Optical detectors for focal plane arrays - Antoni Rogalski

26 Pages·2004·4.63 MB·English

Contributed paper
OPTO-ELECTRONICS REVIEW 12(2), 221–245 (2004)
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A. ROGALSKI*
Institute of Applied Physics, Military University of Technology
2 Kaliskiego Str., 00-908 Warsaw, Poland
Thepaperpresentsprogressinopticaldetectortechnologiesduringthepast25years.Classificationoftwotypesofdetectors
(photondetectorsandthermaldetectors)isdoneonthebasisoftheirprincipleofoperation.Theoverviewofopticalmaterial
systemsanddetectorsispresented.Alsorecentprogressindifferenttechnologiesisdescribed.Discussionisfocusedmainly
oncurrentandthemostrapidlydevelopingfocalplanearraysusing:CdZnTedetectors,AlGaNphotodiodes,visibleCCD
andCMOSimagingsystems,HgCdTeheterostructurephotodiodes,quantumwellAlGaAs/GaAsphotoresistors,andthermal
detectors.Theoutlookfornear-futuretrendsinIRtechnologiesisalsopresented.
Keywords: photondetectors,thermaldetectors,opticaldetectors,focalplanearrays,milti-colourdetectors.
(cid:17)(cid:18) (cid:19)(cid:15)(cid:3)(cid:12)(cid:11)(cid:9)(cid:20)(cid:5)(cid:3)(cid:4)(cid:11)(cid:15)
Hubble space telescope delivered a deep-space picture, a
resultof10day’sintegration,featuringgalaxiesofthe30th
Looking back over the past several hundreds of years we magnitude – an unimaginable figure even for astronomers
noticed that following the invention and evolution of opti- of our generation. Probably, the next effort will be in the
cal systems (telescopes, microscopes, eyeglasses, cameras, big-band age. Thus, photodetectors continue to open to
etc.), the optical image was still formed on the human ret- mankindthemostamazingnewhorizons.
ina, photographic plate, or films. The birth of photode- This paper is a guide over the arrays of detectors sens-
tectors can be dated back to 1873 when Smith discovered ing optical radiation. Optical radiation is considered as a
photoconductivity in selenium. Progress was slow until radiation over the range from vacuum ultraviolet to the
1905, when Einstein explained the newly observed photo- far-infrared or submilimeter wavelength (25 nm to
electric effect in metals, and Planck solved the blackbody 1000µm):
emissionpuzzlebyintroducingthequantahypothesis.Ap-
plications and new devices soon flourished, pushed by the 25–200nm Vacuumultraviolet VUV
dawning technology of vacuum tube sensors developed in 200–400nm Ultraviolet UV
the 1920s and 1930s culminating in the advent of televi- 400–700nm Visible VIS
sion. Zworykin and Morton, the celebrated fathers of 700–1000nm Nearinfrared NIR
videonics, on the last page of their legendary book Televi- 1–3µm Shortwavelengthinfrared SWIR
sion (1939) concluded that: “when rockets will fly to the 3–5µm Mediumwavelengthinfrared MWIR
moonandtoothercelestialbodies,thefirstimageswewill
5–14µm Longwavelengthinfrared LWIR
seeofthemwillbethosetakenbycameratubes,whichwill
14–30µm Verylongwavelengthinfrared VLWIR
open to mankind new horizons”. Their foresight became a
30–100µm Farinfrared FIR
reality with the Apollo and Explorer missions. Photolith-
100–1000µm Submillimeter SubMM
ographyenabledthefabricationofsiliconmonolithicimag-
ing focal planes for the visible spectrum beginning in the (cid:21)(cid:18) (cid:22)(cid:7)(cid:6)(cid:13)(cid:13)(cid:4)(cid:14)(cid:4)(cid:5)(cid:6)(cid:3)(cid:4)(cid:11)(cid:15)(cid:8)(cid:11)(cid:14)(cid:8)(cid:9)(cid:10)(cid:3)(cid:10)(cid:5)(cid:3)(cid:11)(cid:12)(cid:13)
early 1960s. Some of these early developments were in-
tended for a picturephone, other efforts were for television Progress in optical detector technology is connected mainly
