sensors
Article
Planar Indium Tin Oxide Heater for Improved
Thermal Distribution for Metal Oxide
Micromachined Gas Sensors
M. Cihan Çakır 1,2, *, Deniz Çalışkan 1 , Bayram Bütün 1 and Ekmel Özbay 1,3
1
2
3
*
Nanotechnology Research Center, Bilkent University, Ankara 06800, Turkey;
dcaliskan@fen.bilkent.edu.tr (D.Ç.); bbtn@bilkent.edu.tr (B.B.); ozbay@bilkent.edu.tr (E.Ö.)
Department of Nanotechnology and Nanomedicine, Hacettepe University, Ankara 06800, Turkey
Department of Electrical and Electronics Engineering, Department of Physics, Bilkent University,
Ankara 06800, Turkey
Correspondence: ccakir@bilkent.edu.tr; Tel.: +90-312-290-3050
Academic Editors: Sang Sub Kim and Hyoun Woo Kim
Received: 18 July 2016; Accepted: 23 September 2016; Published: 29 September 2016
Abstract: Metal oxide gas sensors with integrated micro-hotplate structures are widely used in
the industry and they are still being investigated and developed. Metal oxide gas sensors have
the advantage of being sensitive to a wide range of organic and inorganic volatile compounds,
although they lack selectivity. To introduce selectivity, the operating temperature of a single sensor
is swept, and the measurements are fed to a discriminating algorithm. The efficiency of those
data processing methods strongly depends on temperature uniformity across the active area of the
sensor. To achieve this, hot plate structures with complex resistor geometries have been designed
and additional heat-spreading structures have been introduced. In this work we designed and
fabricated a metal oxide gas sensor integrated with a simple square planar indium tin oxide (ITO)
heating element, by using conventional micromachining and thin-film deposition techniques. Power
consumption–dependent surface temperature measurements were performed. A 420 ◦ C working
temperature was achieved at 120 mW power consumption. Temperature distribution uniformity
was measured and a 17 ◦ C difference between the hottest and the coldest points of the sensor at an
operating temperature of 290 ◦ C was achieved. Transient heat-up and cool-down cycle durations are
measured as 40 ms and 20 ms, respectively.
Keywords: Indium tin oxide; Metal oxide gas sensor; Micro hot-plate; SnO2 ; Heat distribution
1. Introduction
Metal oxides are widely used in gas sensing applications, such as chemo-resistive [1] and
optoelectronic sensors [2,3]. Metal oxide–resistive gas sensors are being investigated, developed,
and used due to their simplicity, production flexibility, low cost and sensitivity to a wide variety of
volatiles [1]. Due to increasing interest in those sensors, micro-hotplate structures are being developed
with enhanced device performance, decreased power consumption, and low cost [1,4]. Research
activities aimed at enhancing the sensitivity and selectivity as well as lowering the response time
and power consumption are still under active research. Those research studies present innovative
integrated micro-hotplate technologies based on Micro Electro Mechanical Systems (MEMS) to achieve
optimized thermal properties such as low power consumption and good temperature uniformity
across the active layer [4].
With the development of MEMS technologies in the 1990s, new generations of gas sensors with
integrated micro-hotplates were realized. Micromachining processes allow for the precise modification
of the thermal characteristics of those devices. The optimization of device geometry, membrane,
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and resistor materials affects the heat transfer mechanisms, resulting in the enhancement of the
thermal dispersion and Joule heating characteristics of the devices. Besides these enhancements,
the sensor’s heat capacity reduction, which is achieved by thermal mass reduction with the help of
micromachining methods, also reduces the power consumption and improves the response time of the
sensors [5,6]. By improving the response time, selectivity could be enhanced by rapid temperature
cycle measurements to produce additional information by getting the response signature as a variable
of temperature pulses [7–9].
Metal oxide gas sensors are based on the surface reactions between the target gas species and
the sensing metal oxide film. As a result of the surface reactions, gas molecules interact with the film
surface and dope it, the surface band bending alters and then the metal oxide layer resistivity changes.
