NEW HORIZONS FOR UNCOOLED IR SENSORS Charles M. Hanson Raytheon Company, Dallas, TX

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1 Approve for Pulic Release; Distriution is Unlimite. NEW HORIZONS FOR UNCOOLED IR SENSORS Charles M. Hanson Rayeon Company, Dallas, TX ASTRACT The performance requirements for space-ase IR imaging sensors are generally so severe at uncoole etector technologies have historically een ignore an rightly so. The reasons for is have often een inaccurately represente on e asis of eoretical analysis wi unnecessary assumptions. This paper shows at e performance of uncoole or minimallycoole IR etectors can approach or even excee at of coole photon etectors. It also escries e practical arriers to achieving such performance. A etaile analysis of signal an noise shows at e ackgrounlimite performance of a well-esigne ermal etector is not much ifferent from at of a photon etector. The analysis also shows at ese istinct etector types are optimally suite for ifferent types of applications. The arriers to achieving ackgrounlimite sensitivity are quite ifferent for ermal etectors. In is paper we quantify e arriers, an iscuss eir implications. INTRODUCTION Uncoole IR imaging is at last, after some years of evelopment, commonplace. Many companies working wi several ifferent technologies are proucing a iverse variety of proucts in quantity. Some proucts are lower-cost versions of proucts at have een aroun for a long time, while oer proucts were ifficult to imagine prior to e avent of uncoole IR focal plane arrays (FPAs). The initial measure of viaility of ese evices was a sensitivity comparale to e poorest of coole IR sensors. We have seen various technologies mature to e point at FPAs wi 4 pixels an wi pixel sizes on e orer of µm perform significantly etter an at stanar. Larger arrays wi smaller pixels are furer lurring e ounaries etween uncoole an coole technologies. The wors raiation-limite performance are now eing commonly spoken in reference to evelopment activities for uncoole IR FPAs. What is meant y at is usually e oft-quote.8 cm Hz ½ /W ultimate D* limit for ermal etectors. Alough arriers to achieving at level of performance certainly remain, it y no means represents e ultimate limit for ese etectors. The assumptions in e analysis at leas to is magical D* are nonessential, an are in fact not true of moern micromachine evices. This, of course, has serious implications for e future of space-ase IR imaging sensors. REVIEW OF FUNDAMENTALS What we commonly call uncoole etectors are etter characterize as ermal etectors, as oppose to photon etectors. A ermal etector is characterize y e fact at its response is proportional to e amount of energy in e asore photon stream. The response of a photon etector is proportional to e numer of asore photons. In a photon etector, a photon is asore irectly y e IR-sensitive material, an e charge carriers generate y at asorption are sense eier irectly or y a concomitant change in some property of e material. A ermal etector, y contrast, comprises ree istinct parts: An IR asorer, a ermal isolation means, an a temperature sensor or transucer, as illustrate in Figure. The purpose of e IR asorer is to convert IR electromagnetic energy into heat energy. The ermal isolation, moulate y e ermal mass (heat capacity), converts e heat energy to a temperature change. The transucer converts e temperature change to a change in a measurale (usually electrical) parameter. The ree parts of a ermal etector are functionally inepenent, alough e evice may e esigne in such a manner at ey are interepenent. Asorer Transucer Thermal Isolation C IR Raiation Figure Schematic view of e constituent parts of a ermal etector. R The voltage responsivity of is generic etector is

2 Report Documentation Page Report Date 9JUL Report Type N/A Dates Covere (from... to) - Title an Sutitle New Horizons for Uncoole IR Sensors Contract Numer Grant Numer Program Element Numer Auor(s) Project Numer Task Numer Work Unit Numer Performing Organization Name(s) an Aress(es) Rayeon Company Dallas, TX Sponsoring/Monitoring Agency Name(s) an Aress(es) Performing Organization Report Numer Sponsor/Monitor s Acronym(s) Sponsor/Monitor s Report Numer(s) Distriution/Availaility Statement Approve for pulic release, istriution unlimite Supplementary Notes See Also ADM46. Papers from Unclassifie Proceeings from e Annual AIAA/MDA Technology Conference hel 9 July - August in Monterey, CA. Astract Suject Terms Report Classification unclassifie Classification of Astract unclassifie Classification of is page unclassifie Limitation of Astract UU Numer of Pages 9

3 as V R, () G T where as is e asorptivity (asorption efficiency), G is e ermal conuctance (G = /R ), (V/T) is e (voltage) response of e etector material per unit etector temperature change, is e angular frequency of moulation of e signal, an is e ermal time constant. Noise comes from a variety of sources. If e level of e noise at e output of e focal plane is not amplifie to a sufficient level, e sensor may e limite y system noise. This can usually e overcome y conscientious esign of e system an reaout integrate circuit (ROIC). The ROIC itself may contriute sufficient noise to ominate performance. This is especially true for smaller pixels, ecause smaller IC evices ten to have relatively higher noise. Aitionally, several noise sources originate in e etector itself. These inclue Johnson (or ermal) noise, current noise (typically /f), an temperaturefluctuation noise. The last of ese is peculiar to ermal etectors, alough it is not unrelate to shotnoise phenomena in photon etectors. Temperature-fluctuation noise is e ranom variation of e etector temperature resulting from fluctuation in e exchange of heat wi e environment. The spectral ensity of is variation is given y T 4k T. () G Note at is noise is an limite y e ermal time constant in e same manner as e responsivity. When integrate, e net temperature fluctuation is k T C T, () where C is e heat capacity ( = R C = C / G ). The resulting output noise voltage is en v V T k T C, (4) an e noise-equivalent power (NEP) at low frequency is NEP (pw). -µm pixel % fill factor No optical loss = 6.7 ms T = T = K Raiation "Limit",, Thermal Conuctance (nw/k) Figure - Depenence of limiting NETD on ermal isolation. The rate of heat exchange etween e etector an its environment is W G ( T T ) ( T T ) A con 4 4, (6) where G con is e mechanical ermal conuctance, is e Stefan-oltzmann constant, T is e ackgroun/amient temperature, an A is e etector active area. The secon term in Eq. (6) is e raiation term. If raiation ominates, en e conuctance is sai to e raiation-limite, an e effective ermal conuctance is Wra Gra 4 T A. (7) T G (nw/k) R Assumptions No spectral limits T = K % asorption % fill factor 4 6 Pixel Pitch (µm) G,, Figure - Depenence of roaan raiationlimite ermal conuction/resistance on pixel size. R (K/µW) NEP G. () Figure shows how pixel size affects e limiting as k T C as k G T Figure shows e epenence of NEP on G. ermal isolation. o e raiation-limite ermal conuctance an e ermal mass (heat capacity) scale linearly wi etector area, so e ermal time constant is inepenent of area. This is not e case if

4 mechanical conuction ominates, ecause mechanical conuction oes not generally scale wi area. In fact, is is a primary ifficulty in achieving raiationlimite ermal conuctance in small pixels. If we reuce pixel pitch y a factor of two, maintaining constant fill factor, en we must also reuce e ermal conuctance y a factor of four to maintain a given ermal time constant. For a typical microrige structure, scaling y a factor of two affects e support arm leng an wi y e same factor, so e ermal isolation is not affecte. Since e arm wi is often e critical imension for photoliography, it ecomes significantly more ifficult even to maintain e nominal ermal isolation. Furer improvements eyon e asic shrinking of e pixel are necessary to maintain raiation-limite ermal isolation. It is proaly worwhile to point out some analogies wi photon etectors. The raiation limit is, of course, analogous to e ackgroun limit for photon etectors. In e case of photon etectors, ackgroun photons generate charge carriers in e etector material, creating a photocurrent. At e same time, ermal processes wiin e etector material also generate carriers, creating ark current. Each of ese currents has an associate shot noise resulting from ranom fluctuations in e generation rate (an, in some cases, recomination) of ese carriers. If is shot noise ominates, an if e ackgroun photocurrent is greater an e ark current, en e evice is sai to e ackgroun limite. In e case of ermal etectors, we eal wi heat flow an temperature raer an electric current an charge carriers. A ermal etector exchanges raiation wi its environment (ackgroun), resulting in a temperature change. Also, heat may flow to an from e etector y ermal conuction an/or convection. Each of ese heat exchange mechanisms has an associate fluctuation ecause of e ranom nature of e processes involve, an e result is known as temperature fluctuation noise. If is noise ominates, an if e raiant heat exchange is greater an at from conuction an convection, en e evice is sai to e raiation limite. THERMAL VS. PHOTON DETECTORS Some have suggeste e possiility of e operation of photon etectors near normal amient temperature as a technology potentially competitive wi uncoole ermal etectors. There are certain avantages of such a evice, if it is inee feasile. A ermal etector requires a nominal vacuum package for e purpose of eliminating e contriution of air conuction to e total ermal conuctance. An IR photon etector normally requires a vacuum package for oer reasons. Since e etector must e coole (ase on current technologies), e vacuum package is necessary to reuce ermal loaing on e cooling evice an to eliminate e possiility of conensation on e etector itself. An IR photon etector capale of efficient operation wiout cooling woul eliminate o of ese issues an erefore eliminate e nee for a vacuum package. Furermore, such a photon etector woul not require ermal isolation, an it woul likely e capale of operation at much higher frequencies an ermal etectors. Of course, no one really knows how to make an IR photon etector capale of ackgrounlimite operation wiout cooling. A ermal etector has its own avantages, e principle one eing e relative crueness of e etector material requirements. Thermal etectors generally epen upon macroscopic material properties, whereas photon etectors generally epen upon microscopic properties. This has important implications regaring e cost to prouce. Of course, no one really knows yet how to make a ackgroun-limite ermal etector. The intent of is section is to evaluate e limiting performance of ermal etectors as compare to photon etectors. For photon etectors we are concerne wi e numer of photons asore from a lackoy source. The numer of ackgroun photons asore is given y ( ) hc k n ca int e T, (8) 4 where int is e integration time, an () is e spectral asorptivity. For a ermal etector we are concerne wi e rate at which heat is asore from a lackoy source. That rate is W hc ( ) e A hc kt ( ) e hc kt (9) where e secon term accounts for raiation emitte y e etector into its environment. Now e signals for e two etector types are proportional, respectively, to e erivatives of Eqs. (8) an (9) wi respect to ackgroun temperature, n T hc kt an A int ( ) hc k T ()

5 W T h c k T A ( ) 6 hc k T () where we have use hyperolic function notation to simplify e appearance. Equations representing e noise ue to e ranom exchange of photons wi e environment can e erive from consieration of quantum statistical mechanics of e raiation fiel. There is, however, a key ifference etween photon etectors an ermal etectors. In ermal etectors, e ranom emission of photons from e etector affects its temperature fluctuation in e same manner as e ranom asorption of photons emitte y e environment. For a photon etector, at least for a photoioe, only e ranom asorption from e ackgroun contriutes. The equations are an n W ca h c int A f ( ) 4 ( ) 6 ( ) 6 hc k T hc k T () hc k T () since asorptivity equals emissivity. Here f is e noise anwi. The noise anwi is not explicitly note in Eq.() ecause it is implicit in e integration time. It shoul e note at ese two equations assume only one active etector surface. This is vali for a ermal etector at makes use of a resonant asorption structure, ut a photon etector will also asor photons generate y its sustrate, an is will increase Eq. () y approximately a factor of two when T T, unless e acksie is reflective. Clearly, if we reuce e temperature of e ermal etector sufficiently, we can eliminate e contriution of e secon integral in Eq. (). Figure 4 an Figure give an inication of e temperature to which we must reuce e etector to achieve is result. In Figure 4, which plots e integran of Eq.( ), it is clear at e noise peak ecreases wi ecreasing temperature, an it shifts to longer wavelengs. It is easier to make quantitative evaluations using Figure, which is actually a top view of Figure 4 shown wi contours. The contours at e top of e figure represent higher noise values an ose in e lower parts. The minima of e curves occur at wavelengs for which noise is a maximum for e temperature at which e minimum occurs. This ecomes ovious if you raw a horizontal line rough one of e minima, an realize at every point elow a given contour represents lower noise an e points on e contour. The upper arker line is e curve appropriate to e spectral noise peak at K. In general, e noise maxima occur at hc. (4) max.969 k T For T = K, max = 8. µm; at T = 4 K, max =.4, an e peak value is reuce y aout a factor of two. The lower ark contour in Figure is e line for which e noise is exactly half at of e upper ark contour. Its minimum is at aout 4 K an µm. 9.8E- Relative Noise 7.E- 4.9E-.4E-.E+ Waveleng (µm) Temperature (K) Figure 4 - Contour of spectral noise contriutions as a function of temperature an waveleng, for a ermal etector. Temperature (K) Waveleng (µm) Figure - Lines of constant noise, as a function of waveleng an temperature. ol lines inicate two contours whose emission contriution to noise iffers y a factor of two. 4

6 Thus, y reucing e etector temperature to 4 K, we reuce its emission noise y aout one-half. Since is as in quarature wi e asorption noise, e net result is egraation from e ieal ackgroun limit y only aout %. This has important consequences in what follows. The signal-to-noise ratio for a photon etector is otaine y iviing Eq. () y e square-root of Eq. (); an at for a ermal etector, y iviing Eq. () y e square-root of Eq. (). Doing is wouln t give us much insight, ut if we limit e spectral an to a narrow region, we otain n n T ca an W int W T hc ( ) k ca T hc k hc ( ) k T T () (6) hc k T hc k T hc k T In comparing Eqs. () an (6), we oserve a few ifferences. They each epen on a time constant, ut e time constants are ifferent, an ere is an aitional factor of two. These ifferences occur ecause e photon etector is assume to e characterize y an integration time int, whereas e ermal etector is assume to e characterize y e roll-off of a single-pole filter whose time constant is. The anwi for in integrator is int f int, (7) whereas at for a single-pole filter is f, (8) 4 which accounts for e apparent factor of two, when we consier at we shoul compare evices of similar anwi. For an infrare photon etector int is typically very short measure in tens or perhaps hunres of microsecons. For a ermal etector it is longer typically longer an millisecons. The photon etector erefore has a spee avantage; e ermal etector, a sensitivity avantage, in principle. Of course, a system using a photon etector can integrate multiple high-spee frames off focal plane, ut ere will usually e a net loss ue to cycle time efficiencies. Also, a ermal etector can operate at higher spees, wi a concomitant loss of sensitivity very similar to at oserve in photon etectors. Anoer notale ifference is at if e etector temperature is equal to e ackgroun temperature, en e ermal etector signal-to-noise ratio is egrae y relative to at of an ieal photon etector. Note also at if e emission noise term in Eq. (6) is negligile, en e two equations are ientical. The importance of e earlier analysis is at it is only necessary to cool a ermal etector to 4 K to remove is penalty. Of course is is only vali if e raiation limit still applies at e reuce temperature. There is a slight error in comparing Eqs. () an (6) for a roa spectral range, ecause e integral of a ratio is not equal to e ratio of integrals. However, for reasonale spectral ans e error is typically less an a few per cent. Why en oes e conventional wisom maintain at ermal etectors will always e sustantially inferior relative to photon etectors? The answer to is question lies in previous assumptions mae regaring Eq. (). The past pessimistic projections have assume at () is unity, or at least constant, over e entire spectrum. For constant (), Eq. () ecomes W T T A f 8 k. (9) where is e Stefan-oltzmann constant, given y 4 k. () h c Eq. (9) is e usual equation given for raiant noise power in a ermal etector. The NEP in is case is given y NEP W 8 k T T A f () where e / factor is present to convert from asore power to incient power. For = an T = T = K, is results in a lackoy D* of.8 cm Hz / /W. Limiting e spectral an to etween 8. µm an. µm reuces e noise an erey increases D* y more an %. Thus, we see at e key to improving ermal etector performance to e level of photon etector performance is limiting e spectral an over which it asors an emits raiant noise to correspon to at over which it asors signal. A photon etector oes is automatically y virtue of its an gap. In time past, great pains were taken to give ermal etectors

