Figure
3c.approximately
The concentration
of the and
silverit nanowire
inkatused
silver
nanowire
ink used
in thisinwork
was
5 mg/mL
was heated
120 in
°Cthis
for
˝ C for 5 min after the dispersion process.
work
was
approximately
5
mg/mL
and
it
was
heated
at
120
5 min after
the dispersion process.
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The
scanning electron
electronmicroscope
microscope(SEM)
(SEM)image
image
humidity
sensor
before
the
The scanning
of of
thethe
humidity
sensor
before
and and
afterafter
the post
tois350
°C
in in
halfFigure
aninhour
hold4.at Figure
250
°C for
h; cool to
°Cinterdigitated
during
h. At last,
silverelectrodes
nanowires
post
process
is shown
Figure
4a1shows
details
of the~3
interdigitated
before
process
shown
4.;Figure
4a shows
details
of20the
electrodes
before the
post
dispersed
thewidth
polyimide
with
a SonoPlot
GIX Microplotter
instrument
(SonoPlot,
Inc.,
the
postwere
process.
In onto
thethe
structure,
thelength
width
of
each
finger
5 µm
and
3200 µm,
process.
In
the
structure,
and
ofand
eachlength
finger
were
5 μm
and were
3200 μm,
respectively.
The
Middleton, WI, USA) to form the coupling electrode as shown in Figure 3c. The concentration of the
respectively.
The spacing
adjacent
was
3 µm.
The
total area
of the sensitive
capacitor
spacing of adjacent
fingersofwas
3 μm. fingers
The total
area
of the
sensitive
capacitor
was 509 μm
× 3200was
μm.
silver nanowire ink used in this work was approximately 5 mg/mL and it was heated at 120 °C for
509
µm
ˆ
3200
µm.
In
Figure
4b,
polyimide
was
spun
on
the
interdigitated
electrodes
and
silver
In Figure
4b,
polyimide
wasprocess.
spun on the interdigitated electrodes and silver nanowires were
5 min
after
the dispersion
nanowires
were
dispersed
on microscope
the top
of (SEM)
polyimide.
average
diameter
length
the silver
dispersed on
top ofelectron
polyimide.
The
average
diameter
and length
of
theand
silver
nanowires
were
Thethe
scanning
image
ofThe
the humidity
sensor
before
and
after
theof
post
nanowires
were
40respectively.
nminand
20 µm,
respectively.
process
is shown
Figure
4. Figure
4a shows details of the interdigitated electrodes before the post
40 nm and
20 μm,
process. In the structure, the width and length of each finger were 5 μm and 3200 μm, respectively. The
spacing of adjacent fingers was 3 μm. The total area of the sensitive capacitor was 509 μm × 3200 μm.
In Figure 4b, polyimide was spun on the interdigitated electrodes and silver nanowires were
dispersed on the top of polyimide. The average diameter and length of the silver nanowires were
40 nm and 20 μm, respectively.
Figure 4.
4. Scanning
fabricated structure
before and
and after
after
Figure
Scanning electron
electron microscope
microscope (SEM)
(SEM) image
image of
of the
the fabricated
structure before
post-microelectromechanical system
post-microelectromechanical
system (MEMS)
(MEMS) process:
process: (a)
(a)interdigitated
interdigitatedelectrodes
electrodesfabricated
fabricatedbybya
atypical
typicalCMOS
CMOSprocess,
process;(b)
(b)silver
silvernanowires
nanowiresdeposited
depositedon
onthe
thetop
topofofthe
thestructure.
structure.
4. Results
4.
Results
Figure 4. Scanning electron microscope (SEM) image of the fabricated structure before and after
post-microelectromechanical system (MEMS) process: (a) interdigitated electrodes fabricated by a
typical CMOS process, (b) silver nanowires deposited on the top of the structure.
4. Results
4.1.
Measurements
4.1. Static
Static
Measurements
Static
measurements
performed
using
a humidity
calibrator
(Beijing
Great Smart
Static
humidity
measurementswere
were
performed
using
a humidity
calibrator
(Beijing
Great
4.1. humidity
Static
Measurements
New
Co., Ltd.,Co.,
Beijing,
China)
asChina)
shown as
in Figure
5.inThe
humidity
calibrator
consisted
of
SmartTechnology
NewStatic
Technology
Ltd.,
Beijing,
shown
Figure
5.
