Research in the Microelectromagnetic Device Group

The University of Texas at Austin

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Far-Infrared Microbolometer Detectors

D.P. Neikirk, Wayne W. Lam, and D. B. Rutledge

This paper is abstracted from our original paper:

D. P. Neikirk, D. B. Rutledge, and W. Lam, "Far-Infrared Microbolometer Detectors," International Journal of Infrared and Millimeter Waves, vol. 5, pp. 245-277, 1984.

Also see the slide show on bolometers and infrared bolometric detection of various sorts:
"Classical Devices Made Small"

  1. Introduction
  2. Thermal Models
  3. Materials for Antenna-coupled Bolometers
  4. Fabrication
  5. Electrical Testing
  6. Conclusions
  7. References

The bismuth microbolometer is a simple, easily made detector suitable for use throughout the far-infrared, which has been integrated with a variety of planar antennas. The general thermal properties of these devices and some of the constraints on bolometer materials are discussed. The fabrication and performance of several different types of microbolometers and microthermocouples are described. Included here is the air-bridge microbolometer, which was an early version of a surface micromachined bolometer (before we called the process "surface micromachining"!).

Key words: bolometers, far-infrared detectors, integrated detectors.


A great deal of effort is being applied to the development of monolithic millimeter and submillimeter receivers. Because of the increasing loss and mechanical complexity of metallic waveguide at these high frequencies, much of this effort is devoted to quasi-optical Systems coupled to planar antennas with integrated detectors [1][2][3]. These planar antennas have proved to be quite different from their lower frequency counterparts [4][5][6]. The integrated detectors have also presented real fabrication challenges. Planar Schottky diodes, for example, will probably require submicron lithography to avoid excessive parasitic capacitance at wavelengths less than one millimeter. There is one antenna-coupled room temperature detector, however, that can provide reasonably sensitive detection and speed without requiring elaborate fabrication processes: the bismuth microbolometer.

Since the microbolometer is a thermal detector, it works well throughout the far-infrared, without the capacitive roll-off that affects Schottky diodes. It differs from more conventional thermal detectors, however, because of its small size: typically four micrometers square and 100 nm thick. A small device like this has a large thermal impedance, and so by using an antenna to couple power into it large temperature rises can be achieved (fig. 1). This in turn means the microbolometer will have a large responsivity. In addition, since the thermal mass is also small, the detector can be quite fast.

Figure 1: Top view of a typical antenna-fed microbolometer (to view a higher resolution scan, just click on the image).

Two basic types of bolometers have been made: the air-bridge microbolometer (fig. 2a) [7] and the more conventional substrate-supported bolometer (fig. 2b) [8]. A variation that takes advantage of the same thermal properties, but avoids the necessity of biasing, is the bismuth-antimony micro-thermocouple [9]. In this paper we will describe the thermal models that predict microbolometer and microthermocouple performance, the restrictions placed on bolometer materials for antenna-coupled detectors, the electrical measurements which allow accurate detector calibration, and finally present results for a variety of microbolometers we have made.

Figure 2: Cross sectional view of microbolometers; (a) air-bridge bolometer; (b) substrate-supported bolometer.

Thermal Models

The first microbolometers were substrate-supported devices. In these bolometers the conduction of heat out of the detector into both the substrate and the metal antenna are important. An exact solution to the thermal diffusion equation is quite difficult since several interrelated conduction pathways are available. The most obvious path is directly into the substrate material. Another important source of heat loss is direct conduction into the antenna, which is usually a metal with high electrical conductivity, and therefore very high thermal conductivity. Less obvious, but probably important for small detectors with large antennas, is conduction from the bolometer into the substrate, and from there back into the metal antenna. Finally, if the thermal conductivity of the bolometer material itself is small, this may contribute significantly to heat retention in the bolometer.

The original work by Hwang et al. [8] used a considerably simplified but physically helpful thermal model, which we follow here. In order to calculate the conductance into the substrate the presence of the metal antenna is ignored. The contact between bolometer and substrate is taken to be a hemisphere of radius a, which is at a temperature . The thermal diffusion equation giving the substrate temperature reduces to


which is solved subject to the boundary condition that for r = a. Here KS is the substrate thermal conductivity, its density, and CS its specific heat. The solution is


where LS is the complex thermal diffusion length for the substrate, . The total substrate conductance is given by


which is here


The total conductance G is taken to be this substrate conductance plus a frequency independent contribution from the metal antenna, Gm. The thermal impedance for the device is then


where t is the bolometer thickness, w its width, and l its length; is the bolometer material density, and Cb its specific heat. Finally, the responsivity of the detector is given by


where is the temperature coefficient of resistance of the bolometer material, and Vb is the dc bias voltage across the device. From eq. 5 we find


where Gdc is the total dc conductance out of the bolometer, due to both the substrate and metal contacts.