Metal oxide sensors give a response to the wide variety of organic and inorganic gases. However,
this wide range has a disadvantage, which is nonselectivity. This disadvantage is eliminated once a
temperature-dependent measurement is used. When gas species are introduced to the film surface,
the total amount of surface reactions, i.e., the adsorbed and desorbed amount of gas molecules,
is correlated with the resistance change of the thin film, and it is temperature-dependent and specific
to the gas compound type. The base conductance of the sensing film can be defined as the conductance
in the absence of the target gas molecules at a given temperature, and it is also strongly dependent
on the temperature of the sensing film [6]. Different metal oxides react with the same analytes with
different efficiencies at different temperatures [6,10,11]. In other words, the absolute resistivity change
caused by the same concentration of the target gas can reach its peak value at different temperatures
for different metal oxide films. This information can be used to obtain more selective data extraction
from metal oxide gas sensors.
For a given temperature-dependent measurement, the nonuniformity of the surface temperature
distribution means that the total sensor signal will be an integration of signals obtained from metal
oxide sites at different temperatures. Larger nonuniformity in temperature means that the data will be
blurred and the peaks in the resistance change with temperature changes will be broadened. To get a
characteristic temperature-dependent sensor signal at a high resolution, the temperature distribution
on the active sensing film region has to be as uniform as possible [12–14].
To reach good temperature uniformity, power consumption, and working temperature goal
by using conventional metal and metal alloy resistor materials, the hotplates with heat-spreading
plates, micro-bridges, and resistors with complex geometries based on meanders were designed and
fabricated [4–6,14,15].
2. Design and Fabrication
In this work, a planar resistor as a heating element is introduced which can provide high
temperature uniformity across the active region of the thin-film metal oxide sensors. A thin sputtered
indium tin oxide (ITO) film is used as the resistor material, with simple planar resistor geometry. Silicon
nitride (Si3 N4 ) is used for both the membrane material and also as the passivation and insulation layers.
Conventional bulk silicon micromachining methods such as potassium hydroxide (KOH) etching are
used to obtain the micro-hotplate structures. In order to make a realistic thermal characterization, not
only a bare micro-hotplate structure is measured; we also added thermal mass on the hotplate structure
by depositing the contact metals for the sensing metal oxide thin film and the metal connection pads
by e-beam evaporation and the thin tin oxide (SnO2 ) metal oxide sensing layer by sputtering.
The structure of the metal oxide gas sensor is shown in Figure 1, which demonstrates the
cross-section of the gas sensor. As shown in Figure 1, the basic concept of the hotplate is the
planar-structured, continuous ITO resistor that lies underneath the active metal oxide layer, isolated
with a thin Si3 N4 layer.
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Figure 1. Cross-section of the gas sensor.
Figure 1. Cross-section of the gas sensor.
Sensors
on p-type,
double-side,
polished,
300-µm-thick
(100) silicon
(Si)silicon
substrates.
Sensors were
werefabricated
fabricated
on p-type,
double-side,
polished,
300-µm-thick
(100)
(Si)
As
the
first
step
of
the
fabrication,
1.8
µm
thermal
SiO
is
grown
on
both
sides
of
a
wafer
in a
2
substrates. As the first step of the fabrication, 1.8 µm thermal
SiO2 is grown on both sides of a wafer
tube-furnace
(MTI,(MTI,
Richmond,
CA, USA)
oxidation.
This layer
serves
an extra
in a tube-furnace
Richmond,
CA, using
USA) wet
using
wet oxidation.
This
layerasserves
as masking
an extra
layer
during
the
bulk
Si
etching
process
and
also
as
a
mechanical
protection
layer
for
the
fabrication
masking layer during the bulk Si etching process and also as a mechanical protection layer for the
steps
on the steps
backside
silicon substrate,
avoiding
scratches.
A 1-µm-thick
Si3 NA4 layer
is deposited
fabrication
on of
thethe
backside
of the silicon
substrate,
avoiding
scratches.
1-µm-thick
Si3N4
on
the
thermal
SiO
with
plasma-enhanced
chemical
vapor
deposition
(PECVD)
(Samco,
Kyoto,
Japan)
2
layer is deposited on the thermal SiO2 with plasma-enhanced chemical vapor deposition (PECVD)
to
form an
etch stop
layer
for theanwet
anisotropic
etching
process.
