Materials Transactions, Vol. 43, No. 5 (2002) pp. 1065 to 1068
Special Issue on Smart Materials-Fundamentals and Applications
c
2002
The Japan Institute of Metals
Characteristics of Thin-Film NTC Infrared Sensors
Mina Yoo and Moonho Lee ∗
Department of Sensor & System Engineering, Yeungnam University, Kyungsan 712-749, Korea
Characteristics of thin-film NTC infrared sensors fabricated by micromachining technology were studied as a function of the thickness
of membrane. The overall-structure of thermal sensor has a form of Au/Ti/NTC/SiO x /(100)Si. NTC film of Mn1.5 CoNi0.5 O4 with 0.5 µm in
thickness was deposited on SiOx layer (1.2 µm) by PLD (pulsed laser deposition) and annealed at 600–800◦ C in air for 1 h. Au (200 nm)/Ti
(100 nm) electrode was coated on NTC film by dc sputtering. By the results of microstructure, X-ray and NTC analysis, post-annealed NTC
films at 700◦ C for 1 h showed the best characteristics as NTC thermal sensing film. In order to reduce the thermal mass and thermal time
constant of sensor, the sensing element was built-up on a thin membrane with the thickness of 20–65 µm. Sensors with thin sensing membrane
showed the good detecting characteristics.
(Received September 19, 2001; Accepted January 7, 2002)
Keywords: thin-film, negative temperature coefficient (NTC), infrared, thermal sensor, membrane, micromachining technology
1. Introduction
(TCR), defined as:
Thermal sensors can detect the IR source and its size, temperature, moving direction and speed, emissivity, and wavelength.1, 2) Because of these basic abilities, they have many applications as a form of single element or multi-element array.
As a single element, they are applicable to intruder alarms,
fire alarms, pollution monitoring, chemical analysis, energy
metering, laser detection, thermal recording, and so on.1–3)
Thermal detectors, integrated in the membranes on Si
wafer, can be used to measure the thermal irradiations.4) Operation of thermal detector depends on a two-step mechanism:
absorption of radiation; change and readout in sensing parameter. Absorption of radiation raises the temperature of sensor,
which in turn changes some temperature-dependent parameter of sensing materials, such as electrical resistivity, contact
potential, spontaneous polarization, and so on.1)
Thermal sensing materials may involve a thermocouple
(thermopile), thermistor (bolometer), dielectrics (bolometer),
and pyroelectric materials. Bolometer is a thermistor-based
detector that measures temperature change in a thermally isolated membrane heated by infrared radiation absorbed on the
surface. The resulting temperature increase can change the
resistance of embedded thermistor. This resistance change is
then converted into a voltage or current signal.1–3, 5–8)
Among the thermistor materials, composite oxides composed of transition metals, such as manganese, cobalt and
nickel, are widely used for negative temperature coefficient
(NTC) thermistor, because the electrical resistance decreases
greatly with increasing temperature. And, these materials
have a spinel-type crystal structure. The properties, used to
characterize NTC thermistor, are resistance R1 at 25◦ C (T1 )
and R2 at 85◦ C (T2 ). And B, the sensitivity of NTC device
over a given temperature range, is important also.9)
B = T1 T2 /(T2 − T1 ) ln(R1 /R2 )
(1)
One of key parameters characterizing the thermistor material for bolometer is its thermal coefficient of resistance
∗ Correspond
author.
TCR = 1/R(dR/dT )
(2)
where, R and T represent the resistance of material and temperature, respectively.
In order to detect small temperature changes, sensing wafer
should be thermally insulated from the surroundings. This
can be achieved by fabricating thin membrane structures by
means of micromachining.
In this study, thin film NTC thermal sensors were fabricated
by micromachining technique, whose overall-structure had a
form of Au/Ti/NTC/SiOx /(100)Si. Mn1.5 CoNi0.5 O4 was used
as NTC materials, which was deposited on SiOx /Si(substrate)
by PLD. NTC behavior of thin film was measured with TCR
measuring equipment. Thin membranes with 20–65 µm in
thickness were fabricated by Si-processing. And, the characteristics of thermal sensor were monitored by the dynamic
method as a function of the thickness of membrane.
2. Experimental
The overall-structure of thermal sensor has a form of
Au/Ti/NTC/SiOx /(100)Si. Figure 1 shows the schematic flow
diagram for the preparation of device. Silicon oxide (SiOx )
layer of 1.2 µm in thickness was formed at the surface of
(100)Si wafer (thickness: 546 µm) through the reaction of
thermal oxidation at 1050◦ C for 4 h. NTC film (0.5 µm) of
Mn1.5 CoNi0.5 O4 was deposited on silicon oxide layer by PLD
at the oxygen partial pressure (PO2 ) of 300 Pa, where the substrate temperature was 300◦ C. In order to prepare the stable
NTC films, the as-prepared films were annealed at 600, 700
and 800◦ C in air for 1 h. And, Ti (100 nm) and Au (400 nm)
layers, used as the electrode, were coated on NTC film by dc
sputtering.
