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Thin-film resistance bolometer IR detectors
Infrared Phys.Vol.24,No. I,pp.57-64,1984
0020-0891/84 $3.00+0.00
Prmtedin GreatBritain
THIN-FILM
PergamonPressLtd
RESISTANCE
BOLOMETER
IR DETECTORS
K. C. LIDDIARD Electronics
Research
Laboratory, GPO
Defence Science and Technology Organisation, Box 2151, Adelaide, South Australia 5001
Department
of Defence,
(Received 25 October 1983) Abstract-This paper outlines the factors which influence the performance of thin-film resistance bolometer IR detectors. A versatile technology is described which offers the ability to prepare detectors which have a high thermal efficiency, in particular extremely small values of thermal conductance and thermal self-capacitance. The preparation of metal-film bolometers is discussed, and a detectivity of 1.6 x lo* cm Hz’/’ W-’ with a speed of response < 1 msec has been achieved, for detector element sizes as small as 50pm, when operated in uocuo.
INTRODUCTION
Thin-film resistance bolometer IR detectors were first reported more than 25 years ago.” ‘) However, with the exception of recent studies at the Electronics Research Laboratory (South Australia) and the Battelle Institute, Frankfurt (F.R.G.),‘4’ very little interest appears to have evolved since the early work, despite the fact that theory predicts a respectable performance compared to other types of thermal detectors. This paper briefly outlines the major factors which influence the performance of thin-film bolometer detectors. Whilst a more extensive theory has been derived to predict performance and explain the significance of various materials’ properties, an approximate analysis is presented here in order to place emphasis on critical performance parameters. The objective of the research study at the Electronics Research Laboratory is the development of robust, low-cost, uncooled thermal detectors, which have a small element size ( < 0.1 mm), a fast speed of response (Q 1.O msec) and a detectivity exceeding 1 x 10’ cm Hz’/’ W-‘. A versatile process technology which meets this requirement is described in this paper. The technology discussed is for which a maximum detectivity of specifically that of a metal-film bolometer, 2 x 1O’cm Hz’/~ W-’ and a thermal time constant of 60.7 msec can be achieved, with detector element sizes as small as 50 pm. The reader will recognize that the basic technology has applications to other types of thermal detectors, and it is proposed to describe a detector of much higher detectivity in a subsequent paper. THEORY
The responsivity
of a bolometer
detector B=
can be expressed
in the form:‘5’
EBIRc~
(1)
G(l + 4n2f2r2)“”
where E is the emissivity of the absorbing surface, B = R,/(R + RJ is the circuit bridge factor, I is the bias current, R is the electrical resistance of the detector element, R, is the series load resistance, c1 is the temperature coefficient of resistance, G is the total thermal conductance coupling the detector f is the frequency of signal modulation and z is the thermal INF
24,
t
time constant. 51
to its surroundings,
K. C. LIDDIARD
58
If the performance the detectivity is
of the detector is limited by temperature
fluctuations and Johnson noise, then
(2) where A is the receiving area of the detector, Af is the noise bandwidth, P, is the noise-equivalent power associated with temperature fluctuations and V, is the Johnson-noise voltage. In order to simplify the analysis and highlight both material properties and critical performance, it will be assumed that the detector is operated at a temperature T of at most 100 K above the background temperature &, which is taken to be 300 K, and that over this range the total thermal conductance is constant. There are some weaknesses in this assumption of constant conductance (particularly for solid-backed or gas-filled detectors); however, provided the detector is operated below the point of thermal instability, it is found that predictions are in general agreement with experimental results. We can then write
V; = 4kB’R(T
+ RTJRJAj
(4)
and G = f’R/‘(T
where k is the Boltzmann constant. Solving equation (2) for the low-frequency
- To),
(5)
(d.c.) detectivity gives:
Equation (6) is plotted in Fig. 1 for two specific examples, representative of a semiconductor thermistor-flake bolometer and a self-absorbing thin-metal-film bolometer. Also shown are the temperature-fluctuation-limited performance for emissivities of 1.0 and 0.5 and the detectivity of an ideal plane detector of emissivity 1.0 radiating over both surfaces. The bolometer temperature is 325 K. ideal
rY I-
plane
detector
lo@T= 325
g
10’7
K
10-3
10-Z
THERMAL
CONDUCTANCE
1
lo-' PER
UNIT
SURFACE
AREA
( W cm -2K-‘)
Fig. 1. Low-frequency
detectivity
as a function
of thermal
conductance.
Thin-metal-film bolometer detectors
59
It can be seen from Fig. 1 that the performance of the thermistor bolometer detector approaches the temperature-fluctuation noise limit, whereas the metal-film bolometer will be Johnson-noise limited for all values of thermal conductance. This is a direct consequence of the high temperature coefficient of resistance of the semiconductor flake compared to that of a thin-metal film. In order to predict the absolute value of detectivity, an estimate of thermal conductance is required. The lower limit is set by radiation conductance and can be calculated using the linear approximation (assuming radiation from both surfaces) G, = ~AOE T3,
(7)
where (Tis the Stefan-Boltzmann constant. Some care must be taken when using equation (71, but a rigorous treatment shows that a conductance per unit area of 2 x 10m3E W cm-‘K-’ is an acceptable approximation at typical bias temperatures. The major contribution to the total thermal conductance is usually conduction loss, including gaseous conduction to nearby heat sinks. An order of magnitude estimation can be made by applying simplistic models under steady-state conditions. For the purpose of this discussion it is desirable to distinguish between a solid-backed detector, wherein the detector element is deposited or bonded onto a substrate which is in intimate thermal contact with a massive backing, and a pellicle-supported detector where the detector element is deposited onto a thin membrane. In the case of a solid-backed detector such as the thermistor-flake bolometer, it can be shown@) that the substrate conductance is G, = &A ld,,
(81
where K, is the thermal conductivity of the substrate, ci, its thickness, and it is assumed that the detector and substrate are mounted on a perfect heat sink at ambient temperature. Because the substrate contributes to the total thermal capacitance, materials with a large thermal conductivity, such as sapphire (rC, = 0.5 W cm-’ K-‘) or beryllia (KS= 2.5 W cm-’ K-‘), are widely used to achieve a fast speed of response. Thus, for a typical substrate thickness of 1.0 mm, the thermal conductance per unit surface area is at least 5.0 W cm-‘* K-‘. The predicted maximum (low frequency) detectivity is about 2 x 10’ cm Hz”~ W-’ for a temperature coefficient of resistance of 0.04 K-‘, in agreement with typical measured values. The thermal conductance for a pelli~le-supported detector can be estimated from the solution to the equation for heat loss in a flat strip of length t, area A and thickness d, supported at each end by perfect heat sinks maintained at ambient temperature. (QThe average value of conductance due to conduction loss to the heat sinks is given by G, = 12 KdA /12.