The
humidity
calibrator
humidity measurements were performed using a humidity calibrator (Beijing Great
aconsisted
test camber,
a
thermostat
bath
and
a
dual-pressure
humidity
generator.
The
temperature
inside
the
of New
a test
camber,Co.,
a Ltd.,
thermostat
bath and
a dual-pressure
generator. The
Smart
Technology
Beijing, China)
as shown
in Figure 5. Thehumidity
humidity calibrator
test
chamber
was
by
the
bathand
andaby
the
humidity
atmosphere
wasthe
provided
by
temperature
inside
chamber
was stabilized
the
thermostat
bathgenerator.
and
humidity
consisted
ofstabilized
a the
test test
camber,
a thermostat
thermostat
bath
dual-pressure
humidity
The
temperature
inside theby
test
was of
stabilized
by the
thermostat
bath
the were
humidity
the
dual-pressure
humidity
generator.
Both
them
were
controlled
by
a computer.
An LCR
meter
atmosphere
was provided
thechamber
dual-pressure
humidity
generator.
Both
ofand
them
controlled
atmosphere
was
provided
the
dual-pressure
humidity
generator.
Bothused
of them
were controlled
(Changzhou
Applent
Technology
Co.,
Ltd., Changzhou,
China) was
to
measure
the
sensitive
by a computer.
An
LCR
meterby(Changzhou
Applent
Technology
Co.,
Ltd.,
Changzhou,
China)
was
by a computer. An LCR meter (Changzhou Applent Technology Co., Ltd., Changzhou, China) was
capacitance
of the the
humidity
sensor.
The test of
frequency
was 100
kHz and
results were
used to measure
sensitive
capacitance
the humidity
sensor.
Thethe
testtest
frequency
was recorded
100 kHz
used to measure the sensitive capacitance of the humidity sensor. The test frequency was 100 kHz
by
andthe
thecomputer.
testthe
results
werewere
recorded
byby
the
and
test results
recorded
thecomputer.
computer.
Figure 5. Test system of the humidity sensor for static measurements.
Figure 5. Test system of the humidity sensor for static measurements.
Figure 5. Test system of the humidity sensor for static measurements.
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The humidity sensor with a capacitive coupling structure was calibrated from 15 to 90 %RH
The humidity sensor with a capacitive coupling structure was calibrated from 15 to 90 %RH at
at 5, 15, 25 and 35 ˝ C, respectively. Both an adsorption process and a desorption process were carried
5, 15, 25 and 35 °C, respectively. Both an adsorption process and a desorption process were carried
out. The
response
curves
shownininFigure
Figure
The
sensor
exhibited
out. humidity
The humidity
response
curvesofofthe
thesensor
sensor are
are shown
6. 6.
The
sensor
exhibited
goodgood
sensitivity
and
linearity.
The
temperature
coefficient
of
the
sensor
was
not
significant
over
a
range of
sensitivity and linearity. The temperature coefficient of the sensor was not significant over a range
˝ C. The hysteresis of the sensor reduced with the temperature because of
temperatures
from
5
to
35
of temperatures from 5 to 35 °C. The hysteresis of the sensor reduced with
temperature because
the promotion
of diffusion.
It is clear
that water
molecules
cancan
escape
from
thethe
sensitive
of the promotion
of diffusion.
It is clear
that water
molecules
escape
from
sensitivematerial
materialmore
with
higher temperature.
The maximum
hysteresis
3.5 occurred
%RH occurred
75 %RH
easilymore
witheasily
higher
temperature.
The maximum
hysteresis
was 3.5was
%RH
at 75 at
%RH
and 5 ˝ C.
5 °C. Table
1 shows
details
of staticof
properties
of the humidity
Tableand
1 shows
details
of static
properties
the humidity
sensor. sensor.
Figure
6. Output
of the
humidity
ofrelative
relativehumidity
humidity
at different
temperatures.
Figure
6. Output
of the
humiditysensor
sensoras
asaa function
function of
at different
temperatures.