Si3 N4 Si
is etching
preferred
due to its
very
(Samco,
Kyoto,
Japan)
to form
etch
stop layerSifor
the wet
anisotropic
process.
Si3N
4 is
low
etch
rate
in
KOH
solution.
preferred due to its very low etch rate in KOH solution.
The
The thermal
thermal oxide
oxide on
on the
the front
front side
side of
of the
the wafer
wafer is
is removed
removed by
by reactive
reactive ion
ion etching
etching in
in an
an
inductively
coupled
plasma
reactor
(ICP-RIE)
(Samco,
Kyoto,
Japan)
using
fluorine
chemistry.
inductively coupled plasma reactor (ICP-RIE) (Samco, Kyoto, Japan) using fluorine chemistry. The
The
etching
parameters
W100
ICP,W100
bias power,
Pa chamber
and
etching
parameters
were were
75 W 75
ICP,
RF W
biasRFpower,
0.25 Pa 0.25
chamber
pressure,pressure,
and 30 sccm
30
sccm
CHF
process
gas
flow.
Then,
1-µm-thick
Si
N
is
grown
with
PECVD
as
a
membrane
3
3
4
CHF3 process gas flow. Then, 1-µm-thick Si3N4 is grown with PECVD as a membrane of the sensor.
of
the sensor.
Si3 N4 was
preferred
due to its
low thermal[3]
conductivity
[3] in order
to reduce
PECVD
Si3N4 PECVD
was preferred
due to
its low thermal
conductivity
in order to reduce
the heat
losses
the
losses due
to thermal
Si3 N
parameters
130 W
275 ◦ C
4 deposition
dueheat
to thermal
conduction.
Siconduction.
3N4 deposition
parameters
were
130 W RFwere
power,
275RF
°Cpower,
temperature,
temperature,
75 pressure
Pa chamber
andgas
precursor
gasSiH
flow
wassccm)/NH
SiH4 (160 3sccm)/NH
sccm)/N
3 2(6(500
75 Pa chamber
andpressure
precursor
flow was
4 (160
(6 sccm)/N
sccm).2
(500
sccm).
Resistor
byphotolithography.
reversal photolithography.
ITO resistor
is deposited
Resistor
patterns
arepatterns
definedare
bydefined
reversal
The ITOThe
resistor
is deposited
by RF
by
RF
magnetron
sputtering
system
(Leybold
Oerlikon,
Köln,
Germany)
using
an
indium
oxide/tin
magnetron sputtering system (Leybold Oerlikon, Köln, Germany) using an indium oxide/tin oxide
oxide
%)with
target
with a purity.
99.99%Deposition
purity. Deposition
is performed
at power,
75 W RF
(90/10 (90/10
by wt. by
%) wt.
target
a 99.99%
is performed
at 75 W RF
2.2power,
× 10−3
−
3
2.2
× 10
mbarwith
pressure
a rate
of Ar
1.4 atmosphere
Ȧ/s in Ar atmosphere
resulting
an ITO
layer thickness
of
mbar
pressure
a ratewith
of 1.4
Ȧ/s in
resulting in
an ITOin
layer
thickness
of 140 nm.
140
nm.
After
deposition,
lift
off
is
done.
The
1-µm-thick
Si
N
is
grown
with
PECVD
as
a
passivation
3
4
After deposition, lift off is done. The 1-µm-thick Si3N4 is grown with PECVD as a passivation layer.
layer.
Thesilicon
bulk silicon
etch mask
is defined
by photolithography
and patterned
3 NICP-RIE.
4 by ICP-RIE.
The bulk
etch mask
is defined
by photolithography
and patterned
into into
Si3NSi
4 by
The
The
openings
on the
passivation
etched
with
ICP-RIE
using
CHF
A 200-nm-thick
SnO
3 plasma.
2
openings
on the
passivation
are are
etched
with
ICP-RIE
byby
using
CHF
3 plasma.
A 200-nm-thick
SnO
2 is
is
deposited
as
an
active
layer
with
RF
magnetron
sputtering
system
(Nanovak,
Ankara,
Turkey)
by
deposited as an active layer with RF magnetron sputtering system (Nanovak, Ankara, Turkey) by
using
target
withwith
99.99%
purity.purity.