19)
Clearly for a given length the product Kd must be kept as small as possible. It will be shown later that this product can be as low as 8 x 10mRW K-’ for both a thin dielectric pellicle and a self-absorbing metal film. Thus according to equation (9), the thermal conductance per unit area of a 0.1 mm long strip of dielectric material coated with an ultrathin metal film would be 2 x 10e2 W cmm2K-’ (assuming that the individual contributions to the conductance are additive), i.e. about 10 times that of radiation conductance. Referring to Fig. 1, this suggests that a metal-film bolometer operated in U~CUO and having a temperature coefficient of resistance of only 0.0015 I(-’ should have a maximum detectivity of greater than 2 x lo* cm Hz”~ W-‘. The detectivity would be further increased by operating at a higher bias temperature or by increasing the length of the detector. An additional contribution to the thermal conductance must be taken into account when detectors are operated in a gas-filled package. A rough approximation of gaseous.~onduction loss can be obtained from equation (8). Taking air at atmospheric pressure as an example (K = 2.5 x 10m4W cm-’ K-l), the low-frequency thermal conductance per unit surface area for a 1 mm pathlength is 2.5 x low3W cm-* K-l. This order of magnitude is of little importance with solid-backed detectors but is significant for pellicle-supported detectors. The thermal time constant of a bolometer detector is given by z = C/G,
(10)
60
K. C. LIDDIARU
where C is the total thermal capacitance. As stated earlier, the substrate component of a solid-backed detector makes a significant contribution to the total capacitance, and hence it is necessary to increase conductance (at the expense of detectivity) in order to achieve a fast speed of response. Furthermore, both the conductance and capacitance are frequency dependent. with the result that the frequency response departs from that determined by a single unique time constant---characteristic of a simple exponential law. The thermal capacitance of a pellicle-supported detector operated in vacua is the self-capacitance of the detector element and support pellicle. For a thin film C = pcAd.
(11)
where p is the density of the material and c its specific heat. The product pc does not vary greatly for most solid materials, and is typically 2-3 J cm--j K-’ for thin films. If we take the previous value of 2 x 1O-2 W cm-’ K-’ for the thermal conductance of a thin-film vacuum bolometer, then the thermal time constant would be about 1 msec when the thickness is less than 100 nm. Note that it is assumed that the total thermal capacitance of the bolometer is the sum of the individual components. This approximation can be applied on the grounds that the transmission of heat through the thickness of the composite pellicle is effectively instantaneous compared to typical thermal time constants. When a pellicle-supported detector is operated in a gas-filled package there is an increase in both thermal conductance and capacitance, the magnitude of which depends on the type of gas and the gas pressure. It can be readily shown from the theory of gases at reduced pressure”’ that the contribution to the thermal capacitance will not become significant until the pressure approaches atmospheric, whereas the thermal conductance rises sharply at a pressure of about 1 Pa (7.5 mtorr) and becomes constant in the range lo’-IO4 Pa (< 100 torr). It will be shown later that the detectivity and thermal time constant of a typical thin-film bolometer are larger in vacua by a factor of 5510 times that at atmospheric pressure. In summary, it has been shown possible, in theory, to prepare metal-film bolometer detectors with element sizes as small as 0.1 mm, which have a detectivity and speed of response at least comparable with a solid-backed semiconductor thermistor bolometer detector. This is achieved by employing a pellicle-supported design which has a very low thermal conductance; and in practice ultrathin components each having a small thermal conductivity are used. Since the thermal capacitance is also small, a fast speed of response is possible. The following discussions are concerned with detectors of this type. DETECTOR
MATERIALS
The IR sensing element of the thin-film bolometer detector described in this paper comprises an ultrathin metal film deposited onto an amorphous dielectric pellicle. Both components should be as thin as possible and have a low thermal conductivity (the product of thickness and thermal conductivity must be small), and they should be thermally and mechanically stable. The pellicle material must adhere strongly and be a good thermal match to the support substrate. The metal film should be chemically inert, electrically stable and have the maximum possible IR absorptance and temperature coefficient of resistance. A single metal film can absorb at most 50% of incidence radiation.“) The maximum absorption is achieved in theory when the electrical resistance of the film is 189Q/square. A substantial enhancement in absorptance is possible using interferometric techniques; however, since the thickness of the pellicle must be less than 100 nm, the only feasible method is to use a back reflector separated from the pellicle by a gap of n/4, where i is the wavelength of maximum absorption. It may be noted that the single metal film is a broadband IR absorber, whereas interferometric enhancement produces selective absorption. between IR Research in the author’s laboratory, (9’ has shown that the theoretical relationship absorptance and electrical resistance is achieved in practice for metals which have melting point (m.p.) 3 1500 K. The maximum absorptance is observed as a function of thickness for lower m.p. metals, but the correlation with resistance is strongly dependent on deposition conditions. For example, it is possible to obtain Au films of optimum absorptance which have resistances ranging
Thin-metal-film
bolometer
detectors
61
