Third-generation infrared photodetector arrays - Antoni Rogalski

44 Pages·2009·5.06 MB·English

JOURNALOFAPPLIEDPHYSICS105,091101(cid:1)2009(cid:2)
APPLIED PHYSICS REVIEWS—FOCUSED REVIEW
Third-generation infrared photodetector arrays
A. Rogalski,1 J. Antoszewski,2,a(cid:1) and L. Faraone2
1InstituteofAppliedPhysics,MilitaryUniversityofTechnology,2KaliskiegoStreet,00-908Warsaw,
Poland
2SchoolofElectrical,ElectronicandComputerEngineering,TheUniversityofWesternAustralia,
Crawley6009,Australia
(cid:1)Received 30 June 2008; accepted 26 January 2009; published online 11 May 2009(cid:2)
Hitherto, two distinct families of multielement detector arrays have been used for infrared (cid:1)IR(cid:2)
imaging system applications: linear arrays for scanning systems (cid:1)first generation(cid:2) and
two-dimensional arrays for staring systems (cid:1)second generation(cid:2). Nowadays, third-generation IR
systems are being developed which, in the common understanding, provide enhanced capabilities
such as larger numbers of pixels, higher frame rates, better thermal resolution, multicolor
functionality, and/or other on-chip signal-processing functions. In this paper, fundamental and
technologicalissuesassociatedwiththedevelopmentandexploitationofthird-generationIRphoton
detectors are discussed. In this class of detectors the two main competitors, HgCdTe photodiodes
andquantum-wellphotoconductors,areconsidered.Thisisfollowedbydiscussionsfocusedonthe
most recently developed focal plane arrays based on type-II strained-layer superlattices and
quantum dot IR photodetectors.The main challenges facing multicolor devices are concerned with
complicated device structures, thicker and multilayer material growth, and more difficult device
fabrication, especially for large array sizes and/or small pixel dimensions.This paper also presents
and discusses the ongoing detector technology challenges that are being addressed in order to
develop third-generation infrared photodetector arrays. © 2009 American Institute of Physics.
(cid:3)DOI:10.1063/1.3099572(cid:4)
TABLE OF CONTENTS 2. Experimental verification............... 25
VII. HgCdTe MULTICOLOR DETECTORS........ 26
A. HgCdTe dual-band detectors.............. 26
I. INTRODUCTION............................ 1
B. Three-color HgCdTe detectors............. 30
II. BRIEF HISTORYOF IR DETECTORS......... 2
VIII. MULTIBAND QWIPS..................... 32
III. FIRST-AND SECOND-GENERATION IR IX. TYPE-II InAs/GaInSb DUAL-BAND
IMAGING SYSTEMS....................... 3 DETECTORS.............................. 36
IV. BENEFITS OF MULTICOLOR DETECTION.... 5 X. MULTIBAND QDIPS........................ 38
V. REQUIREMENTS OF THIRD-GENERATION XI. ADAPTIVE FPAS.......................... 39
DETECTORS............................... 6 XII. CONCLUSIONS........................... 40
A. Noise equivalent difference temperature..... 6 XIII. NOMENCLATURE........................ 41
B. Pixel and chip sizes..................... 7
C. Uniformity............................. 8 I. INTRODUCTION
D. Identification and detection ranges......... 9
Multicolor detector capabilities are highly desirable for
VI. MATERIALCONSIDERATIONS FOR advanced infrared (cid:1)IR(cid:2) imaging systems, since they provide
THIRD-GENERATION IR DETECTORS....... 9
enhanced target discrimination and identification, combined
A. HgCdTe............................... 10
with lower false-alarm rates. Systems that collect data in
1. Outlook on crystal growth.............. 11
separate IR spectral bands can discriminate both absolute
2. HgCdTe photodiodes.................. 13
temperature as well as unique signatures of objects in the
B. QWIPs................................ 15
scene. By providing this new dimension of contrast, multi-
C. InAs/GaInSb type-II SLS photodiodes...... 19
band detection also offers advanced color processing algo-
1. Material properties.................... 19
rithms to further improve sensitivity above that of single-
2. SLphotodiodes...................... 20
color devices. This is extremely important for identifying
D. QDIP................................. 22
temperature differences between missile targets, warheads,
1. Anticipated advantages of QDIPs........ 23 and decoys. Multispectral IR focal plane arrays (cid:1)FPAs(cid:2) are
highlybeneficialforavarietyofapplicationssuchasmissile
a(cid:2)Electronicmail:[email protected]. warning and guidance, precision strike, airborne surveil-
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091101-2 Rogalski,Antoszewski,andFaraone J.Appl.Phys.105,091101(cid:2)2009(cid:1)
lance, target detection, recognition, acquisition and tracking,
thermal imaging, navigational aids and night vision, etc.1,2
They also play an important role in Earth and planetary re-
mote sensing, astronomy, etc.3
Military surveillance, target detection, and target track-
ing can be undertaken using single-color FPAs if the targets
are easy to identify. However, in the presence of clutter, or
when the target and/or background are uncertain, or in situ-
ations where the target and/or background may change dur-
ing engagement, single-color system design involves com-
promises that can degrade overall capability. It is well
established that in order to reduce clutter and enhance the FIG.1. DevelopmentofIRdetectors:NEDTvsera(cid:1)afterRef.5(cid:2).
desired features/contrast, one will require the use of multi-
spectral FPAs. In such cases, multicolor imaging can greatly
thedistributionofenergyinsunlight.TheearlyhistoryofIR
improve overall system performance.
was reviewed about 40 years ago in two well-known
Currently, multispectral systems rely on cumbersome monographs6,7 and Barr’s paper.8
imaging techniques that either disperse the optical signal
Initially, the development of IR detectors was connected
across multiple IR FPAs or use a filter wheel to spectrally
withthermaldetectors.In1821Seebeckdiscoveredthether-
discriminate the image focused on a single FPA. These sys-
moelectric effect and soon thereafter demonstrated the first
temsincludebeamsplitters,lenses,andbandpassfiltersinthe
thermocouple, but several years later, in 1829, Nobili con-
opticalpathtofocustheimagesontoseparateFPAsrespond-
structed the first thermopile by connecting a number of ther-
ingtodifferentIRbands.Also,complexopticalalignmentis
mocouples in series. Langley’s bolometer appeared in 1880,
requiredtomapthemultispectralimagepixelforpixel.Con-
in which he used two thin ribbons of platinum foil, con-
sequently,theseapproachesarerelativelyhighcostandplace
nected so as to form two arms of a Wheatstone bridge. He
additional burdens on the sensor platform because of their
continued to develop his bolometer over the next 20 years,
extensive size, complexity, and cooling requirements.