7 roa spectral response inee, is was one of eir primary avantages. Moern ermal etectors, however, are mae very small to facilitate highresolution imaging. This necessitates a high egree of ermal isolation to maintain high sensitivity, an is in turn requires low ermal mass. roa-an spectral asorers are not generally compatile wi such lowmass structures. Most moern evices use a resonant optical asorer, which is a low-mass means to provie excellent asorption over a spectrum at can e ajuste y simply changing e gap etween e evice an e sustrate. This provies wiout encumrance a restriction of e spectral an at can contriute to temperature-fluctuation noise. ARRIERS From e earlier iscussions it is clear at minimizing net ermal conuctance is of paramount importance to improving e performance of ermal etectors. In e following susections we will iscuss some of e limitations to such reuctions an e implications on evice an package esign. Thermal Conuctance Just as a change of an gap an col-shieling can improve e performance of photon etectors, so also can a reuction in asore ackgroun raiation improve ermal etectors. There are several straightforwar techniques at will accomplish is, some of which have alreay een mentione. First, limiting e spectral response as nearly as possile to e anwi of interest for signal can have a sustantial effect. For example, For a evice whose signal spectral an is from 8. µm to. µm, eliminating all out-of-an raiation reuces e raiant ermal conuctance y a factor of.6. Secon, limiting e fiel of view of e etector y cool shieling can significantly reuce e effective ermal conuctance. For an f/. optical system, 7% of e raiation at strikes e etector comes from outsie e system fiel of view. For larger f/numers e fraction is even greater. Elimination of raiation from is region coul reuce e ermal conuctance y a factor of 4 or more, epening on e etector temperature. As implie y e term cool shiel (as contraste wi col shiel ), even a moest reuction in e shiel temperature can prouce important improvement. Thir, reucing e etector temperature can eliminate its emissions as a measurale source of noise. All of ese improvements are simple in principle, an mostly in practice. an limiting occurs naturally in most low-mass asorers. Alough e technology for cool shieling is reaily availale, it coul aversely affect cost-sensitive proucts. Reuction of e etector temperature is easily accomplishe y using a two- or ree-stage ermoelectric cooler. This, also, will a to e etector cost. Just as reucing e asore ackgroun raiation improves e performance of photon etectors only if ackgroun current is a significant contriution to total current, so also oes reucing e raiant ermal exchange improve e performance of ermal etectors only if raiant ermal exchange is a significant contriution to total ermal exchange. As one limits e spectral an an implements raiation shieling, e raiant contriution to ermal conuction iminishes. At some point, raiant exchange will e ominate y oer heat exchange processes, an furer reuction in e raiant component will prouce little or no enefit. This is analogous to reucing photoetector ackgroun raiation to e point at ark current strongly ominates. In at case, furer reuction of e ackgroun will not sustantially improve performance. Therefore, e nee to improve e mechanical implementation to minimize ermal conuction ecomes ever more important as one reuces e raiant component. Heat Capacity The ermal time constant is relate to e pixel heat capacity an ermal conuctance as follows: C. () G This means at e heat capacity must e reuce concomitantly wi e ermal conuctance, in orer to maintain a useful time constant. The heat capacity is given y C c p A z, () where is e average ensity of e pixel, c p is e average specific heat, A is e etector physical area, an z is e average etector ickness. Using ese two equations wi Eq. (7), we get c z. (4) p 4 T The only inepenent physical parameter in is equation is z ; so is places a constraint upon e ickness of e etector structure. Figure 6 shows at a time constant of /6 sec ictates at e structure ickness e only 4 Å, assuming an average c p of 6

8 . J/cm. This actually overestimates e allowe ickness, ecause e ermal isolation structure also contriutes half its mass to e heat capacity. Wiin is ickness e pixel must also contain an IR asorption structure. Detector Structure Thickness (Å) 4 Assumptions: c p =. J/cm Thermal Time Constant (ms) Figure 6 - Relationship etween etector structure ickness an ermal time constant for a raiationlimite etector. where N A is Avagaro s numer, an M is e molar mass (8 gm for N ). The net ermal conuctance contriute y e gas is G k L k L g s g w gas A, (7) Ls Lw where L s is e pixel-to-sustrate istance, an L w is e pixel-to-winow istance. Figure 7 elow shows e results for µm pixels. The two contriutions are roughly equal elow aout mtorr. Since e gas conuctance is proportional to e etector area, e ratio of gas conuctance to