Sensors and Materials, Vol. 8, No. 6 (1996) 317-326
MYUTokyo
S &M 0248

A High-Performance Infrared Detector
Using MOS Technology
Tadigadapa A. S. Srinivas, Paul J. Timans 1 and Haroon Ahmed
Microelectronics Research Center, Cavendish Laboratory,
University of Cambridge,
Madingley Road, Cambridge CB3 0HE, United Kingdom
1AG Associates, 4425 Fortran Drive, San Jose,
California 95134-2300, U.S.A.
(Received March 29, 1995; accepted December 11, 1995)
Key words:

infrared detector, microthermopile, free-standing structureThis paper describes a free-standing microthermopile infrared detector. The device
consists of micron-sized copper-constantan free-standing wires which overlap to form the
hot junctions of the thermocouples, while the cold junctions are made from larger metal
strips which are attached to the substrate. Incident radiation absorbed by the device causes
the temperature of the free-standing hot junction to rise relative to the cold junction thereby
resulting in a thermovoltage. These detectors were found to exhibit a response time of-10 µs. The spectral response of the detectors was found to be 3 times higher at awavelength of 10 µm compared to that at 1 µm and this difference is thought to be due toantenna effects. The antenna behavior of free-standing microthermocouples has beenstudied and the results from these experiments are also presented. The device is fabricatedusing standard silicon microfabrication techniques and can be integrated with active
electronic circuits to result in a smart sensor.

1.

Introduction

The ability to fabricate free-standing microstructures with small thermal masses andgood thermal isolation using silicon micromachining technology has led to the develop­
ment of miniaturized sensitive thermal infrared detectors.ci,zJ The traditional approach has
been to fabricate free-standing -1-µm-thick membranes or cantilevers from dielectric
layers of silicon nitride and silicon dioxide. The active structure for transducing the

317

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Sensors and Materials, Vol. 8, No. 6 (1996)

infrared power into electrical signals is then fabricated onto these membranes and cantile­vers. <3-5J An alternative approach is to do away with the supporting membrane altogether
and directly fabricate free-standing structures from the active materials. Since the thermal
isolation of these free-standing structures is better than that of membrane-based devices,
direct use of them is expected to result in performance improvement of the devices.
However, the limitation of this technique is that only materials with sufficiently strong
mechanical properties, which can be fabricated as self-supporting free-standing structures,
can be used. Based on this idea, a copper-constantan free-standing microthermocouple
infrared detector was fabricated and has been reported earlier.<6- l This paper was presented
at the Europhysics Industrial Workshop 10 in Oberhof, Germany along with reports onsome of the aspects of a free-standing microthermopile infrared detector.
Free-standing copper-constantan microthermocouples and microthermopiles have been
fabricated and their thermal and infrared characteristics have been analyzed.<8l A typicalmicrothermocouple is shown in Fig. 1 and consists of a 1-µm-wide and 20-µm-long free­standing wire, one half of which is copper and the other half is constantan. The hot junction
is in the middle of the wire where the copper and constantan wires overlap, whereas themuch larger cold junction is physically and thermally attached to the substrate. Absorptionof the incident radiation by the device causes the hot junction to heat up relative to the cold
junction and therefore produces a thermovoltage. The device has been fabricated on a
1000-A-thick silicon nitride layer on silicon substrate using the "lift-off' process andisotropic CF4 + 8%02 plasma etching of the substrate. <6l Figure 2 shows a SEM micrograph
7

Fig. l. SEM micrograph of a 1-µm-wide, 0.1-µm-thick and 20-µm-long free-standing
microthermocouple. The hot junction is formed at the middle of the free-standing wire where thecopper and constantan wires overlap.

Sensors and Materials, Vol. 8, No. 6 (1996)319

of a microthermopile consisting of 20 thermocouples connected in series. In this case, a U­
shaped copper free-standing structure was chosen for reasons of mechanical robustness
and to accommodate lithographic misalignments during its fabrication. Since the device isfabricated using standard MOS microfabrication techniques, it should be possible tomonolithically integrate the sensor with active circuits to result in a smart sensor.

2.