Prior toPrior
the active
material
Cr/Au/Pt
2 sputtering
using the
theSnO
SnO
2 sputtering
target
99.99%
to the
active deposition,
material deposition,
contact
metals
are
deposited
by
e-beam
evaporation
as
resistor
contacts.
Cr/Au
thick
metal
contact
Cr/Au/Pt contact metals are deposited by e-beam evaporation as resistor contacts. Cr/Au thick
metal
pads
for
the
sensing
layer
are
deposited
with
an
e-beam
evaporator
system
(Leybold
Oerlikon,
Köln,
contact pads for the sensing layer are deposited with an e-beam evaporator system (Leybold
◦
Germany)
as well.
Bulk silicon
etchBulk
was performed
33%
KOH solution
at KOH
90 C solution
for the formation
of
Oerlikon, Köln,
Germany)
as well.
silicon etchin
was
performed
in 33%
at 90 °C for
the
membrane
by
using
a
special
wafer
holder
with
backside
protection
capability.
The
microscope
the formation of the membrane by using a special wafer holder with backside protection capability.
photograph
of thephotograph
final sensor of
device
is shown
in Figure
totalin
heated
which
is also
thearea,
ITO
The microscope
the final
sensor
device2.isThe
shown
Figurearea,
2. The
total
heated
2
2
resistor
820 ×
820 µm
while
membrane
area
was 920
× 920 µmarea
. The
SnO
2 while the
2 active
which isarea,
also was
the ITO
resistor
area,
wasthe
820overall
× 820 µm
overall
membrane
was
920
× 920
2 and positioned geometrically
sensing
area
was
500
×
500
µm
at
the
center
of
both
the
membrane
and
2
2
µm . The SnO2 active sensing area was 500 × 500 µm and positioned geometrically at the center
of
ITO
bothmicro-heater.
the membrane and ITO micro-heater.
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Figure 2. Fabricated sensor’s optical microscope photograph.
Figure 2. Fabricated sensor’s
sensor’s optical
optical microscope
microscope photograph.
3. Results and Discussion
Resultsand
andDiscussion
Discussion
3. Results
The resistance of the square-shaped ITO heater resistor was measured as 35 Ohms by the
The resistance
the square-shaped
ITO heater
resistor
was measured
as 35
by the
four-point
resistanceof measurement
technique.
By using
a thermal
microscope,
theOhms
relationship
resistance
measurement
technique.
By
using
a
thermal
microscope,
the
relationship
between
four-point
resistance
measurement
technique.
By
using
a
thermal
microscope,
the
relationship
between the input power and active area temperature was measured, as shown in Figure 3. The
the
inputthe
power
and
active
area
temperature
was measured,
as The
shown
in
3.in
The
calibration
between
input
power
and
active
area temperature
measured,
asFigure
shown
Figure
3. The
calibration
temperature
used
for
the measurements
waswas
50 C.
graph
implies
that
the
heating
◦
temperature
used
for
the
measurements
was
50
C.
The
graph
implies
that
the
heating
efficiency
calibration
temperature
used
for
the
measurements
was
50
C.
The
graph
implies
that
the
heating
efficiency of the structure is weakly dependent on the temperature. At higher heating powers, the
of
the structure
weakly isdependent
onslope
the temperature.
At higher
powers,
the heating
efficiency
of the is
structure
weakly The
dependent
temperature.
Atheating
higher
heating
powers,
heating
efficiency
decreases
slightly.
of on
the the
linear
fitting
line gives
the heating
efficiency
ofthe
the
efficiency
decreases
slightly.
The
slope
of
the
linear
fitting
line
gives
the
heating
efficiency
of
the
sensor
heating
efficiency
decreases
slightly.
The
slope
of
the
linear
fitting
line
gives
the
heating
efficiency
of
the
sensor as 3 C/mW.
as 3 ◦ C/mW.
sensor
as 3 C/mW.
Figure
Figure 3.
3. Power
Power consumption
consumption versus
versus measured
measured maximum
maximum metal
metal oxide
oxide film
film temperature.
temperature.
Figure 3. Power consumption versus measured maximum metal oxide film temperature.
