of 3
Microscaled and nanoscaled platinum sensors
Aditya Rajagopal,
a

Sameer Walavalkar, Samson Chen, Luke Guo, Tom Gwinn, and
Axel Scherer
Electrical Engineering and Applied Physics, Kavli Nanoscience Institute, Caltech Pasadena,
California 91125, USA

Received 4 April 2010; accepted 5 September 2010; published online 28 September 2010

We show small and robust platinum resistive heaters and thermometers that are defined by
microlithography on silicon substrates. These devices can be used for a wide range of applications,
including thermal sensor arrays, programmable thermal sources, and even incandescent light
emitters. To explore the miniaturization of such devices, we have developed microscaled and
nanoscaled platinum resistor arrays with wire widths as small as 75 nm, fabricated lithographically
to provide highly localized heating and accurate resistance

and hence temperature

measurements.
We present some of these potential applications of microfabricated platinum resistors in sensing and
spectroscopy. ©
2010 American Institute of Physics
.

doi:
10.1063/1.3494088

Microlithography and nanolithography techniques can
now be applied toward the miniaturization of a wide variety
of sensors and actuators, leading to their integration into
chip-based analysis systems. On-chip sensors enable the
monitoring and regulation of many chemical and biological
samples in parallel, and reduce the individual device cost,
following the trend toward more complex and functional mi-
croelectronics through lithographic printing. Specifically,
platinum wires have been used for resistive heaters and in-
candescent light sources since the early 1820s. Platinum does
not oxidize, making it a good candidate for vacuum-free,
miniaturized visible and infrared sources, heaters, and ther-
mometers. Applications of these microscale thermal control
systems include chemical analysis, gas chromatography,
1
4
microcalorimetry as well as thermal regulation of poly-
merase chain reactors
5
and even micropropulsion systems.
6
Miniaturization of these devices is particularly beneficial for
systems that require independent thermal control over many
reactions or wide band spectroscopic light sources; the low
thermal mass of microfabricated heaters enables greater ac-
curacy in measurement, faster heating and cooling rates,
while requiring lower power than macroscopic systems.
Platinum resistance thermometer devices

RTDs

have a
linear temperature response in the range of

−200

–500 °C,
and are well suited for the thermal measurement and control
of wide array of chemical processes.
5
In particular, platinum
RTDs exhibit a high accuracy and repeatability of tempera-
ture measurements when compared with thermocouples for
temperatures below 600 °C.
7
In this paper, we present on-
chip thin-film, micron-sized platinum resistive thermometers
as convenient on-chip thermal control systems and IR light
sources.
We fabricated arrays of platinum microresistors and nan-
oresistors on alumina coated oxidized silicon wafer sub-
strates. Fabrication starts with the growth of 160 nm of
wet thermal oxide on a

100

silicon wafer. Subsequently, a
150 nm layer of alumina

Al
2
O
3

and a 150 nm layer of
platinum were sputter deposited on the surface using a direct
current

dc

magnetron sputtering source.
8
The approximate
film thicknesses were confirmed by scanning electron mi-
croscopy. After the complete stack of materials was depos-
ited, a milling mask pattern was then defined using standard
photo-lithographic techniques. The resistor pattern was trans-
ferred by milling through the platinum and aluminum oxide,
into the glass

to remove shunt thermal resistances

, using a
radio frequency

rf

plasma-based argon mill

Fig.
1
top left
inset

. After the plasma-milling step, the cross-linked resist
milling-mask was removed by exposing the chips to a low-
voltage rf oxygen plasma. These platinum resistors consist of
a series of twenty serpentine platinum wires with widths
ranging from 1–4

m, and cover areas of 100

200

m
2
on-chip area

Fig.
1

. Finally, selected microresistors were
then further postprocessed to create nanometer-wide “nan-
oresistors”

Fig.
2

by using an FEI Nova200 focused ion
beam

FIB

system. The resistor linewidths were reduced
from 1–4

m to dimensions as small as 75 nm. This tech-
nique was utilized since it allows for mesoscaled structures
to be fabricated using the same set of optical lithography
masks.
a

Electronic mail: arajagop@caltech.edu.
100μm
500μm
200μm
300μm
4μm
Silicon
ThermalOxide
Alumina
Platinum
FIG. 1.

Color online

Scanning electron micrograph of platinum microre-
sistors

Inset: top right, resistor element cross section; top left, resistor ele-
ment dimensions

.
APPLIED PHYSICS LETTERS
97
, 133109

2010

0003-6951/2010/97

13

/133109/3/$30.00
© 2010 American Institute of Physics
97
, 133109-1
Downloaded 08 Nov 2010 to 131.215.220.185. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
Before temperature-benchmarking, the microresistors
were annealed by resistive-heating with a constant power of

1.5 W for 10 min to ensure thermal stability. These an-
nealed resistors show considerably better relative tolerances
on each chip than unannealed resistors with a standard de-
viation of resistance of

=2.1%

sample size of n=20

for
annealed resistors and a standard deviation of resistance of

=48%

sample size of n=194

for unannealed resistors. We
believe that this annealing step allows the platinum thin-film
to electromigrate in a controlled way and improves the reli-
ability of the resulting heaters. This helps to minimize resis-
tor failures during subsequent heating and measurement and
has allowed the annealed resistors to be driven at higher
powers than nonannealed resistors.
To test the heating and temperature sensing capabilities
of the resistors, a dc power supply was connected to the
heater resistor and adjusted to deliver power in steps of
250 mW. The resistance of each adjacent resistor was manu-
ally probed and measured in order to characterize the thermal
profile of a platinum resistor bank. By taking advantage of
the linear temperature coefficient of resistance