cameras, satellite surveillance, and digital imaging. Infra- with semiconductor IR detectors which are included in the
red imaging has been vigorously pursed in parallel with classofphotondetectors.Inthisclassofdetectors,theradia-
visible imaging because of its utility in military applica- tion is absorbed within the material by interaction with elec-
tions. More recently (1997), the CCD camera aboard the trons. The observed electrical output signal results from the
changed electronic energy distribution. The photon detectors
show a selective wavelength dependence of the response per
*e-mail:[email protected]
unit incident radiation power. They exhibit both perfect sig-
(cid:1)This paper was presented during the XVII School of
nal-to-noise performance and a very fast response. However,
Optoelectronics: Photovoltaies-Solar Cells and Detectors,
KazimierzDolny,October13–17,2003. toachievethisininfrared(IR)spectralregion,thephotonde-
Opto-Electron.Rev.,12,no.2,2004 A.Rogalski 221
Opticaldetectorsforfocalplanearrays
tectors require cryogenic cooling. Cooling requirements are resolution below 0.1 K is reached because effective noise
the main obstacle to the more widespread use of IR systems bandwidthslessthan100Hzcanbeachieved.
based on semiconductor photodetectors making them bulky,
heavy, expensive and inconvenient to use. Depending on the (cid:23)(cid:18) (cid:24)(cid:10)(cid:12)(cid:14)(cid:11)(cid:12)(cid:25)(cid:6)(cid:15)(cid:5)(cid:10)(cid:8)(cid:13)(cid:2)(cid:10)(cid:5)(cid:4)(cid:14)(cid:4)(cid:5)(cid:6)(cid:3)(cid:4)(cid:11)(cid:15)(cid:13)
nature of interaction, the class of photon detectors is further
sub-divided into different types (see Table 1). The most im- Toprovideeasycomparisonbetweendetectors,certainfig-
portant are: intrinsic detectors, extrinsic detectors, uresofmerit,computedfromthemeasureddata,havebeen
photoemissive(metalsilicideSchottkybarriers)detectors,and defined.
quantumwelldetectors. Thevoltage(oranalogouscurrent)responsivityisgivenby
Thesecondclassofdetectorsiscomposedofthermalde-
Q
tectors. In a thermal detector, the incident radiation is ab- R (cid:2) u, (1)
P
sorbedtochangetemperatureofthematerial,andtheresul-
tant change in some physical properties is used to generate where Q is the output quantity supplied by the detector
u
an electrical output. The detector element is suspended on (e.g., the current I , the voltage V , or any other physical
u u
lags which are connected to the heat sink. Thermal effects quantity)andPistheincidentradiantpower.
are generally wavelength independent; the signal depends At equal responsivity, the detector with the smallest
upon the radiant power (or its rate of change) but not upon output noise Q on the useful signal is the most sensitive.
u
itsspectralcontent.Inpyroelectricdetectorsachangeinthe Therefore,thefirstfigureofmeritforadetectoristheNEP
internal spontaneous polarization is measured, whereas in – noise equivalent power defined as the ratio of output
thecaseofbolometersachangeintheelectricalresistanceis noisetoresponsivity
measured. The thermal detectors typically operate at room
g
temperature. They are usually characterized by modest sen- NEP (cid:2) n. (2)
R
sitivity and slow response but they are cheap and easy to
use.Thegreatestutilityininfraredtechnologyhasfoundbo- So, the NEP represents the input power that gives a unity
lometers,pyroelectricdetectorsandthermopiles. signal to noise ratio, S/N = 1 at the output; that is, a mar-
Up till the nineties last century, thermal detectors have ginalconditionofdetection.