Table
1. Static
properties
thehumidity
humidity sensor
sensor with
coupling
structure.
Table
1. Static
properties
ofofthe
withaacapacitive
capacitive
coupling
structure.
Property
Property
Sensitivity (fF/%RH)
Sensitivity
(fF/%RH)
Linearity
Linearity
Hysteresis
(%RH))
Hysteresis (%RH)
5 °C
5 ˝C
33.9
33.9
1.5%
1.5%
3.5
3.5
15 °C
15 ˝ C
32.7
32.7
3.6%
3.6%
1.9
1.9
25 °C
25 ˝ C
32.8
32.8
2.7%
2.7%
1.0
1.0
35 °C
35 ˝ C
33.9
33.9
0.4%
0.4%1.0
1.0
4.2. Dynamic Measurements
4.2. Dynamic
Measurements
As description
in Figure 7, an experiment set up was constructed to evaluate the dynamic
properties
of the humidity
a response
andconstructed
a recovery process
were performed
As
description
in Figuresensor.
7, an Both
experiment
setprocess
up was
to evaluate
the dynamic
at 25 °C. Take the measurement of response time, for example. First, the relative humidity in the
properties of the humidity sensor. Both a response process and a recovery process were performed
chamber was set to be 15 %RH. A humidity sensor was fixed in the test chamber and allowed to
at 25 ˝ C. Take the measurement of response time, for example. First, the relative humidity in the
stabilize (Figure 7a). Then the cap inside the test chamber was pulled upwards by the connecting rod
chamber
was in
setFigure
to be7b.
15The
%RH.
A humidity
sensor
in atmosphere.
the test chamber
and
allowed to
as shown
humidity
sensor was
sealedwas
in a fixed
15 %RH
Next, the
humidity
stabilize
(Figure
7a).
Then
the
cap
inside
the
test
chamber
was
pulled
upwards
by
the
connecting
outside the cap increased to 90 %RH and kept for 5 min. Finally, the cap was pushed downwards rod
as shown
in Figure
7b.an
The
humidity
sensor
was sealed
in a(15–90
15 %RH
atmosphere.
Next,
the humidity
quickly
to achieve
abrupt
increase
of relative
humidity
%RH).
The sensitive
capacitance
wasthe
measured
by an LCR
and and
recorded
a computer.
outside
cap increased
to meter
90 %RH
keptby
for
5 min. Finally, the cap was pushed downwards
a response
curve
and a recovery
curve
of the humidity
sensor The
are illustrated
in Figure 8. was
quickly toBoth
achieve
an abrupt
increase
of relative
humidity
(15–90 %RH).
sensitive capacitance
The response
timemeter
is defined
as the time
the sensitive capacitance to rise to 90% of its final
measured
by an LCR
and recorded
by for
a computer.
steady-state
value curve
and theand
recovery
time is curve
definedofasthe
thehumidity
time for thesensor
sensitive
to
Both
a response
a recovery
arecapacitance
illustratedtoinfall
Figure
8.
The response time is defined as the time for the sensitive capacitance to rise to 90% of its final
steady-state value and the recovery time is defined as the time for the sensitive capacitance to fall
Micromachines 2016, 7, 74
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10% of
its initial2016,
vale.
As described in Figure 8, the response time of the improved structure 6was
Micromachines
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of 9 10 s
and the corresponding recovery time was 17 s.
10%
initialvale.
vale.As
Asdescribed
described in
time
of of
thethe
improved
structure
was 10
s 10 s
to 10%
of of
itsits
initial
in Figure
Figure8,8,the
theresponse
response
time
improved
structure
was
and
the
corresponding
recovery
time
was
17
s.
and the corresponding recovery time was 17 s.
Figure
7. Test
system
of the
humiditysensor
sensor for
for dynamic
dynamic measurements.
(a) (a)
TheThe
firstfirst
step step
of the
Figure
7. Test
system
of the
humidity
measurements.
of the
measurement;
(b) The
step
the
measurement;
(c)
third
thethe
measurement.
Figure
7. Test (b)
system
of second
the humidity
sensor
for dynamic
measurements.