from SOR/square to MR/square or even non-conducting. These properties are associated with nucleation, growth and structural phenomenon in thin films. It is well known(“)t the electrical resistivity of continuous metal films is larger than that of the bulk materials due to scattering of conduction electrons from grain boundaries, from structural defects and impurities and from the surface of the film. A steep rise in resistivity is predicted and observed when the thickness of the film is less than the mean-free-path of conduction electrons, i.e. less than about 50 nm for most metals. The rise in resistivity is accompanied by a decrease in the temperature coefficient of resistance and, in accordance with the Wiedemann and Franz law, a corresponding decrease in thermal conductivity. Because the resistivity required to achieve optimum IR absorption falls in the region of conduction between initial nucleation of the film and the asymptotic value attained with increasing thickness, the selection of a suitable metal depends on the study and understanding of the nucleation, growth and structural characteristics during film formation. It is also well established,“‘) that thin films of high-m.p. metals are, as a general rule, more finely crystalline than low-m.p. metals prepared under the same deposition conditions. Bearing in mind that the mean-free-path of conduction electrons in metals such as Ni, Pd, Pt and Ir is smaller than that for lower-m.p. metals such as Au and Ag, it is tempting to conclude that high-m.p. metals will form continuous films of the desired electrical resistance at a smaller thickness than films of low-m.p. metals. This trend is in fact observed in practice; for example the thickness of a Pt-film bolometer is roughly one-quarter that of an Au-film bolometer. A number of different metal films have been examined in this study, including those mentioned above. Pt has been chosen on the grounds of thickness, stability and predictability, following an extensive investigation of electrical, optical, structural, mechanical and thermophysical properties. The resistance of vacuum-deposited Pt films is plotted as a function of thickness in Fig. 2. The data are for films prepared by electron-beam deposition and annealed at 250°C. It is seen that the resistance for optimum IR absorption is obtained at a thickness of about 4 nm. The corresponding temperature coefficient of resistance is 0.07-0.1% K-l. Recent research studies indicate that it is possible to enhance the temperature coefficient to 0.15% K-i. As expected from theory, the thermal conductivity of the Pt films is about one-fifth of the bulk value, i.e. 0.15 W cm-l K-l. This value was derived from microradiometer scan records of heat spread in the plane of large bolometer films. Note that the Kd product, which determines thermal conduction loss in the plane of the film, is 6 x lo-’ W K-‘. Turning now to the detector pellicle, thermoplastic materials would appear to be attractive because of very low values of thermal conductivity. However, these materials have a limited temperature range of operation and suffer from other disadvantages such as mechanical instability. A wide range of polymeric and amorphous inorganic dielectric materials have been evaluated, and as the result of this study preference is given to pellicles prepared from thin films of A&O, or S&N,. A&O, pellicles (25 nm thick), prepared by electron-beam deposition, have been used routinely in bolometer preparation. The estimated thermal conductivity of these pellicle films is 500
2
400
s :: $-
300
1
II
"III
II”
01234567
8
THICKNESS
Fig. 2. Electrical
resistance
I nm )
of Pt films.
9
10
62
K. c.
LIDDIARU
SIO,
p+-SI-. n-5 St0
2
-
Fig. 3. ~e~~l-~lln
bolometcr
detector.
0.03 W cm-’ K-‘, giving a iGi value of 7.5 x IO-” W K~-‘. Si,N, pellicles prepared by chemical vapour or glow discharge deposition are under evaluation as an option to Al,@. The total value of the thermal conductance, calculated from equations (7) and (9) for a Pt-film bolometer with an element size of 0.1 x 0.1 mm’ operated in LUCUO,is 1.X x lO--’ W K -I. In practice, other contributions to the conductance must be taken into account, for example heat loss to nearby heat sinks via the uncoated areas of the pellicle. However, the simple model is surprisingly accurate, as will be seen later in the discussion on measured performance. DETECTOR
PREPARATION
The essential components of the bolometer are the sensing element and associated electrical contacts, the detector substrate, and a suitable package fitted with an IR-transmitting window. Various designs which employ a range of detector. pellicle and substrate materials, have been devised.C’2) The technology described in this section is the latest process now under development for the preparation of single-element and arrays of thin-film bolometer detectors. The bolometer detector is illustrated schematically in Fig. 3. A major feature of the technology is the ability to fabricate ultrathin pellicles by means of anisotropic etching of a Si substrate.““’ Pellicle windows are formed by depositing a thin film of the desired material onto the surface of the substrate then etching through the substrate from the rear surface. Detector substrates are (100) surface orientation Si wafers used for microcircuit manufacture. The first steps in the process are the generation of thermal oxide masks, which are used to produce registration markers, Si etch masks, and to selectively B-dope the front surface of the substrate. The B-doping acts as an etch-stop to ensure precise window definition. The pellicle material and metal film are deposited onto the front surface of the wafer, and the detector pattern is formed photolithographically using lift-off or plasma etching. This is followed by deposition and etching of contacts, interconnects and bonding pads. Either Au-based or conventional Al metallizations are suitable. Pellicle windows are then produced by rear-surface etching through a thermal oxide mask, using hydrazine or ethylene diamine/pyrocatechol echants. The etch process produces V-grooves at an angle of 54.7” to the surface plane. Note that vertical side walls can be achieved with (110) orientation Si wafers. Finally, completed detectors are mounted in microcircuit-style packages fitted with IR windows. Low-cost packages which can be evacuated or filled to a predetermined pressure with a selected gas are under development at the Electronics Research Laboratory. Because detector elements are defined by conventional planar photolithography, a variety of shapes and sizes are possible. Both single-element and arrays of detectors can be prepared. and by means of step-and-repeat artwork, a number of detectors can be fabricated on the same w-afer. Detector elements with sizes of 50 ,~rn to l.Omm and resistances from lOO-2500R have been prepared, with shapes ranging from simple strips to fine zig-zag patterns. PERFORMANCE
The responsivity, detectivity and speed of response have been measured for a number of metal-film bolometer detectors under a range of operating conditions. From performance measurements it is possible to calculate other important parameters. such as average temperature rise, thermal conductance, maximum bias current and total effective thermal capacitance. Most of these measurements were made on laboratory test specimens, prepared by depositing the Pt film
Thin-metal-film
I
1
I
0.01 Fig. 4. d.c. Responsivity
of metal-film
bolometer
01
63
detectors
I
I
J
1
10
loo
PRESSURE (torr) bolometer as a function A = 0.075 x 0.1 mm2.