Hwang et al. [8] have also discussed the frequency response of this type of microbolometer. For low frequencies when LS >> a (i.e. ), the thermal impedance is independent of frequency, and is given by 1/Gdc. At higher frequencies when LS << a the impedance varies like LS, that is f-1/2. At still higher frequencies the thermal capacitance of the bolometer itself becomes important, and Zt varies as f-1. Note also that both the low frequency response and the speed of the detector increase as the device size decreases. In contrast, as the substrate thermal conductivity decreases the low frequency response increases, but the speed would be expected to decrease.

A microbolometer's performance can be improved by increasing its thermal impedance. The air-bridge bolometer does this by suspending the device in the air above the substrate. The only conduction path is now out the ends of the detector into the metal antenna. We can model this bolometer in a particularly simple manner: a uniform bar of material in which power is dissipated uniformly, and whose ends are attached to perfect heat sinks (the metal of the antenna). The thermal diffusion equation describing the temperature rise in the device is


where , Cb, t, w, and l are defined as before, Kb is the bolometer material thermal conductivity, and Po is the peak power dissipated in the bolometer. This equation is solved subject to the boundary condition that is zero at the ends of the detector. The solution is integrated over x to obtain the average temperature rise in the device


where Lb is the thermal diffusion length in the bolometer material, . The ratio of the time-varying temperature and time-varying power yields the thermal impedance of the

air-bridge bolometer,


For the air-bridge bolometer, it is possible to plot a universal frequency response curve (fig. 3). For low frequencies when the thermal diffusion length Lb is much larger than the bolometer length l (i.e. ) the thermal impedance is independent of frequency, and . At high frequencies Lb becomes much smaller than l, and . These are the same limiting values as a thermal circuit consisting of a resistance in parallel with thermal capacitance . Unlike the substrate-supported bolometers, the air-bridge response changes quite abruptly from flat to a 1/f roll-off. The speed of the detector is determined by (RtCt)-1 which is . Note that the speed depends on only one dimension of the bolometer, the length l . It should also be noted that for fixed dimensions the thermal conductance out of an air-bridge bolometer is always less than that out of a substrate-supported device.

The microbolometers discussed thus far measure the average temperature of the entire device. We can also make a detector which measures the peak temperature instead. This is done by using two different materials (for instance, bismuth and antimony) to form a microthermocouple (fig. 4). We again assume the silver antenna acts as a perfect heat sink, so the two ends of the device are at the same temperature, To. The center of the detector where the contact between the two materials is located is at a temperature To + . If the thermal-emf of the two materials are different there will be an open circuit voltage across the junction


where a1 is the thermal-emf of one material and a2 the thermal-emf of the other. The output signal then depends on the temperature difference between the center of the device and the ends. From the solution to eq. 8 the ratio Z of the time-varying temperature at the center of the air-bridge to the power dissipated in the device is


where Lb is again the thermal diffusion length, . The low frequency limit is , a factor of 1.5 larger than the low frequency average thermal impedance Zt (eq. 10). The high frequency limit is the same as before, . Thus the peak temperature in the device is described by a thermal equivalent circuit consisting of a resistance in parallel with a capacitance . The responsivity of such a device is then found from eq. 11 to be


Figure 4: Temperature profile in a microthermo-couple. The detector output voltage is proportional to the peak temperature .

Materials for Antenna-coupled Bolometers

The choice of a material for use in a microbolometer is strongly dependent on the electrical impedance desired. If we assume that the antenna is best matched by a resistance Ra for a material with electrical conductivity [[sigma]] we must have device dimensions that satisfy


At the same time we want to maximize the thermal resistance to increase the detector response. For an air-bridge bolometer we found that the thermal resistance is proportional to , but using eq. 14 this is just . Since Ra is fixed by the antenna we should use a material which gives a large value for .