Temperature
with
QFI
InfraScope
II (Quantum
Focus
Istruments,
Vista, CA,
Temperaturemapping
mappingisisperformed
performed
with
QFI
InfraScope
II (Quantum
Focus
Istruments,Vista,
Temperature
mapping
is
performed
with
QFI
InfraScope
II
(Quantum
Focus
Istruments,Vista,
USA),
with with
a liquid
nitrogen–cooled
InSb detector,
using radiance
and uniformity
calibration
targets.
CA USA),
a liquid
nitrogen–cooled
InSb detector,
using radiance
and uniformity
calibration
CA
USA),
with
a liquid
nitrogen–cooled
InSb
using
radiance
and
uniformity
calibration
Measurement
calibration
is done by
keeping
thedetector,
sample
the microscope
stage
with
a high
thermal
targets.
Measurement
calibration
is done
by
keeping
theon
sample
on the microscope
stage
with
a high
◦
targets.
Measurement
calibration
is
done
by
keeping
the
sample
on
the
microscope
stage
with
a
high
conductance
liquid between
the surfaces.
The stage
temperature
is fixed at
C,at
no50bias
thermal conductance
liquid between
the surfaces.
The
stage temperature
is 50
fixed
°C,isnoapplied
bias
is
thermal
conductance
liquid
between
the
surfaces.
The
stage
temperature
is
fixed
at
50
°C,
no
bias
is
to
the micro-heater
and the and
reference
radianceradiance
is taken.isThis
way,
different
emissivity
values values
of the
applied
to the micro-heater
the reference
taken.
This
way, different
emissivity
applied
to the
micro-heater
theand
reference
radiance
isareas
taken.
way, different
emissivity
values
metal
ITO heater
and
dielectric
coated
areas
calibrated
using
measured
radianceradiance
data
as
of the electrodes,
metal
electrodes,
ITO and
heater
dielectric
coatedare
areThis
calibrated
using measured
of
the
metal
electrodes,
ITO
heater
and
dielectric
coated
areas
are
calibrated
using
measured
radiance
references.
Then biasThen
is applied
to applied
the micro-sensor,
and then temperature
is
data as references.
bias is
to the micro-sensor,
and then distribution
temperatureuniformity
distribution
data
as references.
Then
bias
is applied
to theto micro-sensor,
and then temperature distribution
measured
and
corrected
according
to the
reference
map.
uniformity
is measured
and
corrected
according
the
reference map.
uniformity
corrected
according
to of
thethe
reference
map.
Figure
shows theand
thermal
microscope
view
operating
device at a 80 mW
mW heating
heating power.
power.
Figureis4 measured
Figure
4
shows
the
thermal
microscope
view
of
the
operating
device
at
a
80
mW
heating
power.
As
As can
can be
be seen
seen from
from the
the line
line scan
scan through
through the
the center
center of
of the
the heated
heated zone,
zone, the
the temperature
temperature difference
difference
As
can bethe
seen
fromresistor
the linecontacts
scan through
the30%
center
heated zone,
the temperature
difference
between
planar
is about
of of
thethe
maximum
temperature
at the center
of the
between the planar resistor contacts is about 30% of the maximum temperature at the center of the
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heater. However, the temperature change in the SnO2 active layer area was measured to be lower
thanSensors
7% with
a maximum 17 C temperature difference between the hottest and coldest
points.
2016,
16, 1612 resistor contacts is about 30% of the maximum temperature at the center
5 of 7of the
between the
planar
Sudden temperature jumps on the sensor electrodes and heater electrodes were probably linked to
heater. However, the temperature change in the SnO2 active layer area was measured to be lower
heater. However,
the temperature
changeemissivities,
in the SnO2 whereas
active layer
was measured
to be
lower
the temperature
dependence
of the metal
thearea
thermal
microscope
uses
a single
than than
7% withwith
a maximum
17 ◦ C temperature
differencebetween
between
hottest
coldest
points.
a maximum
difference
thethe
hottest
and and
coldest
points.
emissivity7%
value for
each pixel17atC
thetemperature
reference radiance
measurement
temperature.