of pressure.
R = 360R; I = 0.75 mA;
through a micromechanical evaporation mask onto a pre-mounted pellicle. It is convenient to choose one of these specimens, to illustrate the performance characteristics of a self-absorbing metal-film bolometer detector. The maximum (low frequency) responsivity of a typical detector of size 100 x 75 pm (I x w) is shown in Fig. 4. The responsivity is plotted as a function of pressure at a bias current of 0.75 mA. The detector resistance is 3600. It is seen that the maximum responsivity of 50 V W-‘, which is achieved in DUCUO,falls to 4.5 V W-’ in air at 100 torr pressure, and is then constant as the pressure is further increased to atmospheric pressure. The responsivity of a gas-filled detector can be enhanced by using a low thermal conductivity gas such as Freon F-22 or Xe. In the latter case the responsivity is twice that obtained with air. Assuming Johnson-noise-limited performance, which has been verified for frequencies greater than 10 Hz, the maximum detectivity of the detector is 1.6 x 108cm Hz’12W-’ when operated in U~CUO and 3 x 1O’cm Hz”’ W-’ in Xe at atmospheric pressure. The respective average bias temperatures, referred to a 300 K background, are 400 and 315 K. By increasing the bias current for the Xe-filled detector, so as to attain the same bias temperature as the vacuum bolometer, it is possible to double the detectivity, i.e. 6 x 10’ cm Hz ‘j2 W-’ . This is roughly the maximum safe bias condition for which a substantial enhancement in detectivity can be achieved, since a further increase in current rapidly leads to detector burnout. An optical study of deliberately-damaged detectors suggests that failure is caused by electromigration, despite the use of a bridge factor approaching unity, when failure would be expected to occur by thermal runaway. The thermal conductance of the above detector specimen, calculated from experimental data, is 2 x 1O-6 W K-’ in ZIUCUOand 1 x 10m5W K-’ for Xe at pressures greater than 10 torr. The measured speed of response was 0.7 msec for operation in vucuo and 0.15 msec for Xe at l&100 torr. Thus the total effective self-capacitance of the detector is 1.4 x 10m9J K-l. These results are in good agreement with theoretical predictions. However, the measured temperature coefficient of resistance was O.OS’AK-l, and it is anticipated that a higher temperature coefficient will be achieved using the recently-developed technology described in this paper, giving a 1.5 times predicted increase in detectivity. In general the performance characteristics can be varied over a wide range by virtue of changes in design, choice of materials and selection of operating conditions. The element size and electrical resistance can also be readily changed within the limits imposed by conventional photolithography. Acknowledgemenfs-The author wishes to acknowledge the contributions of his colleagues, J. Hlava, Powell, to the work described in this paper, and is indebted to the staff of the Electronics Workshop Instrumentation Groups of the Advanced Engineering Laboratory, South Australia, for their support. AWA Microelectronics, Sydney, N.S.W., is also greatly appreciated. REFERENCES 1. Aiken 2. Billings
C. B., Carter W. H. and Philips F. S., Rev. scient. Instrum. 17, 377 (1946) B. H., Barr E. E. and Hyde W. L., Rev. scienr. Instrum. 18, 429 (1947).
P. Girdler and I. and Mechanisms The assistance of
64
K. C. LIDDIARD
3. Yosihara K., Sci. LI, Tokyo 5, 29 (1956). 4. Hartmann R. and Selders M., Sensor 82 ConJ Proc. Battelle Institute. Frankfurt. F.R.G., 5. p. 26. 5. Smith R. A., Jones F. E. and Chasmar R. F., The Detection and Measurement of Iyfiared Radiation. Clarendon Oxford (1957). 6. Carslaw H. S. and Jaeger J. C., Conduction o/ Hear in Solids. Clarendon Press, Oxford (1959). 7. Dushman S., Scien@ Foundations qf’ Vacuum Technique, Wiley, New York (1962). 8. Hadley L. N. and Dennison D. M., J. opt. Sot. Am. 37, 451 (1947). 9. Liddiard K. C., MSc. Thesis. University of Adelaide, Australia (1973). IO. Coutts T. J., EIec/rica/ Conduction in Thin Metal Films. Elsevier. Amsterdam (1974). Il. Chopra K. L., Thin Film Phenomena. McGraw-Hill, New York (1969). 12. Liddiard K. C., Australian Patent Application 75842/81 (1981). 13. Petersen K. E.. Proc. IEEE 70, 420 (1982).