Experimental Results

Responsivity, time constant and the spectral response of the microthermopiles de­scribed above have been measured in the 1-12 µm wavelength range. In an experimentreported earlier the time constant of the device was measured using an acoustooptic
modulator in conjunction with a CO2 laser. C7l The response of the microthermopile to aradiation step was measured and the time constant, re, defined as the time taken by theoutput signal to rise to (1- e-1) or 63% of the peak value was found to be -11 µs.<9) An
alternative definition of re is given byC 10l(1)where f3ctb is the modulation frequency at which the output is 3 db below the steady stateFig. 2. SEM micrograph of a copper-constantan free-standing microthermopile. The cold junctions
are the overlapping large pads; the hot junctions are formed at the intersection of the free-standing
wires. The U-shaped free-standing structures are made of copper and the single wires are ofconstantan.

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Sensors and Materials, Vol. 8, No. 6 (1996)

value, i.e., the voltage is 0.707 of the steady state voltage. In order to employ this definitionof 'l"c, the output of the detector was monitored as the CO2 laser beam was sinusoidally
modulated. This was achieved by sinusoidally modulating the laser cavity length using a
piezoelectric drive in the grating mount. Keeping the peak-to-peak incident infrared power
constant, the modulation frequency was varied from 100 Hz to 80 kHz. The detector output
was monitored as a function of the modulation frequency and is shown in Fig. 3. At afrequency of 40 kHz the output is approximately 3 db below the output at 2 kHz and, using
eq. (1), this implies 'l"c of -4 µs. Since .hctb has not been accurately measured, the error is
expected to be large and explains the discrepancy in the re's obtained by the two methods.
The responsivity of the microthermopile at 10.6 µm measured using a CO2 laser was
found to be 12.3 V/W.<9l In another experiment a 1.3-µm-wavelength buried-heterostructure
laser was used to calibrate the detector. The laser gave a continuous output of 4 mW when
operated at 25 mA over the threshold current. In order to determine the incident power
density, a sharp blade edge was scanned across the laser to determine its beam profile.
Figure 4 shows the microthermopile output as a function of incident power density fromwhich the reponsivity of the device was deduced to be 3.4 V/W. The area of the detector
was taken to be the cross-sectional area of the free-standing wires with respect to theincident beam which, for the device described here, is 1.1 x 10-9 m 2 . The reponsivity of theFig. 3. Microthermopile output in response to an incident CO2 laser beam which was modulated
sinusoidally at the frequencies: (a) 2 kHz, (b) 20 kHz, (c) 40 kHz and (d) 80 kHz.

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Sensors and Materials, Vol. 8, No. 6 (1996)

10
Slope=3.76 X 10·3 (µV m2/W)

-

8

0

6

C.

�

"ii
0

-

4

G)

.c:
0

2

(.)

:E

0
0

500

1000

1500

2000

Incident power density (W/m2)

2500

Fig. 4. Output of a free-standing microthermopile as a function of the incident laser power density
at a wavelength of 1.3 µm. Dividing the slope of the curve by the detector area, i.e., the total surface
area of the free-standing wires, which is 1.1 x 10-9 m2, a responsivity of 3.4 V/Wis obtained.

detector at 1.3 µrn wavelength is about 3 times smaller than that at 10.6 µm. Such abehavior was also observed in the wavelength dependence of the relative response in the 112 µm wavelength range.<7l The reason for the rise in the relative response is not exactly
understood but is thought to be due to the antenna effects arising frorn the interaction of theinfrared radiation with the free-standing structures of the rnicrothermopile. In order tostudy these antenna effects, microthermocouples of three different free-standing wire
lengths of 20 µm, 40 µm and 60 µm and width 1 µm were fabricated and the results of theinvestigation are presented in the next section.

3.

Analysis of a Free-Standing Microthermocouple as an Infrared
Antenna

Infrared radiation incident on the free-standing metal structures induces electric cur­rents in them at infrared frequencies. These electric currents are dissipated in the free­standing wire structures, causing them to heat up. The efficiency of this "transduction
process" determines the responsivity of the device. Since the physical dimensions of thefree-standing structures are smaller than or comparable to the wavelength of the radiation
being detected, the problem of the interaction of the electromagnetic fields with the free­standing structures has been treated using the techniques of antenna theory. The structure322