been considerably less exploited in commercial and mili- Thebetterthedetectorperformanceis,sincethesmaller
taryinfraredsystemsincomparisonwithphotondetectors. theNEPis.Thereforeitismoreconvenienttodefineitsin-
The reason for this disparity is that thermal detectors were verse as a merit figure. In addition, it should be taken into
popularly believed to be rather slow and insensitive in consideration that whatever the noise source is, it can be
comparison with photon detectors. As a result, the world- expectedthatthenoisequadratictotalvalueisproportional
wide effort to develop thermal detectors was extremely toobservationbandwidth(cid:1)fandthedetectorareaA.Thus,
smallrelativetothatofphotondetector.Inthelastdecade, is even better to take, as the intrinsic noise parameter of a
however, it has been shown that extremely good imagery detector,theratioNEP/(A(cid:1)f)1/2normalizedtounitareaand
can be obtained from large thermal detector arrays operat- bandwidth.Inordertosimplifythecomparisonofdifferent
ing uncooled at TV frame rates. The speed of thermal de- detectorsandtohaveaparameterthatincreasesastheper-
tectors is quite adequate for non-scanned imagers with formanceimproves,thedetectivityD*(calledD-star)isde-
two-dimensional (2D) detectors. The moderate sensitivity finedas
ofthermaldetectorscanbecompensatedbyalargenumber
ofelementsin2Delectronicallyscannedarrays.Withlarge (A(cid:1)f)12
D*(cid:2) . (3)
arrays of thermal detectors the best values of temperature NEP
Table1.Photondetectors.
Type Transition Electricaloutput Example
Intrinsic Interband Photoconductive AlGaN,Si,GaAs,PbSe,InSb,HgCdTe
Photovoltaic AlGaN,Si,InGaAs,InSb,HgCdTe
Capacitance Si,GaAs,InSb,HgCdTe
PEM InSb,HgCdTe
Extrinsic Impuritytoband Photoconductive Si:In,Si:Ga,Ge:Cu,Ge:Hg
Freecarriers Intraband Photoemissive PtSi,Pt Si,IrSiSchottkybarriers
2
GaAs/CsO
Photoconductive InSbelectronbolometer
Photon-drag Ge
Quantumwells Toand/orfromspatially Photoconductive GaAs/GaAlAs,InSbnipi
quantisedlevels Photovoltaic InAs/InGaSbSLS
222 Opto-Electron.Rev.,12,no.2,2004 ©2004COSiWSEP,Warsaw
Contributed paper
This is the fundamental figure of merit used for detectors. trated.ThereadercanconverttotheD*valuesappropriate
Itcanbetransformedtothefollowingequation to the photoresistors and photovoltaic detectors by multi-
plyingthedetectivityvalueillustratedbythesquarerootof
(A(cid:1)f)12 S the detector area. The signal fluctuation limit shown in the
D*(cid:2) . (4)
figureisindependentofarea[seeEq.(5)].
P N
Table2.Thermaldetectors.
The ultimate performance of detectors is reached when
the detector and amplifier noise is low compared to the Detector Methodofoperation
photonnoise.Thephotonnoiseisfundamentalinthesense
Bolometer Changeinelectrical
thatitarisesnotfromimperfectioninthedetectororitsas-
Metal conductivity
sociated electronics but rather from the detection process
Semiconductor
itself,asaresultofthediscretenatureoftheradiationfield. Superconductor
The radiation falling on the detector is a composite of that Ferroelectric
fromthetargetandthatfromthebackground. Hotelectron
When photodetectors are operated in conditions where
Thermocouple/Thermopile Voltagegeneration,causedby
the background flux is less than the optical (signal) flux, changeintemperatureofthe
the ultimate performance of detectors is determined by the junctionoftwodissimilar
materials
signal fluctuation limit (SFL). It is achieved in practice
with photomultipliers operating in the visible and ultravio- Pyroelectric Changesinspontaneous
letregion,butitisrarelyachievedwithsolid-statedevices, electricalpolarization
which are normally detector-noise or electronic noise lim-
Golaycell/Gasmicrophone Thermalexpansionofagas
ited.TheNEPofdetectorsoperatinginthislimithavebeen
derivedbyanumberofauthors(seee.g.Kruseetal.[1,2]).
Table3.AreasofdetectorsillustratedinFig.1.
