ieee nuclear science symposium nov. 8 2001, san diego, ca professor priscilla cushman university of...

IEEE Nuclear Science SymposiumNov. 8 2001, San Diego, CA

Professor Priscilla CushmanUniversity of Minnesota

Problems and Solutions Problems and Solutions in high-rate multi-channel Hybrid Photodiode design in high-rate multi-channel Hybrid Photodiode design

HPD’s for the CMS Hadronic CalorimeterHPD’s for the CMS Hadronic Calorimeter

Professor Priscilla CushmanProfessor Priscilla CushmanUniversity of MinnesotaUniversity of Minnesota

The US-CMS HCAL Collaboration

Fermilab Florida State Purdue Notre Dame University of Illinois (Chicago) University of Mississippi University of Maryland Rochester University of Minnesota



HCAL

The CMS Tile/Fiber Hadronic CalorimeterThe CMS Tile/Fiber Hadronic Calorimeter



Reading out the Towers of Tiles with WLS fiber and HPD’sReading out the Towers of Tiles with WLS fiber and HPD’s



Optical Decoder UnitOptical Decoder Unit

HPD mount aligned to cookie and plate

Fiber Optic Cables attach to a patch panel

Optical decoding from layer into tower bundles occurs at

the readout boxes.



Stringent Photodetector RequirementsStringent Photodetector Requirements

• Magnetic Field of 4 Tesla

• Linear Response from MIP to 3 TeV Shower

• DC Calibration to 2% using Radioactive Source

• Integrated Neutron Dose of up to 5 x 1010 n/cm2

• Integrated Output Charge up to 3 Coulombs

• Use same system in HO for ease of integration

Outer Barrel and Endcap have relaxed constraints

• tag muons (10 p.e./MIP) and measure shower tails

• many channels => low cost

• fringe field



The Hybrid PhotodiodeThe Hybrid Photodiode

Tube Fabrication by DDelft EElectronic PProducts (Netherlands) Subcontracts: Canberra (Belgium) diodes Schott Glass (USA) fiber optic windows Kyocera (Japan) vacuum feedthru/ceramic carrier

19 x 5.4mm 73 x 2.68mm

CMS diode design

• 12 kV across 3.3 mm gap with Vth < 3 kV => Gain of 2500

• Silicon PIN diode array, T-type. Operated at 80 V reverse bias

• Thin (200 m) with 100 V reverse bias for fast charge (holes) collection

• Aluminized surface and AR Coating



How They WorkHow They Work

PIN Diode arrayCeramic feedthrough

Fiber-OpticWindow

PhotocathodeElectrons from the photocathode are accelerated in a high electric field and stop in the diode where they generate electron hole pairs.Detect current as holes move across the depletion region in the back-illuminated version.

e

4

16 kV

Gain

0

0

4000

80

1000

2000

3000

4000

5000

6000

7000

0 50 100 150 200 250 300



APD vs HPD decision (1995-6 CERN test beams)System Integration Issues NIM A387 (1997) 107Signal/Noise at low light-levels

AC: 1 MIP=7-10 pe DC: Radioactive Source

Validation B-field Studies at 4-5 T NIM A418 (1998) 300 Radiation Damage at Oak Ridge NIM A411 (1998) 304

Extensive Bench Studies

Project Approval and CMS-specific design Negotiate specifications and priceAcquire prototypes and test NIM A442 (2000) 289

From Validation to Quality Assurance and YieldThis has been LONGER than we expected !

Iterate

Anatomy of a DecisionAnatomy of a Decision



Magnetic Field Issues

Performance measured at 4 TeslaDoesn’t breakImage shift = gap * tan

Small gain shifts (angular effects and backscattter)

Align tube axis parallel to fieldField locally uniform: ~ 5o (6o) inclination in HB (HE)Minimum gap (eases tolerance) vs HV (maintain gain)

Maintain sufficient space between fiber bundlesMechanically robust cookiesMaximize photocathode active area

Pixel position must be measured (and aligned) to 50 m



Larger active area: Less room for HV connection, possible field distortions

Minimize gap: Improve tube components

This has been a development projectThis has been a development project (the tube)

HV coax cable : Reliability and compatibility with RBX

RBX mountings and cookies must be rubber insulated

Gold-plated pins: Enables us to use ZIF sockets - questions of gold diffusion

FIBERS

RBX

HPD

RBXCookie

HV Cable

Electronics Interface Board



-50

-40

-30

-20

-10

0

0 0.1 0.2 0.3 0.4 0.5 0.6

HV Discharge to HPD MountingHV Discharge to HPD Mounting

Current across mount at 12 kVwith normal RBX configuration

nAm

ps

Lifetime Setup modified for HV Monitor

Trigger: 1 nA for spikes + 1.5 minute interval

Time (hrs)

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

After silicon rubber potting, at 15 kV

nAm

ps

Time (hrs)



Richardson-Dushman Equation Jth = AoT2exp(-eff/kT)eff = 1.09 for this fit.

Photocathode has a typical ePhotocathode has a typical e--dependence on Temperaturedependence on Temperature

Keep red sensitivity low. Fitted eff ranged from 1 - 1.33Dark counts at 25oC are 50 Hz – 1 kHz

Peltier CoolerControl

Thermocouple

Aluminum BlockPeltier Cooler

HPD

HPD DifferentialOutput

HPD Serial # R9851087 5-3-99

-100

400

900

1400

1900

2400

-10 0 10 20 30 40

Dar

k P

ulse

s pe

r S

econ

d

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-22.3E+20 2.4E+20 2.5E+20 2.6E+20 2.7E+20 2.8E+20

1/kT

ln(J

/T^2

)

Temperature (oC)



Custom Pixel Design: 19 ch (towers) and 73 ch (short stacks)

This has been a development projectThis has been a development project ( the diode)

2 side-contacts (100 nm thick Al)

Bump-bonded vacuum feedthru

n++ contact

n++ n++p+ p+ p+ p+ p+

n+ bulk (200 m thick)

AR (16 nm sputtered Si)Metal (25 nm AL)Barrier (25 nm SiO2)

Higher pin-out density: wire-bonds =>glass feedthrus => ceramic from Kyocera

Alignment to 50 m: manufacturer tolerances tightened, new measurement procedures

Improved rise time: Thinner silicon: 200 m replaces 300 m

Guard ring and drain structure: lower leakage current and better uniformity for edge pixels

Lower depletion voltage and better control of process: higher breakdown voltage

Surface aluminization and edge traces: Reduce negative crosstalk: 300 /sq => 1.7 /sq

Anti-reflective coating: Reduce positive crosstalk from reflected light



Capacitance as a function of reverse bias voltageCapacitance as a function of reverse bias voltage

33 pF

18 pF

Depletion at 43 V and 25 V

C73 = 5 pF/pixel C19 = 20 pF/pixel C(interface card and leads) = 13 pF



pe’s

Internal Electric Field in the bulk n-type siliconInternal Electric Field in the bulk n-type siliconOutput current and pulse width can be calculated with this simple modelOutput current and pulse width can be calculated with this simple model

2

2

/)()(2

dVVNqetI db

td

Vd

from t = 0 to t =

V=VbV=0

n++ p+ n+

E(0) =Vb-Vd

d

x

E

x=dx=0

E(d) = 2Vd + E(0) = Vb+Vd

d d

d

VV

d

xVxE dbd )(2)(

2

h hhh h



Drift time of holes translates into pulse width Drift time of holes translates into pulse width Simple form for over-depletion matches dataSimple form for over-depletion matches data

d

db

db VVV

VVdns

ln)( 2

bV V

dd

2

0lim

Pulse Width

72

1098.6 d

From fit: Consistency check: Hole mobility in silicon is = 450 cm2/Vs

cm d018 . 0 10 98 . 67



300 m thick 200 m thick

Pulse width can be shortened by reducing wafer thickness d Pulse width can be shortened by reducing wafer thickness d or by increasing bias voltage, Vor by increasing bias voltage, Vbb

Drift time is approximately given by

and the shape of the plateau mirrors the internal electric field

bV V

dd

2

0lim



200 100 66 50 40 33 28.5 25 Volts Bias Voltage

d

db

db uVVV

VVdns

ln)( 2

For higher depletion (lower ohmic silicon) VFor higher depletion (lower ohmic silicon) Vbb-V-Vdd ~ V ~ Vb b is not trueis not true

Fit data to modelearly (5 k-cm silicon) diodes 12 k-cm diodes

Operating bias voltage = 80 V

We now SPECIFY > 8 k-cm P

uls

e W

idth

50 ns

40 ns

30 ns

20 ns

10 ns

0 1/Vb



Total Leakage current for all 73 pixels HPD R0003241 (new-style diode)

0.00E+00

5.00E-09

1.00E-08

1.50E-08

2.00E-08

2.50E-08

0 50 100 150 200 250 300 350 400 450 500

Bias (Volts)

Cu

rre

nt

(am

ps

)

No evidence of breakdown, even at 500 volts!



Pulse Width in nsec

Flatter plateau and higher breakdown for new diodesFlatter plateau and higher breakdown for new diodes

R0003241 (200 micron, 73-channel new-style diode)

Reminder: on the same scale



Crosstalk in non-Aluminized 73-ch tube (B=1.5)

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

200 250 300 350 400

time[ns]

I/50o

hms[

mv]

pixel 36pixel 35pixel 34pixel 33pixel 37 /100

Crosstalk in Aluminized 73-ch tube (B=1.5)

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

200 250 300 350 400time[ns]

I/50o

hms[

mv]

pixel 36

pixel 35pixel 34

pixel 33pixel 37 /100

AC Crosstalk eliminated by AluminizationAC Crosstalk eliminated by Aluminization

33 34 35 36 37

Pixels in center row

Positive crossstalk now observed !



0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7 8

Distance d from edge of pixel (mm)

% C

ros

sta

lkBackscatter crosstalk for a 19-channel Aluminized HPDBackscatter crosstalk for a 19-channel Aluminized HPD

Move fiberRead outnearest neighbor

Convolution of hexagonal pixel shape with backscatter radial distribution

d



0o 45o 90o

Initial scattering angle of backscattered electron

Rad

ial D

ista

nce

fro

m im

pac

t p

oin

t (m

m)

7

6

5

4

3

2

1

Ballistic model of Backscattering. 10 keV electrons Ballistic model of Backscattering. 10 keV electrons



Trajectories of Backscattered ElectronsTrajectories of Backscattered Electrons

B = 0 T = 45o

B = 0.15 T = 45o

B = 4 T = 75o

Radial distance

Radial distance is a minimum here



Number of backscattered e’s Number of backscattered e’s as a function of radial distance from impact pointas a function of radial distance from impact point

0 1 2 3 4 5 6 7 mm 0 0.5 1 1.5 2 2.5 3 3.5 4 mm

0 0.5 1 1.5 2 2.5 3 mm 0 .025 .05 .075 .1 .125 .15 mm

B = 0 T B = 0.15 T

B = 4 TB = 0.2 T



0.0%0.2%0.4%0.6%0.8%1.0%1.2%1.4%1.6%1.8%2.0%

0 10 20 30 40 50 60 70 80Pixel Number

% C

ross

talk

B=0 Aluminized B=1.5T Aluminized B=1.5T Al + AR

Positive crosstalk from backscatter can be removed by B-Positive crosstalk from backscatter can be removed by B-fieldfield

73-ch tube Total DC Crosstalk(2.7 mm pixels) B=0 B=1.5 T

Bare silicon 18% 7%Aluminized 29% 16%Al with AR 12.4% 4.3%

STILL some positive crosstalkand Al is worse than silicon

33-41

1-2

72-73

Pixel number



photoelectrons

Light

Re-emitted photoelectrons

APD views reflected light

pe backscatter focussed by B

Light injected thru fiber

Test Confirms Reflected LightTest Confirms Reflected Light

DIODE ARRAY

FIBER OPTIC & PHOTOCATHODE



Compare residual HPD crosstalk in B-FieldCompare residual HPD crosstalk in B-Fieldwith APD measurement of optical reflection with APD measurement of optical reflection



Study Problem at Minnesota, then export technology to DEPStudy Problem at Minnesota, then export technology to DEP

IMD - optical modeling package for multilayer structures

(by David L. Windt, http://cletus.phys.columbia.edu/windt/idl)

Ag oxidizes quickly Au not available

Model

Data monochrometer

PIN diode

Samples: glass slides with various coatings (PECVD)

10 nm Ag120 nm SiO2

8 nm A-Si15 nm Ag50 nm SiO2

16 nm A-Si25 nm Al25 nm SiO2

Some Options

DEP makes samples on old

diodes using sputtering



Deposition rate a-Si:H at 3 Watts, T=300 C, P=1.1 Torr

y = 25.631x

R2 = 0.9999

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70Deposition time (minutes)

La

ye

r th

ick

ne

ss

(A

ng

str

om

s)

5.4 min10.8 nm

20 min52.1 nm

30 min76.7 nm

60 min153.5 nm

Comparison of Model vs Data calibrates PE-CVD ProcessComparison of Model vs Data calibrates PE-CVD Processa-Si:H (SiH4 gas) deposited on glass slides



Measured Reflectance of 14nm A-SiH on top of thin Al layer

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

400 450 500 550 600 650 700

Wavelength (nm)

Re

fle

cta

nc

e

a-Si:H before bakeout

a-Si:H after bake-out

Bare Aluminum (25 nm)

Glass slides + 25 nm Al + 14 nm a-Si:HGlass slides + 25 nm Al + 14 nm a-Si:H



Minnesota test slides

14 nm a-SiH 25 nm Al 25 nm SiO2

First DEP attempt at 16 nm a-Si

MC for 10 nm

We tell them to add another 6 nm

MC for 16 nm

Reflectance Studies

Angular Dependenceof test slides



Specifications are finalized in ContractQuality Assurance protocol detailed

4.1 Photocathode Fiber Optic Window with green-sensitive Photocathode

Minimum Typical Maximum UnitQuantum Efficiency at 520nm 11 %Dark counts 50 kHz/cm2

Non-Uniformity in PC response 8 %Operating voltage 12 13 -kV

4.2 Diode Silicon PIN diode array, T-type

Minimum Typical Maximum Unit Threshold for 10kev electon 500 2500 3300 V

Thickness non-uniformity ±1 µmSilicon resistivity 5 kΩ cm

Response non-uniformity 10 %Resistance between pixels 100 MΩDepletion depth 185 200 215 µm

Reversed diode characteristics: Operating voltage 80 80 V

Breakdown voltage 100 Full Depletion voltage 20 30 V Guard Ring Reverse current (80 V bias) 500 nA

Reverse current per pixel (80V bias) 1 10* nA * For each HPD one pixel may have a reverse current of 50 nA.

19-channel Pattern

Total active area 487 mm2

Active area per channel 25.6 mm2

Flat to flat hex 5.4 mmGap between pixel contacts 40 µm

Capacitance 14 pF



Specifications are finalized in Contract...continued

4.3 Overall Tube Performance:Minimum Typical Maximum Unit

ELECTRON GAIN Gain 2300 e/pe

Gain non-linearity 5 % (signal range 1 - 70000 photo-electrons)

CROSSTALKTotal optical crosstalk to sum of all pixels 3 %Total capacitive crosstalk to sum of all pixels 3 %

TIMINGFull Width Half Maximum at 80 V 20 nsBaseline Maximum at 80 V 30 ns

MAGNETIC FIELDResistant to axial B-fields to 4 Tesla

Gap between PC and diode 3.55 mmCarrier size tolerance with in one batch ±40 µm

PIXEL FAILURE No disconnected pixels allowed

OPERATING CONDITIONS Operating temperature -20 20 +40 °C

Radiation Environment 108 109 1010 n/cm2/yr



Quality AssuranceQuality Assurance

fail pass

Return to Return to DEP DEP

Bake-out at 13 kV for 2 weeks

Evaluate 500 tubes, automated procedure, complete web-accessible databaseEvaluate 500 tubes, automated procedure, complete web-accessible database

Leakage current for each pixel and guard ring @ 80V

HV gain, reverse bias curve

DC StationAlignment measurements for 50 micron tolerance Machine

custom ring

crosstalk checks alignment

2-D response scans (10kV, 80 V)

pe spectra, AC xtalk capacitance vs bias

To FNAL for installation in readout boxes

AC Station Lifetime: Q, Cf252, HV

High Rate and B-Field testssubset



Precision RegistrationPrecision Registration

• Test fixture = Standard Mount + metal plate with 3 alignment holes

• Scan to find centroid of alignment holes

• Same scan finds pixel intersections above and below metal piece by iterative sector equalization

• Machine shop uses measured x, y, to produce Custom Ring

Stabilized light source

HP

D

Mou

nt

Scanning Table

Integratingsphere Focussing

optics

Green filter

Metal alignment

plate



FIBERS

Plate

HPD

Alignment Pins

Plate Cookie

Ring HV Cable

Electronics Interface Board

Optical Decoding Unit

• Each ring is registered to its HPD via alignment pins

• Plate and Cookies are universal

Precision Registration (in assembly)Precision Registration (in assembly)



Universal Cookie + PlateUniversal Cookie + Plate

73-channel (HB) 19-channel (HB right)



HPD alignment after correction for 8 tubesHPD alignment after correction for 8 tubes



Samples from DC database Samples from DC database

Pixel Number Dark Current

1 8.6827e-0092 9.2811e-0093 2.4709e-0084 8.6967e-0095 8.995e-0096 2.2894e-0087 8.2391e-0098 2.1001e-0089 9.1783e-00910 2.2305e-008

11 1.36793e-00812 8.7693e-00913 2.0977e-00814 3.0818e-00815 1.78352e-00816 7.1035e-00917 7.2061e-00918 8.2694e-00919 7.2356e-009

Total Current 2.6605e-007



AC Station: Viking chip serial readout at 10 MHz

HPD +interface card

AC-Coupling2 chip

128 channel PARepeater card

Laptop with ADC card

128 Mu

ltiplexer

ShaperSample& hold

10 MHzreadout



Individual spectra for each pixel from AC StationIndividual spectra for each pixel from AC Station19-channel tube at low light levels19-channel tube at low light levels



Lifetime Issues

PIN reference diodes

1.0 mm diam. WLS fibers

Blue LED’sPixel 1

HPD

Pixel 2

Lifetime Monitoring Stations monitor current (PIN diodes, HPD) and temperature

Radiation Damage: (10 CMS years = 5 x 1010 n/cm2 in worst region)

Expose samples to Cf252 Oak Ridge: Early HPD version to 1013 n/cm2 in 1997 tests.Minnesota: Low flux drawer instrumented

Aluminized new HPD to >1011 n/cm2 in 2001

Integrated Charge: (10 CMS years = 3 C over 25.6 mm2 pixel at high

Expose to accelerated rate plus control pixel at CMS rate.

Surface scans done before and after exposure distinguish between photocathode degradation and silicon damage.



Red light with HV=0 Scans only silicon

Green light with HV = 8 kV Scans total tube response

Concentrated light in a small fiber will damage photocathodeConcentrated light in a small fiber will damage photocathodeThis is far beyond the CMS rate which allows for photocathode self-annealingThis is far beyond the CMS rate which allows for photocathode self-annealing



6 CMS months for high towerat expected CMS rate

6 CMS years for high towerat 13 x expected rate

Accelerated aging testsAccelerated aging tests: Red (upper curves) = 73-ch aluminized HPD Blue (lower curves) = old tube with poor potting

Lifetime setup traps sparking by triggering on current spikes from High Voltage Supply

All curves normalized to reference diode and corrected for temperature shifts



Irradiate 73-channel aluminized HPD - leakage current increasesIrradiate 73-channel aluminized HPD - leakage current increases

Leakage current rising at 46 pA/hr

PIN diode => Injected light is constant

HPD (light-dark)

hours

Flux = 7 x 109 n/(cm2 hr)MeV neutrons from Cf252

Day 1 Day 2 Day 3 Day 4

Integrated dose is geometrically equivalent to 4.5 x 1011 n/cm2 head on

10 CMS years



ConclusionsConclusions

5 years ago, no existing technology could satisfy our specifications.

Development project was initiated with one Company - DEPwith backup plans which included Hamamatsu and Litton

Rigorous evaluation must include accelerated aging and test beams and enough prototype detectors to understand the yield.

The anticipated problems are not the ones that really bite you.

This takes time! In the last year, the HPD subsystem has approached “critical path” in the CMS Project.

Final Result: CMS HCAL gets what it needs (so we can find the Higgs) and a better product is offered to the general public.

ieee nuclear science symposium nov. 8 2001, san diego, ca professor priscilla cushman university of...

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