400 times more sensitive than in his first efforts. His latest
Inthefuture,multispectralimagingsystemswillinclude
bolometer could detect the heat from a cow at a distance of
verylargesensorsfeedinganenormousamountofdatatothe
quarter of a mile.
digitalmissionprocessingsubsystem.FPAswiththenumber
The initial spectacular applications of thermal detectors
of pixels above 1(cid:1)106 are now available.As these imaging
in astronomy are noted in Fig. 1. In 1856, Charles Piazzi
arraysgrowindetectornumberforhigherresolution,sowill Smyth,9,10fromthepeakofGuajaraonTenerife,detectedIR
the computing requirements for the embedded digital image
radiation from the Moon using a thermocouple. In the early
processing system. One approach to solving this processing
1900s, IR radiation was successfully detected from the plan-
bottleneck problem could be to incorporate a certain amount
ets Jupiter and Saturn and from some bright stars such as
of pixel-level processing within the detector pixel, similar to Vega and Arcturus. In 1915, William Coblentz9 at the U.S.
the technique implemented in biological sensor information
National Bureau of Standards developed thermopile detec-
processing systems. Currently, several scientific groups in
tors, which he uses to measure the IR radiation from 110
the world have turned to the biological retina for answers as
stars. However, the low sensitivity of early IR instruments
to how to improve man-made sensors.4,5
preventedthedetectionofothernear-IRsources.WorkinIR
In this paper, we will review the state-of-the-art muti-
astronomyremainedatalowleveluntilbreakthroughsinthe
colordetectortechnologiesoverawideIRspectralrange.In
developmentofnew,sensitiveIRdetectorswereachievedin
the beginning we will outline the historical evolution of IR
the late 1950s.
detectortechnologyshowingwhycertaindevicedesignsand
Photon detectors were developed during the 20th cen-
architectures have emerged as successful candidates in the
tury. The first IR photoconductor was developed by Case in
development of multicolor detectors. Next, discussion is fo- 1917.11 In 1933, Kutzscher at the University of Berlin dis-
cused on the most recently developed FPAs containing such covered that lead sulfide (cid:1)from natural galena found in Sar-
material systems as HgCdTe photodiodes, type-II strained- dinia(cid:2) was photoconductive and had a response out to about
layer superlattices (cid:1)SLS(cid:2), quantum-well IR photodetectors 3 (cid:2)m.12
(cid:1)QWIPs(cid:2), and quantum dot IR photodetectors (cid:1)QDIPs(cid:2). Fi-
IR detector technology development was and continues
nally,wediscusstheongoingdetectortechnologyeffortsbe-
to be primarily driven by military applications. Many of
ing undertaken to realize third-generation FPAs.
these advances were transferred to IR astronomy from U.S.
Department of Defense research. In the mid-1960s, the first
IR survey of the sky was made at the Mount Wilson Obser-
II. BRIEF HISTORYOF IR DETECTORS
vatory using liquid nitrogen cooled PbS photoconductors,
Looking back over the past 1000 years, it is noted that which were most sensitive at 2.2 (cid:2)m. The survey covered
IR radiation itself was unknown until just under 200 years approximately 75% of the sky and found about 20000 IR
ago when Herschel’s experiments with thermometers was sources.9 Many of these sources were stars which had never
first reported. He built a crude monochromator that used a been seen before in visible light.
thermometer as a thermal detector so that he could measure Hitherto, many materials have been investigated in the
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091101-3 Rogalski,Antoszewski,andFaraone J.Appl.Phys.105,091101(cid:2)2009(cid:1)
FIG.2. (cid:1)Coloronline(cid:2)HistoryofthedevelopmentofIRdetectorsandsystems.Threegenerationsystemscanbeconsideredforprincipalmilitaryandcivilian
applications:Firstgeneration(cid:1)scanningsystems(cid:2),secondgeneration(cid:1)staringsystems—electronicallyscanned(cid:2),andthirdgeneration(cid:1)multicolorfunctionality
andotheron-chipfunctions(cid:2).
IR field. Observing a history of the development of IR de- staringsystems(cid:1)seeupperpartofFig.2(cid:2).Scanningsystems,
tector technology, a simple theorem, after Norton,13 can be which do not include multiplexing functions in the focal
stated: “All physical phenomena in the range of about 0.1–1 plane, belong to first-generation systems.Atypical example
eVcanbeproposedforIRdetectors.”Amongthesephenom- of this kind of detector is a linear photoconductive array in
ena are thermoelectric power (cid:1)thermocouples(cid:2), change in whichanelectricalcontacttoeachelementofamultielement
electrical conductivity (cid:1)bolometers(cid:2), gas expansion (cid:1)Golay arrayisconnectedfromthecryogenicallycooledfocalplane
cell(cid:2), pyroelectricity (cid:1)pyroelectric detectors(cid:2), photon drag, totheoutside,wherethereisoneelectronicamplifyingchan-
Josephson effect (cid:1)Josephson junctions, superconducting nel at ambient temperature for each detector element. The
quantum interference devices(cid:2), internal emission (cid:1)PtSi U.S. common-module HgCdTe arrays employ 60, 120, or
Schottky barriers(cid:2), fundamental absorption (cid:1)intrinsic photo-
180photoconductiveelements,dependingontheapplication.
detectors(cid:2), impurity absorption (cid:1)extrinsic photodetectors(cid:2),
HgCdTe photoconductors entered production in the late
low dimensional solids (cid:3)superlattice (cid:1)SL(cid:2), quantum well
1970s following the establishment of reproducible bulk
(cid:1)QW(cid:2), and quantum dot (cid:1)QD(cid:2) detectors(cid:4), different types of
growth techniques and anodic oxide surface passivation.Al-
phase transitions, etc.