The ratio of the electrical conductivity to the thermal conductivity, , is very nearly the same constant for most metals; the two properties are fundamentally related. This relation is embodied in the Wiedemann-Franz law, which gives


where kB is Boltzmann's constant, e the electron charge, and T the absolute temperature [10]. Because of this, for fixed device resistance, almost all metals would give the same bolometer thermal resistance.

The other material constant that enters into the responsivity of the detector is the temperature coefficient of resistance [[alpha]]; the larger [[alpha]] the larger the responsivity. Once again, however, this is very nearly the same for all metals because the resistivity [[rho]] near room temperature is proportional to temperature. Using the definition of [[alpha]],


we find that [11]. For 300 K this gives a temperature coefficient of about 0.003K-1; almost every metallic element is within a factor of two of this value.

A search for materials with a large temperature coefficient usually leads to a consideration of semiconducting materials. For intrinsic semiconductors the carrier concentration varies exponentially with temperature, and their resistivity is proportional to exp(E/2kBT). From eq. 16 this gives , where Eg is the band gap of the material. At room temperature for a 1 eV band gap this is 0.06K-1, about twenty times larger than that of a metal. Unfortunately this increase in a is more than offset by a decrease in the material conductivity [[sigma]], since the quantity we must really maximize is . The conductivity in an intrinsic semiconductor is usually 103 to 106 times smaller than that of a typical metal, with a thermal conductivity 2 to 10 times smaller. This yields a figure of merit that is actually smaller than a typical metal. If the semiconductor is doped to increase its conductivity the resistivity is no longer proportional to exp(Eg/2kBT). The carrier concentration is now set by the dopant concentration, and is only weakly temperature dependent at room temperature. From fairly basic considerations, then, a semiconducting material is unlikely to provide any advantages as a microbolometer material.

It would seem that almost any metallic material would make an equally good bolometer. There is one practical constraint, however, that has not been addressed. Generally the smaller the dimensions of a bolometer the better. Using a photolithographic process capable of producing a minimum feature size wmin the best microbolometer will be roughly wmin wide and wmin long (i.e. one square). Since the desired resistance is Ra, we must have a thickness that yields a resistance per square of Ra. There is usually a minimum thickness, tmin, below which good deposited layers are very difficult to produce, so the conductivity is constrained by


For a typical matching resistance of 100 Ohm and minimum thickness of 20nm this gives [[sigma]] < 5x103 (Ohm-cm)-1. In comparison, the conductivity of copper is 6x105 (Ohm-cm)-1, and for lead is 4.8x104 (Ohm-cm)-1. This constraint is therefore quite serious, eliminating all the more common metals.

An examination of the elements shows very few with a conductivity low enough to satisfy the restriction above. Since it is also advantageous to avoid extremely large thicknesses (which increase the thermal mass and complicate the fabrication process), we can also find a lower bound on [[sigma]]. Assuming this maximum thickness to be 0.5um, the minimum conductivity is approximately 200 (Ohm-cm)-1. This eliminates several of the elements that satisfied eq. 17. One material which does cover this range of conductivities is thin-film bismuth.

The properties of thin-film bismuth are quite different from those of the bulk material. Its conductivity is typically two to ten times lower than the bulk, falling into the range desired for a microbolometer. The exact value of its conductivity depends strongly on film thickness, substrate material, and substrate temperature during deposition, and somewhat less on evaporation rate [12][13][14][15][16]. The range of resistivities obtained by different authors is fairly large, but under our deposition conditions we have found our values of the resistivity to be repeatable.


Bismuth microbolometers have been made in several different ways. These techniques fall into two general categories, the two-step process and the single-step process. In a two-step process the antenna metalization is first defined. The bolometer photoresist pattern is then aligned to the antenna, the bismuth evaporated, and finally the lift-off performed. Single-step processing uses a photoresist-bridge [17][18][19] or groove [20] so that both the antenna and the bolometer can be formed with a single pattern in one vacuum evaporation step.

Two-step processing has two major disadvantages. The first of these is technological: for small antennas and small detectors the alignment between them becomes very critical. The second is more fundamental: the low frequency 1/f-noise in a two-step bolometer is usually significantly larger than in a single-step detector. This is probably due to contamination of the first level metalization during the second photolithographic step.

The photoresist-bridge technique has been widely used to fabricate a variety of devices [21][22]. Figure 7 illustrates the general principle. In this process the detector is formed under a bridge by evaporating bismuth at an angle from both sides of the bridge. By evaporating different materials from each side it is also possible to form bi-metallic junctions, such as the bismuth-antimony microthermocouple (fig. 8) [9]. Note that this type of process is self-registering; that is, the bolometer is aligned precisely to the antenna since the same photoresist structure patterns both.