Sensors and Materials, Vol. 8, No. 6 (1996)of the free-standing wire is analyzed as a transmitting antenna since, by the reciprocity
theorem in antenna theory,O tl the conclusions remain unaffected when its role is reversed tothat of a receiving antenna.
For the purposes of analysis, the free-standing wire forming the microthermocouple hot
junction, Fig. 1, is assumed to be a dipole antenna, aligned along the z-axis of a right­
handed Cartesian coordinate system with the hot junction at z = 0. The wires of copper andconstantan forming the free-standing structure are assumed to be the two legs of atransmitting dipole antenna and the hot junction is replaced by a narrow gap across which
the infrared signal is fed. The wider strip of metal leading to the cold junction, Fig. 1, isapproximated by four wires tracing the perimeter of the metal strip. The assumption of thegap A to be infinitesimally small can be justified in this case, since Af)., = 1 µm/10 µm =0.1 < l.
Infrared planar antennas fabricated on substrates exhibit properties different from thoseof antennas at lower frequencies.< 12-14l In the infrared, metals are characterized by theirsurface impedance. The surface impedance of metals at infrared frequencies is muchhigher than at microwave frequencies because of the skin effect. This causes losses anddamps the antenna currents, thereby degrading antenna performance. If -r denotes therelaxation time of the electrons in the metal, then at infrared frequencies co-r >> 1 and in thiscase the surface impedance Z, is given by<15l
1 µoPo. �
Zs = - �-- + JCO-V µoPo ,
T
2

(2)

where co is the frequency,� is the permeability of free space, and p0 is the de resistivity ofthe metal. It can be seen from eq. (2) that, in the infrared, the resistive part of Z, is aconstant, but the reactance increases as the first power of the frequency. Thus at infrared
frequencies the reactance is much more important than the resistance. Using the measured
values of de resistivities for the thin films of copper and constantanC9l and the value for therelaxation time of 2.7 x 10- 14 s for Cu,06l the surface impedances for copper and constantan
can be calculated to bez, (copper) = (1.6 + j 15.5) n,Z, (constantan) = (3.5 + j 33.7) n.

(3)

(4)

From the surface impedance, the loss impedance of any wire structure can be calculatedusing the expressionC11l
ZL = z,--

2na

(5)

where ZL is the loss impedance for the given wire structure, L is the length of the wire anda is its radius.
Since the wire forming the hot junction of the microthermocouple is free-standing, inSensors and Materials, Vol. 8, No. 6 (1996)

323

this simple analysis, the wire is assumed to be surrounded by free space and the effects ofthe substrate on the far field patterns have not been considered. The far field radiation
pattern was computed for the three lengths L = 20 µm, 40 µm and 60 µm of the free­standing wires using a software package AWAS (Analysis of Wire Antennas and Scatter­ers). < 17l AWAS is based on the two-potential equation for the distribution of antenna
current. This integro-differential equation is solved numerically, using the method of
moments with a polynomial (power series) approximation for the current distribution. Far
field patterns have been evaluated considering the structure as a transmitting antenna. The
antenna is assumed to be driven by a voltage source of frequency 28.3 THz, corresponding
to a wavelength of 10.6 µm. A lumped impedance of magnitude equal to that of the
impedance of the hot junction has been assumed across the input terminals of the source. A
distributed impedance along the copper and constantan wire lengths of magnitude equal to
the loss impedance was also assumed. The antenna dimensions in the model have beenvery closely approximated to those of the fabricated devices and the value of the far-zone
electric field as a function of the azimuth· angle 0 was computed. The wire for z > 0 was
assumed to be of copper while for z < 0 the wire was assumed to be made of constantan.
In order to experimentally investigate the microthermocouple output as a function of
the angle 0, an x-z translation mount was fixed on a precision rotation stage with a least
count of 1 minute of arc. The orientation of the microthermocouple in the socket was suchthat the rotations about the mounting post corresponded to variations in the angle 0. The
experimental arrangement is schematically shown in Fig. 5. In practice, it was very
difficult to mount the free-standing microthermocouple precisely on the axis of rotation of
the optical mount. Thus, for every rotation, the device was found to have moved out of theline of the incident infrared beam and had to be corrected by translating it back into thelaser beam. This was done by maximizing the output of the device for each angle by using
the x-z translation micrometer screws. An error of - 5% was estimated in the process of
maximization, due to the difficulty in aligning a 1-µm-wide free-standing wire to the exact
maximum of the Gaussian intensity profile of an infrared laser-the experimental condi­tion which would result in a maximum output. The output was measured at intervals of 5°
from normal incidence. The experimental arrangement permitted a maximum rotation of
50° . Another limitation of the experimental arrangement was that the measurements had tobe performed at an angle <f>- 15°, where <f> is the angle from the normal to the wire surface
in the plane perpendicular to the wire. The experimental results and the theoreticalbehavior of the free-standing structure as a function of 0, for <f> = 15° and for the threedifferent lengths of the free-standing wires, are plotted in Fig. 6. The error bars have been
calculated as the standard deviation of four different measurements made at each angle.
Agreement between the predictions of the model and the experiments is not perfect but
a number of features may be noted. There are striking periodicities in the measured outputs
and these are mimicked quite well by the antenna calculation, particularly for the 20 and 60µm devices as shown in Figs. 6(a) and 6(c), if a small translation of 2-4° is allowed in thepatterns. Agreement for the 40 µm device is not as good although the peak at 0 = 100° is
predicted. The experimental setup has several limitations which include: the uncertainty inthe exact orientation of the wire structure, the difficulty in signal maximization and theallowed maximum angular scan. Given the experimental constraints and the simplicity of
the antenna modelling, the comparisons are quite encouraging. They are certainly good