thoughfirst-generationsystemsarenowbeingsupplantedby
Figure 2 gives approximate dates of significant develop-
second-generation photovoltaic-based arrays, the production
ment efforts for the above-mentioned materials. The years
of such devices will continue for many years to come. It
during World War II saw the origins of modern IR detector
should be noted that many photovoltaic detector arrays are
technology. Photon IR technology combined with semicon-
also used in scanning formats.
ductor material science, photolithography technology devel-
Second-generation systems (cid:1)full-framing systems(cid:2) typi-
oped for integrated circuits, and the impetus of Cold War
cally have three orders of magnitude more elements (cid:1)(cid:3)106(cid:2)
militarypreparednesshavepropelledextraordinaryadvances
on the focal plane than first-generation systems, and the de-
in IR capabilities within a short time period during the last
century.14 tector elements are configured in a two-dimensional (cid:1)2D(cid:2)
array format.These staring arrays are scanned electronically
by readout integrated circuits (cid:1)ROICs(cid:2) that are hybrid pack-
III. FIRST-AND SECOND-GENERATION IR IMAGING
aged with the arrays. These modules are 2D arrays of pho-
SYSTEMS
todiodes connected with indium bumps to a ROIC chip as a
Twodistinctfamiliesofmultielementdetectorarrayscan hybrid structure, often called a sensor chip assembly (cid:1)SCA(cid:2)
be considered for military and civilian IR imaging applica- (cid:1)see Fig. 3(cid:2). These ROICs include, e.g., pixel deselecting,
tions; one used for scanning systems and the other used for antiblooming on each pixel, subframe imaging, output
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091101-4 Rogalski,Antoszewski,andFaraone J.Appl.Phys.105,091101(cid:2)2009(cid:1)
FIG.3. HybridIRFPAwithindependentlyoptimizedsignaldetectionandreadout:(cid:1)a(cid:2)indiumbumptechnique;(cid:1)b(cid:2)loopholetechnique.
preamplifiers, and many other functions in such structures. holeinterconnectiontechnologyoffersmorestablemechani-
The detector material and multiplexer can be optimized in- cal and thermal features than the flip-chip indium bump-
dependently. Other advantages of hybrid-packaged FPAs are bonded hybrid architecture.
near-100% fill factors and increased signal-processing area The 2D second-generation detector arrays are usually il-
on the multiplexer chip. Photodiodes with their very low luminated from the back side with photons passing through
powerdissipation,inherentlyhighimpedance,negligible1/f the transparent detector array substrate. In HgCdTe hybrid
noise,andeasymultiplexingviatheROICcanbeassembled FPAs, photovoltaic detectors are usually formed on a thin
in 2D arrays containing a very large number of pixels, lim- HgCdTeepitaxiallayergrownonatransparentCdZnTesub-
strate.FortheHgCdTeflip-chiphybridtechnology,themaxi-
ited only by existing technologies. Photodiodes can be re-
mum chip size is on the order of 20 mm square due to ther-
verse biased for even higher impedance, and can therefore
malmismatchbetweentheFPAandROIC.Toovercomethis
better match electrically with compact low-noise silicon
problem, alternatives to CdTe for epitaxy technology was
readout preamplifier circuits. The photoresponse of photo-
developed with sapphire or silicon as the substrate for the
diodes remains linear for significantly higher photon flux
HgCdTelayer.Whenusingopaquematerials,substratesmust
levels than that of photoconductors, primarily because of
be thinned to below 10 (cid:2)m to obtain sufficient quantum
higher doping levels in the photodiode absorber layer and
efficiencies and minimize cross-talk.
because the photogenerated carriers are collected rapidly by
Intermediatesystemsinusearealsofabricatedwithmul-
the junction. Development of hybrid packaging technology
tiplexed scanned photodetector linear arrays and with, as a
beganinthelate1970s(cid:1)Ref.15(cid:2)andtookthenextdecadeto
rule, time delay and integration (cid:1)TDI(cid:2) functions.Typical ex-
reach volume production. In the early 1990s, fully 2D imag- amples of these systems are HgCdTe multilinear 288(cid:1)4 ar-
ing arrays provided a means for staring sensor systems to rays, fabricated by Sofradir for both 3–5 and 8–10.5 (cid:2)m
entertheproductionstage.Inthehybridarchitecture,indium
bands with signal processing on the focal plane.
bump bonding with readout electronics provides for multi- Complementary metal oxide semiconductor (cid:1)CMOS(cid:2)
plexingthesignalsfromthousandsormillionsofpixelsonto
multiplexers are the best choice to perform the integration
a few output lines, greatly simplifying the interface between
and signal processing for 2D arrays. The advantages of
the vacuum-enclosed cryogenic sensor and the system elec-
CMOS are that existing foundries, which fabricate applica-
tronics. tionspecificintegratedcircuits,canbereadilyusedbyadapt-
Hybridized FPAdetectors and multiplexers are also fab- ingtheirdesignrules.Designrulesof0.07 (cid:2)marecurrently
ricated using a loophole interconnection structure.16 In this in production, with preproduction runs of 0.045 (cid:2)m design
case, the IR detector material and the multiplexer chip are rules.Asaresultofsuchfinedesignrules,morefunctionality
glued together to form a single bonded unit before detector has been designed into the unit cells of IR and visible mul-
fabrication. The photovoltaic detector elements are then tiplexers with smaller unit cells, leading to large array sizes.
formed by a combination of ion implantation and an etching Figure 4 shows the timelines for minimum circuit features
process, with the loopholes drilled by ion milling.The loop- andtheresultingcharge-coupleddevice(cid:1)CCD(cid:2),IRFPA,and
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091101-5 Rogalski,Antoszewski,andFaraone J.Appl.Phys.105,091101(cid:2)2009(cid:1)
FIG.5. (cid:1)Coloronline(cid:2)Sixteen2048(cid:1)2048HgCdTeSCAswereassembled
fortheVISTAtelescope.TheSCAsareattachedtoaprecisiongroundplate
that ensures that all pixels are within 12 (cid:2)m of the desired focus. The
detectorsarereadytobeplacedinthetelescopecamera’svacuumchamber
andcooledto72K(cid:1)afterRef.17(cid:2).