Press.
0020-0891/84 $3.00+0.00
Prmtedin GreatBritain
THIN-FILM
PergamonPressLtd
RESISTANCE
BOLOMETER
IR DETECTORS
K. C. LIDDIARD Electronics
Research
Laboratory, GPO
Defence Science and Technology Organisation, Box 2151, Adelaide, South Australia 5001
Department
of Defence,
(Received 25 October 1983) Abstract-This paper outlines the factors which influence the performance of thin-film resistance bolometer IR detectors. A versatile technology is described which offers the ability to prepare detectors which have a high thermal efficiency, in particular extremely small values of thermal conductance and thermal self-capacitance. The preparation of metal-film bolometers is discussed, and a detectivity of 1.6 x lo* cm Hz’/’ W-’ with a speed of response < 1 msec has been achieved, for detector element sizes as small as 50pm, when operated in uocuo.
INTRODUCTION
Thin-film resistance bolometer IR detectors were first reported more than 25 years ago.” ‘) However, with the exception of recent studies at the Electronics Research Laboratory (South Australia) and the Battelle Institute, Frankfurt (F.R.G.),‘4’ very little interest appears to have evolved since the early work, despite the fact that theory predicts a respectable performance compared to other types of thermal detectors. This paper briefly outlines the major factors which influence the performance of thin-film bolometer detectors. Whilst a more extensive theory has been derived to predict performance and explain the significance of various materials’ properties, an approximate analysis is presented here in order to place emphasis on critical performance parameters. The objective of the research study at the Electronics Research Laboratory is the development of robust, low-cost, uncooled thermal detectors, which have a small element size ( < 0.1 mm), a fast speed of response (Q 1.O msec) and a detectivity exceeding 1 x 10’ cm Hz’/’ W-‘. A versatile process technology which meets this requirement is described in this paper. The technology discussed is for which a maximum detectivity of specifically that of a metal-film bolometer, 2 x 1O’cm Hz’/~ W-’ and a thermal time constant of 60.7 msec can be achieved, with detector element sizes as small as 50 pm. The reader will recognize that the basic technology has applications to other types of thermal detectors, and it is proposed to describe a detector of much higher detectivity in a subsequent paper. THEORY
The responsivity
of a bolometer
detector B=
can be expressed
in the form:‘5’
EBIRc~
(1)
G(l + 4n2f2r2)“”
where E is the emissivity of the absorbing surface, B = R,/(R + RJ is the circuit bridge factor, I is the bias current, R is the electrical resistance of the detector element, R, is the series load resistance, c1 is the temperature coefficient of resistance, G is the total thermal conductance coupling the detector f is the frequency of signal modulation and z is the thermal INF
24,
t
time constant. 51
to its surroundings,
K. C. LIDDIARD
58
If the performance the detectivity is
of the detector is limited by temperature
fluctuations and Johnson noise, then
(2) where A is the receiving area of the detector, Af is the noise bandwidth, P, is the noise-equivalent power associated with temperature fluctuations and V, is the Johnson-noise voltage. In order to simplify the analysis and highlight both material properties and critical performance, it will be assumed that the detector is operated at a temperature T of at most 100 K above the background temperature &, which is taken to be 300 K, and that over this range the total thermal conductance is constant. There are some weaknesses in this assumption of constant conductance (particularly for solid-backed or gas-filled detectors); however, provided the detector is operated below the point of thermal instability, it is found that predictions are in general agreement with experimental results. We can then write
V; = 4kB’R(T
+ RTJRJAj
(4)
and G = f’R/‘(T
where k is the Boltzmann constant. Solving equation (2) for the low-frequency
- To),
(5)
(d.c.) detectivity gives:
Equation (6) is plotted in Fig. 1 for two specific examples, representative of a semiconductor thermistor-flake bolometer and a self-absorbing thin-metal-film bolometer. Also shown are the temperature-fluctuation-limited performance for emissivities of 1.0 and 0.5 and the detectivity of an ideal plane detector of emissivity 1.0 radiating over both surfaces. The bolometer temperature is 325 K. ideal
rY I-
plane
detector
lo@T= 325
g
10’7
K
10-3
10-Z
THERMAL
CONDUCTANCE
1
lo-' PER
UNIT
SURFACE
AREA
( W cm -2K-‘)
Fig. 1. Low-frequency
detectivity
as a function
of thermal
conductance.
Thin-metal-film bolometer detectors
59
It can be seen from Fig. 1 that the performance of the thermistor bolometer detector approaches the temperature-fluctuation noise limit, whereas the metal-film bolometer will be Johnson-noise limited for all values of thermal conductance. This is a direct consequence of the high temperature coefficient of resistance of the semiconductor flake compared to that of a thin-metal film. In order to predict the absolute value of detectivity, an estimate of thermal conductance is required. The lower limit is set by radiation conductance and can be calculated using the linear approximation (assuming radiation from both surfaces) G, = ~AOE T3,
(7)
where (Tis the Stefan-Boltzmann constant. Some care must be taken when using equation (71, but a rigorous treatment shows that a conductance per unit area of 2 x 10m3E W cm-‘K-’ is an acceptable approximation at typical bias temperatures. The major contribution to the total thermal conductance is usually conduction loss, including gaseous conduction to nearby heat sinks. An order of magnitude estimation can be made by applying simplistic models under steady-state conditions. For the purpose of this discussion it is desirable to distinguish between a solid-backed detector, wherein the detector element is deposited or bonded onto a substrate which is in intimate thermal contact with a massive backing, and a pellicle-supported detector where the detector element is deposited onto a thin membrane. In the case of a solid-backed detector such as the thermistor-flake bolometer, it can be shown@) that the substrate conductance is G, = &A ld,,
(81
where K, is the thermal conductivity of the substrate, ci, its thickness, and it is assumed that the detector and substrate are mounted on a perfect heat sink at ambient temperature. Because the substrate contributes to the total thermal capacitance, materials with a large thermal conductivity, such as sapphire (rC, = 0.5 W cm-’ K-‘) or beryllia (KS= 2.5 W cm-’ K-‘), are widely used to achieve a fast speed of response. Thus, for a typical substrate thickness of 1.0 mm, the thermal conductance per unit surface area is at least 5.0 W cm-‘* K-‘. The predicted maximum (low frequency) detectivity is about 2 x 10’ cm Hz”~ W-’ for a temperature coefficient of resistance of 0.04 K-‘, in agreement with typical measured values. The thermal conductance for a pelli~le-supported detector can be estimated from the solution to the equation for heat loss in a flat strip of length t, area A and thickness d, supported at each end by perfect heat sinks maintained at ambient temperature. (QThe average value of conductance due to conduction loss to the heat sinks is given by G, = 12 KdA /12.