By Si-processing, NTC and electrode layers were patterned, and thin membranes with 20–65 µm in thickness were
fabricated. Then, the electrodes were connected with leads in
the package by using Au wire.
NTC behavior of Mn1.5 CoNi0.5 O4 film in the temperature
range of 25–85◦ C was measured with TCR measuring equip-
1066
M. Yoo and M. Lee
increasing temperature. Among them, more or less big pores
were observed for the film annealed at 700–800◦ C. Films annealed at 600–700◦ C show more denser structure than the film
post-annealed at 800◦ C.
Figure 5 shows the NTC behavior of as-prepared and postannealed NTC thin films. All of them showed the good
NTC behavior: the logarithmic resistivity (ln ρ) shows a linear dependence of reciprocal temperature (1/T ). And the asprepared film showed the highest resistivity. Figure 6 shows
the variations of B and resistivity at room temperature, obtained from Fig. 5. All the values are suitable for the bolometer applications. Especially, the film post-annealed at 700◦ C
showed the lowest B and resistivity, then this film was chosen
as the sensing film of NTC thermal sensor.
S i - Wafer
Si O X Formation
NTC Film
Au/ T i
Electrode
Membrane
Fig. 1 Schematic flow diagram for the preparation of device.
Intensity, cps
as-prepared
(311)
(220)
(422)
600 C
(440)
(511)
700C
800 C
30
40
50
60
Angle, 2
Fig. 2 Variations of XRD patterns for the as-prepared and post-annealed
NTC thin films.
ment (SA 2500E, EMO Co., Korea). Most of the sensors
showed 2–3 M of resistance. And, the characteristics of
thermal sensor were monitored by the dynamic method by
using of He–Ne laser, chopper and Merlin unit (Oriel Co.).
3. Results and Discussion
3.1 Characteristics of NTC thin films
Figure 2 shows the XRD patterns for the post annealed
NTC films at 600–800◦ C, prepared at the substrate temperature of 300◦ C under Po2 of 300 Pa. The as-prepared (300◦ C)
and post-annealed films show the well developed spinel crystalline structure. The degree of crystallization increased with
increasing the post-annealing temperature. Figures 3 and 4
show the scanning electron micrographs and variation of grain
size for the as-prepared and post-annealed films. After postannealed, the grain size and porosity of films increased with
3.2 Characteristics of thin-film NTC thermal sensors
In general, the thermal sensor shows the unwanted outputs (noises) due to the environmental effects such as temperature fluctuation, mechanical vibration, electromagnetic interference, and sudden relief of stress.1) In this study, all of
sensors had a structure of serially compensated sensing elements, to eliminate this unwanted output signal. As shown
in Fig. 7, showing the overall feature of sensing membrane,
two NTC films are serially connected. With this sensing
membrane, TO-5 package (head and can) and AR-coated Siwindow, NTC thermal sensors were fabricated under the nitrogen gas atmosphere, so the inner spaces of sensor is filled
with nitrogen gas, which can reduce the level of thermal fluctuation noise. Figure 8 shows the overview of thermal sensors. And, the voltages, taken from the R-bridge circuit, were
used as the output signals.
Figure 9 shows the dependence of output waveform upon
the chopping frequency for the sensor with membrane thickness of 20 mm. At 0.1 Hz, the output voltage showed a square
wave with small noises including thermal drift. With increasing frequency, the shape of output voltage changed from the
square to smoothen and rigid triangles. From these Figures,
variations of peak-to-peak voltage (Vp-p ) and rising/falling
time with chopping frequency and membrane thickness of
sensor could be obtained. As shown in Fig. 10, Vp-p decreased with increasing membrane thickness in the range of
20–65 mm. With increasing the membrane thickness, bad
thermal isolation would be obtained, and the low output signal
was taken. Vp-p showed the constant value in the frequency
range of <3 Hz, then it decreased with increasing chopping
frequency. This is caused by the large heat capacity, which
gives rise to large thermal time constant. Figure 11 shows the
variations of rising and falling time with membrane thickness.
As presupposed, time constants were increased with increasing thickness of membrane. This can be explained as follows:
as the membrane thickness increased, the heat capacity C T of
sensing part increased, and its thermal conductivity G T decreased, so the thermal time constant τT = C T /G T increased.
