rainer wallny silicon detector workshop at ucsb may 11th, 2006

05/11/2006 CDF Silicon Workshop at UCSB 1

Rainer Wallny

Silicon Detector Workshop at UCSBMay 11th, 2006

Slides ruthlessly stolen from: Paula Collins, CERN Alan Honma, CERN Christian Joram, CERN Michael Moll, CERN Steve Worm, RAL

Silicon Detectors – How They Work


o Why Silicon ?o Semiconductor Basics

– Band-gap, PN junction– Silicon strip detectors

o Some Technicalities - Wafer Production

- Wire Bondingo Radiation Damage

– Effect on Vd

– Effect on Leakage Currentso Conclusions

Outline


Tracking Chambers with Solid Media

o Ionization chamber medium could be gas, liquid, or solid – Some technologies (ie. bubble chambers) not applicable in collider

environments

“Solid-state detectors require high-technology devices built by specialists and appear as black boxes with unchangeable characteristics.”

-Tom Ferbel, 1987

LowModerateModerateIonization Energy

FastModerateModerateSignal Speed

ModerateModerateLowAtomic number

HighModerateLowDensity

SolidLiquidGas

o High-precision tracking advantages with solid media– Easily ionized, relatively large amount of charge– Locally high density means less charge spreading– Fast readout possible


o Electrical properties are good– Forms a native oxide with excellent electrical properties– Ionization energy is small enough for easy ionization, yet large enough to maintain

a low dark currento Mechanical properties are good

– Easily patterned and read out at small dimensions– Can be operated in air and at room temperature – Can assemble into complex geometries

o Availability and experience– Significant industrial experience and commercial applications– Readily available at your nearest beach

Why Silicon?


The Idea is Not Quite New …

Semiconductors used since 50’s for energy measurement in nuclear physics

o Precision position measurements up until 70’s

done with emulsions or bubble chambers

-> limited rates, no triggering

o Traditional gas detectors: limited to 50-100 μm

o First silicon usage for precision position

measurement: NA11 at CERN, 1981


fan out to readout electronics

E706 (FNAL 1987)NA11 (CERN 1981)

sensor

o 24x36 mm2 active areao 8 layers of silicono 1m2 readout electronics!

o 50x50 mm2 active areaSilicon sensor and readout electronics technology closely coupled with electronics miniaturization (transistors, ICs, ASICs …) silicon quickly took off …

Pioneering Silicon Strip Detectors


CDF SVX IIa half-ladder: two silicon sensors with readout electronics (SVX3b analog readout chip) mounted on first sensor

ATLAS SCT barrel module: four silicon sensors with center-tapped readout electronics (ABCD binary readout chip)

Silicon sensor and readout chip development intimately relatedBUT will concentrate on silicon only here …

Contemporary Silicon Modules


CDF SVX IIa (2001-)

~ 11m2 silicon area

~ 750 000 readout channels

DELPHI (1996)

~ 1.8m2 silicon area

175 000 readout channels

Large ‘Contemporary’ Silicon Systems

CMS Silicon Tracker (~2007)

~12,000 modules

~ 223 m2 silicon area

~25,000 silicon wafers

~ 10M readout channels


evolution : DELPHI (888 detectors, 8 geometries) CDF (8000 sensors, 8 geometries) CMS (25000 sensors, 15 geometries)

Each sensor treated individually, nurtured into life in many hours of careful handling

Systematic assembly line production with decent QA systems

New Production Paradigm

P. Collins,Warwick 2006


Moore’s Law: Exponential growth of sensitive area and number of electronic channels with time(from Computer Science: doubling of IC integration capacity every 18 months)

‘Moore’s Law’ for Silicon Detector Systems


LEP Tevatron LHC

Whoops… P.Collins, ICHEP 2002

Large Silicon Detector Systems ….


The Basics ……


o If the gap is large, the solid is an insulator. If there is no gap, it is a conductor. A semiconductor results when the gap is small.

o For silicon, the band gap is 1.1 eV, but it takes 3.6 eV to ionize an atom. The rest of the energy goes to phonon exitations (heat).

o In a gas, electron energy levels are discrete. In a solid, energy levels split and form a nearly-continuous band.

Semiconductor Basics – Band Gap


Semiconductor Basics – Principle of Operation- Basic motivation: charged particle

position measurement- Use ionization signal (dE/dx) left

behind by charged particle passageEf

E

valence band

conductance band

h

e

++

++

__

__

- In a semiconductor, ionization produces electron hole pairs- Electric fields drift electrons and holes to oppositely electrodesBUT:- In pure intrinsic (undoped) silicon, many more free charge

carriers than those produced by a charged particle. Have 4.5x108 free charge carriers; only 3.2x104 produced by MIP- Electron –hole pairs quickly re-combine …

Need to deplete free charge carriers and separate e-holes ‘quickly’!

300 m

1 cm

1 cm


Ef

E

VB

CBe

n-type: • In an n-type semiconductor, negative charge carriers (electrons) are obtained by adding impurities of donor ions (eg. Phosphorus (type V))• Donors introduce energy levels close to conduction band thus almost fully ionized => Fermi Level near CB

Electrons are the majority carriers.

Ef

E

VB

CB

h

p-type: • In a p-type semiconductor, positive charge carriers (holes) are obtained by adding impurities of acceptor ions (eg. Boron (type III))

• Acceptors introduce energy levels close to valence band thus ‘absorb’ electrons fromVB, creating holes => Fermi Level near VB.