19)
Clearly for a given length the product Kd must be kept as small as possible. It will be shown later that this product can be as low as 8 x 10mRW K-’ for both a thin dielectric pellicle and a self-absorbing metal film. Thus according to equation (9), the thermal conductance per unit area of a 0.1 mm long strip of dielectric material coated with an ultrathin metal film would be 2 x 10e2 W cmm2K-’ (assuming that the individual contributions to the conductance are additive), i.e. about 10 times that of radiation conductance. Referring to Fig. 1, this suggests that a metal-film bolometer operated in U~CUO and having a temperature coefficient of resistance of only 0.0015 I(-’ should have a maximum detectivity of greater than 2 x lo* cm Hz”~ W-‘. The detectivity would be further increased by operating at a higher bias temperature or by increasing the length of the detector. An additional contribution to the thermal conductance must be taken into account when detectors are operated in a gas-filled package. A rough approximation of gaseous.~onduction loss can be obtained from equation (8). Taking air at atmospheric pressure as an example (K = 2.5 x 10m4W cm-’ K-l), the low-frequency thermal conductance per unit surface area for a 1 mm pathlength is 2.5 x low3W cm-* K-l. This order of magnitude is of little importance with solid-backed detectors but is significant for pellicle-supported detectors. The thermal time constant of a bolometer detector is given by z = C/G,
(10)
60
K. C. LIDDIARU
where C is the total thermal capacitance. As stated earlier, the substrate component of a solid-backed detector makes a significant contribution to the total capacitance, and hence it is necessary to increase conductance (at the expense of detectivity) in order to achieve a fast speed of response. Furthermore, both the conductance and capacitance are frequency dependent. with the result that the frequency response departs from that determined by a single unique time constant---characteristic of a simple exponential law. The thermal capacitance of a pellicle-supported detector operated in vacua is the self-capacitance of the detector element and support pellicle. For a thin film C = pcAd.
(11)
where p is the density of the material and c its specific heat. The product pc does not vary greatly for most solid materials, and is typically 2-3 J cm--j K-’ for thin films. If we take the previous value of 2 x 1O-2 W cm-’ K-’ for the thermal conductance of a thin-film vacuum bolometer, then the thermal time constant would be about 1 msec when the thickness is less than 100 nm. Note that it is assumed that the total thermal capacitance of the bolometer is the sum of the individual components. This approximation can be applied on the grounds that the transmission of heat through the thickness of the composite pellicle is effectively instantaneous compared to typical thermal time constants. When a pellicle-supported detector is operated in a gas-filled package there is an increase in both thermal conductance and capacitance, the magnitude of which depends on the type of gas and the gas pressure. It can be readily shown from the theory of gases at reduced pressure”’ that the contribution to the thermal capacitance will not become significant until the pressure approaches atmospheric, whereas the thermal conductance rises sharply at a pressure of about 1 Pa (7.5 mtorr) and becomes constant in the range lo’-IO4 Pa (< 100 torr). It will be shown later that the detectivity and thermal time constant of a typical thin-film bolometer are larger in vacua by a factor of 5510 times that at atmospheric pressure. In summary, it has been shown possible, in theory, to prepare metal-film bolometer detectors with element sizes as small as 0.1 mm, which have a detectivity and speed of response at least comparable with a solid-backed semiconductor thermistor bolometer detector. This is achieved by employing a pellicle-supported design which has a very low thermal conductance; and in practice ultrathin components each having a small thermal conductivity are used. Since the thermal capacitance is also small, a fast speed of response is possible. The following discussions are concerned with detectors of this type. DETECTOR
MATERIALS
The IR sensing element of the thin-film bolometer detector described in this paper comprises an ultrathin metal film deposited onto an amorphous dielectric pellicle. Both components should be as thin as possible and have a low thermal conductivity (the product of thickness and thermal conductivity must be small), and they should be thermally and mechanically stable. The pellicle material must adhere strongly and be a good thermal match to the support substrate. The metal film should be chemically inert, electrically stable and have the maximum possible IR absorptance and temperature coefficient of resistance. A single metal film can absorb at most 50% of incidence radiation.“) The maximum absorption is achieved in theory when the electrical resistance of the film is 189Q/square. A substantial enhancement in absorptance is possible using interferometric techniques; however, since the thickness of the pellicle must be less than 100 nm, the only feasible method is to use a back reflector separated from the pellicle by a gap of n/4, where i is the wavelength of maximum absorption. It may be noted that the single metal film is a broadband IR absorber, whereas interferometric enhancement produces selective absorption. between IR Research in the author’s laboratory, (9’ has shown that the theoretical relationship absorptance and electrical resistance is achieved in practice for metals which have melting point (m.p.) 3 1500 K. The maximum absorptance is observed as a function of thickness for lower m.p. metals, but the correlation with resistance is strongly dependent on deposition conditions. For example, it is possible to obtain Au films of optimum absorptance which have resistances ranging
Thin-metal-film
bolometer
detectors
61