FIG.4. (cid:1)Coloronline(cid:2)Imagingarrayformatscomparedwiththecomplexity function of the year first used in astronomy for medium-
ofmicroprocessortechnologyasindicatedbytransistorcount.Thetimeline wavelength IR (cid:1)MWIR(cid:2) SCAs. At present arrays exceed
designruleofMOS/CMOSfeaturesisshownatthebottom(cid:1)afterRef.17
withcompletions(cid:2).ThenumberofpixelsonanIRarrayhasbeengrowing 4K(cid:1)4K format—16(cid:1)106 pixels—this about a year later
exponentially,inaccordancewithMoore’slawfor25yearswithadoubling than the Moore’s law prediction.
timeofapproximately19months(cid:1)insertoffigure—afterRef.18(cid:2). Itshouldbenotedthatthedramaticincreaseinthenum-
ber of pixels is observed in short- and medium-wavelength
applications where most or all of the sensing time can be
CMOS visible imager sizes with respect to number of imag-
usedforsignalintegration.InthelogwavelengthIR(cid:1)LWIR(cid:2)
ingpixels.Alongthehorizontalaxisisalsoascaledepicting
spectral region, the photon flux from earth scenes fills the
the general availability of various MOS and CMOS pro-
unit cell charge-storage capacitor in just a fraction of the
cesses.The ongoing migration to even finer lithography will
available time (cid:1)especially in the case of HgCdTe photo-
thus enable the rapid development of CMOS-based imagers
diodes(cid:2), limiting the full-signal averaging advantage of star-
having even higher resolution, better image quality, higher
ing technology. Even so, the improvement over LWIR
levels of integration, and lower overall imaging system cost
scanned sensors is significant.
thanCCD-basedsolutions.Atpresent,CMOSwithminimum
The trend of increasing pixel numbers is likely to con-
features of (cid:4)0.5 (cid:2)m makes possible monolithic visible
tinueintheareaoflarge-formatarrays.Thisincreasewillbe
CMOS imagers because the denser photolithography allows
continued using a close-butted mosaic of several SCAs as
for low-noise signal extraction and high-performance detec-
shown in Fig. 5. Raytheon has recently manufactured a 4
tion with high optical fill factor within each pixel. The sili-
(cid:1)4 mosaic of 2K(cid:1)2K HgCdTe SCAs into the final focal
con wafer production infrastructure which has put high-
plane configuration to survey the entire sky in the southern
performance personal computers into many homes makes
hemisphere at four IR wavelengths. With 67(cid:1)106 pixels,
CMOS-based imaging in consumer products such as video
this is currently the world’s largest IR FPA. Although there
and digital still cameras widely available.
arecurrentlytechnologicallimitationstoreducingthesizeof
Large 2D IR detector arrays, that meet the demanding
the gaps between active detectors on adjacent SCAs, many
requirements of astronomy and civil space applications, are
of these can be overcome. It is predicted that FPAs of 100
alsoavailabletoday.IRradiation,havinglongerwavelengths
megapixels and larger will be possible in the near future,
and lower energy than visible light, is not suitable for use constrained only by budgets, but not technology.19
with the photographic plates which are used in visible light
astronomy. Astronomers, in particular, have eagerly waited
IV. BENEFITS OF MULTICOLOR DETECTION
for the day when optoelectronic arrays could match the size
ofphotographicfilm.Developmentoflargeformat,highsen- IR multispectral imaging, sometimes referred to in the
sitivity,mosaicIRsensorsforground-basedastronomyisthe literature as hyperspectral imaging, is a relatively recent de-
goal of many observatories around the world, since large velopment that combines the information available from
arrays dramatically multiply the data output of a telescope spectroscopy with the ability to acquire this information in a
system. spatially resolved manner.20 Instrumentally, an IR camera is
Forthepast25yearsarraysizehasbeenincreasingatan used to record the spatial distribution of IR radiation in the
exponential rate, following the Moore’s law grow path (cid:1)see scene, and the spectral information is gained by scanning a
insertofFig.4(cid:2),withthenumberofpixelsdoublingevery19 dispersive element to record spectra for each image. Single-
months.ThegraphshowsthenumberofpixelsperSCAasa color FPAs in conjunction with spectral filters, grating spec-
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091101-6 Rogalski,Antoszewski,andFaraone J.Appl.Phys.105,091101(cid:2)2009(cid:1)
trometers or Fourier transform spectrometers have been de- C
T= 2
ployed for a variety of NASA spaceborne remote-sensing
lnR−lnC
1
applications utilizing push-broom scanning to record hyper-
spectral images of the Earth over the visible through very = (cid:1)hc/k(cid:2)(cid:3)(cid:1)1/(cid:6)2(cid:2)−(cid:1)1/(cid:6)1(cid:2)(cid:4) .
LWIR (cid:1)VLWIR(cid:2) spectral range. Dispersive devices based on lnR+ln(cid:1)(cid:5) /(cid:5) (cid:2)+5ln(cid:1)(cid:6) /(cid:6) (cid:2)+ln(cid:1)(cid:8)(cid:9)(cid:6) /(cid:8)(cid:9)(cid:6) (cid:2)
2 1 2 1 1 1 2 2
mechanical scanning (cid:1)e.g., filter wheels, monochromators(cid:2) (cid:1)5(cid:2)
arenotdesirablebecause,inadditiontotheirrelativelylarge
sizetheyarepronetovibrationsandarespectrallytunedina The two-color detection technique is beneficial because
relatively narrow range at a relatively slow speed.21 Recent the temperature becomes independent of the object emissiv-
advances in material, electronic, and optical technologies ity, providing that the emissivity does not vary between (cid:6)
1
have led to the development of novel types of electronically and (cid:6) and is inherently self-calibrated. This method can be
2
tunable filters, including the so-called adaptive FPAs.22 extremely useful in missile detection where there is a large
As mentioned in Sec. I, simultaneous detection in mul- differencebetweenthetemperatureofthesurfaceofthemis-
tiwavelength bands with a single FPAresults in reduction or sile and the missile exhaust plume.