(a)
The first step of the
measurement;
The
second
step
ofof
the
measurement;
(c) The
The
thirdstep
stepofof
measurement.
measurement; (b) The second step of the measurement; (c) The third step of the measurement.
Figure 8. Dynamic test results of the sensor at 25 °C.
5. Discussion
Figure 8. Dynamic test results of the sensor at 25 ˝°C.
Figure
8. Dynamic
results of the
sensor at 25 capacitive
C.
As described in our
previous
work, atest
conventional
interdigitated
humidity sensor
suffered from low sensitivity (1.46 fF/%RH at 25 °C) and significant hysteresis. An on-chip heater
5. Discussion
was introduced to reduce the hysteresis of the sensor (decreased to 1 %RH at 25 °C) but it had little
5. Discussion
effect
on the sensitivity
[24]. To evaluate
sensitivity of different
structures,
the output
should be
As described
in our previous
work, the
a conventional
interdigitated
capacitive
humidity
sensor
As
describedby
inaour
previous
a conventional
interdigitated
capacitive humidity sensor
characterized
relative
changework,
of capacitance
ΔC/Cdry, where
C
dry is the sensitive capacitance in a
suffered from low sensitivity (1.46 fF/%RH at 25 °C) and significant hysteresis. An on-chip heater
suffered from low sensitivity (1.46 fF/%RH at 25 ˝ C) and significant hysteresis. An on-chip heater
was introduced to reduce the hysteresis of the sensor (decreased to 1 %RH at 25 °C) but it had little
was introduced to reduce the hysteresis of the sensor (decreased to 1 %RH at 25 ˝ C) but it had little
effect on the sensitivity [24]. To evaluate the sensitivity of different structures, the output should be
effect on the sensitivity [24]. To evaluate the sensitivity of different structures, the output should be
characterized by a relative change of capacitance ΔC/Cdry, where Cdry is the sensitive capacitance in a
characterized by a relative change of capacitance ∆C/Cdry , where Cdry is the sensitive capacitance
Micromachines 2016, 7, 74
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dry
air atmosphere. In a conventional interdigitated structure, Cdry is about 1.4 pF and in a
in a dry air atmosphere. In a conventional interdigitated structure, Cdry is about 1.4 pF and in
capacitive coupling structure, Cdry is about 11.3 pF. Figure 9 shows the relationships between
a capacitive coupling structure, Cdry is about 11.3 pF. Figure 9 shows the relationships between relative
relative change of capacitance and relative humidity of both structures ˝at 25 °C. In percentage terms,
change of capacitance and relative humidity of both structures at 25 C. In percentage terms, the
the sensitivity increases from 0.1 %/%RH to 0.29 %/%RH with the aid of a capacitive coupling effect.
sensitivity increases from 0.1 %/%RH to 0.29 %/%RH with the aid of a capacitive coupling effect.
In other words, the sensitivity of the capacitive coupling structure is 2.9 times higher than that of a
In other words, the sensitivity of the capacitive coupling structure is 2.9 times higher than that of
conventional structure. Additionally, the response time of a conventional structure is 38 s (without
a conventional structure. Additionally, the response time of a conventional structure is 38 s (without
self-heating) and 25 s (with self-heating). The response time of a capacitive coupling structure is 10 s,
self-heating) and 25 s (with self-heating). The response time of a capacitive coupling structure is 10 s,
which is about 3.8 times faster than that of a conventional structure and 2.5 times faster than that of a
which is about 3.8 times faster than that of a conventional structure and 2.5 times faster than that of
self-heating structure.
a self-heating structure.
Figure 9. Relationship between relative change of capacitance and relative humidity at 25 ˝ C.
Figure 9. Relationship between relative change of capacitance and relative humidity at 25 °C.
The capacitive
capacitive coupling
coupling structure
structure is
is compared
metal structure
structure and
and
The
compared with
with both
both aa multi-stacked
multi-stacked metal
an increased
as as
shown
in Table
2. For2.allFor
of the
the sensitivity
is enhanced
an
increasedheight
heightstructure
structure
shown
in Table
all structures
of the structures
the sensitivity
is
and the capacitive
structure
is structure
more efficient
thanefficient
the otherthan
twothe
structures.