from SOR/square to MR/square or even non-conducting. These properties are associated with nucleation, growth and structural phenomenon in thin films. It is well known(“)t the electrical resistivity of continuous metal films is larger than that of the bulk materials due to scattering of conduction electrons from grain boundaries, from structural defects and impurities and from the surface of the film. A steep rise in resistivity is predicted and observed when the thickness of the film is less than the mean-free-path of conduction electrons, i.e. less than about 50 nm for most metals. The rise in resistivity is accompanied by a decrease in the temperature coefficient of resistance and, in accordance with the Wiedemann and Franz law, a corresponding decrease in thermal conductivity. Because the resistivity required to achieve optimum IR absorption falls in the region of conduction between initial nucleation of the film and the asymptotic value attained with increasing thickness, the selection of a suitable metal depends on the study and understanding of the nucleation, growth and structural characteristics during film formation. It is also well established,“‘) that thin films of high-m.p. metals are, as a general rule, more finely crystalline than low-m.p. metals prepared under the same deposition conditions. Bearing in mind that the mean-free-path of conduction electrons in metals such as Ni, Pd, Pt and Ir is smaller than that for lower-m.p. metals such as Au and Ag, it is tempting to conclude that high-m.p. metals will form continuous films of the desired electrical resistance at a smaller thickness than films of low-m.p. metals. This trend is in fact observed in practice; for example the thickness of a Pt-film bolometer is roughly one-quarter that of an Au-film bolometer. A number of different metal films have been examined in this study, including those mentioned above. Pt has been chosen on the grounds of thickness, stability and predictability, following an extensive investigation of electrical, optical, structural, mechanical and thermophysical properties. The resistance of vacuum-deposited Pt films is plotted as a function of thickness in Fig. 2. The data are for films prepared by electron-beam deposition and annealed at 250°C. It is seen that the resistance for optimum IR absorption is obtained at a thickness of about 4 nm. The corresponding temperature coefficient of resistance is 0.07-0.1% K-l. Recent research studies indicate that it is possible to enhance the temperature coefficient to 0.15% K-i. As expected from theory, the thermal conductivity of the Pt films is about one-fifth of the bulk value, i.e. 0.15 W cm-l K-l. This value was derived from microradiometer scan records of heat spread in the plane of large bolometer films. Note that the Kd product, which determines thermal conduction loss in the plane of the film, is 6 x lo-’ W K-‘. Turning now to the detector pellicle, thermoplastic materials would appear to be attractive because of very low values of thermal conductivity. However, these materials have a limited temperature range of operation and suffer from other disadvantages such as mechanical instability. A wide range of polymeric and amorphous inorganic dielectric materials have been evaluated, and as the result of this study preference is given to pellicles prepared from thin films of A&O, or S&N,. A&O, pellicles (25 nm thick), prepared by electron-beam deposition, have been used routinely in bolometer preparation. The estimated thermal conductivity of these pellicle films is 500
2
400
s :: $-
300
1
II
"III
II”
01234567
8
THICKNESS
Fig. 2. Electrical
resistance
I nm )
of Pt films.
9
10
62
K. c.
LIDDIARU
SIO,
p+-SI-. n-5 St0
2
-
Fig. 3. ~e~~l-~lln
bolometcr
detector.
0.03 W cm-’ K-‘, giving a iGi value of 7.5 x IO-” W K~-‘. Si,N, pellicles prepared by chemical vapour or glow discharge deposition are under evaluation as an option to Al,@. The total value of the thermal conductance, calculated from equations (7) and (9) for a Pt-film bolometer with an element size of 0.1 x 0.1 mm’ operated in LUCUO,is 1.X x lO--’ W K -I. In practice, other contributions to the conductance must be taken into account, for example heat loss to nearby heat sinks via the uncoated areas of the pellicle. However, the simple model is surprisingly accurate, as will be seen later in the discussion on measured performance. DETECTOR
PREPARATION
The essential components of the bolometer are the sensing element and associated electrical contacts, the detector substrate, and a suitable package fitted with an IR-transmitting window. Various designs which employ a range of detector. pellicle and substrate materials, have been devised.C’2) The technology described in this section is the latest process now under development for the preparation of single-element and arrays of thin-film bolometer detectors. The bolometer detector is illustrated schematically in Fig. 3. A major feature of the technology is the ability to fabricate ultrathin pellicles by means of anisotropic etching of a Si substrate.““’ Pellicle windows are formed by depositing a thin film of the desired material onto the surface of the substrate then etching through the substrate from the rear surface. Detector substrates are (100) surface orientation Si wafers used for microcircuit manufacture. The first steps in the process are the generation of thermal oxide masks, which are used to produce registration markers, Si etch masks, and to selectively B-dope the front surface of the substrate. The B-doping acts as an etch-stop to ensure precise window definition. The pellicle material and metal film are deposited onto the front surface of the wafer, and the detector pattern is formed photolithographically using lift-off or plasma etching. This is followed by deposition and etching of contacts, interconnects and bonding pads. Either Au-based or conventional Al metallizations are suitable. Pellicle windows are then produced by rear-surface etching through a thermal oxide mask, using hydrazine or ethylene diamine/pyrocatechol echants. The etch process produces V-grooves at an angle of 54.7” to the surface plane. Note that vertical side walls can be achieved with (110) orientation Si wafers. Finally, completed detectors are mounted in microcircuit-style packages fitted with IR windows. Low-cost packages which can be evacuated or filled to a predetermined pressure with a selected gas are under development at the Electronics Research Laboratory. Because detector elements are defined by conventional planar photolithography, a variety of shapes and sizes are possible. Both single-element and arrays of detectors can be prepared. and by means of step-and-repeat artwork, a number of detectors can be fabricated on the same w-afer. Detector elements with sizes of 50 ,~rn to l.Omm and resistances from lOO-2500R have been prepared, with shapes ranging from simple strips to fine zig-zag patterns. PERFORMANCE
The responsivity, detectivity and speed of response have been measured for a number of metal-film bolometer detectors under a range of operating conditions. From performance measurements it is possible to calculate other important parameters. such as average temperature rise, thermal conductance, maximum bias current and total effective thermal capacitance. Most of these measurements were made on laboratory test specimens, prepared by depositing the Pt film
Thin-metal-film
I
1
I
0.01 Fig. 4. d.c. Responsivity
of metal-film
bolometer
01
63
detectors
I
I
J
1
10
loo
PRESSURE (torr) bolometer as a function A = 0.075 x 0.1 mm2.