elimination of heavy and complex optical components now
requiredforwavelengthdifferentiationinremotesensorsand V. REQUIREMENTS OF THIRD-GENERATION
leads to smaller, lighter, simpler instruments with better per- DETECTORS
formance. In the 1990s (cid:1)see Fig. 2(cid:2) third-generation IR detectors
Whenever the target to be detected behaves as a black-
emerged after the tremendous impetus provided by detector
body,thetruetemperatureinferredfromthebodyisaccurate
developments. The definition of third-generation IR systems
and reliable. On the other hand, when it exhibits a behavior
is not particularly well established. In the common under-
different from a blackbody, emissivity compensation needs
standing, third-generation IR systems provide enhanced ca-
to be undertaken. In the case of a known emissivity, (cid:5), a
pabilitiessuchaslargernumberofpixels,higherframerates,
single wavelength system can be used, whereas for gray
better thermal resolution, as well as multicolor functionality
body (cid:1)unknown but constant emissivity in a narrow band-
and other on-chip signal-processing functions.According to
width(cid:2) a dual-color system is more likely to be utilized. Reago et al.,23 the third generation is defined by the require-
Let us consider detectors, which are selected so as to
ment to maintain the current advantage enjoyed by the U.S.
cover the spectral range of blackbody emission from the tar-
and allied armed forces. This class of devices includes both
get surface at the desired temperature. Thus, their cutoff cooled and uncooled FPAs:1,23 (cid:1)i(cid:2) high performance, high
wavelength will be shorter for higher temperature objects. resolution cooled imagers having multicolor bands; (cid:1)ii(cid:2)
Dependingontheselecteddetectors,thetwo-bandtechnique medium- to high-performance uncooled imagers; (cid:1)iii(cid:2) very
allows for temperature measurements, for example, with ei-
low cost, expendable uncooled imagers.
ther MWIR detectors (cid:1)higher temperature range(cid:2) or LWIR
When developing third-generation imagers, the IR com-
detectors (cid:1)lower temperature range(cid:2). The detector is aimed
munity is faced with many challenges. Some of them are
towardthescenewhichisatatemperatureT,whichinitially considered here: (cid:1)i(cid:2) noise equivalent temperature difference
can be taken as a blackbody given by Planck’s law, (cid:1)NEDT(cid:2), (cid:1)ii(cid:2) pixel and chip size issues, (cid:1)iii(cid:2) uniformity, and
2h(cid:7)2 (cid:1)iv(cid:2) identification and detection ranges.
r(cid:1)(cid:6)(cid:2)= , (cid:1)1(cid:2)
(cid:6)3exp(cid:3)(cid:1)h(cid:7)/kT(cid:2)−1(cid:4)
A. Noise equivalent difference temperature
where r is the radiance per unit wavelength and (cid:7) is the
For FPAs the relevant figure of merit is the NEDT, the
radiation frequency.
temperature change of a scene required to produce a signal
Thetechniqueoftwo-colordetectionconsistsofmaking
equal to the rms noise. It can be shown that24
two measurements of the collected power at two separate
(cid:9)
wavelengths (cid:6)1 and (cid:6)2. It can be shown that the ratio of the NEDT=(cid:1)(cid:10)C(cid:11)BLIP Nw(cid:2)−1, (cid:1)6(cid:2)
detected signals is equal to
(cid:5) (cid:6)(cid:5) (cid:6) (cid:5) (cid:6) (cid:7) (cid:5) (cid:6)(cid:8) where (cid:10)is the optics transmission spectrum and C is the
R= (cid:5)1 (cid:6)1 5 (cid:8)2(cid:9)(cid:6)2 exp hc 1 − 1 . (cid:1)2(cid:2) thermalcontrast.Nwisthenumberofphotogeneratedcarriers
(cid:5) (cid:6) (cid:8)(cid:9)(cid:6) kT (cid:6) (cid:6) integrated for one integration time t and (cid:12) is the photon
2 2 1 1 2 1 int B
flux density incident on detector area A ,
The signal is now of the type d
(cid:5) (cid:6) N =(cid:11)A t (cid:12) . (cid:1)7(cid:2)
w d int B
C
R=C1exp T2 , (cid:1)3(cid:2) Percentage of background-limited performance (cid:1)BLIP(cid:2),
(cid:11) , is simply the ratio of photon noise to composite FPA
BLIP
where C and C are constants of the instrument.Taking the noise,
1 2 (cid:5) (cid:6)
logarithm of R we have N2 1/2
(cid:11) = photon . (cid:1)8(cid:2)
lnR=lnC + C2, (cid:1)4(cid:2) BLIP Np2hoton+NF2PA
1 T
From the above formulae, an important result is that the
and finally solving for T charge handling capacity of the readout, the integration time
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091101-7 Rogalski,Antoszewski,andFaraone J.Appl.Phys.105,091101(cid:2)2009(cid:1)
linked to the frame time, and dark current of the sensitive
material become the major issues limiting the performance
of IR FPAs. The NEDT is inversely proportional to the
squarerootoftheintegratedcharge,andthereforethegreater
the charge, the higher the performance.
The distinction between integration time and the FPA
frame time must be noted. At high backgrounds it is often
impossible to handle the large amount of carriers generated
within a frame time compatible with standard video rates.
Off-FPA frame integration can be used to attain a level of
sensor sensitivity that is commensurate with the detector-
limited D(cid:1) and not the charge-handling-limited D(cid:1). Even
thoughthedetectivityofLWIRdetectorsisbackgroundlim-
ited, the ROIC can collect only about 1% of the charge FIG. 6. Trends for design rule minimum and maximum bias voltage of
within the unit cell for a flux of 1016–1017 photons/cm2s. siliconfoundryrequirements(cid:1)afterRef.1(cid:2).
Unit cell capacitors fill up in about 100 (cid:2)s, while the frame
time is on the order of 10 ms. B. Pixel and chip sizes
The well charge capacity is the maximum amount of
Pixel and chip sizes are important issues in association
charge that can be stored on the storage capacitor of each
with multicolor imager formats. Small pixels reduce cost by
unitcell.Thesizeoftheunitcellislimitedtothedimensions
increasing the number of readout and detector dice poten-
of the detector element in the array. Usually, it is assumed
tially available from processed wafers. Small pixels also al-
thattheintegrationtimeissuchthatthereadoutnodecapac-
low smaller, light-weight optics to be used.
ity is maintained at half full. For a 30(cid:1)30 (cid:2)m2 pixel size,
The fundamental limit to pixel size is determined by
thestoragecapacitiesarelimitedto1–5(cid:1)107 electrons.For
diffraction.The size of the diffraction-limited optical spot or
LWIR HgCdTe FPAs the integration time is usually below
Airy disk is given by
100 (cid:2)s. Since the noise power bandwidth (cid:9)f=1/2t , a
int
shortintegrationtimeresultsinextranoiseintheintegration d=2.44(cid:6)f, (cid:1)9(cid:2)
process.