Figure 7: Photoresist bridge fabrication of bolometers. The antenna metalization (silver) is evaporated at normal incidence, followed by angle evaporation of the detector materials.

Figure 8: Bismuth-antimony microthermocouple. The SEM was taken at a 60o angle to the substrate (to view a higher resolution scan, just click on the image).

For substrate-supported detectors reductions in substrate thermal conductivity offer a simple way to increase response. One possibility is the use of a plastic to insulate the detector from the substrate. This approach has been used in a 119um antenna array [23] that was fabricated on a silicon substrate. Since silicon has a large thermal conductivity (about one hundred times larger than fused quartz) bolometers made directly on it have very low responsivity. By using a 0.5um thick layer of DuPont Pyralin 2555 (a polyimide) between the microbolometer and silicon substrate a detector responsivity of 3V/W at 0.1V bias was obtained. Comparable size devices on fused quartz gave 5V/W. From this we can see the excellent insulating properties of Pyralin, with a thermal conductivity of only 0.15W(m K)-1 [24]. With this in mind, we have fabricated substrate-supported bolometers both directly on glass substrates, and with a 2um thick layer of Pyralin between the device and the glass.

A somewhat more elaborate bridge process is used to fabricate the air-bridge microbolometer. In the usual process the bridge is suspended above the substrate by another layer of uniformly exposed photoresist. This layer is then undercut during development to leave the bridge above the substrate. In order to make the air-bridge bolometer three layers of resist are used, with only the middle layer flood exposed. Since we use transparent plasma-formed buffer layers [25][19] when the tri-layer resist is contact printed to form the bridge pattern an identical pattern is produced in the bottom layer. A finished photoresist structure is shown in fig. 9. The antenna is formed by evaporating silver at normal incidence to the substrate. Bismuth is then evaporated at a 500 angle from each side of the bridge. The bolometer is thus formed under the bridge, but is supported above the substrate by the bottom resist layer (fig. 10). After evaporation the substrate is soaked in acetone for approximately one hour, which dissolves all the photoresist. Unwanted metal on the top layer of resist is removed, and the bolometer is left suspended by its ends above the substrate when the resist below it dissolves away. A finished air-bridge detector is shown in fig. 11.

Figure 9: Photo-resist bridge pattern used to fabricate airbridge microbolometers (to view a higher resolution scan, just click on the image).

Figure 10: Evaporation sequence for air-bridge bolometers.


Figure 11: SEM of an air-bridge bolometer (to view a higher resolution scan, just click on the image).

Electrical Testing

Unlike many far-infrared detectors it is possible to accurately calibrate the bismuth microbolometer. This is done by first measuring the dc responsivity of the bolometer from its dc I-V curve. An ideal bolometer has a resistance R that is just a linear function of the power P dissipated in it,


The low frequency voltage responsivity Rdc of such a device with a constant bias current Ib applied to it is then


We can find [[beta]] and Ro by measuring the dc I-V curve of the bolometer, since eq. 18 gives


Typical R-P plot for an airbridge bolometer produce linear regression fits to the data with correlation coefficients usually better than 0.999. Note that we can also find the dc thermal impedance Zdc of the device once b is known using


The NEP of a thermal detector has a fundamental limit, set by statistical fluctuations in the power flow between the bolometer and its environment. The mean square value of this fluctuation is given by


where G is the thermal conductance out of the bolometer and [[Delta]]f is the detection bandwidth [26]. In terms of the complex thermal impedance Zt this gives the minimum noise equivalent power as


For the two thermal models discussed earlier Zt is independent of frequency for low frequencies, so NEPmin is independent of frequency. At high frequencies Re(1/Zt) varies like f1/2 so the minimum NEP increases like f1/4. Note that the detector responsivity decreases like f-1, so its actual NEP will increase as f, faster than the fluctuation limit.


In conclusion, we have found the bismuth microbolometer to be a very useful detector. The simple fabrication techniques used make it quite easy to integrate into different antennas. Because it can be accurately calibrated it is useful in measuring the coupling efficiency of our antennas in conjunction with their quasi-optical systems. Finally, for wavelengths shorter than one millimeter, the microbolometer's sensitivity as a video detector is quite competitive with what can be obtained from other integrated detectors currently available.


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