of pressure.
R = 360R; I = 0.75 mA;
through a micromechanical evaporation mask onto a pre-mounted pellicle. It is convenient to choose one of these specimens, to illustrate the performance characteristics of a self-absorbing metal-film bolometer detector. The maximum (low frequency) responsivity of a typical detector of size 100 x 75 pm (I x w) is shown in Fig. 4. The responsivity is plotted as a function of pressure at a bias current of 0.75 mA. The detector resistance is 3600. It is seen that the maximum responsivity of 50 V W-‘, which is achieved in DUCUO,falls to 4.5 V W-’ in air at 100 torr pressure, and is then constant as the pressure is further increased to atmospheric pressure. The responsivity of a gas-filled detector can be enhanced by using a low thermal conductivity gas such as Freon F-22 or Xe. In the latter case the responsivity is twice that obtained with air. Assuming Johnson-noise-limited performance, which has been verified for frequencies greater than 10 Hz, the maximum detectivity of the detector is 1.6 x 108cm Hz’12W-’ when operated in U~CUO and 3 x 1O’cm Hz”’ W-’ in Xe at atmospheric pressure. The respective average bias temperatures, referred to a 300 K background, are 400 and 315 K. By increasing the bias current for the Xe-filled detector, so as to attain the same bias temperature as the vacuum bolometer, it is possible to double the detectivity, i.e. 6 x 10’ cm Hz ‘j2 W-’ . This is roughly the maximum safe bias condition for which a substantial enhancement in detectivity can be achieved, since a further increase in current rapidly leads to detector burnout. An optical study of deliberately-damaged detectors suggests that failure is caused by electromigration, despite the use of a bridge factor approaching unity, when failure would be expected to occur by thermal runaway. The thermal conductance of the above detector specimen, calculated from experimental data, is 2 x 1O-6 W K-’ in ZIUCUOand 1 x 10m5W K-’ for Xe at pressures greater than 10 torr. The measured speed of response was 0.7 msec for operation in vucuo and 0.15 msec for Xe at l&100 torr. Thus the total effective self-capacitance of the detector is 1.4 x 10m9J K-l. These results are in good agreement with theoretical predictions. However, the measured temperature coefficient of resistance was O.OS’AK-l, and it is anticipated that a higher temperature coefficient will be achieved using the recently-developed technology described in this paper, giving a 1.5 times predicted increase in detectivity. In general the performance characteristics can be varied over a wide range by virtue of changes in design, choice of materials and selection of operating conditions. The element size and electrical resistance can also be readily changed within the limits imposed by conventional photolithography. Acknowledgemenfs-The author wishes to acknowledge the contributions of his colleagues, J. Hlava, Powell, to the work described in this paper, and is indebted to the staff of the Electronics Workshop Instrumentation Groups of the Advanced Engineering Laboratory, South Australia, for their support. AWA Microelectronics, Sydney, N.S.W., is also greatly appreciated. REFERENCES 1. Aiken 2. Billings
C. B., Carter W. H. and Philips F. S., Rev. scient. Instrum. 17, 377 (1946) B. H., Barr E. E. and Hyde W. L., Rev. scienr. Instrum. 18, 429 (1947).
P. Girdler and I. and Mechanisms The assistance of
64
K. C. LIDDIARD
3. Yosihara K., Sci. LI, Tokyo 5, 29 (1956). 4. Hartmann R. and Selders M., Sensor 82 ConJ Proc. Battelle Institute. Frankfurt. F.R.G., 5. p. 26. 5. Smith R. A., Jones F. E. and Chasmar R. F., The Detection and Measurement of Iyfiared Radiation. Clarendon Oxford (1957). 6. Carslaw H. S. and Jaeger J. C., Conduction o/ Hear in Solids. Clarendon Press, Oxford (1959). 7. Dushman S., Scien@ Foundations qf’ Vacuum Technique, Wiley, New York (1962). 8. Hadley L. N. and Dennison D. M., J. opt. Sot. Am. 37, 451 (1947). 9. Liddiard K. C., MSc. Thesis. University of Adelaide, Australia (1973). IO. Coutts T. J., EIec/rica/ Conduction in Thin Metal Films. Elsevier. Amsterdam (1974). Il. Chopra K. L., Thin Film Phenomena. McGraw-Hill, New York (1969). 12. Liddiard K. C., Australian Patent Application 75842/81 (1981). 13. Petersen K. E.. Proc. IEEE 70, 420 (1982).
Press.