In sensor applications, one of the most important things is
the signal-to-noise (S/N) ratio. The S/N ratios were calculated
from the observed output waveforms. As shown in Fig. 12,
the S/N ratio decreased with increasing the thickness of membrane. The sensor with smallest thickness showed the highest
S/N ratio, because the thermal time constant decreased with
decreasing the thickness of membrane. In order to obtain the
Characteristics of Thin-Film NTC Infrared Sensors
(a) as-prepared
1067
(b) 600OC
(d) 800OC
(c) 700OC
Fig. 3 SEM for as-prepared and post-annealed NTC thin films: (a) as-prepared, (b) 600◦ C, (c) 700◦ C and (d) 800◦ C.
5000
B/K
55
4500
4000
4000
400
300
40
200
100
35
cm
as-prepared
45
Resistivity, /
Grain Size, g /nm
50
0
300
400
500
600
700
800
Annealing Temperature, T/ C
30
300
400
500
600
700
800
Fig. 6 Variations of B and resistivity with annealing temperature.
Annealing Temperature, T / C
SiOx/Si Substrate
Fig. 4 Variations of grain size with post-annealed temperature.
6.0
Log Resistivity, /
cm
5.5
as-prepared
600 C
700 C
800 C
5.0
4.5
as-prepared
4.0
Au/Ti Electrode
3.5
NTC Film
3.0
2.5
2.7
2.8
2.9
3.0
3.1
Temperature,T /K
3.2
3.3
3.4
-1
Fig. 7 Overall feature of sensing membrane.
Fig. 5 Variations of resistivity with temperature for NTC thin films annealed at 600–800◦ C for 1 h.
4. Conclusion
good thermal sensor with excellent detectivity, the more precise micromachining technology is needed to obtain thinner
membrane.
By post-annealing the PLD film at 700◦ C for 1 h, a well
crystallized and good NTC Mn1.5 CoNi0.5 O4 thin film was
obtained. And, the thermal sensor prepared with this NTC
1068
M. Yoo and M. Lee
1200
1000
20 m
Vp-p/mV
800
600
35 m
400
65 m
200
0.1
Fig. 8 Overview of manufactured thermal sensors with TO-5 package.
(a) 0.1Hz
1
5
10
Chopping Frequency, f /Hz
Fig. 10 Variations of Vp-p with chopping frequency for NTC thermal sensors with membrane thickness of 20, 35 and 65 µmm.
Thermal Time Constant, t/ms
240
Output Voltage [200mv/div]
Time [1.00s/div]
(b) 0.5Hz
200
rising time
160
falling time
120
20
40
60
Membrane Thickness, t / m
Fig. 11 Variations of thermal time constants with membrane thickness for
NTC thermal sensors.
Time [500ms/div]
60
(c) 1.0Hz
S/N Ratio
40
20
0
Time [200ms/div]
20
Fig. 9 Output waveforms for NTC thermal sensor with TO-5 package,
measured at (a) 0.1 Hz, (b) 0.5 Hz and (c) 1.0 Hz.
Acknowledgements
The authors express sincere appreciation for the financial
support of KISTEP (contract No. 06-03-01) in the performance of this work.
60
Membrane Thickness, t / m
Fig. 12
film showed the good sensor characteristics for IR detection.
Sensing element was built-up on a thin membrane with the
thickness of 20–65 µm to reduce the thermal mass and thermal time constant, and sensors with thin sensing membrane
revealed the good sensor characteristics. In order to obtain
the good thermal sensor with excellent sensitivity, the thinner membrane and precise micromachining technology are
needed.
40
Variation of S/N ratio with membrane thickness.
REFERENCES
1)
2)
3)
4)
5)
6)
7)
8)
9)
R. W. Whatmore: Rep. Prog. Phys. 49 (1986) 1335–1386.
M. Lee and S. Bae: Opt. Eng. 39 (2000).
E. H. Putley: Ferroelectrics 33 (1981) 207–216.
P. Kleinschmidt and W. Hanrieder: Sensors and Actuators A 33 (1992)
5–17.
P. T. Lai and B. Li.: IEEE Elec. Dev. Lett. 20 (1999) 589–591.
T. Yokoyama: J. Mater. Sci. 30 (1995) 1845–1848.
S. Baliga and A. L. Jain: Appl. Phys. 50 (1990) 473–477.
A. Dziedzic, L. Golonka and H. Kozlowski: Meas. Sci.Technol. 8 (1996)
78–85.
H. Jerominek, F. Picard and N. Swart: SPIE. 2746 (1996) 60–64.