Current readout technology is based on CMOS circuitry whered isthediameterofthespot,(cid:6) isthewavelength,and
that has benefited from dramatic and continuing progress in f is the f-number of the focusing lens. For typical f/2.0
miniaturizingcircuitdimensions.Second-generationimagers opticsat5 (cid:2)mwavelength,thespotsizeis25 (cid:2)m.Because
provideNEDTofabout20–30mKwith f/2optics.Agoalof system users prefer some degree of oversampling, the pixel
third-generation imagers is to achieve sensitivity improve- size may be reduced for MWIR applications to dimensions
mentcorrespondingtoNEDTofabout1mK.FromEq.(cid:1)6(cid:2)it on the order of 12 (cid:2)m. Given the track record of human
canbedeterminedthatina300KsceneintheLWIRregion nature, Norton13 predicted that MWIR pixel size will even-
with thermal contrast of 0.02, the required charge-storage tually be reduced to about 10 (cid:2)m at some point, just to
capacity is above 109 electrons. This high charge-storage achieve the smaller pixel record. Short-wavelength IR
densitycannotbeobtainedwithinthesmallpixeldimensions (cid:1)SWIR(cid:2) pixel size will shrink to correspondingly smaller di-
using standard CMOS capacitors.1Although the reduced ox- mensions, for applications seeking maximum spatial reso-
ide thickness of submicrometer CMOS design rules gives lution. LWIR pixels are not likely to shrink much below
large capacitance per unit area, the reduced bias voltage, as 20 (cid:2)m. However, it is anticipated that LWIR pixels will be
illustrated in Fig. 6, largely cancels any improvement in madeassmallasMWIRpixels,sincethiswillallowasingle
charge-storage density. Ferroelectric capacitors may provide readout design then to be used with both MWIR and LWIR
much greater charge-storage densities than the oxide-on- FPAs.
silicon capacitors now used. However, such a technology is Readout circuit wafers are processed in standard com-
not yet incorporated into standard CMOS foundries. mercial foundries and can be constrained in size by the die-
To provide an opportunity to significantly increase both, size limits of the photolithography step and repeat printers.1
the charge-storage capacity and the dynamic range, the ver- This limit is currently on the order of 22(cid:1)22 mm2 for sub-
tically integrated sensor array (cid:1)VISA(cid:2) program has been micron lithography. Thus, the array itself can only occupy
sponsored by DARPA.25–27 The approach being developed 18(cid:1)18 mm2 assuming one needs about 2 mm on each side
buildsonthetraditional“hybrid”structureofadetectorwith for the peripheral circuitry such as bias supplies, shift regis-
a2Darrayofindium-bumpinterconnectstothesiliconread- ters,columnamplifiers,andoutputdrivers.Underthesecon-
out. VISA allows additional layers of silicon processing ditions, a 1024(cid:1)1024 array would need to have pixels no
chips to be connected below the readout to provide more larger than 18 (cid:2)m.
complex functionality. It will allow the use of smaller and To build larger sensor arrays, a new photolithographic
multicolor detectors without compromising storage capacity. technique called stitching can be used to fabricate detector
Signal-to-noiseratioswillincreaseformulticolorFPAs.This arrays larger than the reticle field of photolithographic step-
willpermitLWIRFPAstoimprovethesensitivitybyafactor pers.Thelargearrayisdividedintosmallersubblocks.Later,
of 10. the complete sensor chips are stitched together from the
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091101-8 Rogalski,Antoszewski,andFaraone J.Appl.Phys.105,091101(cid:2)2009(cid:1)
FIG. 8. (cid:1)Color online(cid:2) NEDT as a function of detectivity. The effects of
FIG.7. (cid:1)Coloronline(cid:2)Thephotocompositionofadetectorarraydieusing
nonuniformityareincludedforu=0.01%,0.1%,0.2%,and0.5%.Notethat
arraystitchingbasedonphotolithographicstepper. forD(cid:1)(cid:3)1010 cmHz1/2/W,detectivityisnottherelevantfigureofmerit.
building blocks in the reticle as shown in Fig. 7. Each block
able pixels in an array. For a system operating in the LWIR
canbephotocomposedonthewaferbymultipleexposuresat
band, the scene contrast is about 2%/K of change in scene
appropriate locations. Single blocks of the detector array are
temperature. Thus, to obtain a pixel to pixel variation in ap-
exposed at one time, as the optical system allows shuttering,
parent temperature to less than, e.g., 20 mK, the nonunifor-
or selectively exposing only a desired section of the reticle.
mity in response must be less than 0.04%. This is almost
III-V compound semiconductors are available in large
impossible to obtain in the uncorrected response of the FPA,
diameter wafers, up to 8 –in. in the case of GaAs wafers.
so a two-point correction is typically used.
Thus, focal plane technologies such as InSb, QWIP, and
FPA uniformity influences the IR system complexity.
type-II SLS are potential candidates for development of
The uniformity is important for accurate temperature mea-
large-format arrays such as 4096(cid:1)4096 and larger.
surements, background subtraction, and threshold sensing.
Gunapala et al.28 have discussed the possibility of extending
Nonuniformities require elaborate compensation algorithms
the array size up to 16 megapixels.
tocorrecttheimage,andbyconsuminganumberofanalog-
It should be noted that stitching creates a seamless de-
to-digital bits they also reduce the system dynamic range.
tector array, as opposed to an assembly of closely butted
Tactical IR FPAs usually require operation in the LWIR
subarrays. The butting technique is commonly used in the
window, with a smaller number of applications in the
fabrication of very large-format HgCdTe hybrid sensor ar-
3–5 (cid:2)m window. Ranges from the sensor to the target are
rays due to the limited size of substrate wafers (cid:1)usually
typically short, allowing for use of imaging sensors with
CdZnTe(cid:2). For example, Teledyne Scientific & Imaging has
large FPAs where precise radiometry is not critical. Imaging
developed the world’s largest HgCdTe SWIR FPA for as-
arrays can usually tolerate some percentage of dead or de-
tronomyandlow-backgroundapplications.Theformatofthe
graded pixels without jeopardizing mission performance.