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Sensors and Materials, Vol. 8, No. 6 (1996)

CO21aser

Die cast box
Electric field vector of
the incident laser beamparallel to z-axis

Fig. 5. Schematic diagram showing the experimental arrangement used for the measurement of thedetector output as a function of the angle of incidence 0. Also illustrated is the relative orientation of
the free-standing wire with respect to the incident electric field vector of the laser beam. Rotations
about the mounting post correspond to variation in the angle 0.

:::s0-- Theoretical (arb. units)·····•···· Experimental (µV)

10

Q)

8

:::s
0
u
0

6

ii
(a)

12

... 4
...u

-

Q)

:§

2

0

90

120
110
100
Angle of incidence 0

130(de g)

140

Fig. 6. Dependence of the output for (a) 20-µm-, (b) 40-µm- and (c) 60-µm-long free-standing wirernicrothermocouples on the angle of incidence 0. Also plotted is the variation of the signal output
predicted using a simple model. Both the measurements and the calculations have been performedfor</)=15° .

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Sensors and Materials, Vol. 8, No. 6 (1996)-

25-- Theoretical (arb. units)......... Experimental (µV)

:::,

.S20
:::,
0

c.a, 15
:::,

0

..
-..
-�
()

(b)

0

E 10
a,

.c
0

:!=

5
0

-

90

100

110

120

Angle of incidence e

130

140

120

130

(deg)

30

[ 25
:::,

0

a, 20
C.
::::,
0
15
(c) 0
E...
(I)
.c 10
()

-..

0

()

i

5
0

-- Theoretical (arb. units)----,•--- Experimental (µV)

80

90

100

110

Angle of incidence e
Fig. 6.

(deg)(b) and (c).

enough to conclude that the devices are behaving as antennas at 10 µm. A more exact
theoretical model should take the presence of the silicon substrate under the free-standing
wires into consideration. In other experiments, the output of the microthermocouples was
also found to be a function of the polaiization of the incident radiation, which has alreadybeen reported. <8>

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4.

Sensors and Materials, Vol. 8, No. 6 (1996)

Conclusions

Free-standing microthermocouples and microthermopiles made from copper and con­stantan exhibit characteristics which are desirable for detecting infrared radiation and forCO2 laser power measurements. The free-standing microthermopile infrared detector was
found to be very fast with re -10 µs. The interaction of infrared radiation with the free­standing wires has been investigated using antenna theory. It should be possible to tailor
the antenna properties exhibited by these devices to result in a high directional sensitivityof the output. Staring arrays consisting of these directional detectors can be employed,
without the use of conventional optics, in infrared imagers and beam profilers. Besides
power measurements, these detectors can also be used for the simultaneous determinationof the polarization of CO2 and other infrared lasers, which is of immense significance inspectroscopic and industrial applications of these lasers.

Acknowledgments
The authors would like to thank Dr. R. Padman for the use of AWAS software and
Dr. R. J. Butcher for the use of the CO2 laser.

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