Both
the capacitive
enhanced
and the coupling
capacitive
coupling
is more
other two
structures.
Both
coupling
structure
and the
multi-stacked
metal structure
exhibit
better linearity
than linearity
the increased
the
capacitive
coupling
structure
and the multi-stacked
metal
structure
exhibit better
than
height
structure.
Thestructure.
hysteresisThe
of the
increased
structure
is larger
than that
of thethan
capacitive
the
increased
height
hysteresis
of height
the increased
height
structure
is larger
that of
coupling
structure
because
the former
one
a thicker
sensitive
layer.
The recovery
time
of the
the
capacitive
coupling
structure
because
theuses
former
one uses
a thicker
sensitive
layer. The
recovery
multi-stacked
metal
structure
is
slower
than
that
of
the
capacitive
coupling
structure.
Both
the
time of the multi-stacked metal structure is slower than that of the capacitive coupling structure.
capacitive
coupling structure
the multi-stacked
metal structure
are structure
fabricatedare
by afabricated
CMOS MEMS
Both
the capacitive
couplingand
structure
and the multi-stacked
metal
by a
process
but
the
latter
one
is
much
more
complex.
CMOS MEMS process but the latter one is much more complex.
Table 2.
Comparison of
of different
different optimized
Table
2. Comparison
optimized structures.
structures.
Structure
Structure
Improvement
Improvement
of Sensitivity
Capacitive coupling structure
Multi-stacked
structure
[23]
Capacitivemetal
coupling
structure
Increased height structure [24]
Multi-stacked metal structure [23]
Increased height structure [24]
6. Conclusions
of 2.9
Sensitivity
times
2 times
2.9
times
2.3 times
2 times
2.3 times
Linearity
Hysteresis
Linearity
Hysteresis
good
poor
Not reported
2.87 %RH
good
good
good
poor
1.0 %RH
Not%RH
reported
1.0
2.87 %RH
Recovery
Recovery
Time
Time
17 s
>20
17 ss
Not reported
>20 s
Not reported
6. Conclusions
A capacitive coupling technique was used to improve the performances of an interdigitated
capacitive
structure
in a CMOS
MEMS
humidity
capacitance
of the sensor
A capacitive
coupling
technique
was
used to sensor.
improveThe
thesensitive
performances
of an interdigitated
was
enhanced
by
the
coupling
capacitance.
A
thinner
sensitive
layer
was
used
to
the
capacitive structure in a CMOS MEMS humidity sensor. The sensitive capacitance of theincrease
sensor was
coupling capacitance
and capacitance.
the sensitivity.
The humidity
wasused
fabricated
by a the
post-CMOS
enhanced
by the coupling
A thinner
sensitivesensor
layer was
to increase
coupling
process.
Both
static
and
dynamic
measurements
were
carried
out.
According
to
the
measurements,
the
capacitance and the sensitivity. The humidity sensor was fabricated by a post-CMOS process. Both
sensitivity
of
the
sensor
was
32.8
fF/%RH
(0.29
%/%RH
in
percentage
terms)
and
the
hysteresis
of
the
static and dynamic measurements were carried out. According to the measurements, the sensitivity
˝ C. The temperature coefficient of the sensor was not significant over a range
sensor
was 1 %RH
at 25fF/%RH
of
the sensor
was 32.8
(0.29 %/%RH in percentage terms) and the hysteresis of the sensor
was 1 %RH at 25 °C. The temperature coefficient of the sensor was not significant over a range of
Micromachines 2016, 7, 74
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of temperature from 5 to 35 ˝ C. The response time of the improved structure was 10 s and the recovery
time was 17 s at 25 ˝ C. With the aid of the capacitive coupling structure, the sensitivity increases
2.9 times and the response time reduces 3.8 times as compared to that of a conventional structure.
In the future work, the humidity sensor will be further improved by optimizing the thickness of films,
the concentration of the nanowire ink and the sizes of nanowires.
Acknowledgments: The project is supported by the National Natural Science Foundation of China under contract
No. 61302021.
Author Contributions: Jian-Qiu Huang designed the sensor and the test systems, and also prepared and approved
the manuscript. Baoye Li and Wenhao Chen carried out test experiments. All authors reviewed the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
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