device is a hybrid 2048(cid:1)2048 with a unit cell size of 18
Tactical backgrounds in the IR windows are relatively high,
(cid:1)18 (cid:2)m2andwithactivesizeof37mm.Setsoffourarrays
with about 1016 photons/cm2s reaching the detector.
are “tiled” into a 2(cid:1)2 mosaic configuration giving 4096
Table I shows the uncertainty in cutoff wavelength of
(cid:1)4096 pixels.29
Hg Cd Te for x variations of 0.1%. For short-wavelength
Recently, the first large-format MWIR FPAs with pixel 1−x x
IR (cid:1)(cid:10)3 (cid:2)m(cid:2) and MWIR (cid:1)(cid:10)5 (cid:2)m(cid:2) materials, the variation
dimension of 15 (cid:2)m have been demonstrated.30,31 It will be
in cutoff wavelength is not large. However, the nonunifor-
anextremechallengetodeployatwo-orthree-colordetector
mity is a serious problem in the case of LWIR HgCdTe de-
structure into a small pixel such as 18(cid:1)18 (cid:2)m2. Current
tectors. The variation in x across the Hg Cd Te wafer
two-colorsimultaneousmodepixelswithtwoindiumbumps 1−x x
causes much larger spectral nonuniformity; e.g., at 77 K, a
perpixelhavenotbeenbuiltwithpixelssmallerthan25 (cid:2)m
variation of (cid:9)x=0.2% gives a (cid:9)(cid:6) =0.064 (cid:2)m at (cid:6)
on a side. c c
TABLEI. Cutoffwavelengthforxvariationsof0.1%andthecorresponding
C. Uniformity cutoffwavelengthshiftforHg CdTeat77K.
1−x x
It is well known that, when the detectivity is approach-
Cutoffwavelength(cid:6) Uncertainty(cid:9)(cid:6)
ingavalueabove1010 cmHz1/2/W,theFPAperformanceis Compositionx (cid:1)(cid:2)m(cid:2) c (cid:1)(cid:2)m(cid:2) c
uniformity limited prior to correction and thus essentially
independent of the detectivity (cid:1)see Fig. 8(cid:2).An improvement 0.395 3 0.012
0.295 5 0.032
in nonuniformity from 0.1% to 0.01% after correction could
0.210 10 0.131
lower the NEDT from 63 to 6.3 mK.
0.196 14 0.257
The nonuniformity value is usually calculated using the
0.187 20 0.527
standard deviation over mean, counting the number of oper-
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091101-9 Rogalski,Antoszewski,andFaraone J.Appl.Phys.105,091101(cid:2)2009(cid:1)
FIG. 9. (cid:1)Color online(cid:2) Composition and wavelength diagram of Sb-based
III-Vmaterialsystems.
FIG. 10. Target and background contrast reversal in the MWIR spectral
range(cid:1)afterRef.32(cid:2).
=5 (cid:2)m, but (cid:9)(cid:6) =0.51 (cid:2)m at 14 (cid:2)m, which cannot be
c
fully corrected by either two or three point corrections.2
detection mode, the optical system provides a wide field of
Therefore material composition control is much more strin-
view (cid:1)f/2.5(cid:2) since third-generation systems will operate as
gent for LWIR than for MWIR. For applications that require
anon-the-movewideareastepscannerwithautomatedtarget
operation in the LWIR band as well as two-color LWIR/
recognition (cid:1)second-generation systems rely on manual tar-
VLWIR bands, most probably HgCdTe will not be the opti-
getsearching(cid:2).33MWIRoffershigherspatialresolutionsens-
mal solution.
ing and has an advantage for long-range identification when
Alternative candidate for third-generation IR detectors
used with telephoto optics narrow field-of-view (cid:1)NFOV(cid:2)
are the Sb-based III-V material system. These materials are
(cid:1)f/6(cid:2).
mechanically robust and have fairly weak dependence of
band gap on composition (cid:1)see Fig. 9(cid:2).
VI. MATERIALCONSIDERATIONS FOR THIRD-
GENERATION IR DETECTORS
D. Identification and detection ranges
In the wavelength regions of interest such as SWIR,
Thermal imaging systems are used first to detect an ob-
MWIR,andLWIR,fourdetectortechnologiesthataredevel-
ject, and then to identify it. Typically, identification ranges
oping multicolor capability are visited here: HgCdTe,
are between two and three times shorter than detection
QWIPs, antimonide based type-II SLs, and QDIPs.
ranges.32Toincreaseranges,betterresolutionandsensitivity Both HgCdTe photodiodes31,34–38 and QWIPs2,28,32,39–44
of the IR systems (cid:1)and hence the detectors(cid:2) are required.
offer multicolor capability in the SWIR, MWIR, and LWIR
Third-generation cooled imagers are being developed to ex-
ranges. The performance figures of merit of state-of-the-art
tend the range of target detection and identification and to
QWIPand HgCdTe FPAs are similar because the main limi-
ensure that defense forces maintain a technological advan-
tations are related to the readout circuits. A more detailed
tage in night operations over any opposing force.
comparison of both technologies has been given by Tidrow
Identification ranges can be further increased by using at el.2 and Rogalski.40,45
multispectral detection to correlate the images at different Recently, type-II InAs/GaInSb SLs42,46–49 and
wavelengths.Forexample,itappearsthatintheMWIRspec- QDIPs50–55 have emerged as possible candidates for third-
tral range, the IR image is washed out to the point that the
targetandthebackgroundcannotbedistinguishedfromeach
other (cid:1)see Fig. 10(cid:2). Detectors that cover the entire spectral
rangewillsufferfromwashoutbecausethebackgroundcon-
trast changes from positive to negative. Alternatively, using
twobanddetectors(cid:1)upto3.8 (cid:2)mandfrom3.8upto5 (cid:2)m(cid:2)
and summing the inverse of the second band and the output
of the first band will yield a contrast enhancement that is
impossible to achieve if an integrated response of the entire
spectral range is used.
Figure 11 compares the relative detection and identifica-
tion ranges modeled for third-generation imagers using
NVESD’s(cid:1)FortBelvoir,VA22060(cid:2)NVTHERMprogram.Asa
range criterion, the standard 70% probability of detection or
identificationisassumed.Notethattheidentificationrangein FIG.11. Comparisonofthedetectionandidentificationrangebetweencur-
rent second-generation TDI scanned LWIR imagers and the LWIR and
the MWIR range is almost 70% of the LWIR detection MWIR bands of third-generation imager in a 1280(cid:1)720 format with
range. For detection, LWIR provides superior range. In the 20 (cid:2)mpixels(cid:1)afterRef.33(cid:2).