The NEP in the SFL is given (when Poisson statistics
areapplicable)by Detector Area(cm2)
CdSphotoconductor(PC) 1.00
2hc(cid:1)f
NEP (cid:2) , (5) CdSephotoconductor(PC) 1.00
(cid:2)(cid:3)
SiSchottkybarrierphotodiode 0.03
wherehisthePlanck’sconstant,cisthelightvelocity,(cid:2)is Sip-njunctionphotodiode 0.25
thequantumefficiency,and (cid:3)isthewavelength.
Siphotoconductor 0.25
Thepracticaloperatinglimitformostinfrareddetectors
is not the SFL but the background fluctuation limit, also Siavalanchephotodiode 0.07
known as the background limited infrared photodetector Gephotoconductor(PC) 0.20
(BLIP) limit. In this approximation the NEP is given by
Geacbiasphotoconductor(PC) 2.4×10–5
[1,2]
Photomultipliers(PM) 1.00
12
(cid:3)2A(cid:4) (cid:1)f(cid:6)
NEP (cid:2)hv(cid:5) b (cid:8) , (6) Forinfraredfocalplanearrays(FPAs),therelevantfig-
(cid:4) (cid:2) (cid:7)
ure of merit is the noise equivalent temperature difference
(NEDT). Noise equivalent difference temperature of a de-
where (cid:4) is the total background photon flux density
b tectorrepresentsthetemperaturechange,forincidentradia-
reaching the detector and (cid:1)f is the electrical bandwidth of
tion, that gives an output signal equal to the rms noise
the receiver. The background photon flux density received
level.NEDTisdefined
by the detector depends on its angular view of the back-
V ((cid:5)T (cid:5)Q) (cid:1)T
ground and on its ability to respond to the wavelengths NEDT (cid:2) n (cid:2)V , (7)
containedinthissource. ((cid:5)V (cid:5)Q) n (cid:1)V
s s
Typical D* values for available optical detectors are
shown in Figs. 1 and 2. Figure 1 shows the spectral where V is the rms noise and (cid:1)V is the signal measured
n s
detectivity of optical detectors responding in 0.1–1.2 µm for the temperature difference (cid:1)T. It can be approximated
region.NotethatdetectivityisnotD*,butratherreciprocal that
of NEP for a 1-Hz bandwidth. This figure of merit is em-
ployed to include photomultipliers whose noise does not NEDT (cid:2)(C(cid:2)BLIP Nw)(cid:9)1, (8)
depend in all cases upon the square root of the photo-
cathode area. Table 3 lists the areas which Seib and where C is the thermal contrast, N is the number of photo-
w
Aukerman[3]statearepropertothevariousdetectorsillus- generatedcarriersintegratedforoneintegrationtime,t
int
Opto-Electron.Rev.,12,no.2,2004 A.Rogalski 223
Opticaldetectorsforfocalplanearrays
N (cid:2)(cid:2)At Q . (9)
w int B
whereQ isthephotonfluxdensityincidentonthedetector
B
areaA.
PercentageofBLIP,(cid:2) ,issimplytheratioofphoton
BLIP
noisetocompositeFPAnoise
12
(cid:3) N2 (cid:6)
(cid:5) photon (cid:8)
(cid:2) (cid:2) . (10)
BLIP (cid:4)(cid:5) N2photon (cid:10)NF2PA(cid:7)(cid:8)
It results from the above formulas that the charge han-
dlingcapacityofthereadout,theintegrationtimelinkedto
the frame time, and dark current of the sensitive material
becomes the major issues of IR FPAs. The NEDT is in-
versely proportional to the square root of the integrated
charge and therefore the greater the charge, the higher the
performance.
(cid:26)(cid:18) (cid:27)(cid:11)(cid:5)(cid:6)(cid:7)(cid:8)(cid:2)(cid:7)(cid:6)(cid:15)(cid:10)(cid:8)(cid:6)(cid:12)(cid:12)(cid:6)(cid:16)(cid:8)(cid:6)(cid:12)(cid:5)(cid:28)(cid:4)(cid:3)(cid:10)(cid:5)(cid:3)(cid:20)(cid:12)(cid:10)(cid:13)
Theterm“focalplanearray”(FPA)referstoanassemblage
ofindividualdetectorpictureelements(“pixels”)locatedat
the focal plane of an imaging system. Although the defini-
tion could include one-dimensional (“linear”) arrays as
well as two-dimensional (2D) arrays, it is frequently ap-
plied to the latter. Usually, the optics part of an optoelec-
Fig. 1. Detectivity vs wavelength values of 0.1–1.2 µm photo- tronic images device is limited only to focusing of the im-
detectors.PRindicatesaphotoresistorsandPMindicatesaphoto- age onto the detectors array. These so-called “staring ar-
multiplies.DetectorareasaregiveninTable2(afterRef.3). rays”arescannedelectronicallyusuallyusingcircuitsinte-
Fig.2.ComparisonoftheD*ofvariouscommerciallyavailableinfrareddetectorswhenoperatedattheindicatedtemperature.Chopping
frequencyis1000Hzforalldetectorsexceptthethermopile(10Hz),thermocouple(10Hz),thermistorbolometer(10Hz),Golaycell(10
Hz)andpyroelectricdetector(10Hz).Eachdetectorisassumedtoviewahemisphericalsurroundatatemperatureof300K.Theoretical
curvesforthebackground-limitedD*foridealphotovoltaic(PV)detectors,photoresistors(PR)andthermaldetectorsarealsoshown.
224 Opto-Electron.Rev.,12,no.2,2004 ©2004COSiWSEP,Warsaw
Contributed paper
Fig.3.Increaseinarrayformatsizeoverthepastthirtyyears(afterRef.4).
(cid:26)(cid:18)(cid:17)(cid:18) (cid:29)(cid:11)(cid:15)(cid:11)(cid:7)(cid:4)(cid:3)(cid:28)(cid:4)(cid:5)(cid:8)(cid:6)(cid:12)(cid:12)(cid:6)(cid:16)(cid:13)
gratedwiththearrays.Thearchitectureofdetector-readout
assemblies has assumed a number of forms which are dis-
cussed below. The types of readout integrated circuits In general, the architectures of FPAs may be classified as
(ROICs) include the function of pixel deselecting, anti- monolithicandhybrid.Whenthedetectormaterialiseither
blooming on each pixel, subframe imaging, output pream- siliconorasiliconderivative(suchase.g.platinumsilicide
plifiers, and may include yet other functions. Infrared im- PtSi),thedetectorandROICcanbebuiltonasinglewafer
aging systems, which use 2D arrays, belong to so-called (seeFig.4).EffortstodevelopmonolithicFPAsusingnar-
“secondgeneration”systems. row-gapsemiconductorshavefailed.Thereareafewobvi-
Development of detector FPA technology has revolu- ous advantages to this structure, principally in the simplic-
tionized many kinds of imaging in the past twenty five ity and lower cost associated with a directly integrated
structure. Common examples of these FPAs in the visible
years[4].From(cid:11)raystotheinfraredandevenradiowaves,
andnearinfrared(0.7–1.0µm)arefoundincamcordersand
the rate at which images can be acquired has increased by
more than a factor of a million in many cases. Figure 3 il- digital cameras. Two generic types of silicon technology
provide the bulk of devices in these markets: charge cou-
lustrates the trend in array size over the past thirty years.
pled devices (CCDs) and complementary metal-oxide-
Imaging FPAs have developed in proportion to the ability
-semiconductor (CMOS) imagers. CCD technology has
of silicon integrated circuit (ICs) technology to read and
processthearraysignals,andwithabilitytodisplaythere- achieved the highest pixel counts or largest formats with
the numbers approaching 108 (see Fig. 5). CMOS imagers
sulting image. FPAs have nominally the same growth rate
are also rapidly moving to large formats and are expected
as dynamic random access memory (DRAM) ICs (which
to compete with CCDs for the large format applications
have had a doubling-rate period of approximately 18
within a few years. Because the CCD imager market is
months;itisaconsequenceofMoore’slaw,whichpredicts
much smaller than that for CMOS devices in general, it
theabilitytodoubletransistorintegrationoneachICabout
maybedifficultforCCDtoremaincompetitiveinthelong
every 18 months) but lag behind in size by about 5–10
term.
years. ROICs are somewhat analogous to DRAM-only
CCD technology is very mature in respect to both the
readouts, but require a minimum of three transistors per
fabricationyieldandtheattainmentofnear-theoreticalsen-
pixel,comparedtooneforeachmemorycell.Readoutsare
sitivity. Figure 6 shows the schematic circuit for a typical
alsoanalogousintermsofanemphasisonlownoiseinputs
CCD imager. The monolithic array is based on a
and generally maximum charge storage capacity. Charge
metal-insulator-semiconductor(MIS)structure.Incidentra-
coupled devices (CCDs) with close to 100 M pixels offer
the largest formats. PtSi, InSb and HgCdTe have been fol- diationgenerateselectron-holepairsinthedepletionregion
lowing the pace of DRAM. In the infrared, 4 M pixel ar- of the MIS structure. The photogenerated carriers are first
integrated in an electronic well at the pixel and subse-
raysarenowavailableforastronomyapplications.
Opto-Electron.Rev.,12,no.2,2004 A.Rogalski 225
Opticaldetectorsforfocalplanearrays
Fig.4.MonolithicIRFPAs:(a)all-silicon;(b)heteroepitaxy-on-silicon;(c)non-silicon(e.g.,HgCdTeCCD);and(d)microbolometer.
quently transferred to slow and fast CCD shift registers. 0.18 µm are in production, with pre-production runs of
This is done step by step as the gate biases are clocked to 0.13µmdesignrulesalreadyunderway.Asaresultofsuch
movethechargewithminimalloss.Thefigureofmeritfor fine design rules, more functionality has been put into the
the effectiveness of this process is called the charge trans- unit cells of multiplexers and smaller unit cells, leading to
ferefficiency(CTE).Channelstopsbetweencolumnshelp large array sizes. Figure 5 shows the timelines for mini-
topreventchargesfromstrayinglaterally.Attheendofthe mum circuit features and the resulting CCD, IR FPA and
CCD register, a charge carrying information on the re- CMOSvisibleimagersizeswithrespecttoimagingpixels.
ceived signal can be readout and converted into a useful Along the horizontal axis is also a scale depicting the gen-
signal. eralavailabilityofvariousMOSandCMOSprocesses.The
The configuration of CCD devices requires specialized ongoingmigrationtoevenfinerlithographieswillthusen-
processing, unlike CMOS imagers which can be built on able the rapid development of CMOS-based imagers hav-
fabricationlinesdesignedforcommercialmicroprocessors. ingevenhigherresolution,betterimagequality,higherlev-
CMOShavetheadvantagethatexistingfoundries,intended els of integration and lower overall imaging system cost
for application specific integrated circuits (ASICs), can be than CCD-based solutions. At present, CMOS having with
readilyusedbyadaptingtheirdesignrules.Designrulesof minimum features of (cid:12) 0.5 µm is also enabling monolithic
226 Opto-Electron.Rev.,12,no.2,2004 ©2004COSiWSEP,Warsaw
Contributed paper
Fig.5.Imagingarrayformatscomparedwiththecomplexityofmicroprocessortechnologyasindicatedbytransistorcount.Thetimeline
designruleofMOS/CMOSfeaturesisshownatthebottom(afterRef.4).
visible CMOS imagers, because the denser photolithogra- A typical CMOS multiplexer architecture (see Fig. 7)
phy allows low-noise signal extraction and high perfor- consists of fast (column) and slow (row) shift registers at
mance detection with the optical fill factor within each the edges of the active area, and pixels are addressed one
pixel. The silicon wafer production infrastructure which by one through the selection of a slow register, while the
has put personal computers into many homes is now en- fast register scans through a column, and so on. Each
ablingCMOS-basedimaginginconsumerproductssuchas photodiode is connected in parallel to a storage capacitor
digitalstillandvideocameras. locatedintheunitcell.Acolumnofdiodesandstorageca-
Fig.6.ArchitectureoftypicalCCDimager(afterRef.5).
Opto-Electron.Rev.,12,no.2,2004 A.Rogalski 227
Opticaldetectorsforfocalplanearrays
Fig.7.CMOSmultiplexingreadoutwithCTIAdetectorinterface(afterRef.6).
pacitors is selected one at a time by a digital horizontal
scan register and a row bus is selected by the vertical scan
register. Therefore each pixel can be individually ad-
dressed.
CMOS-basedimagersuseactiveorpassivepixels[6–8]
asshown,insimplifiedform,inFig.8.Incomparisonwith
passive pixel sensors (PPSs), active pixel sensors (APSs)
apart from read functions exploit some form of amplifica-
tionateachpixel.PPSshavesimplepixelsconsistingofas
few as two components (a photodiode and a MOSFET
switch).Asaresult,circuitoverheadislowandtheoptical
collection efficiency [fill factor (FF)] is high even for
monolithicdevices.AlargeopticalFFofupto80%maxi-
mises signal selection and minimises fabrication cost by
obviating the need for microlenses. Microlenses, typically
used in CCD and CMOS APS imagers for visible applica-
tion,concentratetheincominglightintothephotosensitive
region when they are accurately deposited over each pixel
(see Fig. 9). When the FF is low and microlenses are not
used, the light falling elsewhere is either lost or, in some
cases,createsartifactsintheimagerybygeneratingelectri-
calcurrentsintheactivecircuitry.
APSsincorporatetransistorsineachpixeltoconvertthe
photogeneratedchargetoavoltage,amplifythesignalvolt-
age,andreducethenoise.Addingthesecomponents,how-
ever, reduces the FF of monolithic imagers to about
30–50% in 0.5-µm processes at a 5–6-µm pixel pitch or in
0.25-µmprocessesata3.3–4.0-µmpixelpitch[6]. Fig.8.Passive(a)andactive(b)pixelsensor(afterRef.6).
228 Opto-Electron.Rev.,12,no.2,2004 ©2004COSiWSEP,Warsaw
Contributed paper
(cid:26)(cid:18)(cid:21)(cid:18) (cid:30)(cid:16)(cid:31)(cid:12)(cid:4)(cid:9)(cid:8)(cid:6)(cid:12)(cid:12)(cid:6)(cid:16)(cid:13)
Ultraviolet and infrared imagers are most commonly built
with a hybrid structure. Visible hybrids have also been
built for specific applications. Hybrid FPAs detectors and
multiplexers are fabricated on different substrates and
matedwitheachotherbyflip-chipbondingorloopholein-
terconnection (see Fig. 10). In this case, we can optimise
the detector material and multiplexer independently. Other
advantages of the hybrid FPAs are near 100% fill factor
and increased signal-processing area on the multiplexer
chip. Indium bump bonding of readout electronics, first
demonstrated in the mid-1970s, provides for multiplexing
the signals from thousands of pixels onto a few output
lines, greatly simplifying the interface between the sensor
andthesystemelectronics.
The detector array can be illuminated from either the
frontside(withthephotonspassingthroughthetransparent
silicon multiplexer) or backside (with photons passing
through the transparent detector array substrate). In gen-
eral,thelatterapproachismostadvantageousasthemulti-
plexerwilltypicallyhaveareasofmetallizationsandother
opaqueregions,whichcanreducetheeffectiveopticalarea
of the structure. When using opaque materials, substrates
must be thinned to 10–20 µm in order to obtain sufficient
quantumefficienciesandreducethecrosstalk.
Hybrids readouts are usually built with silicon, al-
thoughafewdemonstrationsofreadoutsusingothermate-
rialshavebeenexperimentallystudied.Shiftregistersclock
Fig. 9. Micrograph and cross-sectional drawing of microlensed the signals from each row in turn, while all columns are
hybridFPAs(afterRef.6). typically read in parallel. The readout may have as few as
Fig.10.HybridFPAwithindependentlyoptimizedsignaldetectionandreadout:(a)indiumbumptechniques,(b)loopholetechnique.
Opto-Electron.Rev.,12,no.2,2004 A.Rogalski 229
Opticaldetectorsforfocalplanearrays
Fig.11.QuantumefficiencyofUV,visible,andinfrareddetectorarrays(afterRef.4).
one, and as many as 64 outputs, depending on the format HgCdTe detectors – is the high-density vertically-integra-
size and frame rate. A typical output can provide tedphotodiode,orloopholephotodiode[13].
5–20MHzdatarates[4]. Key to the development of ROICs has been the evolu-
Awidevarietyofdetectormaterialshavebeenadapted tion in input preamplifier technology. This evolution has
to the monolithic and hybrid format [9–12]. Figure 11 been driven by increased performance requirements and
shows the quantum efficiency of some of the detector ma- silicon processing technology improvements. A brief dis-
terials used to fabricate arrays of ultraviolet (UV), visible cussionofthevariouscircuitsisgiveninRef.14.
and infrared detectors. AlGaN detectors are being devel-
opedintheUVregion.Siliconp-i-ndiodesareshownwith (cid:18) (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:14)(cid:11)(cid:5)(cid:6)(cid:7)(cid:8)(cid:2)(cid:7)(cid:6)(cid:15)(cid:10)(cid:8)(cid:6)(cid:12)(cid:12)(cid:6)(cid:16)(cid:13)
and without antireflection coating. Lead salts (PbS and
PbSe) have intermediate quantum efficiencies, while PtSi
Detectorarraysareavailableinwidespectralrangeofelec-
Schottky barrier types and quantum well infrared photo-
tromagnetic spectrum. A variety of detector array formats
detectors(QWIPs)havelowvalues.InSbcanrespondfrom
areelaboratedinthevisibleandinfraredregions.Feverop-
thenearUVoutto5.5µmat80K.Asuitabledetectorma-
tions are available in the shorter or longer wavelength re-
terial for near-IR (1.0–1.7-µm) spectral range is InGaAs
gions. Below we will survey a sample of the types that
latticematchedtotheInP.VariousHgCdTealloys,inboth
have been built to address the various portions of the opti-
photovoltaic and photoconductive configurations, cover
calspectrum.
from 0.7 µm to over 20 µm. Impurity-doped (Sb, As, and
Ga) silicon impurity-blocked conduction (IBC) detectors
(cid:18)(cid:17)(cid:18) !"(cid:12)(cid:6)(cid:16)(cid:8)(cid:9)(cid:10)(cid:3)(cid:10)(cid:5)(cid:3)(cid:11)(cid:12)(cid:8)(cid:6)(cid:12)(cid:12)(cid:6)(cid:16)(cid:13)
operating at 10 K have a spectral response cut-off in the
rangeof16to30µm.Impurity-dopedGedetectorscanex-
tendtheresponseoutto100–200µm. InthecourseofthepasthundredyearstheX-raydetection
has migrated from film to digital cameras for dental and
UV, visible, and infrared arrays most commonly em-
ploy a photodiode structure. Photodiodes are preferred to medical applications. Several classes of X-ray sensor ar-
photoconductors because of their relatively high imped- rayshavebeendevelopedincluding[4]:
ance,whichmatchesdirectlyintothehighinputimpedance (cid:127) phosphors,
stage of an FET readout circuit and also allows lower (cid:127) scintilators,
power dissipation. Mesa photodiodes are used in AlGaN, (cid:127) microchannelplates,
InSb, and HgCdTe detectors, whereas planar photodiodes (cid:127) silicon detector arrays (CCDs, hybrid p-i-n structures,
areusedinSi,PtSi,Ge,HgCdTe,InGaAs,andInSbdetec- thinfilmsiliconpanels),and
tors. A third photodiode structure – used exclusively with (cid:127) CdZnTehybriddetectorarrays.
230 Opto-Electron.Rev.,12,no.2,2004 ©2004COSiWSEP,Warsaw