Uncooled Microbolometer Infrared Sensor Arrays

Chapter 6

UNCOOLEDMUCROBOLOMETERINFRARED SENSOR ARRAYS

R.A. Wood

6.1 INTRODUCTION

The first man-made infrared (IR) sensor was a thermometer, demonstrated by Herschel in 1800 [1]. Such an IR sensor is now termed a "thermal" sensor, since it operates by sensing the temperature rise caused by absorbed infrared radiation (in contrast to other types of infrared detectors [2] which operate at fixed temperatures). Figure 1 shows the fundamental form of a thermal IR sensor: an IR-absorbing plate (area A) is suspended from a large thermal mass (supporting substrate) by supporting "legs". The supporting legs are long and narrow, and are made from a material with low thermal conductivity, so that the IR heat energy dissipated in the IRabsorbing plate does not quickly leak away to the supporting substrate. Some type of temperature-sensitive device is placed on the absorbing plate, to measure the temperature changes produced by incident infrared radiation.

From Fig. 1, it is intuitively reasonable to expect that the greatest sensitivity to infrared radiation will be attained from a thermal sensor if:

1) the thermal mass ofthe absorbing plate is as small as possible (so that small amounts of energy induce a high temperature rise);

2) the absorbing plate is highly thermally isolated from the supporting structure (so that the absorbed heat does not readily leak away to the substrate).

If a two-dimensional (20) array of such thermal sensors is placed at the focal plane of an IR transmitting lens, each of the thermal sensors in the 20 array can provide one picture element (pixel) of an infrared image of a scene. Such an imager is called a "staring" imager (to distinguish it from imagers [3] which use a scanning mechanism). For such an application, it is intuitive to further expect that

P. Capper et al. (eds.), Infrared Detectors and Emitters: Materials and Devices© Kluwer Academic Publishers 2001

150 Infrared Detectors and Emitters:Materials and Devices

3) very large numbers of thermal sensors are desirable, to generate a finely detailed image (a reasonable-quality image requires a minimum of about 1 OOx 100 pixels);

4) with tens of thousands of thermal sensors in a 2D array, each thermal sensor must be as small as possible, to avoid an impractically large array;

5) if an electrical method is used to measure the sensors' temperature signals, it is impractical to provide individual electrical leads to tens of thousands of individual sensors, so the electrical signals must be efficiently passed out along a much smaller number of electrical leads.

6) it is clearly impractical to assemble tens of thousands of individual tiny thermal sensors by hand, so some easy method of mass-fabrication is necessary;

7) from the description of the operating mechanism of a thermal sensor, we can expect that the array will operate well at room temperature ("uncooled" operation).

Each of these expectations will be verified and quantified later in this chapter, but for the moment we shall take these on trust, and use them to guide the design of suitable 2D arrays of thermal sensors. Using these guidelines, we shall describe how two-dimensional arrays of thermal sensors may be constructed, and show how to calculate their performance in an infrared imaging system. These calculations will justify the above intuitive guidelines.

Infrared Radiation

C981~.1

Area A

Figure 1 Fundamental form of a thermal IR sensor.

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6.2 FABRICATION OF ARRAYS OF THERMAL SENSORS

151

A very suitable method of making 2D arrays of thermal sensors is the process of "silicon micromachining", which is the technology of shaping microscopic structures by etching silicon wafers. Large 2D arrays can be formed using photolithographic masks with repeated features. Although silicon micromachining can be used to fabricate most types of thermal sensors, the particular type of thermal sensor we will discuss in this chapter will be a microminiature bolometer ("microbolometer"). Bolometers were first demonstrated effectively by Langley in 1882 [1] and are thermal sensors which use a temperature-dependent resistor attached to the IRabsorbing mass to sense the temperature signal. To sense the signal electrically, an electrical bias voltage is applied across a thin-film resistor deposited on the IR-absorbing plate, to produce an electrical signal current which varies as the incident IR radiation varies. We shall show below that micromachined microbolometer arrays offer very good sensitivity for practical infrared imaging applications, without the need to be cooled below room temperature. This useful performance without the need for a cryogenic cooler, and the low production costs available with silicon fabrication techniques, are the two principal factors which make micro bolometer arrays valuable.

6.3 MICROMACHINED MICROBOLOMETER DESIGN AND FABRICATION

All of the desired features listed above (low th~rmal mass, high thermal isolation, small size, large numbers of sensors, efficient electrical readout etc.) are attainable, to a near-ideal level, with silicon "micromachining", which produces microscopic suspended bridge-like structures ("microbridges") on silicon wafers. Micromachining is possible by several low-cost techniques, and micromachining is now a wide technical area which cannot be adequately described here: the major techniques are well described in other publications (e.g., [4]). Here we will only outline the particular micromachining techniques and materials typically used to fabricate microbolometer arrays.

Figure 2 shows a drawing of a typical micromachined microbolometer. This consists of a 35 J.lm-square plate of silicon nitride, a hard, electrically insulating material, elevated above a semiconducting silicon substrate which contains electronic readout circuits. The silicon nitride plate is supported


over the substrate by long narrow "legs" of silicon nitride, which carry metal-film conductors between the plate and the substrate. The thickness of the elevated silicon nitride plate and silicon nitride legs is typically 0.5 J.U1l, and the elevation above the underlying silicon substrate is typically 2.5 J.l.m. The unit cell is about 50 J.U1l square, and the thin plate occupies about half of the unit cell area. A thin film (typically 0.05 J.U1l) of a resistive material with high thermal coefficient of resistivity (TCR, a=dRJ Rdn is deposited upon the square plate, and the thin-film metal layers deposited on the supporting legs provide electrical contact to the underlying silicon, which contains readout electronic circuits. As discussed in detail later, the infrared absorption of the microbridge is enhanced by a reflecting thin-film metal layer deposited on the silicon substrate underneath the suspended plate. The legs are intentionally made narrow (typically 3 J.l.m), and the conduction metallizations made thin (typically 0.05 J.U1l), in order to provide as little thermal conduction along the legs to the substrate as possible. The complete unit cell area of the structure (suspended plate, legs, narrow spacing to the adjacent sensor) of Fig. 2 is typically 50 J.U1l square. The ratio of the area of the bolometer plate to the unit cell area is termed the "fill factor" of the bolometer, and is typically 50--75%. Such unit cells may be fabricated by micromachining in a two-dimensional array (Fig. 3) which may be placed at the focal plane of an infrared-transmitting lens to produce an infrared image, in the same way that photographic film receives an image when placed at the focal plane of a camera lens (Fig. 4). Figure 5 shows photographs of typical microbolometers.

Figure 2 Drawing of a microbolometer.

R.A. Wood

Figure 3 Drawing of a 2D array of microbolometers.

lnframd Radiation

\

Lens

Figure 4 Use of a 2D array in an imaging system.

153


Figure 5a Micrograph of a 50 ~m microbolometer unit cell similar to that shown in Fig. 2. In this unit cell, each supporting metalized leg terminates at a vertical metal pillar. seen as sixsided areas with a central circular feature, rather than the downsloping leg termination shown in Fig. 2. The microbridge is the central area, with a fill factor of about 50%, with thin film metal contacts at left and right edges.

Figure 5b Micrograph of a larger region of a two-dimensional array of 50 ~m microbolometer unit cells.

R.A. Wood

Figure 5c Scanning electron micrograph of microbolometer unit cell.

6.4 TEMPERATURE-SENSITIVE RESISTOR MATERIALS FOR MICROBOLOMETERS

155

Metals show little change in free carrier density with temperature, but the mobility of the free carriers reduces with increasing temperature, producing a small, positive a., typically about +0.002 K-1• The a. of metal films usually varies slowly with temperature T, so that for metals near room temperature (Ts), R(D can be described well by equation (1).

R(T) = R(Ts)(1 + a.(T - 1'.,» (1)

Semiconductor materials have mobile charge carrier densities which increase with increasing temperature, as well as carrier mobilities which change with temperature, producing a larger, negative, more strongly temperature-dependent a.. A typical R(D behavior for a semiconductor whose mobile charge carrier density is controlled by thermal excitation across a bandgap Eg is

R(D oc exp(Egl2kbD i.e., a. = dRJRdT= -Egl2kbr (2)

Thin films (typically 500 A) of mixed vanadium oxides (VOx) are commonly used for microbolometer resistors, because these semiconductor oxides have been found to be able to be deposited in thin films with good resistor qualities. An a. value of about -0.02 K-1 is achievable at 25 °e, which is five to ten times better than the a. of most metals.


6.5 MICRO BOLOMETER MICRO MACHINING SEQUENCE

Figure 6 illustrates the typical micromachining fabrication steps of a microbolometer array (many different micromachining processes are possible, [4]). Table 1 summarizes the properties of typical materials used in the process. Fabrication begins with implantation of the required readout electronics and conducting metallizations in the silicon wafer. The wafer is then planarized with a material, such as spun-on polyimide, which can be photolithographically patterned to form "sacrificial" mesas. Silicon nitride layers are sputtered over the sacrificial mesas, together with a 500 A thick vanadium oxide resistor, and 500 A nickel-chromium connecting metallizations. As a final step, the sacrificial mesas are removed by a material-selective etch, e.g., oxygen plasma etching, to leave a selfsupporting two-level structure like that of Figs. 2 and 5.

t.F~~ ~in$lftoon war(~ and Imarcon~ metaII~)

2.~ ~layet islands

Figure 6 Typical fabrication sequence for a microbolometer array.

Figure 7 shows a photograph of a completed 4" diameter silicon wafer with 12 completed 320x240 microbolometer arrays of 50 JlIIl square pixels. Each array is functionally complete, having monolithic multiplexing and control electronics in the underlying silicon substrate.

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Figure 7 Photograph of a completed 4" diameter silicon wafer with 12 completed 320x240 microbolometer arrays of 50 J.l.m pixels.

Table 1 Typical microbolometer materials and their parameters

Material Density Thermal Specific a Electrical (g cm-3) conductivity heat

(W cm-1K-1) (J cm-3 K-1) (K-1) conductivity

(Qcmr l

Silicon nitride 3.2 0.0185 2.3 N/A N/A

Nickel-chromium 8.5 0.05 3.3 +0.0028 Sxl04

Vanadium oxide 4.6 0.05 3.0 -0.02 10

6.6 TYPICAL MICROBOLOMETER PARAMETERS

In Fig. Sa, the suspended plate (microbridge) is a silicon nitride plate of dimensions approximately 25 J.l11l x 45 J.l11l x 0.8 J.l11l, suspended over the silicon substrate, with a gap of approximately 2.5 J.l11l between the micro bridge and the silicon. The sensing material is a 500 A layer of vanadium oxide, with (l = -0.02 K-' at 25 °C, sandwiched between upper and lower insulating layers of silicon nitride. The two supporting legs are silicon nitride approximately 2 Jlm wide and 48 Jlm long, with 500 A Ni:Cr conductive films.

Using the material parameters of Table 1, we calculate the microbridge thermal capacity c is 2.1 x I 0-9 J K-', and total leg thermal conductance g (two legs) is l.4x 1 0-7 W K-'. The thermal (exponential) response time of the microbolometer is then 't = c1g = 15 ms. We can similarly calculate the other parameters of a typical microbolometer unit cell like that of Fig. Sa. These


typical values are summarized in Table 2. For completeness, Table 2 also includes other typical microbolometer parameters which have not been discussed yet, but will be introduced later in this chapter.

Table 2 Summary of typical values for microbolometer parameters. The upper part of the table lists typical parameters of single microbolometer unit cells like that of Fig. Sa. The lower part lists typical parameters of two-dimensional arrays and the array operating conditions employed in a typical infrared camera. These typical values are used in the illustrative numerical calculations of this chapter.

Parameter Symbol Typical value

Area of unit cell Ac 2.5xI0-5 cm2 (SO J.1Ill square)

Fill factor of unit cell Ff O.S

IR-absorbing (microbridge) area A=FjAc 1.2x10-5 cm2

Mass of micro bridge m 2.3xI0-9 g

Thermal capacity of micro bridge c 2.1 x 10-9 J K- I

Thermal conductance of supporting legs g 1.4x10-7 W K-I

Thermal time constant of bolometer (elg) t IS ms

Infrared absorption of micro bridge E 0.8

Resistance of temperature-sensitive resistor Rb SO kn at 300 K

TCR of temperature-sensitive resistor a -0.02 K-I

lifnoise parameter of temperature-sensitive resistor k IxlO-13 S·I

1C Sx10-24 cm3 S·I

Applied bias voltage (pulsed) Vb IV

Bias current (Vbl&) Ib 20 j.lA

Bias voltage pulse time Tp 6S J.IS

Frametime Tf 33 ms (30 Hz framerate)

Upper bandwidth limit Ji 7.7 kHz

Lower bandwidth limit fi 0.0001 Hz

Optical fnumber Fn 1.0

Range of infrared wavelengths transmitted by lens A2. AI 12 J.1Ill to 8 J.1Ill

Number of rows in array M 240

Number of columns in array N 320

Temperature of microbolometer Tb 300K

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6.7 THERMAL ISOLATION OF MICROBOLOMETERS

159

If the temperature of a microbolometer increases, the IR power radiated from it also increases according to Stefan's law, producing a "radiative" thermal conductance

d I 4) 3 grad = - \(2A)EcrTb = 4(2A)EcrTb

dTb (3)

where cr = 5.67xlO-12 W cm-2 K-4 (Stefan's constant). Since this radiative thermal conductance is present even if the legs are perfectly thermally insulating, grad represents the lowest possible thermal conductance a microbolometer can have. For the typical microbolometer parameters of Table 2, we calculate

(4)

which is about an order of magnitude lower than the typical thermal conductance of the supporting legs (Table 2).

6.8 INFRARED ABSORPTION IN MICROBOLOMETERS

The use of a vacuum gap of approximately 2.5 Jlm, together with a thinfilm metal reflector layer on the underlying substrate, produces a "114 wave" resonant optical cavity for wavelengths near 10 Jlffi. The infrared absorption of this multilayer structure can be computed using computer programs adapted from those used for dielectric multilayer interference filters, and a typical absorption in the microbridge can be shown to be about 80% in the 8-12 Jlm wavelength band [5].

6.9 READOUT OF TWO-DIMENSIONAL ARRAYS OF MICROBOLOMETERS

As noted above, the signal current from each individual microbolometer may be time-multiplexed onto a single output signal line. This may be done in many ways familiar to electronic engineers. Figure 8 illustrates a typical electronic method employed to read out a two-dimensional array of


microbolometers in a TV frametime (about 1130 s). In Fig. 8 the microbolometers are interconnected in a "row and column" structure, with an "on/off' CMOS FET switch connected in series with a bolometer at the intersection of each row and column. The bias voltage is applied to each row and column in sequence by means of CMOS multiplexers at the periphery of the array. In Fig. 8, the signal currents from each microbolometer in tum flow down the corresponding column to individual CMOS integrators and a time-multiplexer located at the bottom of each column. All electronic components are fabricated by standard silicon CMOS techniques in the underlying silicon substrate, and the microbolometer fabrication process includes deposition of metallizations which connect the microbolometers to the underlying electronics.

substrate

R

Integrators

transfer gate

row store

output multiplexer

serial output

Figure 8 Typical monolithic readout circuit for a microbolometer array.

In Fig. 8, all columns are held continuously at zero voltage by the virtual-ground nature of the integrators at the foot of each column. The substrate is held continuously at the desired bias voltage of the microbolometers. All rows, except one, are also held at zero volts by the row multiplexer, which maintains the CMOS FET switches in those rows in an "off' state. At any instant, one single row is raised above the tum-on voltage of the FET switches, so turning all the FETs in that row to the "on" condition, and so applying the bias voltage across all the microbolometers in that row. The current which flows through each microbolometer in that row flows individually down each column, and is accumulated on the charge storage capacitor at the base of each column for the time that that row is "on". Accumulated charges are then transferred to storage capacitors, and

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time multiplexed onto a single output line whilst the current integration sequence is repeated with another row of the array.

In this scheme, the bias voltage can only be applied for a maximum time Trow=TJM = 137 IlS (Table 2). In practice, a shorter bias pulse is used, to allow free time for background circuit operations, and a typical value of Tp might be 65 IJ.S (Table 2). This pulsed bias voltage causes a self-heating in each bias pulse, causing the temperature of a microbolometer to rise during each bias pulse, by an amount

(5)

In the frametime interval 1j between bias pulses, each bolometer will cool down exponentially, to close to the substrate temperature.

It is a curious feature of the operation of micro bolometer arrays that the temperature of each bolometer is varying by several degrees by self-heating caused by the applied bias pulses. This heating is, however, reproducible from frame to frame (Fig. 9), and hence temperature signals induced by incident radiation can be distinguished by frame-subtraction.

Vb

time

bolometer temperature

Figure 9 Illustration of micro bolometer temperature variation (lower plot) due to bias voltage pulses (upper plot) as used in readout circuit operation (Fig. 8).

Using the typical values for microbolometer arrays listed in Table 2 numerical values of the bias-induced peak temperature rise in each bias pulse (equation (5» are shown in Fig. 10.

The rate of rise of microbolometer temperature during a typical bias pulse (1 V, 651ls - Table 2) is astonishingly high (about 9500K-1 S-I).


I

K

1

I+---~~~------~------~----~ 1 11

Figure 10 Computed microbolometer temperature increase during a bias pulse.

6.10 CALCULATION OF THE PERFORMANCE OF BOLOMETER ARRAYS

We will first derive the basic equations for responsivity and noise of a single bolometer, then extend this to operation in a two-dimensional microbolometer array. In these calculations we will use the typical parameters of Table 2 to calculate numerical values. We will also use these equations and numerical values to justify the assumptions made in Sect. 6.1.

To calculate the performance of a microbolometer, we use the simple physical model shown in Fig. 1. This model represents a microbolometer as a small thermal mass (c, J K-1) at temperature Tb (K), with an electrical resistance R(Tb) (0) and a = dRlRdTb. As shown in Fig. 1, the thermal mass c is assumed to be suspended from some supporting structure at fixed temperature Ts by legs which provide a very low total thermal conductance (g, W K-1) to heat flowing from the thermal mass c to the supporting structure, and also provide electrical contact to the temperature-sensitive resistance R(Tb). The total unit cell area of the device in Fig. 4 is Ac, and within this area the absorbing frontal area of the microbolometer is A=F .rAc. We assume a steady infrared power 8Q is being dissipated by optical absorption in the thermal mass c: this absorbed power 8Q flows through the two supporting legs to the supporting silicon substrate, producing a temperature elevation of 8n = Tb-T., = 8Q /g in the microbolometer.

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6.10.1 Responsivity to Changes in Absorbed Infrared Power

With a constant voltage Vb applied across the microbolometer, the "internal" current responsivity (Ri' current change per unit power absorbed in the bolometer) can be directly written:

(6)

where Ib and Vb are the applied bias current and voltage. A more detailed analysis of microbolometer operation can be performed

[5] which takes into account the small changes in heat re-radiated from the microbolometer when the bolometer temperature changes, and the fact that the microbolometer bias-induced heating is temperature-dependent (the "electro-thermal" effect). These effects are not numerically significant under the operating conditions we assume here.

Using the typical microbolometer parameters provided in Table 2, the internal responsivity of the microbolometer is

R = _ IbU = _ (20 X 10-6 x- 0.02) = 2.9 I g 1.4 x 10-7

AW-l (7)

6.10.2 Responsivity to Changes in Target Temperature

In a typical night-vision camera, a bolometer array is coupled to a target by a lens as shown in Fig. 4, so that infrared radiation from the target scene is imaged onto the array. Each bolometer then captures infrared radiation from one corresponding picture-element (pixel) of the target scene. It can be shown that with this arrangement, the infrared power Q absorbed by each bolometer absorbing area A is [5]

(8)

where L is the radiance of the corresponding target pixel, and Fn is the Fnumber of the lens.

The change in temperature of a bolometer induced by a change m temperature of a black body target at temperature 1'r is then given by


(9)

Because of the above equation, numerical values of dL/d1't are especially useful in night-vision imaging system calculations. Numerical values of dL/d1't are calculated from Planck's law, and are listed in Table 3 for black-body targets at 290 K, 300 K and 310 K, assuming several typical wavelength bands transmitted by the lens.

Table 3 dLldT, values (W cm-2 sr-I K-I) for targets (assumed to be black bodies) with temperatures of T, = 290, 300 and 310 K

Wavelength range 290K 300K 310K

Al to A2 (1J.IIl)

3to4 8.9xlO-7 1.3 x 10-6 1.9xlO-6

4to 5 4.0xI0-6 S.SxIO-6 7.2xI0-6

8 to 10 3.1 x 10-5 3.SxI0-5 3.9xI0-5

10 to 12 2.6xlO-5 2.8xI0-5 3.1 x 10-5

12 to 14 1.9xlO-5 2.lxI0-5 2.2xI0-5

As a numerical example, using the typical parameter values of Table 2, if a typical micro bolometer views a 290 K blackbody target via a 8-14 Jlm F /1 lens, and the temperature of the target changes by 1.0K, the microbolometer will change temperature by

(10)

Using the typical parameters of Table 2, we find that this temperature change in a microbolometer will cause a resistance change of dRb = RbaoTb = -4 n. The function of the readout circuit (Fig. 8) is to measure these small changes in microbolometer resistance, for each individual microbolometer in the array, in a frametime of about 1130 s, whilst adding as little additional electrical noise as possible. The circuit design (Fig. 8) is therefore not trivial, but is possible using established circuit principles. Space here does not allow the calculation of the noise added by the readout circuits, so in this discussion, we shall assume that no noise is added by the readout circuit.

It is worth noting that the signal-induced temperature signals (equation (10» are much less than the bias-induced temperature pulses (Figs. 9 and

RA. Wood 165

10). The bias-induced temperature pulses may be removed 1D camera operation by frame subtraction, as described later in this chapter.

6.10.3 Response Time of a Bolometer

By analogy with an "RC" electrical circuit, the temperature of a thermal mass c linked to a heat reservoir via thermal conductance g will change exponentially with time, with a ''thermal time constant" given by

't = c/g (s) (II)

Using typical values for c and g (Table 1) we find a typical microbolometer thermal time constant is:

't = 2.1 x 10-9/1.4x 10-7 = 15 ms (12)

6.10.4 Noise Level of a Microbolometer

The minimum infrared signal that can be detected by a bolometer is determined by the electrical noise on the signal current. The "internal" noise equivalent power (NEP) of the microbolometer is defined as the infrared power absorbed that induces a signal current equal to the rms current noise, i.e.,

NEP = rms noise current/current responsivity (13)

There are three principal sources of noise which must be considered: 1) electrical noise from the microbolometer resistor (Johnson noise), 2) resistance fluctuation noise in the bolometer resistor (lif noise) and 3) temperature fluctuation noise of the microbolometer.

6.10.4.1 Resistor Noise As described above, a bolometer contains a temperature-dependent

resistor, whose value is measured by the application of a bias voltage, which produces a signal current. In addition to the signal current, there is a small noise current [5], with a mean square value in a 1 Hz interval at frequency f (i.e., noise power density) given by


(14)

The first term is the Johnson current noise due to thermal agitation of charge carriers in the resistor, and the second term is "1/j' fluctuation in the resistor. IIj noise in microbolometer resistors is often characterized by the "k parameter" [6]. Figure 11 shows typical values of the Iljnoise parameter (k) for various resistors [6], including measured values for vanadium oxide thin film resistors [5].

i ~ f ~ ~ ~ ~MetaIF!lmS~

I I I

Figure 11 Typical k value for resistors.

Assuming that 1If noise power density is inversely proportional to the resistor volume [7], we can also define a volume-independent figure of merit K = kxvolume. For the typical bolometer parameters of Table 1, with vanadium oxide resistors 500 A thick and 23 J.1m by 43 J.1m in area, the k value is typically 1 x 1 0-13 Hz, corresponding to a K value of 5 xl 0-24 cm3 s -1.

6.10.4.2 Temperature Noise It can be shown by thermodynamic arguments that, in thermal

equilibrium, a thermal capacity c connected to a heat reservoir via a thermal conductance g, will fluctuate in temperature with a power density [5]:

(15)

The zero-frequency temperature noise density is

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The equivalent-noise-bandwidth 11/ [6] is the same as an electrical "Re" filter i.e., 11/ = 1I4't Hz. (Equivalent-noise-bandwidth is the "rectangular" noise bandwidth that would produce the real noise.)

With a bias voltage Vb applied, the current noise density is then

(16)

Using the concept of equivalent-noise-bandwidth described above, and the zero-frequency temperature noise density, we can immediately write the total rms temperature noise:

(17)

With the parameters of Table 2, this has the numerical value:

(18)

6.10.4.3 Total rms Noise With the parameters of Table 2, the total current noise density is as

shown in Fig. 12. With the pulsed-bias readout scheme described in Sect. 6.9, the

microbolometer signal current may be integrated electronically over part or all of the duration of each bias pulse tp.

The upper noise bandwidth limit h of an ideal integrator with integration time M is [5]

1 f:::::-2 2t

p

(19)

which is 7.7 kHz for tp=65 fls (Table 2). The lower bandwidth limit.li is determined by the "staring time" Tstare. by the engineering relation

1 J;:::::--

4Tstare

(20)


which is 0.0001 Hz for Tstare = 40 min (Table 2). Thus the total mean square microbolometer current noise is

(21)

The three individual terms in equation (21) are, respectively, Johnson, l/j and thermal noise. Using the typical parameter values of Table 2, the rms magnitudes of these terms are, respectively, 58 (Johnson), 27 (11j) and 10pA (thermal).

rll1l boIon'IBter noise

1&10.----------------------------,

1&11~--.. ~--------------------~

Arrps per root Hz 1&12 -I-----------~~o:::":""----------__1

1&13~----------------~~~----~

1&14+---~--__ --__ --__ --__ ~ __ --~

0.01 0.1 10 100 1000 10000 100000

Hz

.....-johnson ____ 111

......-thermal

-M-total

Figure 12 Plot of noise current density versus frequency for a bolometer with parameters in Table 2. Curves show resistor noise (Johnson and Ilf), thermal noise, and total noise density.

6.10.5 Noise Equivalent Power

From equation (14) the "internal" NEP of each microbolometer in a twodimensional array is then given by

(22)

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The equation for the NEP has been written in a way that illustrates that the best possible value for the noise equivalent power dissipated within the microbolometer is

(23)

which is the ideal thermodynamic value of a thermal sensor. For a microbolometer, the ideal NEP value is attained if Johnson and lifnoise are negligible compared with thermal noise. Using equation (11) we can rewrite the ideal NEP as

(24)

Since the maximum value of 't is set by the framerate of the camera, equation (24) illustrates that the ideal performance limit of a thermal sensor is determined by the smallest value of g that can be attained [8]. If a maximum thermal response time is required for an application, this in tum places an upper limit on the maximum acceptable value of c.

At first sight, it may seem that the NEP cannot approach the ideal NEP value if pulsed bias is used, since the system bandwidth (fi-Ji) required for pulsed bias is necessarily large, with a consequent increase in total noise (equation (21». However, if we assume lifnoise is negligible, and use the relation.h»Ji (Table 2), we can manipulate equation (22) to the form:

(25)

so that, with these assumptions, NEP is in fact dependent on dTb rather than (fi-Ji). Thus, provided that the bias current is increased as the bias pulses are made shorter, so as to keep dTb constant, then the NEP is not dependent on (fi-Ji).

In general, we can define an operating "quality factor" as the ratio of the expected NEP of a microbolometer array to the ideal NEP (Johnson and lif noise negligible compared with thermal noise):

170

Q= NEP = NEPideal

Infrared Detectors and Emitters:Materials and Devices

(26)

Figures 13 and 14 show the computed variation ofNEP and Q for the typical array having parameters listed in Table 2.

1.E-10

W S.E-11

O.E+OO ~---~----~----_---~ o 10 20

uA

30 40

Figure 13 Computed variation of NEP with applied bias for the typical array parameters listed in Table 2.

20

Q10

O~---~---~----~---~ o 10 20

uA

30 40

Figure 14 Computed values of Q with applied bias for an array with parameters in Table 2.

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6.10.6 Noise equivalent Temperature Difference (NETD)

We define NETD as ''the temperature change at the target that produces a signal in the microbolometer equal to the total (rms) noise". From equation (8) we can write directly:

(27)

NETD is a widely used figure of merit for an infrared imager. For typical every-day scenes, a good quality infrared image requires an NETD of 300mK or less [3,9]. Using equation (27), Fig. IS shows the computed NETD of an array with the typical parameters of Table 2. dLldT is obtained from Table 3 assuming a black body target at 300 K.

200

mK100

O~--------~----------~--------~--------~ o 10 20

uA

30 40

Figure 15 Computed variation ofNETD with applied bias for an array with parameters listed in Table 2 and Table 3 (target temperature 300 K).

6.11 PRACTICAL INFRARED CAMERAS USING MICROBOLOMETER ARRAY

6.11.1 Microbolometer Packaging

The thermal conductivity of STP air is 2.Sxl0-4 W cm-1 K-1, so a microbolometer of 50 !-lm2 area suspended 2.5 !-lm above an underlying substrate has a thermal conductance in STP air of about 2.SxlO-5 W K-1•

This is much greater than the thermal conductance of typical supporting legs


(Table 2), so that if operated in air, the microbolometer responsivity is greatly reduced (equation (8».

As air pressure is reduced, the thermal conductance of a typical microbolometer eventually becomes limited by the leg thermal conductance, which typically occurs at an air pressure of about 50 mTorr [5]. Further reduction in air pressure provides negligible reduction in the microbolometer g value. Since the responsivity of a microbolometer is proportional to lIg, full sensitivity requires an air pressure of about 50 mTorr or less. Little improvement in sensitivity is attained by operating at lower pressures.

Although 50 mTorr air pressure is easy to obtain with air pumps, maintenance of this pressure for many years in a small-volume package requires careful attention to leaks and outgassing phenomena. Long-lived (multi-year) sealed vacuum packages have been demonstrated using packages constructed of brazed and soldered materials, carefully cleaned and baked before sealing to produce low outgassing. Some package designs use internal getters to absorb outgassing from internal package walls and so provide a longer package vacuum life. Figure 16 shows the construction of a sealed vacuum package designed for 240x336 arrays, operating in a chopperless mode [5]. In this package the microbolometer array is held at a constant (arbitrary) temperature by use of a thermoelectric (TE) stabilizer mounted inside the package, controlled by a thermistor placed near the array. Temperature stabilization of the microbolometer array is one way to eliminate false signals that would otherwise occur as the array temperature changed.

lower Periphery MetaJized to Permit Soldering

Pads for AI Bonds

Thermistor -Metanzed for Solder -

TE StabilIzer (BeO Plates)

Mounting Holes

OFHC COpper __ Pumpout Tube

Mlerobotometer Array

- } TE Power Leads

Peds for AI Bonds 88 Pins

AIO Frame

r-:-'loo..__ Qopperlfungsteo BaHpiate

Zr Getter Inside Pumpout Tube

Crimp Seal

Figure 16 Long-life sealed vacuum package for a 2D microbolometer array.

R.A. Wood 173

6.11.2 Offset Signal Removal

Each microbolometer array has some intrinsic variation in microbolometer resistance, due to fabrication process variations from microbolometer to microbolometer. A typical microbolometer array might have a standard deviation of 1 % in the as-fabricated microbolometer resistance values, i.e., a typical standard deviation of 500 0 for a typical bolometer resitance of 50 kn (Table 2). This is, however, very large compared to the bolometer signals induced by thermal radiation in a typical night-vision camera application: equation (10) has been shown to predict a thermal signal of about 4 mK if the scene increases by 1.0 K, and assuming a typical a (Table 2) the equivalent resistance change would be given by (4x 10-3)(-0.02)(50 000) = 40.

If not removed in the camera, the large fixed resistance non-uniformity of the array would produce a large fixed pattern on the image. The fixed resistance non-uniformity of the array can be removed in a camera by use of an intermittent shutter which blocks radiation from the lens: whilst the shutter is closed, several frames are averaged (to reduce time-dependent noise) and stored in a digital memory in the camera. The shutter is then opened, and the stored memory frame is subtracted from subsequent frames, on a pixel-by-pixel basis. This process also removes the fixed pattern caused by bias-pulse heating.

6.11.3 Typical Microbolometer Camera Performance

Figure 17 shows measurements [5] made with a microbolometer array, with an average NETD of 39 mK with an FIl lens, 8-12 J.U1l wavelength range, 300 K black body target. Figure 18 shows typical infrared images obtained with a 240x240 microbolometer array.

6.12 CONCLUSION

Micromachined thermal infrared sensors are capable of operation close to the room-temperature thermodynamic limit. This limit is set by the thermal conductance between the individual sensors and the supporting substrate, which in a micromachined sensor can approach the radiative limit, provided the sensor is operated in a vacuum. Silicon micromachining offers cost-effective batch processing of two-dimensional thermal arrays, complete with integrated readout electronics. The combination of silicon batch processing and uncooled operation provides the potential for the lowest cost infrared cameras. Whilst the above comments generally apply to all


micromachined thermal sensors, bolometers are an attractive thermal sensor, since the sensing device (a resistor) is comparatively simple to fabricate, and the sensing mechanism has some intrinsic advantages (e.g., a wide dynamic range).

Pixel Count

.01 .03

Pixel NETD

.05

i--NETD @1I1.0

I Mean ~ 39.4 mK Median ~ 38.8 mK St. Dev. ~ 8.3 mK

.07 .09

°C Figure 17 Histogram of NETD values measured for pixels of a 320x240 microbolometer array (F II lens, 8-I2!lm wavelength range, 300 K black body target, 30 Hz framerate).

ACKNOWLEDGMENTS

The author wishes to thank his many colleagues at Honeywell, both present and past, who played major roles in the achievement of uncooled IR imaging with microbolometer arrays, and whose work is described here. A large part of the development work at Honeywell was directed by the staff of the US Night Vision and Electronic Sensors Directorate and the Advanced Research Projects Agency. Government funding has been provided to Honeywell under the following programs: ASP Sensor Development DAALOI-85-C-0153, High Density Array Development DAAB07-87-C-F024, Low Cost Uncooled Sensor Prototype DAAB07-90-C-F300, Technology Reinvestment Program MDA972-95-3-0022.

REFERENCES

1. Barr, S. (1961) Infrared Phys., 1, 1: (1963) Infrared Phys., 3, 195. 2. Kruse, P.W., McGlauchlin, L.D and McQuistan, R.B. (1963) "Elements of Infrared

Technology" Wiley. 3. Lloyd, 1M. (1979) "Thermal Imaging Systems" Plenum, NY. 4. Middelhoek, S. and Audet, S.A. (1989) "Silicon Sensors" Academic Press. 5. Wood, R.A. (1997) Semicond. Semimet., 47.

RA. Wood 175

6. Motchenbacher, C.D. and Fitchen, F.C. (l973) "Low noise electronic design" Wiley. 7. Hooge, F.N. (1976) Physica B 83, 14. 8. Kruse, P.W. (1997) Semicond. Semimet., 47. 9. Schumaker, D.L. et at (eds), (1988) "Infrared Imaging Systems Analysis" ERIM

Series in Infrared and Electro Optics.

Figure 18 IR images obtained with a 240x340 micro bolometer array with parameters similar to those of Table 2.

Documents

Uncooled Microbolometer Infrared Sensor Arrays