raiative conuctance is inepenent of pixel size. For N, gas conuctance is equal to raiative conuctance at a pressure of aout 8 mtorr. Since, accoring to Figure 7, e gas ermal conuctance is proportional to pressure at low pressure, e package pressure must e elow aout mtorr to keep e NETD egraation ue to gas conuction at aout %. (Conuctances a linearly, an NETD varies as e square-root of total conuctance.) Vacuum Package The etector pixel can exchange heat wi its environment y ermal conuction rough e gas in its container. This imposes a constraint on e package vacuum level. It is apparent at e conuction pa etween e pixel an e ROIC will e most important, ecause e spacing is typically only aout µm. However, at low pressures, e mean-free pa can e significantly longer an e spacing etween e pixel an e winow, which is typically aout µm. Therefore, uner low-pressure conitions e pixel-towinow conuction pa will contriute equally wi e pixel-to-roic pa. The effective mean-free pa is given y P ( L), () kt L where P is e pressure, is e molecular iameter of e conucting gas (.4 Å for N ), an L is e spacing etween e pixel an e surface uner consieration. Aing e /L term is an approximation at assumes e inter-molecular an surface collision rates are aitive. The effective ermal conuctivity for N is k g L 7 8k N A P L, (6) 4 M T Thermal Conuctance (nw/k). Pixel-to-sustrate. - 4 Log Pressure (mtorr) Pixel-to-w inow Figure 7 Contriutions of gas ermal conuctance as a function of pressure. Electrical Resistance The structure at provies ermal isolation must also provie mechanical streng as well as an electrically conuctive pa for accessing signals generate y e etector. These are not inepenent features, an we will consier in particular e relationship etween electrical conuctance an ermal conuctance. The support structure generally comprises two elements, an electrically insulating mechanical layer an an 7

9 electrically conuctive layer. For e purposes of is analysis we will assume at e electrically conuctive layer ominates e ermal conuctance. This is proaly a goo approximation for well-esigne small structures in which e mechanical emans are not too severe. In e electrically conuctive layer e ermal conuctance an e electrical resistance are linke y e Wieemann-Franz relationship, G R k e k µw Ω T 7. q K, (8) where G is e ermal conuctance, R e is e electrical resistance, k is e ermal conuctivity, an is e electrical resistivity. The > is present instea of e = sign ecause e Wieemann-Franz law applies strictly only to electron transport. Since phonons can contriute significantly to ermal transport ut not irectly to electron transport, e Wieemann-Franz law is a lower limit to e ermal conuctivity. The expression G R e reuces to k ecause geometrical factors exactly cancel. We require at e ermal conuctance of e support structure e much less an at ue to raiative exchange, so G 4 T A R e µw Ω 7. K 4 T A.9 kω If we allow e support structure to contriute % to e total ermal conuctance, en we have R e > k for a % impact to NETD. The resistance nees to e even higher if e electrically conuctive material is a poor conuctor or if e ermal conuctance contriution of e mechanical structure is nonnegligile. This has serious consequences for low-impeance etectors such as ermocouples or low-resistance olometers. For a olometer of resistance R, a series resistance of R s egraes e effective temperature coefficient of resistance (TCR) y eff R R R s R s T R T R R s R R R s (9) Therefore a k olometer wi a k series resistance woul have an effective TCR of half its nominal value. For pyroelectric etectors or olometers wi resistance greater an aout k, e impact is negligile. Spatial Noise Detector pixel signal-to-noise ratio is not e only parameter important to target acquisition an surveillance. As sensitivity improves, oer issues ecome more important. The issue of paramount importance in most moern uncoole IR imaging sensors is spatial noise. Spatial noise has ree funamentally ifferent causes: Offset non-uniformity, response non-uniformity, an low-frequency noise. Offset variation among e etector pixels is e greatest contriutor to raw non-uniformity. In olometers is results primarily from pixel-to-pixel resistance variation. This non-uniformity is reaily correcte, ut it greatly increases e electronic complexity of e evice. Sutle changes such as slight rift of e focal plane temperature or ias voltage give rise to unacceptale non-uniformity even after e aseline correction, so e correction coefficients must e upate perioically, which requires oscuration of e image for a short time every few minutes. Alternatively, offset non-uniformity can e oviate y use of a chopper wi a etector having AC-couple pixels. A chopper can also e effective wi a DCcouple etector, ut only wi sustantial performance penalty. The prolems arising from gain non-uniformity are not usually as severe a prolem as ose from offset nonuniformity, ut it must also e correcte. Its greatest impact to performance arises when e average temperature of a region of e scene is sustantially ifferent from at for which e sensor was initially calirate. If such a scene were uniform, en e nonuniform response of e array woul make e image appear non-uniform or cluttere. If such a scene were complex, e non-uniformity woul a clutter or shaing. Gain uniformity is usually very easily correcte unless e etector response is significantly non-linear, an e correction coefficients are usually highly stale. Low-frequency noise is manifest as slowly-changing spatial noise, an is is particularly so for /f noise. Signal an noise measurements are generally mae y taking ata samples over a perio of up to one secon. Noise components at frequencies sustantially lower an one Hertz are not at all evient in such measurements. If e same measurements were taken a minute or an hour later, e noise woul appear e same, ut e offsets woul appear to e ifferent. The low-frequency noise components woul have change etween measurements, ut ey o not change uring e measurements. ecause each pixel is an inepenent noise source, e changes are not uniform. If e ominant noise source were column amps or row- 8

10 aress circuitry, en e spatial noise woul appear column- or row-correlate. The nature of /f noise in is regar is at it creates spatial noise at increases logarimically wi time. ecause low-frequency noise generates effects at change slowly wi time, use of a chopper effectively suppresses it. Spatial noise is proaly e limiting noise for all current DC-couple uncoole IR sensors. Only ACcouple ferroelectric evices have sufficiently low spatial noise to e limite y temporal noise. As e sensitivity of ermal etectors improves, e removal of spatial noise will ecome even more challenging. SUMMARY AND CONCLUSIONS Development of ackgroun-limite ermal etectors will e a sustantial challenge. It will require clever esign to inclue an efficient IR asorer in a very lowmass pixel structure. The etector evice will have to e of sufficiently high impeance to avoi egraation of performance as e result of high-resistance in e electrical conuits. Success of ackgroun-limite evices will require evelopment of low-cost vacuum technology capale of maintaining an amient pressure of aroun mtorr for e life of e prouct. Oer issues, such as excessive self-heating in a olometer of very low mass an very high ermal isolation, coul prove etrimental to achieving e ultimate performance. Many challenges lie ahea for continue progress in uncoole ermal etectors, ut so also o many opportunities to ramatically improve e performance an utility of ese evices. Wi sustaine investment, ermal etectors will e ale to serve e most stringent IR imaging applications currently serve y low-temperature photon etectors, ut ermal etectors will o it wi sustantially reuce cost, weight, an power. C.M. Hanson, Analysis of e Effects of /f Noise an Choppers on e Performance of DC-Couple Thermal Imaging Systems, Infrare Technology an Applications XXVII, SPIE Vol. 469, p. 6. However, provie we are willing to moestly cool our ermal etectors, ey potentially have essentially e same performance limits as photon etectors. Spectral an limiting y e IR asorer reuces e ackgroun noise limit in a manner not consiere in past projections at showe ermal etectors to have a significant performance isavantage. It is apparent at ermal etectors may even have an avantage for low-frequency applications, whereas photon etectors offer an avantage for high frequencies. Once ermal etectors are coole, ey are suject to e same enefits of col shieling as are photon etectors. Note, however, at reucing e raiant emission of e etector an e ackgroun reuces o ermal conuctance an noise. This will put furer stress on etector mass, package vacuum an oer noise components. This is not unexpecte, of course, ecause reucing e limiting noise always makes oer noise components more important. To take avantage of e improvements, ese future etectors must effectively suppress spatial noise. This means at such ultra-high-performance sensors will likely use AC-couple etectors such as in-film ferroelectric evices. 9