TCR


0.003927


cm

/
C

Ref.
9

of platinum, we can relate
resistance measurements of the platinum structure to the
temperature of the substrate. The resulting temperature pro-
file across the resistor array

Fig.
3

shows that the tempera-
ture decreases with distance from the heating element and
with decreasing heater power. Thermal isolation, provided by
the SiO
2
film, ensures that the heating is confined to the
surface Al
2
O
3
.
Furthermore, the platinum resistor structures can be
driven with enough power to exhibit luminescence in the
visible range

Fig.
4

. This process can be enhanced by cre-
ating small, highly resistive regions within the platinum con-
ductors. The intense localized heating of the platinum struc-
tures caused them to emit as blackbodies with components in
the orange-red region of the visible spectrum. Spectra of
light emission from these incandescent Pt filaments were ex-
tracted using an Acton cooled charge-coupled device camera.
We compensated for the spectral sensitivity of the camera
and optical system using a 3100 K near-blackbody light
source. Furthermore, we were able to characterize the peak
unnormalized

i.e., not corrected for detector optics

emis-
sion frequencies as a function of applied filament power
across an individual resistor, and these have been summa-
rized in Table
I
.
Using standard finite element analysis techniques, we
were able to validate these spectral measurements. We mod-
eled conduction, radiation, convection, and Joule heating
using a nominal 1 cm

1 cm chip with microresistors and a
5 W power source. Our model includes the chip mount used
during the measurements, which was modeled as a heat sink
2
4
6
8
10
12
14
16
18
R
es
i
stor #
300
600
900
1200
1500
1800
2100
2400
2700
0
5
10
15
20
25
30
35
Distance
(
micrometers
)
Delta T
(
K
)
230mW
478mW
721mW
984mW
1236mW
1526mW
1720mW
2014mW
2225mW
2517mW
2716mW
Heater Power
FIG. 3.

Color online

Temperature vs distance for resistor array

R0 is the
heating element

located at distance 0

, R1–R19 are measurements ele-
ments

for various driving powers.
FIG. 2. Scanning electron micrograph of platinum nanobulb

FIB thinned
microresistor

.
300
400
500
600
700
800
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Wavelen
g
th
(
nanometers
)
NormalizedRelativeLuminosity
Mi
cro
b
u
lb
spectrum:s
i
mu
l
at
i
onan
d
measurement
Original
Compensated
SimulatedMicrobulbFit
BlackbodyFit
Blackbodyr
2
:
0.99225
BlackbodyTemperature:
1522K
FEMSimulationFitr
2
:
0.95573
FIG. 4.

Color online

Normalized blackbody emission spectrum of plati-
num microbulb, and finite element methods extracted spectrum.
TABLE I. Peak emission wavelength vs power dissipation.
Measured peak wavelength

nm

Input power

W

719.28
a
5.40
718.02
a
5.92
706.67
a
6.62
701.62
a
7.22
733.36
b
38.57
a
Denotes nanobulb.
b
Denotes macrobulb.
133109-2 Rajagopal
etal.
Appl. Phys. Lett.
97
, 133109

2010

Downloaded 08 Nov 2010 to 131.215.220.185. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
with 9000 W
/
Km
2

as measured

. The simulated blackbody
emission spectrum, produced using the surface temperatures
created by the chip is shown in Fig.
4
. The simulated peak
temperature was approximately 1900 K, slightly above the
melting point of platinum. After optical compensation for our
measured spectra, we found that the simulated peak, and the
predicted peak from Wein’s law matched well with our mea-
surements

see Fig.
4

.
10
,
11
From this data, we conclude that
the platinum filaments are heated slightly beyond their melt-
ing temperature, causing them to radiate in the near-visible
and visible frequencies.
These high temperatures eventually led the bulb to fail-
ure, most likely caused by the melting and evaporation of the
platinum from the substrate material. In the future, active
cooling or physical confinement of the platinum through en-
capsulation might be employed to mitigate this failure.
The on-silicon fabrication of the resistive elements al-
lows for integration of complex control circuitry for thermal
control. For example, we envision that such a device can be
used for applications such as microcalorimetry. These resis-
tor arrays can be used to quantify the exothermic or endo-
thermic nature of reactions. Ultimately, when electronics and
fluidics are integrated with these platinum heaters, we envi-
sion these platinum resistance thermometers within chip-
based gas chromatography systems, accurate thermal con-
trollers for microscopic polymerase chain reactors,
12
14
and
black-body emitter light sources for visible and mid-IR
spectroscopy.
15
18
The authors would like to acknowledge Michael David
Henry, Bophan Chhim, Melissa Melendes, and the Kavli
Nanoscience Institute for help with fabrication of the de-
vices. Furthermore, the authors want to acknowledge Profes-
sor Joseph Shepherd and Philipp Boettcher for the use of an
IR pyrometer that was used to calibrate temperature mea-
surements. Furthermore, we would like to thank Claudia
Shin, Raymond Jimenez, Teresa Emery, and Greg Lutrell
for helpful insights. This work was supported by a grant
from the Boeing Corporation for the investigations into
miniaturized sensor systems for mobile platforms

BOEING
CT-BA-GTA-1

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133109-3 Rajagopal
etal.
Appl. Phys. Lett.
97
, 133109

2010

Downloaded 08 Nov 2010 to 131.215.220.185. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions