A Novel test structure for automated measurement of charge transfer efficiency in charge coupled imaging devices

Rochester Institute of Technology

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Theses

Thesis/Dissertation Collections

8-1-1991

A Novel test structure for automated measurement

of charge transfer efficiency in charge coupled

imaging devices

Herbert J. Erhardt

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Recommended Citation

A NOVEL TEST smucrURE FOR

AUTOMATED MEASUREMENT OF CHARGE TRANSFER EFFICIENCY

IN CHARGE COUPLED IMAGING DEVICES

by

Herbert

J.

Erhardt

A Thesis Submitted

in

Partial Fulfillment

of the

Requirements for the Degree of

MASTER OF SCIENCE

in Electrical Engineering

Approved by:

Prof.

Lynn

Fuller

(Thesis Advisor)

Prof.

Name Illegible

Prof.

Name Illegible

Prof.

_

(Department Head)

DEPARTMENT OF ELECTRICAL ENGINEERING

COLLEGE OF ENGINEERING

ROCHESTER INSTITUTE OF TECHNOLOGY

ROCHESTER, NEW YORK

August_8,1991

Thesis Title:

A NOVEL TEST STRUCTURE FOR AUTOMATEDMEASUREMENTOF

CHARGE TRANSFER EFFICIENCY IN CHARGE COUPLED IMAGING

DEVICES.

I,

HerbertJ.

Erhardt,

hereby

grantpermissionto theWallaceMemorial

Library

of

Rochester Instituteof

Technology

toreproduce_mythesisinwhole orin part,provided

Preface

The conceptofthecharge coupleddevice (CCD)was

initially

establishedand

developed in the_early1970s. Inthepastseveral_years,solid statedevice

technology

has

movedthese typesofdevicesforwardintoadvanceddesigns fora _varietyofapplications

rangingfrom

imaging

to_analoganddigitalsignal processing.

Scaling

ofthe

technology

has

allowedfor increased

density

ofdevicecellsandtheemergenceofhighresolutionarrays such

asthe 4million pixelsensor1

reportedin1990. _{Other processing}andphotolithographic

advanceshavealsopaved the_{way for linear}arrays_{exceeding 8000}elements,^

withdevice

lengthsinexcessof72mm. As

technology

enablesfurtheradvancementindevicesize and

architecture,similarimprovements inperformance,speed anddeviceyieldcontinuetoexpand

theapplications ofCCD

imaging

devices intosystems

including

camcorders,documentcopiers,

facsimilemachines, filmscanners,andevenhand-heldscannersfor

desktop

publishing. The

resulting demand forthesedevicesrequiresthe_{ability for}automated testofdevice

characteristicsina_{manufacturing}environment. Buthigher_clockingspeeds,

increasing

numbers

oftransfersand smaller signalpackets,increasethe_complexityoftesting. Oneofthemost

importantparametersthatmustbemonitoredforacharge coupleddevice isthatofcharge

transfer_efficiency(CTE). Theobject ofthis thesisistopresentamethod of_monitoringcharge

transfer_efficiencyof charge coupled imagersina_reliable,repeatable

fashion,

suitable for

Abstract

Thisworkdiscussesthedesignand operationof ateststructure usedforthe

measurementofchargetransfer_{efficiency (CTE)}ofCCDimagers. Themethod of operation

makesuse ofexposure controlgates,integral to thebasic devicearchitecture,to_{form optically}

introducedsignalchargeintoa pulsetrainfor injection_into,and subsequenttransfer_through,

theCCDshiftregisterundertest. Thismethod eliminatestheneedforelectricalinjection

circuitry makingitusefulfor highspeed readoutapplications. Asno charge_meteringgatesare

required, thestructure_{is relatively immune}tochannel potential_{variations,making it}idealfor implementation inautomated

testing

ofa_{manufacturing}setting. Theconcepthas beenverified

on a2048elementlinearCCD

imaging

_{array using}true_{two-phase, NMOS buried-channel}

technology

withno additional_processingorlithographicsteps required. Chargetransfer

efficiencyofthemono-linear_readout,4096stageshiftregisterhasbeenmeasured over arange

of_{clocking frequencies}and output signal

levels,

withmeasuredefficienciesinexcessof0.99999

pertransfer. Thestructurehasalsobeen found tobeusefulfor

determining

thediffusion

crosstalkbetweenadjacent photodetectors within the array, thus the_primarycomponents

TableofContents

ListofTables vi

ListofFigures vii

List of Symbols x

1. Introduction 1

2. Historical Review 1

2.1 TheCharge Coupled Concept 1

2.2 Charge Transfer Dynamics 6

2.3 Basic CCD Imager Architecture 10

2.4 Imager Performance Limitations 13

2.4.1 Aperture Limited Response 14

2.4.2 Diffusion Crosstalk Effects 16

2.4.3 Charge Transfer

Inefficiency

Degradation 20

2.5 CTE Measurement Techniques 23

2.5.1 Electrical Charge Input Techniques 25

3. Experimental Implementation 31

3.1 OpticalInjectionfor CTE Measurement 31

3.2 Test Structure

Theory

Utilizing

ExposureControl Gate 34

3.3 FabricationandProcess

Technology

38

3.4 Actual Implementation of Design and Layout 43

3.4.1 Detector and Shift Register Architecture 44

3.4.2 Optical Test

Array

(OTA) Architecture 57

4. Experimental Results 65

4.2 Basic Device Performance 68

4.3 Optical InjectionStructurePerformance 72

4.4 Alternative Use of OTA For Diffusion Crosstalk Measurement 82

4.4.1 Diffusion MTF from Adjacent Crosstalk

Theory

84

4.4.2 Diffusion Sigma andModified MTFCalculations 88

4.5 Complete Device MTF Calculations 94

5. Conclusions 96

6. Acknowledgment 99

7. References 100

8. Appendix A: Charge Transport Calculations for 9umCell 102

9. Appendix B: Aperture MTF Derivation 103

10. AppendixC: DiffusionMTFCalculations 107

11. Appendix D: Charge Transfer

Inefficiency

MTF Derivations 109

12. Appendix E: ConvolutionofGaussianand Box Functionfor Diffusion Profile 112

ListofTahles

Table 1 Linear Imager MaskSequencefor Fabrication 38

Table2 FET _{Options for Linear TGI Process} 43

Table 3 Output StructureFETParameters 56

Table4 Gaussian and Box Function Convolution Solution 85

Table 5 Comparison of Diffusion Spread Values vs. Sigma 86

Table 6 Diffusion Crosstalk Measurement Data 90

Table 7 Calculated Diffusion Sigma Valuesvs. Wavelength 92

Table 8 Calculated Diffusion MTF Values

Using

Measured Crosstalk

andGaussian Function 92

ListofFigures

Figure 2.1a MOS Structure and Equivalent Circuit 2

Figure 2.1b

Energy

Band Diagram for MOS Structure Under Positive Bias 3

Figure 2.2 Charge

Coupling

andRequired

Timing

foraThree-Phase CCD 4

Figure 2.3 BCCD Structure and

Resulting

Channel PotentialProfiles 5

Figure 2.4 Two-Phase CCD Channel Potentials 7

Figure 2.5 ChargeTransferComponentsfor 4.5umPhase 9

Figure 2.6 Basic CCD Imager Architecture 10

Figure 2.7 CCD Output

Circuitry

and

Timing

11

Figure 2.8

Antiblooming

and Exposure Control Implementation 12

Figure 2.9

Sampling

of Sine Wave Input Signal 14

Figure 2.10 Aperture MTF for Different Aperture/PitchRatios 15

Figure2.11 Silicon AbsorptionCoefficientvs.Wavelength @ 300 K 17

Figure 2.12 DiffusionOptions for

Long

WavelengthGenerated Carriers 18

Figure 2.13 Diffusion MTFfora9urnCellLengthatVarious Input Wavelengths 19

Figure 2.14 ComparisonofDepletion DepthandCell Size Increases 20

Figure 2.15 SignalRepresentationatStage k-l of aCCD Shift Register 21

Figure 2.16 Transfer MTF for a 2048 Stage Shift Register 23

Figure 2.17

Resulting

PulseTrainDueto ProportionalLoss 24

Figure 2.18

Resulting

PulseTrainDuetoFixed Loss 25

Figure 2.19 Dynamic Current Injection 26

Figure 2.20 Potential EquilibrationCharge Injection 27

Figure 2.21 Threshold In-sensitive InjectionCircuit 29

Figure 2.22 Potentialsand

Timing

fortheCircuitofFigure 2.21 30

Figure 3.2 Schematic ofOptical Test

Array

35

Figure 3.3

Biasing

and _{Channel Potential for Test Structure} 36

Figure 3.4 A Pulse Train

Suffering

from Edge Diffusion LossesandResult from

Compensated TestStructure withActive-Drained Adjacent Cells 37

Figure 35 Channel Potential

Doping

Profiles for Storage RegionofShift Register 39

Figure 3.6 Channel Potential

Doping

Profiles for Barrier RegionofShift Register 40

Figure 3.7 Channel Potential

Doping

Profiles for Pinned Photodiode Structure 41

Figure 3.8 Line

Spacing

Between Adjacent ChannelsofaMultilinear

Array

44

Figure 3.9 Shift Register Cell Cross-Section 45

Figure 3.10 2-Dimensional GEMINI Solution ofPotential Within Shift Register 46

Figure 3.11 Photodiode Cross-Section (halfcell)

Doping

Profilesas a ResultofSUPRA 49

Figure3.12 GEMINI

Modeling

ofPinned DiodePotential 50

Figure 3.13 ImagerCell Sketch

Showing Primary

Sections 53

Figure 3.14 Actual ImagerCellImplementation_{using KIC Layout Tools}(2500X) 54

Figure 3.15 OutputDetectionCircuitand BufferSchematic 56

Figure 3.16 Channel Potential Diagram for Simplified Structure 59

Figure 3.17 SchematicofSimplified Optical Injection Structure 60

Figure 3.18 ActualTest CellImplementation_{using KIC Layout Tools}(2500X) 61

Figure 3.19 Actual Test

Array

Implementation _{using KIC Layout Tools}(1000X) 62

Figure 3.20 CCD Imager Single Channel Schematic Representation 63

Figure 3.21 CCDImager

Timing

Diagrams:(a)Line

Timing

(b)DetectortoCCD

Timing

64

Figure 4.1 Sensor SupportElectronicsfor Device OperationandOTA Verification 66

Figure 4.2 Dual In-line Package Sketch and Pinout 67

Figure4.3 Device Output foranEntire Line 68

Figure 4.5 Reset Current vs. Output Voltage Plot for CCD Imager 70

Figure 4.6 Sensor Output Voltage Variation vs. EXP Gate Duration 71

Figure 4.7 OperationofOptical Test

Array

at 1 MHz 72

Figure 4.8 OTA Operationat(a)4

MHz,

(b)10 MHzand (c) 10 MHz Magnified 73

Figure 4.9a OTA Pattern

Non-uniformity

at5.5V LOD 75

Figure 4.9b OTA Pattern

Non-uniformity

at5.5V LOD- Expanded 75

Figure 4.10

Pinning

Potential Parametric Test Monitor 76

Figure 4.11a

Biasing

oftheTest Monitor forEXPPotential Measurement 77

Figure 4.11b Channel

Potential, Vg,

as aFunctionofAppliedGate Voltage 77

Figure 4.12 Test Monitor Potential Diagram

Including

Parasitic Barrier 79

Figure 4.13 Expanded View of Channel Potential vs. Gate Voltage 79

Figure4.14 OTA Outputat withLOD Biasat6.5 Volts 80

Figure 4.15 OnsetofOTA Operationat anLODBiasof6.9 Volts 81

Figure 4.16 Tail EndofImagerandOTAOutput in Normal

Imaging

83

Figure 4.17 Graphical Example (inPart)ofConvolutionofGaussianand Box Functions 85

Figure 4.18 ExampleDiffusionProfiles forT=_{9with (a)c}=_{2, (b)a}=_1and(c)a=_{0.5 86}

Figure 4.19 Diffusion Crosstalk Measurementat(a)450nm(b)550nmand(c)650nm 89

Figure 4.20a Calculated Diffusion Profile at 450 nm 90

Figure 4.20b Calculated Diffusion Profile at 550 nm 91

Figure 4.20c Calculated Diffusion Profile at 650 nm 91

Figure 4.21 DiffusionMTF

Using

Measured SigmaandGaussian Approx. for 9umCell 93

Figure4.22 2048ElementCCDImager MTF ComponentsandTotal MTFfor 10MHz

Clocking

and550nmIllumination 95

Figure 4.23 2048 Element CCD Imager Total MTFfor 10MHz

Clocking

and_450,

550,

and

ListofSymhnk

AC:

Alternating

_Current

As: Arsenic

A

-10

Angstrom (10 meters)

a: AbsorptionCoefficient

BAR1: Polysilicon Level 1Electrode Barrier Region

BAR2: Polysilicon Level 2Electrode Barrier Region

BCCD: Buried-Channel Charge Coupled Device

CCD: Charge Coupled Device

CMOS:

Complementary

_{Metal Oxide Semiconductor}

CTE: Charge Transfer

Efficiency

CTF: Contrast Transfer Function

Crj>Ep: Depletion Capacitance

Cefj: Effective Capacitance

Cpp:

Floating

Diffusion Capacitance

Cq: Gate Capacitance

Cqx: Oxide Capacitance

DC: Static orDirectCurrent

Dn

: Electron Thermal Diffusion Coefficient

AO: Changein Potential

AOs: ChangeinSurface Potential

AQS: Changein SignalCharge

Ax: Aperture Width

Ec

: _Conduction

Energy

Level

Ep: Fermi

Energy

Level

Ej: Intrinsic

Energy

Level

Emin: _{Minimum Electric Field}

Ey: Valence

Energy

Level

eV: _{Electron-Volt}

e: Transfer

Inefficiency

perCellorPhase

e0x: _{Silicon Dioxide}

Permittivity

= 3.45-10'11 _Farads/Meter

Si: Silicon

Permittivity

= 3.45-10'11 _Farads/Meter

FET: Field Effect Transistor

Fqy

: Charge-to-Voltage Conversion Factor

J

: Spatial

Frequency

_(cycles/mm)

fcik

: CCD

Clocking

Frequency

/

nyq Nyquist

Frequency

(cycles/mm)

Js

:

Sampling Frequency

_(cycles/mm)

HDTV: High Definition Television

ID: Electrical InjectionInput Diode

IG: ElectricalInjection InputGate

KIC:

Berkeley

Graphical Layout Editor Software

k: BoltzmannConstant=1.38 lfT23Joule/K

kT/q: ThermalVoltageatTemperature T(K) =_{0.0256 Voltsat300 K}

LDD:

Lightly

Doped Drain

LHS: Left Hand SideofEquation

LOD: Lateral Overflow/Exposure Drain

L0

= Carrier Diffusion Length

X: Wavelength

MHz: Megahertz= IO6_Hertz

MOS: Metal-Oxide-Semiconductor Structure

MTF: Modulation Transfer Function

um: Micronormicrometer(IO"6 meters)

Hn: Electron

Mobility

ND: Neutral

Density

N-Si: Donor Doped Silicon

N+:

Highly

Doped Donor Region

nm Nanometer(IO"9

meters)

v: Transfer

Efficiency

perCellorPhase (1-e)

OG: Output GateofCCD ShiftRegister

OTA: Optical Injection Test

Array

PRNU: Photoresponse

Non-uniformity

P-Si: AcceptorDoped Silicon

P+:

Highly

Doped Acceptor Region

poly: Polysilicon

P: Numberof clock phases perCCDcell *bi= Built-in Junction Potential

OR: Reset Clock

Ol: Phase 1

Clocking

PhaseofCCD ShiftRegister

02: Phase 2

Clocking

PhaseofCCDShiftRegister

QS: Signal Charge

q; ElectronCharge=1.6

IO'19

RHS: Right Hand SideofEquation

SG: _{Electrical Injection Signal Gate}

Si: Silicon

Si02: Silicon Dioxide

TG: Transfer Gate

TGI: ThroughtheGate Implant

TTL: Transistor-Transistor Logic

*ox: Silicon Dioxide Thickness

VCC: Upper-most Circuit

Supply

Voltage

VDD

= Upper-most Device

Supply

Voltage

VDS: Drain-to-Source Voltage

vG= Gate Voltage

vGS: Gate-to-Source Voltage

VPINN: Photodiode

Pinning

Potential

Vrd= Reset Drain BiasofCCDOutput Circuit

VT: Threshold Voltage

WSix: TungstenSilicide

co:

Frequency

inradians/sec

Xz: Channel Potential Minimum

Xdn: Depletion Region#n

1. Introduction

The_primaryfunctionof aCharge-Coupled Device(CCD) imageristo_accurately

convert

impinging

photonsintoelectrical charge carriersand transferthecarriers_off-chipinan

equallyprecisemannerfor downstreamsignal_processingandeventualdisplay. Thefigureof

merit forchargetransportistermed thechargetransfer_{efficiency (CTE)}ofthedevice.

Technological innovation hasspiritedthedevelopmentoftheCCD imagerover thepast

decade,

allowingfor

larger, faster,

andhigher_performingdevices.

But,

as speed andthe

number oftransfersincreasesthenumber of signalcarriersgoesdownyet,therequired_accuracy

forsystems_continually

increases,

hence CTE degradationeffectson performance canbe

enhanced.

Developing

ina similar mannerarelowercost marketswithhighervolumessuch as

facsimile,

copier,andhandheldscanner_markets,wherelargenumbers of sensorsmustbe

consistentlyproduced. _{The expanding}useinboth highend andhighvolumemarketspresents

theneedforaccurateand reliableautomated measurement ofCTE.

The popularity ofCCD imagers hasdeveloped theirfeaturesas wellastheir

performance. Multiplereadoutstructures,on-chip line

delays,

anti-blooming,exposurecontrol

means,sample/holdcircuits,and clockdriverscanbe found integratedwith_manysensors.

Thesefeatures

typically

involvethedevelopmentof newstructures and enhanced_processing

options,which canthenbeusedinotherareas ofdevice designand architecture. Theextension

ofthesefeaturestoward

improving

device

testability

isalogicalprogressionand onesuch

implementationis discussedherein.

2.Historical Review

Zl. TheChargeCoupled Concept

The underlyingstructure upon whichCCDoperationisbased isthewellknown MOS

capacitor,3

the

deep

depletion_mode,a potentialwell canbe formedwithinthesilicon substratethat

attractsandstores_minoritycarriers. Whenthedevice is builtupona_{uniformly doped}

Metal Oxide

CG=(1/COX+ _1/CDEP>

ox

Si-SiO Interface

Figure 2.1a MOS StructureandEquivalent Circuit.

substrate,it istermeda surface channeldevice. This isdue,asshowninFigure

2.1b,

tothefact

that thepotential minimum ofthebiasedcapacitoris atthesurfaceof_{the substrate,}atthe

silicon/silicon-dioxideinterface. Thetotalcapacitance as measured at node

Vq

isgiven as

Q-;

andisequalto theseriescombination of

Cqx

anc*

Crj

asshowninFigure 2.1a.

During

charge-coupled operation, thesubstrate and gatepotentials appear static(DC)with respect tothe

potentialatthe interfacewherethechargestorage and transferoccurs.

Hence,

theeffective

storage well capacitanceis given astheparallel combination ofthecapacitances

Cqx

+

Cf>

Withcommongate oxidethicknessesof severalhundredangstroms andsubstrate

doping

in the

1015/cm3

range,

Cp

is

typically

much smaller than

Cqx

anc* tnetotal storage capacitance is

approximated

by

Cqx

itself. Hencethepotentialchange atthesemiconductor surfaceas a

functionofthesignalcharge

Qg,

isgivenas:

AOo=

_

s C

AQs

ox _(2.1.1)

By

providinga succession ofthese these capacitors,andsequentiallyclockingthem,carriersare

stageswithviewsoftherequired clock

timing

andchannel potentials. Thedegreetowhich

chargescanbetransferredbetweenpotentialwellsgivesrise to the fundamentalmeasure of

captured electrons

V.

Figure 2.1b

Energy

Band Diagram for MOS Structure Under Positive Bias.

CCD performance,chargetransfer_{efficiency(CTE). The minority}carrierstransported through

theCCDcanbe injectedeither_opticallyor_electricallyand are an accurate

linear,

representationoftheinputsignal. Tomaintainsignal_integrity,itisdesirable fortheCTEof a

device toapproachunity, where100% ofthechargeistransferred betweenpotential wells

eachtimea transferoccurs.

Realistically

thisis not possibledue to physicallimitationsof

carriertransportand _processingcapabilities, butonecan_easilysee thatwithCCDs

having

severalthousand stages, transferefficiencies of0.99999per stage mustbeattainedtomaintain

signal_accuracytowithin several percent. Becauseofits basicstructure, the surface channel

CCD isthesimplesttomanufacture,butthere aredrawbacksofdevicesthatoperatein this

mode. Limitationsontransfer_efficiencyof surface channeldevices have beenshown4

tooccur

dueto

trapping

effects caused

by

interfacestatesatthesemiconductor-oxideboundary. These

limitations_surroundingtheperformance of surface channeldevices led tothedevelopmentof

the buried-channeldevicecharge coupleddevice5 (hereafterreferred to as BCCD) which

t=t2

\ffimz##0&#l

\

I

iMdWdi/iuiti t'l'i'i'iTi'i'l l-.-.-.-.-.-.-.-.-.-.--:7.-,-.-.-.---:-;J

f=f3

f=M

1=15^*

T_J~L

01

02

03

\

\

\

\

rf t2 f3 f4 ts

(a) (b)

Figure 2.2 Charge

Coupling

(a)andRequired

Timing

(b)fora3-phase CCD.

Suchastructureisshownin Figure 2.3awhereitcanbeseenthatan additional

doping

layerof

opposing conductivitytype,n-rypeinthis case, isplaced atthesurface ofthesemiconductor.

This layer isof_{sufficiently low}

doping

suchthatitcanbe depletedof all _majoritycarriers

by

applyingareversebiasacrossacontactto then-type regionandthesubstrate. Withan

additionalbiasplaced onthegateof_{the structure,}onecanenvisiontwodepletionregions,one

extending downward fromtheoxide-semiconductor

interface,

and asecondextendingoutward

fromthemetallurgicaljunctionofthe twodopantspecies. Asthebiasonthesurface electrodeis

increased,the two depletionregions merge_resultingin thepotentialprofile showninFigure

withinthen-typeregion. Providedthecharge packetis notexcessive, the_minoritycarriersin

such adevicecanbe transferred between potential wells without_contactingthe

Xd1 Xd2 Xd3

V

V =0V-G

V >0V G

<-.

W

N-Si

X

K

N

P-Si (a)

(b)

Figure 2.3 BCCD Structure(a) and

Resulting

Channel Potential Profiles (b).

semiconductor-oxide

interface,

and hence averting

trapping

losses. Anadditionalbenefitof

thisstructurehas beenshowntobea markedincreasein theextension of

fringing

electricfields

betweenadjacent phases. Thisallowsdrifttodominatetransportofcharge

during

thelatter

stagesof_transfer,wheretheelectronconcentrationissmall anddiffusiontransportwould

normallyberelied upon. Theabove changesgive risetoa substantialimprovementin transfer

(nearly

10X)efficiency,at evenhigher_{operating frequencies. Limitations}on performancedue

to

trapping

lossesalsooccurinBCCDs duetobulktrapswithin_{the semiconductor,but}these

traps,duetoimpuritiesanddamagesites,have beenshowntooccur withinisolated_energy

levelswithinthesilicon

bandgap,

andoccurat reduceddensities_resultinginlowerlevelsof

CTEdegradation.

Still,

thedevicecapabilitiesare affectedand thedistributionof_energy

levels leadstovariationincaptureandemissionratesof_{carriers,giving}riseto transfer

2.2_{Charge Transfer Dynamics}

Thecharge-coupledconcept relies onthe transferofsignal carriersbetween potential wells

createdbeneath_{sequentiallypulsed electrodes. As}

mentioned earlierinSection

2.1,

limitations

onthechargetransportarerealized

depending

on_clocking

frequency,

devicestructure,and

processingconditions. Transportitself isgoverned

by

three phenomena;

diffusion,

self-induced

driftand

fringing

fieldaided drift.

Diffusiontransportarisesfromthegradientofthecarrierdistributionacrossthe

transferarea and isproportional to the thermal diffusioncoefficient

Dn

=

u^ kT/q. Fourier

analysisofthediffusionprocess6

allowsforthecalculationofthe_remainingchargeundera

transferring

electrodeoflength

L,

at atimet,with anoriginal signalpacket_{containing n0}

electrons,usingtheexpression:

n(t) 8

f

t

1

= -TexF1

"o 71 V xdiff/

(2.2.1)

where

f

4L2

1 (

L2

)

^U2DnJH2,Dj

Thecharged natureof_{the carriers,}againcoupledwiththegradient inthecarrier

distribution,

developsachargegradient_givingrisetoanelectricfield. Asadjacent potential wells are

formed,

and chargemovesfromafullwell toan_{empty well,}thecarriergradientcanbecome

very steepandactsto

help

movecarriersinthedirectionoftransfer. This isreferredtoas

self-induced drift. Since theself-induced fieldvaries with carrier

density

and hencewith timeas

thechargemoves,itcanbethoughtofintermsof adiffusionprocess7

as well with a carrier

concentrationdependentdiffusioncoefficient given as:

DSI=

where

Ceff

istheeffective storagecapacitancein thechannel and the_quantity

[q

. n(x,t)/Ceff]

isthechangein thechannelpotentialdueto thechargeredistribution. Thechannel potential

difference between fulland_emptygatesis_normallyseveral voltsand _comparingthisto

kT/q,

which is_{approximately}0.026 volts(at 300

K),

wenotethattheself-induceddriftcanfar

outweighthediffusioncomponent

during

the_earlystagesoftransfer. Asthetransfercontinues

andthecharge equilibratesbetween thepotential _{wells, the}self-inducedfieldsvanish. Dueto

the_{dynamic property}ofthediffusioncoefficient,_exactingsolutionsforself-inducedtransferis

difficult toobtain with other thannumerical calculations,butBarbe8presents useful

approximations. Intheformsimilarto _2.2.1,thecharge percentage_{remaining is}

where

2LZC

*sr

eff

(2.2.4)

nVrfPo. (2.2.5)

Fringing

field aided transportresultsfromtheelectric fieldsgenerated

by

adifference

in thegate potentials of adjacent electrodes. Figure 2.4 illustrates thechannel potential

V////A

02 -off Ol 'on'

"*\>//A

I

Vf't*

02 -off

1_

SO. o UJ O.

Ideal

Actual

diagrams foratwo-phaseCCD in boththeidealcaseand therealistic case,where

fringing

betweenelectrodes modifiesthepotential acrossthecell. Theextension ofthe electricfield

further intothe cell,providesdrift fora greaterdurationofthe transportperiod _allowingfor

greatlyimprovedtransferratios. Anapproximateform forthe transferpercentagein the

presenceofa

fringing

field is

n(t)

ft}

=exp

"o V iwJ _(2.2.6)

where

(2.2.7)

lFF-p

Onenotesthelessthanor equal signinequation2.2.7withrespectto theminimumelectricfield

^min-

Referring

toFigure2.4 again, theslopeof_{the potential,}andhence theelectricfield is

variedacrossthecelland

by

taking

theminimumvalue,a worst casetransfertimeconstantis

obtained. Barbeprovides an approximationfortheminimum

field,

whichingeneralis

structure

dependent,

as

2 AO rcSi

(2.2.8)

3 L2

Ceff

Thechannel potentialdifferencebetweenthe

forwarding

and _receivingelectrodes,AO,is

controlled

by

channel

doping

levelsand theapplied gate voltagesandis

typically

several

volts. Foraburied-channel

CCD,

Ceff

isgiven as

( Ut)^"1

Ceff-

tox

"ox + "n V no

(2.2.9)

V _eox eSi

2eSi

>

where_xnis thedepth oftheburied-channel layerand

tox

istheoxide thickness. Equation 2.2.9

canbeusedboth foruseintheself-induceddriftcase

during

the_earlystagesoftransferwhere

n(t)canbeconsidered equaltoi^,,andinthe

fringing

fieldcase,whichdominatesinthelatter

stagesoftransferwheretheselfinduced fields havecollapsed and thechargein thetransfer

C-eff- rox ₊ *n

-ox Si J _(2.2.10)

Therelativecontributionof eachofthe transportmechanismsis dependentuponthe

cell_geometryand_processingconditionsandcanbest be determined

by

_calculatingthe

components_separatelyand _comparingtheirrelativevalues with respectto thetransferperiod.

4 6

Time_(nsec)

Figure 2.5 Chargetransfercomponentsfor 4.5um phase.

Thetotaltransferequation canthenbeapproximated

by

superposition ofthegivenequations

overseparateintervals. Figure2.5showsthetransferratio with respectto transfertime for

each ofthecomponentsfora4.5urnphaselength(9umcell). A_summaryoftheinput

parameters and resultsforequations2.2.1 through2.2.10 forthefirst12nanosecondsoftransfer

aregiveninAppendixA. The data showsthecrossoverto

fringing

field dominatedtransport

afterthefirst 2nanoseconds oftransfer. Moreover,

fringing

fieldtransportallows_upto50 MHz

limitedtransportpredicts_{approximately}5 MHzoperation. Precisesolutions can ofcoursebe

obtainedthroughtheuseof2-dimensionalsimulationprograms ofthe_processing (i.e., SUPRA)

and electrostatics

(i.e.,

GEMINI)wherenumericalcalculationsare employed. However,the

above approximations providefirst-orderapproximationsandallowone tosimulatetheeffects

ofstructure changesina moreexpedientfashion.

23Basic CCDImagerArchitecture

CCD imagersare constructed_utilizingthestructurediscussedaboveas ashift register

forthereadoutof signal carriers. _{In addition,}meansfor receivingand_{storing image}charge,

and_circuitry foroutputconversionarerequiredtofromacompletedevice. Figure 2.6showsthe

essentialpartsofalinear CCD imager

including

aphotodetectorarray,shiftregister,transfer

gate,andassociatedoutputcircuitry. _{Photodetectors generally}consist of either

Transfer Gate

Photodetector

Array

020

<J>1 o

p^ii^llilili

Buffer

Amplifier

CCD Shift Register Output Detector

Figure 2.6 Basic CCDImagerArchitecture

photocapacitors orphotodiodes,which servetoreceive

imaging

photonsand,afterconversion

toelectron-hole pairs,storethe resultant_minoritycarriers. Isolationof thecharge carriers

fromtheCCDshift registerisperformed

by

means ofthe transfer gate,whichformspotential

barrier between the tworegions

during

the integrationperiod. Readout fromthedetectors

increased,removingthebarrierand _allowingthechargetoentertheCCDshift register.

Clocking

oftheshiftregistergates transfers thechargeto an outputstructure,

typically

a

resettable

floating

diffusion,

which is followed

by

a bufferamplifier.

Referring

toFigure2.7,

onecanseethatconversionfromthechargedomaintothevoltagedomainoccursatthe

floating

diffusiononeachclockcycle. This isaccomplished onthe

falling

edgeofthe02phase,inthis

illustration,

wherechargeistransferredoverabarrierset

by

theoutput gate(OG)bias

potential. Theconversionfactor is _simplythesignalchargedivided

by

theeffectivetotalnode

capacitance ofthediffusionanditsassociated structure. Source followerbufferamplifierslend

powergaintodriveexternal circuitry.

Buffer

Amplifier

Section

Ol <E>2

OG(DC)

IXI.

ES

ResetJL

f=Ceff

fl

Baa^saas^^

1

-^

Vrd

(DC)

Reset "off level

*._nL^r

Reset

I \

Vout

n

1

I

Vsig=A

A = Qsig/Ceff *

Av,buffer

Reset 'on'

level

Figure 2.7 CCD Output

Circuitry

and

Timing

Controlling

themaximumamount of chargethat theimagerseesisrequiredtoprevent

Anti-blooming

orexposure controlcanbe_{implementedwhenthestructureofFigure 2.6 is}

augmented

by

alateraloverflowstructure. Figure2.8showsthe_positioningoftheadditional gate

Antiblooming/ Exposure Control

t=t1

t=t2

t=t3

t=t4

Ol

EXP

TG

t1 t2t3 t4

inrOin

T,. line

exp

Figure 2.8

Antiblooming

andExposure ControlImplementation.

anddrainonthealternateside ofthephotodetector.

Referring

to the tl

timing

diagram

portion ofFigure

2.8,

we notetheEXPgateisassertive

during

the_earlyperiodoftheline_time,

diverting

anygeneratedchargeto theLOD drainstructure,whichis_positivelybiased and

functionsasan electron sink. Chargeintegrationbeginsonthe

falling

edgeoftheEXPgate

pulse wherethe potentialbeneath EXP is shifted toalevelabove thedepleteddiode potential

but belowthe transfergate potentialasshown

during

t= _tl. Charge_isallowedto

during

thebalanceofthelineperioduntil apotential isreachedin thephotodiode,which

matchesthepotentialbeneath EXPas shown

during

t=_t3.

Any

furthercharge collection

resultsinchargeflowfromthediode intotheLODonceagain,

thereby

limiting

thestored

charge. AdjustmentoftheEXP 'off levelwithrespect to thedepleted diode level determines

themaximum signal charge. Attheend ofthe

line-time,

the transfergate

TG,

ispulsed

positive_allowingforthechargeto transfer to theshift registerasshown

during

t= _{t4. The}

effective exposure period ends uponthe

falling

edge oftheTGpulsewhenchargeisonce again

isolatedfromtheshift register.

Clocking

oftheregistertoreadoutthe_previouslyimaged line

occursbetweensuccessiveTGpulses. TheEXP'on'

durationcanbevariedtoinclude_anylengthof

thelinetimefromstatic(DC)operationtosub-clock cycle periodsif

desired,

_allowingfora

wideexposurelatitude.

2.4 ImagerPerformance Limitations

Many

sensorapplicationshavecomeintoexistencethrough theirexploitation ofCCD

imaging

technology

and

they

stretchtheperformance capabilities ofthesedevices. The

emergenceofHDTV_scanningpresents such anexample9

withCCD dataratesinexcessin 120

millionsamplespersecondanddynamicrange requirements_extendingtoover60dB. Thiscase

representstheextremeindevicerequirements yetillustratestheimportanceof

determining

deviceperformancelimitations. The primaryattributethatdefines the

imaging

performance

ofthedevice istermedthemodulationtransferfunction (MTF). The MTF isameasureofthe

magnituderesponseas afunctionof spatial

frequency

toa sinewave_{varying input}signal. There

arethreemain components of

MTF;

aperturelimitedresponse,diffusion limitedresponse,and

chargetransfer_efficiencyresponse.

Cascading

thesecomponentsgivestheoveralldevice

2.4.1Aperture Limited Response

Thestructureof chargecoupledimagerscontainsa_{finite array}ofdetectorelements

witha pitchPandwithapertureAx. Figure 2.9 illustratesthe_samplingof asinewaveinput

signalof

frequency

_co0=1/T

by

_{such an array. The}modulationisdefinedasthemaximum signal

minustheminimum signalandis_unityfortheinput,

(i.e.,

Thesignalscaleisoffset and

normalizedwith_f(x)=0

correspondingtozeroimage light leveland one_{corresponding}toa

saturatedsignal). Forthe output, themodulationisthedifference betweenthesignal samples

thataretheaveragevaluesoff(x)takenoverthe_{sampling intervals}Axj. The MTF is defined

astheoutput modulationovertheinputmodulation. Thephysical_sampling

f(x)

1

-T 12 -T /4 T /4 T 12

\+-Sampling

Pitch ?

|

= P= T 12

j

Figure 2.9

Sampling

ofSine Wave Input Signal.

arrangement ofthe imager

inherently

limitsthesystemresponse with atransferfunction

derived as(seeAppendixA):

sin

MTF,

f Ax

f Ax

5C~

Sampling

theory

dictatesthemaximum allowableinput

frequency

limit toavert_aliasingand

definesthis

frequency

astheNyquist

frequency

where

jnyq

isequaltoone-halfthe_sampling

frequency. Thus_assuminga systembandlimitedto

fmax

=

Jnyq

,wecalculatean MTFof

approximately63.67%atthemaximuminputfrequency. TheMTFcan

theoretically

be

improved

by

_scalingtosmaller apertures asshowninFigure 2.10wherea50%reductionin the

aperturesizeimprovestheMTFto90%atNyquist. Wenoteherethatequation2.4.1.1

1.0"

0.9;

0.8;

0.7"

u. AX/P=0.5

2

-6*

0.5"

ZJ

I

0.4.

**

0.3

Ax/P=1

0.2"

0.1

n n > _I i . _!

\

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

f/fnyq

Figure2.10ApertureMTF forDifferent Aperture/PitchRatios.

assumesthe idealcase and_actuallyprovides an upperbound fortheapertureMTFin that

phaseshiftbetweentheinputpeaks andtheactualimagersample pointsisnotaccountedforin

the derivation.

Referring

toFigure 2.9again,we notethat_shiftingthesample points

by

T/4

tends togive sample values of0.5 forallsamples,resulting inzero net outputmodulation.

Also,

the_narrowingoftheapertures resultsinareductioninthemagnitude ofthesignallevels dueto

thereduced_sensingarea.

Recovery

ofthemagnitude canbeachieved withhighersignal

processinggain providedthenoiseflooroftheimagerisnotreached,otherwiseMTFwillbe

inputs is difficultand_{may limit}the_accuracyofmeasurementofthe imager. Analternative

function,

termedcontrast modulation

(CTF),

isameasureofsquarewave responseandisa

somewhatpreferred approachfroma practicalimplementationstandpoint. Sinewave

performancecalculationsand _{manipulations,}

however,

are somewhat more_{mathematically}

convenienttodealwith. Coltman10addressestheconversionbetween CTFandMTFand

provides_{the methodology,}

involving

thesubtractionoftermsfromtheFourierseriesforthe

squarewaveinputresponsetoachievethesine waveresponseto thedesired degreeof accuracy.

2.4.2DiffusionCrosstalk Effects

Thecollection ofcharge carriersinsiliconbased imagers is baseduponabsorptionof

incoming

photons,conversionintoelectron-hole pairs and

finally,

captureofthe_minoritycarriers

(typically

electrons)inthedepletionregions ofthephotodetector. Theabove processeslimit

thequantum_efficiencyofthedeviceas afunctionofthephoton wavelength. Theseprocesses

willalsolimittheMTFoftheimager,

depending

onthedevicestructure. Asphotonsenterthe

siliconsurface

they

penetratetoa physicaldepth dependentonthephoton_energyandthe

interactionwithsilicon lattice. Theabsorptioncoefficient,a,giveninunitsofcm"1, varies

with wavelength and temperature. It is

fundamentally

coupled to thebandstructureofthe

semiconductor,with minimum conversion_{energy limited}

by

the

bandgap

potential.

Hence,

for

silicon, the

bandgap

of1.1 eVlimitsabsorptiontoa maximum wavelength of_nearly1150

nanometers. Acurvefittomeasured values ofDashandNewman11has beencalculatedover

thevisible and nearinfraredspectraas

a(nm-1)=

5.54595-1010

exp(-1.01472-10_1X- 1.38044

-10~V+6.66205-10~\3) _(2.4.2.I)

whereX representsthewavelengthinnanometers. Thecurveisplottedalongsidethemeasured

valuesin Figure2.11. Thisplot showsthattheabsorptiondepthvariesfrom lessthan

one-tenthofa umtotensofumoverthegivenwavelengthrange. _Thus,

depending

onthedepletion

thedevicesubstrateforsomedistance before

they

are collected

by

adepletionregionorare

annihilated throughrecombination. Figure 2.12shows thepotential pathsfor

100-1

xxcccx

w -WW**

^^^^ ..,,0^ J *

-^,r,,

Dash&Newmanuaia

;=-

10-Approx. i-n E ^*~ c 0> N~s~3 5S3 o O ...^-< ~ c o O. O 10 ₁.

S5S5S5_^s

< ^....MMl..^o^^ srrr

1

300 400 500 600 700 800 900

Wavelength_(nm)

Figure 2.11 Silicon Absorption Coefficientvs.Wavelength @ 300 K

diffusionafter generation of anelectron-holepair

by

a photon of wavelengthX afteritpasses

throughtheaperture areaofdetectorN. Case 1 showsthecollectionoftheelectroninthe

intended depletionregion. Ifacarrier,generatedin thearea ofdetector

N,

managestodiffuse

to thedepletionregion of adjacentdetectorN+l andiscollectedthereasincase

2,

itthen

degradestheeffectiveMTFofthesystem. The diffusion MTF functionhas beenshown12

by

Seib

tobeoftheform:

l-[exp(-aLu)/( 1+ aL)]

where

MTFn= D

l-texrX-aLuJAl+aL,,)]

L2 L2

2.4.2.2

The diffusion lengthofthecarriersis

L0

[=

(Dnxn)^/2)

_and

LD

is definedasthedepletion depth

extensionofthephotodetectorintothesilicon. Case3representsthecase wheretheelectron

recombinesinthesubstrate. ThisoptiondoesnotimpacttheMTF

directly

buttends tolowerthe

overallcollection _efficiencyofthe

device,

_{potentially reducing}thesignal-to-noise ratio and

limiting

theresolutionofthemodulation.

[\

, Detector N

' _{Detector N+1 >}

Figure 2.12 DiffusionOptions for

Long

WavelengthGenerated Carriers.

The diffusion lengthofthesignal carriersis dependentonthe_qualityofthesilicon_starting

material,subsequent elevatedtemperature_processingandthe_processing materials

forming

the

CCD structures. Thisparameter_{is routinely} measured andis

typically

in therangeof50to100

umforimagerprocesses.

Anticipating

theirnegativeMTF effects,oneshouldnotethata

long

diffusion lengthissynonymous withthelow darkcurrent one

typically

strivestoachievefor

CCD imagers inordertoobtainahighdynamicrangedevices. Equations 2.4.2.2and2.4.2.3

assumethat the detectorsurfaceis uniformly depletedacross_{the array,}andhence arefree

from terms

involving

thecell pitchandaperture. Thisisa simplified viewinthatatypical

imagerwillhavesomeisolation betweenadjacent_detectors,in theformofdepletionor

isolationwill affectthedepletion profileacross the detector surface,_complicatingthe

diffusionprocess. Imagersarealso, in_manycases,builtina'well'or upon an epitaxiallayer.

Thesecases provide a carriersinkor_reflecting

boundary

_{respectively,}whichmodifiesthe

effective carrier collectionwithrespect towavelength. Theabove equations

do, however,

provideafirst-orderapproximation oftheMTFand serveto illustratetheeffects of_varying

depletion

depth,

diffusion lengthand imagercell size. Figure 2.13 plotsthecalculated

diffusion MTF forthecase of

Lp

=_{3.5um andfor Lo}=50_urn,and100_um. Thecellsizechosenis

9umwitha_{corresponding Nyquist}

frequency

of55.56cycles/mm.

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

f/fnyq

Figure 2.13 Diffusion MTFfora9umCellLengthatVarious Input Wavelengths.

Thedegradation in MTF becomessignificantatwavelengths above500nm and canimpose

serious systemimpact beyond800nmfora cell ofthissize.

Scaling

toalargercellsizeand

perhaps moreimportantly,adeeper depletionregion allowfor MTFimprovementas shown

in Figure2.14where resultsfordepletiondepthsof3.5um and5um are comparedfora9umcell

c

3

0.3

0.2

0.1

0.0

O 9umCell,Ld=3.5|j.m

9 9umCell,Ld=5um

D 18um

Cell',

Ld=3.5um

Lq=50Lq=50umun

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

f/fnyq

Figure2.14 ComparisonofDepletionDepthandCell Size Increases.

Thechangetoan18umcelllengthprovidesMTF improvementover abroaderrange of

frequenciesthandoestheextension ofthedepletion region,butthisalsoimpliesanincreasein

theimagerlengthof

2X,

whichleadstoreview of systemopticsand cost considerations.

2.43 ChargeTransfer

Inefficiency

Degradation

Ideally,

theoperation of a charge-coupledshift registerinvolves thecomplete

transferof signal packetsfromstage-to-stagethrough to thedeviceoutput,withthesignal

being

aprecise,

linearly

related representation ofthe input image. As discussedinSections2.1

and2.2aboveinefficienciesinthechargetransportexist,dueto fundamentallimitationsin the

transportmechanisms or

trapping

states withinthesemiconductor structure. Withshift

registers of several thousand stages

being

typically

realized, significantdispersionof the

signal packets can result. Consider

injecting

anormalized chargepacketinone endof aCCD

register withNcells and_pphases per cell. Thetotalnumber oftransfersisgiven asK =_N p,

phaseisgiven as e'

suchthate= p

e'

isthe

inefficiency

per cell. JoyceandBertram13have

provided rigorous analysis

utilizing Green's theorem, resultinginan expression thatdescribes

thedispersion oftheoriginalsignal. Analternativeanalysis14

makes use oftheZ-transformto

reacha similar_result,inamore workableform. The CCDshiftregister canbethoughtofasa

two-portsystem wheretheoutputisseparatedfromtheinput

by

afinitenumber of

delay

stages

equaltoNasdescribedabove. Z-transform

theory

definesaunit

delay

ofa

k-1 k-2 _Unit

Delay

k-1 -Unit

Delay

Figure 2.15 Signal RepresentationatStage k-1 ofaCCD Shift Register.

signal V(z)asV(z) z"1

. Again_notingthe

inefficiency

perstage ase,onecandescribetheoutput

ofthekthstageafter n cycles asVk(n). Afterthe

following

clockcycle, theoutput atthisstage

isgivenasVk(n+1). Thesignalatthispointismade_upof_{two components, the}residualamount

left fromtheprevious signal givenase-Vk(n)and theforwardedsignalfromthe(k-l)thstage

givenas (l-e)-Vk_;[(n).

Thus,

wehave:

Vk(n+1)= _e-Vk(n)+

(l-e)-VH

(n).

Applying

thez-transformof2.4.3.1 wehave

z-Vk(z)=_E-Vk(z)+(\-E)-Vk.i(z)

and_multiplying

by

z"1

and_{collecting like}termsgives

Vk(z)

-z^-e-V^z)=z"1

(l-e)-Vk_i(z),

Vk(z)(1-z"1

e)=z'1

(l-e)Vk_;i

(z),

(2.4.3.1)

(2.4.3.2)

which simplifies to:

Vk(z)=

(1-e) (l-z-1e)

z^V^z)

(2.4.3.3)

Sincethetotalnumber of stagesisN,theoutput oftheNthand finalstage occursafterN

VJz)=

(1-e)

N

Z^VjCz)

(2.4.3.4)

.(l-z_1e).

The_clockingoftheshift registerwith

frequency

_fclkimpliesa

delay

perstageof

Tclk

=

l//clk

IfweletzrepresenttheFouriertransformof this

delay,

then

HKi)

= _{cos 2k}

elk

+jsin 27C

elk _(2.4.3.5)

and_substitutinginto 2.4.3.4and_{simplifying, the}

frequency

responseofthesystemcanbe

calculatedtobe:

H(f)=

Vq(f)

V,(f)

(1-e)

1

-ecos

N

(2.4.3.6)

Themagnitude response IH(f)I providesuswiththeeffectiveMTF dueto theinefficiency. This functionand the_resultingphase shiftA<|>, lesstheN-stage

delay,

have been determined from 2.4.3.6 (see AppendixD)tobe

MTFe=

|H(f)H

_{(1-e) l-2ecos} 27i-

|

+

e2

V rclk

-i-4-i N

(2.4.3.7)

and

A<)>=_-Ntan

esi_inf 2n-V rclk

1-_{e co}

<*)

(2.4.3.8)

*? c ro

1.0

0.9

0.8

0.7-0.6

0.5

0.4

0.3

0.2

0.1

0.0 1

*-v 0 .

V

N

>*vwwww.pvwnwmvv

Ne=0.041

0 Ne=0.082

^"""""-l

* _Ne=0.41

.__ * Ne=0.82

-J

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

f/fnyq

Figure 2.16 Transfer MTF fora2048Stage Shift Register.

frequency

equalsthedatarate and correspondsto thespatial_sampling

frequency

ofthedevice

allowingfor interchangeof Jcik=

J

s- Inobservingthe

figure,

weseethat tomaintainMTF

values above

90%,

onemust realize anNeproduct of greaterthan0.5. Thisvalueis

typically

required whenoneconsiders theeffectsofthepreviousMTFdiscussionsandthetotalresponseof

thesystemasthecascadedproduct ofthe MTF.

23 CTEMeasurement Techniques

Traditionalmeasurement ofCTEconsists ofthecontrolledinjectionof apulsetrain15

intothe

leading

endofaCCDregister,followed

by

the transferofthecharge packetsthrough

theentirearray,andevaluationofthe_resultingsignal afteroutput conversion. Ifa uniform

seriesofkpulses ofsize

Vs

isinjectedandtransferred throughanNstage shift_register,with_p

device,

givingrisetolosscomponents

Aj

fromeach ofthe

leading

pulsesasillustratedinFigure

2.17. Itcanbeshown16

that thelossper_transfer,z,isgivenby:

e= ₁

-< _exp pN

ln

VS

J

which canbeapproximated

by

where

e= pN-Vs

(2.5.1)

(2.5.2)

k

vL=XAi

i=1 . (2.5.3)

Thepulseseriesmustincludea sufficientnumber of pulses suchthat the totalinjectedsignalis

undistortedfortwoormore ofthepulses_allowingforanaccuratemeasurement ofthe'lost'

charge. The analysisofthe

leading

edgedistortionand

trailing

edge

distortion,

intheformof

extra pulses

following

theinput train,allowsforthedeterminationofthelossmechanism.

Vi

Original Pulse Train

|

_Xail

Figure2.17

Resulting

PulseTrainDueto ProportionalLoss.

Proportional_losses,dueto thefundamental limitationsonchargetransport(diffusionanddrift

aided),

typically

resultinamirrored

leading

edgelossand

trailing

edge_recovery,whereasas

pulse(orpulses,untilthe

trapping

statesarefilled) and a non-symmetric_resulting

trailing

edge,withslow

trap

emission_resultingina

long

tailofpulseswithlowmagnitudeasseenin

Figure 2.18. Othertypesoflossescanbedependentonsignallevel. These

losses,

termed 'non

linear,'

can resultfrom barriersorwellsinthechannel potential profiles caused

by

deficiencies

in processingor celldesign. The distortion inthepulsetraincanbeacombination oftheabove

effectsor altogether separate. Onenotesthat in_anycase,allthechargeisconserved

hence,

the totalloss in the

leading

pixelswillequal the total recovered signalin theadditional

trailing

pulses.

T

-i

A9 2'

-1-A4

Vi

A1

Th-^

Original PulseTrain p^ Tail

Figure 2.18

Resulting

PulseTrainDuetoFixedLoss.

2.5.1 ElectricalChargeInput Techniques

To formthepulsetrainsfor CTEmeasurementdiscussedabove,onemusthavea means

forchargeinjection. Electricalchargeinputtechniqueshaveevolvedthroughinjectionmethods

utilized _{in analog CCD}signalprocessing. Forthese applications,an_analogsignalvoltagemust

be accuratelyconvertedintothechargedomain,and_{appropriately injected} intothe

device,

for

subsequent operations

by

theCCD. Unfortunately,normal variationsindeviceprocessing

requiretheuse of several control signalsorrelativelyelaboratefeedbackschemestocontrol the

chargeinjection. For

testing

purposesonegenerally doesnothavetobeaspreciseaboutthe

signallevel. Inautomated_{testing,theapparatus}

mustbe_set-uptolookina specified_range,

unless recursivemethodsare usedtoadjustthelevel foreach

device,

which_may

undesirably

extend the requiredtest timeand _{repeatability.}

Thesimplestformofcharge

inputting

isdynamiccurrentinjectionas showninFigure

2.19. HereaMOSFETtypestructureisformedfromareversebiased junction

ID,

whichacts as

the source,electrodeIGwhich

t=t1 t=t2 t=t3 II c 3 K

j> _{(3 C}

1 3 i

r

I MWM&tim

n

\"

<D1P

|

(hip'

l_

<&1 ID VID,L IG

Tinj

t1 t2 t3

Qs =

f(Tinj,

V |D L

)

Figure 2.19 Dynamic Current Injection

actsthegate and potential well<MP thatprovidesforthedrain. Electrode <D1 isthefirst

stageoftheCCDshiftregisterandisclockedina mannerconsistentwith_{normal readout.}

Two-phase_clockingwitha50%

duty

cycleisshownin thiscase. TheIDjunctioncanbepulsedor

heldat a constantbiasabovetheIGchannel potential suchthat

during

theOl 'on'

period,

chargeis injected through the _MOSFET,whichis_effectively ina saturated state. Thesignal

level isgivenas :

where

Tinj

isthedurationofthe IDpulse. The deeperpotential oftheOIP'regionactsto

retardbackward flowofcharge onthe_ensuing'off periodof<X>1andalsoservesto

keep Vds

constant_{eliminating X}(channel lengthmodulation)effectsfrom

influencing

thecurrent.

Unfortunately,

thecurrentisanon-linearfunctionoftheIGchannel potential toIDvoltage,

hence,

control ofthesignallevel

by

_alteringtheIDpulse amplitude giveslimitedcontrol

accuracy. Modulationoftheinjectionperiod givesbetterresultsbutrequires

transforming

a

control signallevel intopulsewidthmodulated signal.

Furthermore,

_anyvariationsin the

effective IG threshold levelcan _{significantly}alter the injection level for different devices

withinafabrication lotor evenagivendevicewafer.

Theaddition ofa second charge_meteringgateto theabove structure providesfora

muchimprovedinputstructure. Thesurface potential equilibrationmethod,17

modifiedforuse

with

buried-channel,

two-phaseCCDstructures,isshownin Figure2.20. Inthis

ID

1

IG 33 <I>1

O Q O

J-L-r^-t=t1

<D1

t=t2 S3

IG

t=t3

t1 t2 t3

configuration, thefirstgateIG isheldat a DC

bias,

while thesecond gatereceivestheinput

signal andiscalledthesignal gate(SG).

Pulsing

IDtoalevelabovethatoftheIGchannel

potential whileOl is in an'off _state,will fill theSGandIG areaswithcharge.

By

_{pulsing ID}

backtoadeeperpotentialstatepriorto<&1

turning

'on',

chargeisdrained back intotheID

region

leaving

afixed charge packetintheSGregion,whoselevel is determined

by

the

difference betweentheSGandIGchannel potentials. TheSGchannelpotential canbe

linearly

controlled

by

thegatevoltage, therefore

by

proper_sizingof_{the gates,}onecan_varytheinjected

chargefromzeroto the_capacity oftheCCDregisteritselfwith

fairly

goodresolution. This

method

is,

in

fact,

themostcommonstructurefoundontodaysCCD_{imagers,allowing for}

determinationofbasic device

functionality,

charge_capacitymeasurements andCTE

characterization. _{The primary drawback}of thisstructureisvariationin the thresholdsofthe

electrodes. Withburied-channel thresholdvoltages

typically

_{ranging from}-3to-10V

nominally,tolerancesof0.3V or greater canberealizedinmanufacturing. Sincetheformation

oftheIG andSG gatesis

typically

in two differentlevelsof polysilicondueto theiradjacency,

the totalpotentialdifference may beon theorderof0.5-0.6V. ForCCDs with_{low clocking}

levels,

and_consideringthe tolerancesinvolved in_settinguptheinjection

levels,

thislevelof

potential difference_maytranslate intoasignificant amount ofthedevicecharge_capacityand

can modulatethe_resultingoutput signal

by

20-50%or greater.

Toalleviatetheabove problem,a thresholdinsensitivestructure18

has beenproposed

andisshown withitsschematicrepresentationinFigure221. Thisstructure utilizesa common

electrodeCG for the_chargingand

discharging

ofthecapacitanceassociated with

floating

diffusionCFD. Switches

SWS

and

SWR

couplethepotentialonCGtoeither

VSIG

or

Vref

whilea thirdswitch_INJ, coupled with aninjectiondiode

ID,

provides forthechargingcurrent.

?

SWR

9

SWS

SW

"f

j^lH^r

_TL

sws

I

vSIG

REF

lizrwi

<5^

4

1|

_{o_TL INJ}

ID

Figure 2.21 AThreshold In-sensitive Injection Circuit

that

by

actuation of

SWS

and

INJ

andthe_pulsingof_ID,

CFD

ischargedtoa level

SIG'

by

the

potential ongateCG. Thepotential on<E>1 is heldat abarrier

during

thisperiod

(tl,

t2)to

prevent chargeflow intotheCCDshiftregister.

Subsequently,

SrEF

isturned

'on',

couplingCG

to the

VRgp

potential and

discharging

the

floating

diffusionto the potentialREF. This injects

carriersbackacrossIGto the Ol phase nowinits

'on'

state. Thetotal injectedchargeisgiven

by:

QSIC=CFD-Ap

(2.5.1.2)

with

Ap

as thedifference betweenpotentials

SIG'

(atthepoint ofIDinjection)andREF'. These

potentials scale

linearly

with the applied voltages

VsiG

an<^

^REF

while _any variation in

thresholdvoltageiscommonmodetobothresultantpotentialson

Cprj),

thereby

_reducingthe

sensitivityof thisstructure to process variation. While the threshold toleranceofthis

structurelooksattractive,it comes withtheadded expense oftwoadditionalclockedcontrol

signalsand an additional DC

bias,

incomparisonto thatofthesimpler potential equilibration

method. The_on-chipcircuitryalso translatesintoadded areafor local interconnectand

ID 'low' level

SIG'

level

t=t2

INJ "on'

level

INJ 'off level <j>i

ID 'high' level

SIG' level

INJ

SW,

ID

SWr

:j

FEF

SIG

IT

t1 t2 t3 t4

Figure 2.22 Potentialsand

Timing

fortheCircuitofFigure 2.21

Whilethecircuitsabove provide means forchargeinjection withreasonable_accuracy

andcontrol,

they

are stillbestsuited foracharacterization environment. Wenotethatina

manufacturingsetting, therequirements onthe

flexibility

andcontroloftheinputsignal could

be somewhatrelaxed, withcircuits_providingamore qualitativeindicationofdevice

functionality

as opposedtoquantitative measurements. Butratherthan

compromisingthe

results,onecan takeadifferentapproachtotheproblem. Oneapproachisto incorporate

methods onthedeviceitself toallow forself-determinedcompensationforvariationsindevice

biascircuitsand feedbackamplifiersfabricated_entirelyon-chip.19

This,ofcourse requires

considerabledesigneffortdirectedtoward theinjection_circuitryitself

(depending

ondevice

architecture and available_processing),consumes valuable_chiparea and _{may limit}eventual

deviceyieldthroughdefectiveoperationorspuriousinjection dueto_processingdefects.