Sudden
temperature
jumps
on
the
sensor
electrodes
and
heater
electrodes
were
probably
linked
to
Sudden temperature jumps on the sensor electrodes and heater electrodes were probably linked to
the temperature
dependence
of
the
metal
emissivities,
whereas
the
thermal
microscope
uses
a
single
the temperature dependence of the metal emissivities, whereas the thermal microscope uses a single
emissivity
value
for for
each
pixel
atatthe
measurement
temperature.
emissivity
value
each
pixel
thereference
reference radiance
radiance measurement
temperature.
Figure
4. Temperaturedistribution
distribution image
image taken
(a) (a)
and and
temperature
Figure
4.
Temperature
distribution
image
taken by
bya athermal
thermalmicroscope
microscope
temperature
4. Temperature
taken
by
and
temperature
distribution
line
scan
through
center
of
the
device
at
80
mW
(b).
distribution
distribution line
line scan
scan through
through center
center of
of the
the device
device at
at 80
80 mW
mW (b).
(b).
Figure 5 shows the results of transient measurements taken with the thermal microscope.
Figure 55 shows
shows the
the results
results of
of transient measurements
measurements taken with
the thermal microscope.
Figure
Measurements
were performed
at 2 transient
Hz with a pulse width of 200taken
ms. with the thermal microscope.
Measurements were
were performed
performed at
at 22 Hz
Hz with
with aa pulse
pulse width
width of
of 200
200ms.
ms.
Measurements
Figure 5. Heat-up and cool-down times as measured at 100 C, 150 C, and 200 C average active area
temperature.
◦ C average
Figure 5.
5. Heat-up
at at
100
C,◦ C,
150150
C,◦and
200 200
C average
active
area
Figure
Heat-upand
andcool-down
cool-downtimes
timesasasmeasured
measured
100
C, and
active
temperature.
area temperature.
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As is commonly used, the rise time can be defined as the time for the sensor to heat up from 10%
to 90% of the maximum stabilized temperature. Similarly, the fall time is defined as the time for the
sensor to cold down from 90% to 10% of its maximum temperature. Transient measurements were
made for three different working temperatures. Heat-up and cool-down times of 40 ms and 20 ms were
obtained, respectively. These values can be improved by reducing the size of the heater areas inside
the thermally insulating Si3 N4 membranes to reduce heat leakages. Also, by using a low-pressure
PECVD (LPECVD) membrane and passivation layers with low intrinsic stress, the film thicknesses
can be further reduced. In this way the thermal mass can be reduced and the rise-fall time values can
be improved.
4. Conclusions
This work describes the design and fabrication of a novel micro-hotplate structure for thin-film
metal oxide gas sensors, using silicon micromachining methods. ITO is used as the resistor material
with a square planar geometry. The structure and fabrication techniques are simple but they meet the
required performance specifications of low power consumption and linearity of heating efficiency for
such devices. Further, the demonstrated heating element achieves high temperature uniformity with a
17 ◦ C gradient at a 300 ◦ C working temperature on the active film area with 80 mW power consumption.
The rise and fall times are measured to be 40 ms and 20 ms, respectively. The hotplate structure with
the square planar ITO geometry that we introduced solves the temperature nonuniformity problem.
Cold spots at elevated temperatures, which arise from the non-continuous nature of conventional
resistor meanders [14], are avoided. Key advantages of the planar ITO resistors are their continuity
underneath the active metal oxide layers and the lacking necessity of extra microstructures, such
as heat-spreading plates. The micro-hotplate with a planar ITO resistor can serve as the platform
for integrated thin-film gas sensors. It brings simplicity to sensor design and fabrication. Both the
membrane structure and the resistor are transparent; it could be a promising technology for sensor
applications based on both of the electrical and optical measurements.
Acknowledgments: This work is supported by the project DPT-HAMIT and the Turkish Science Industry and
Technology Ministry SANTEZ Program. One of the authors (E.O.) also acknowledges partial support from the
Turkish Academy of Sciences.
Author Contributions: M.C.Ç. and D.C. conceived, designed the experiments and made the measurements.
M.C.Ç. made the fabrication. M.C.Ç. and B.B. prepared the manuscript and figures. E.Ö. provided consumables,
processing equipment and characterization setup, followed all the work from the beginning, including the design,
fabrication and measurements, read the manuscript, commented on the design and manuscript, and made
corrections on the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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