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091101-10 Rogalski,Antoszewski,andFaraone J.Appl.Phys.105,091101(cid:2)2009(cid:1)
TABLEII. SummaryofthematerialpropertiesfortheHg CdTeternaryalloy,listedforthebinarycompo-
1−x x
nentsHgTeandCdTe,andforseveraltechnologicallyimportantalloycompositions(cid:1)afterRef.56(cid:2).(cid:10) and(cid:10)
R A1
calculatedforn-typeHgCdTewithN =1(cid:1)1015 cm−3.Thelastfourmaterialpropertiesareindependentofor
d
relativelyinsensitivetoalloycomposition
Hg CdTe
1−x x
Property HgTe CdTe
x 0 0.194 0.205 0.225 0.31 0.44 0.62 1.0
a(cid:1)Å(cid:2) 6.461 6.464 6.464 6.464 6.465 6.468 6.472 6.481
77K 77K 77K 77K 140K 200K 250K 300K
E (cid:1)eV(cid:2) (cid:13)0.261 0.073 0.091 0.123 0.272 0.474 0.749 1.490
g
(cid:6)c(cid:1)(cid:2)m(cid:2) ¯ 16.9 13.6 10.1 4.6 2.6 1.7 0.8
ni(cid:1)cm−3(cid:2) ¯ 1.9(cid:1)1014 5.8(cid:1)1013 6.3(cid:1)1012 3.7(cid:1)1012 7.1(cid:1)1011 3.1(cid:1)1010 4.1(cid:1)105
mc/mo ¯ 0.006 0.007 0.010 0.021 0.035 0.053 0.102
gc ¯ (cid:13)150 (cid:13)118 (cid:13)84 (cid:13)33 (cid:13)15 (cid:13)7 (cid:13)1.2
(cid:5) /(cid:5) 20.0 18.2 18.1 17.9 17.1 15.9 14.2 10.6
s o
(cid:5)(cid:14)/(cid:5)o 14.4 12.8 12.7 12.5 11.9 10.8 9.3 6.2
n 3.79 3.58 3.57 3.54 3.44 3.29 3.06 2.50
r
(cid:2)e(cid:1)cm2/Vs(cid:2) ¯ 4.5(cid:1)105 3.0(cid:1)105 1.0(cid:1)105 ¯ ¯ ¯ ¯
(cid:2)hh(cid:1)cm2/Vs(cid:2) ¯ 450 450 450 ¯ ¯ ¯ ¯
b=(cid:2)e/(cid:2)(cid:11) ¯ 1000 667 222 ¯ ¯ ¯ ¯
(cid:10)R(cid:1)(cid:2)s(cid:2) ¯ 16.5 13.9 10.4 11.3 11.2 10.6 2
(cid:10)A1(cid:1)(cid:2)s(cid:2) ¯ 0.45 0.85 1.8 39.6 453 4.75(cid:1)103
(cid:10)typical(cid:1)(cid:2)s(cid:2) ¯ 0.4 0.8 1 7 ¯ ¯ ¯
E (cid:1)eV(cid:2) 19
p
(cid:9)(cid:1)eV(cid:2) 0.93
m /m 0.40–0.53
hh o
(cid:9)E (cid:1)eV(cid:2) 0.35–0.55
v
generation IR detectors. Whether the low dimensional solid • large optical coefficients that enable high quantum ef-
IR photodetectors can outperform the “bulk” narrow gap ficiency, and
HgCdTedetectorsisoneofthemostimportantquestionsthat • favorable inherent recombination mechanisms that
needs to be addressed for the future of IR photodetectors. lead to long carrier lifetime and high operating
The subsections below describe issues associated with temperature.
the development and exploitation of materials used in the
These properties are a direct consequence of the energy
fabrication of multicolor IR detectors.
band structure of this zinc-blende semiconductor. Moreover,
additional specific advantages of HgCdTe are the ability to
obtain both low and high carrier concentrations, high mobil-
A. HgCdTe
ity of electrons, and low dielectric constant. The extremely
The HgCdTe ternary alloy is a close to ideal IR detector
small change in lattice constant with composition makes it
material system. Its unique position is dependent on three
possible to grow high quality layered and graded gap struc-
key features:
tures.Asaresult,HgCdTecanbeusedfordetectorsoperated
• composition-dependent tailorable energy band gap in various modes, photoconductor, photodiode, or metal-
over the entire 1–30 (cid:2)m range, insulator-semiconductor detector.
TABLEIII. Somephysicalpropertiesofnarrowgapsemiconductors.
E n (cid:2) (cid:2)
g i e h
(cid:1)eV(cid:2) (cid:1)cm−3(cid:2) (cid:1)104 cm2/Vs(cid:2) (cid:1)104 cm2/Vs(cid:2)
Material 77K 300K 77K 300K (cid:5) 77K 300K 77K 300K
InAs 0.414 0.359 6.5(cid:1)103 9.3(cid:1)1014 14.5 8 3 0.07 0.02
InSb 0.228 0.18 2.6(cid:1)109 1.9(cid:1)1016 17.9 100 8 1 0.08
In Ga As 0.66 0.75 5.4(cid:1)1011 14.6 7 1.38 0.05
0.53 0.47
PbS 0.31 0.42 3(cid:1)107 1.0(cid:1)1015 172 1.5 0.05 1.5 0.06
PbSe 0.17 0.28 6(cid:1)1011 2.0(cid:1)1016 227 3 0.10 3 0.10
PbTe 0.22 0.31 1.5(cid:1)1010 1.5(cid:1)1016 428 3 0.17 2 0.08
Pb SnTe 0.1 0.1 3.0(cid:1)1013 2.0(cid:1)1016 400 3 0.12 2 0.08
1−x x
Hg CdTe 0.1 0.1 3.2(cid:1)1013 2.3(cid:1)1016 18.0 20 1 0.044 0.01
1−x x
Hg CdTe 0.25 0.25 7.2(cid:1)108 2.3(cid:1)1015 16.7 8 0.6 0.044 0.01
1−x x
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