Also,

typicalCCD_processinglimitstheamountofhightemperaturestepsandoptimizesvarious

parameters aroundthe imagercell,

thereby

limiting

the

flexibility

in termsofavailable

devicecharacteristicsfortheadditionalcircuitry. Anotheroption of

injection,

whichlooks to

retain control oftheinjected signal level while_simplifyinginputmeans isthatof_optically

introducing

thecarriersfor test,muchin thesamemanner as

imaging

thedeviceitself. This

alternativetechniqueprovidesthefoundation forthis thesisand isdiscussed inthe

following

sections.

3. Experimental Implementation

3.1 Optical InjectionforCTE Measurement

Theintroductionof signalcarriersintothephotodetectorregions of siliconbasedsensors

iscaused

by

_energyconversionfrom

impinging

photonsasdiscussed inSection2.4.1. _{A properly}

focusedpattern of photons allowsCCDsensorsto'image'ascene,_collectingthegenerated

minoritycarriers and_clockingthemto thedeviceoutput while maintaining localizedcharge

packetisolation. This fundamental

imaging

operation can alsobeusedin

testing

the

performance ofadevice

by

optically

inputting

aknownsignal patternand_observingthe

distortionofthispatternatthedeviceoutput. In thecase of chargetransfer_efficiency

measurements,injectionof pulsetrainssimilarto theelectricalinjectionmethodsdiscussed

above canbeemployed.

The

imaging

ofaslit provides a meansfor measuringthesquarewave responseof a

restofthe_arrayis kept dark. Examinationofthe

leading

and

trailing

edgesignal signatures

providesinformationaboutthe

efficiency

oftransfer. Wenotethatfrom equation_2.5.2,the

signalloss is

linearly

proportionalto thenumberoftransfers.

Therefore,

by imaging

different

sections ofthe

device,

at_varyingtransferdistancesfrom_{the output,}onecan_verifytheeffects

onthepattern withrespectto thenumberoftransfers. Precise

imaging

ofpatternscanbe

accomplished_usingoptical grade slitsor_pinholes,whicharedefinedinsizetoahigh degree

ofaccuracy,and_projectingthemontothedevicewithefficientlensessuch as microscope

objectives. Sincetheimage is_verysmall,aconfined sourcecanbeusedtomaximizethe_energy

through theoptical system.

Unfortunately,

opticalconstraints and deviceabsorptioncharacteristicsplace

limitationsontheimplementationofslitimaging. Themost obviousoftheseisthatthe

introduction of a projectionlenssystemimpliesafall-offofimage

intensity

fromcenter-to-edge.

This isknown^0 as

'cosine4'

fall-off dueto thefactthat the image

intensity

isreduced toward

theedgesoftheprojectionas a cosinefunctionto thefourthpower. Thiseffectsthesignal

patternin themost critical areas ofobservation, the_rising and

falling

edges ofthepulsetrain.

Theeffectsoftheopticscan_{be empirically}subtracted

by

_measuringthedeviceresponseinan

areaofthedevicenearthe output,wherethenumber oftransfersisfewandCTEeffects canbe

presumedtobeinsignificant. Oncethedistortionpatternisobtained,theresponsecanbe

normalized

by

use of acorrection

look-up

table.

Patterning

on severalsectionsof_{the array,}

with_varyingnumbers oftransferswillprovideinformationonthetransferefficiency.

Asecond problem occursin thealignment of patterns with respectto the_sensingarray. Ifwe

considera contiguous_arrayof photodetectors arranged ontheCCDsurface,delineationofthe

exact startand exact end oftheprojected slit issomewhatdifficult. Withpixelsizes_rangingto

lessthan7um_{in many}cases,control oftheslitwidthanditspositional_accuracymustbe

ILLUMINATION

mmmzzmm

N N+1 N+2 N+3 N+4 N+5 N+6 N+7 N+8 N+9

Relative Device Output

Figure3.1 ProjectionofSlitontoDetector

Array

illuminationof pixels_mayresult, as showninFigure3.1,wherethe_resultingpulsetrainis

disturbed. Correction forthese

imaging

errorsissomewhat moredifficultasinthecase

discussed above,inthat

imaging

theslit ontodifferentsectionsofthe_{device may}resultina

differentpositional errorateach point. Ifthe focal distance isvaried from_{point-to-point,}due

to flatness variationofthedevice or nonparallel track movement ofthe

device,

additional

errors will result and compoundthe inaccuracy. The diffusioncrosstalkbetweenthe

photodetectors themselves,

intrinsically

limits thepatterndefinitionas well. Onemethodof

circumventingthealignmentand magnification effectsistoincorporateaperturesof widthAx

over_{the pixels, shrinking}theireffective_sensingarea_{but providing} a toleranceinthe slit

projectionof

P-Ax,

whereP is thepixel pitch. Diffusioneffects, coupledwiththeinevitable

inaccuracies in_aligningtheapertures_{themselves, may}continuetolimitprecisioninthecaseof

apertures,whichagain,inthecaseofsmallcells, _may prove tobe variable.

Still,

for larger

cell

designs,

thismethod_mayprovetobeacceptable.

Optical injectionthenappears tobeanalternative approachto electricalinjection for

measurementofCTEperformance. It is

desirable, however,

to

develop

a methodtoovercome

thelimitationsimposed

by

projectionoptics,intermsoftherestrictionson pixel_{size, the}use of

sub-pitch apertures and thehardware

itself,

required toimagea pattern onthedevice. Sucha

method,_utilizing thecharge_couplingcapabilitiesinherentinCCDs ispresentedinthe

following

sections.

3.2 TestStructure

Theory

Utilizing

Exposure Control Gate

As discussed in Section

2.3,

thebasicarchitectureofa chargecoupledimagerconsistsof

adetectorregionandCCDshiftregisterseparated

by

atransfergateregion,whichisolatesor

allowsinteractionbetweenthe tworegions. Exposurecompensation, through theuse of an

additionalcontrolgatethatdivertssignal charges toadrain

during

a portion oftheintegration

period,was alsopresented. Thecapabilities ofthelatterfeaturecoupledwith a unique

arrangement of_similarly

functioning

structures with modifiedthresholdsprovidesfora novel

opticalinjection test_array(OTA). Thisarrangement canbeused toforma signal pulsetrainin

theimager detectorarray,andallowsforinjectionintoand subsequenttransfer_through,the

CCDshiftregisterinordertomonitorchargetransferefficiency. Implementationof such a

structureforalinear

imaging

_{array is}showninFigure3.2 below. Ontheend ofthe_array

furthermost fromtheoutput structure(tomaximizethenumberof_transfers)an_arrayof

additional cellsisincluded. This_arraywould containNcellswhich wouldprovideaninjected

pulsetrainofNpixelstobetransported through theshift register

during

thetestmode of

operation. Thesepixelsarecoupledtoan exposure controlgate,

EXP2,

ofsimilar natureto the

Lateral Drains EXP2

Exposure

Gates

EXP1

Photodiode

Array

<Dlo-

<D20-I

I

I

CCD Shift Register

VrYrWYrV

T

0

N Pixel Optical Injection into CCD

During

Test Mode

Figure 3.2Schematic ofOptical Test

Array

Ateach end oftheseNcellsisplaced one or more additional cells_resemblingtheimager itself.

Outsideof_{these cells,}a covered cellisplaced, whichserves todelineatethe teststructurefrom

therest oftheactive_arrayandalso,as we shalldiscuss_later, provides foroptional use ofthe

test_arrayfor diffusioncrosstalkmeasurements.

Fornormal_imaging,theEXP1gateis pulsed'on'forthe

leading

portionofthelineto

sweepcarriers_awaytothelateraldrainuntil thedesiredexposure periodbegins.

During

this

time, theEXP2gate canfunction ina'don'tcare'

state of either

being

held_'on', 'offorpulsed

withEXP1. Inthetest mode, theEXP2gate potentialisloweredto allowforcharge collection

intotheNtestcells. Meanwhile,asshowninFigure

3.3,

theexposurecontrolgate_EXP1,forthe

balanceofthearray,wouldberaised toa

'high'

or

'on'

levelwhichwould servetodrainall

photo-generated chargeto the lateral drains. At theend of givenline cycle, thetransfergate

is pulsed,resultingina parallel transferoftheNpulsesto the CCDshiftregister.

Ideally,

all

othercellsin theregister remainempty dueto theprevious actuation ofEXP1.

But,

fromthe

Exposure Control

Gates

MMBHMll

EXPlJr"

LOD EXP2

0 0

t=t1

EXP1

t=t1

EXP2

t=t2

EXP2

t=t3

EXP2

/

TG Ol

Photodiode

LI

CCD

Shift

Registei

XfflMWiWWflfflM

'Dl

t1 t2 t

Mil

4

EXP1