Holes are the majority carriers.

Doping Silicon


The pn-JunctionExploit the properties of a p-n junction (diode) to collect ionization charges

+ –

+–

+

+

+

+

+

+

+

+

+

+ –

– –+

+–

+

+

–

–

–p nWhen brought together to form a junction, a gradient of electron and hole densities results in a diffuse migration of majority carriers across the junction. Migration leaves a region of net charge of opposite sign on each side, called the depletion region (depleted of charge carriers).

Electric field set up prevents further migration of carriers resulting in potential difference Vbi

Another way to look at it:

Fermi-Levels need to be adjustedso thus energy bands get distorted => potential Vbi

Ef

E

VB

CBe

Ef

E

VB

CB

hEf

E

VB

CB

p n

e.V

Funky cartoon from Brazil: http://www.agostinhorosa.com.br/artigos/transistor-6.html


+ –

+–

+

+

+

+

+

+

+

+ –

– –++

–

+

+

–

–

–

p n

Dopantconcentration

Space chargedensity

Carrierdensity

Electricfield

Electricpotential

o p-type and n-type doped silicon forms a region that is depleted of free charge carriers

o The depleted region contains a non-zero fixed charge and an electric field. In the depletionzone, electron – hole pairs won’t recombine but rather drift along field lines

o Artificially increasing this depleted region by applying a reversed bias voltage allow charge collection from a larger volume

pn - Junction


pp p

n

If we make the p-n junction at the surface of a silicon wafer with the bulk being n-type (you could also do it the opposite way), we then need to extend the depletion region throughout the n bulk to get maximum charge collection by applying a reverse bias voltage.

+

–

h+ e-

How to Build a Silicon Detector


Properties of the Depletion Zone

– Depletion width is a function of the bulk resistivity , charge carrier mobility and the magnitude of reverse bias voltage Vb:

+

–Depletion zone

undepleted zone

– The bias voltage needed to completely deplete a device of thickness d is called the depletion voltage, Vd

Vb

wd

– Need a higher voltage to fully deplete a low resistivity material.– A higher voltage is needed for a p-type bulk since the carrier

mobility of holes is lower than for electrons (450 vs 1350 cm2/ V·s)

Vd = d2 /(2)

w = 2Vb

where = 1/ q N for doped material where N is the doping concentration and q is the charge of the electron and is the carrier mobility (v= E)


C = A / 2Vb

Properties of the Depletion Zone (cont’d)

– One normally measures the depletion behavior (finds the depletion voltage) by measuring the capacitance versus reverse bias voltage. The capacitance is simply the parallel plate capacity of the depletion zone.

capacitance vs voltage1/C2 vs voltage

Vd


Leakage Current- Two main sources of (unwanted) current

flow in reversed-biased diode:– Diffusion current, charge generated in

undepleted zone adjacent to depletion zone diffuses into depletion zone (otherwise would quickly recombine)

– Generation current Jg, charge generated in depletion zone by defects/contaminants

negligible in a fully depleted device

Jg exp(-b/kT)

Exponential dependence on temperature due to thermal dependence of e-h pair creation by defects in bulk. Rate is determined by nature and concentration of defects.


Bias Resistor and AC Coupling

– Need to isolate strips from each other and collect/measure charge on each strip => high impedance bias connection (resistor or equivalent)

– Usually want to AC (capacitatively) couple input amplifier to avoid large DC input from leakage current.

– Both of these structures are often integrated directly on the silicon sensor. Bias resistors via deposition of doped polycrystalline silicon, and capacitors via metal readout lines over the implants but separated by an insulating dielectric layer (SiO2 , Si3N4).

+

–h+ e-


• Collected charge usually given for Minimum Ionizing Particle (MIP)

dE/dx)Si = 3.88 MeV/cm, for 300 m thick = 116 keVThis is mean loss, for silicon detectors use most probable loss (0.7 mean) = 81 keV

3.6eV needed to make e-h pairCollected charge 22500 e (=3.6 fC)

Mean charge

Most probable charge ≈ 0.7 x mean

The Charge Signal


Landau distribution has significant low energy tail which becomes even lower with noise broadening.

Noise sources:o Capacitance ENC ~ Cd

o Leakage Current ENC ~ √ Io Thermal Noise ENC ~ √( kT/R)

Landau distribution

with noise

noise distribution

One usually has low occupancy in silicon sensors most channels have no signal. Don’t want noise to produce fake hits so need to cut high above noise tail to define good hits. But if too high you lose efficiency for real signals.

Figure of Merit: Signal-to-Noise Ratio S/N.

Typical Values ~ 10-15, people get nervous below 10. Radiation Damage can degrade the S/N. Thus S/N determines detector lifetime in radiation environment.

But There Is Noizzzzzz …..


– Diffusion of charge “cloud” caused by scattering of drifting charge carriers, radius of distribution after time td:

Charge Collection and Diffusion

– Drift velocity of charge carriers v = E, so drift time,td = d/v = d/E

– Typical values: d=300 m, E= 2.5kV/cm, e= 1350;h= 450 cm2 / V·s, gives: td(e)= 9ns , td(h)= 27ns

= 2D td , where D is the diffusion constant, D = kT/q

– Typical charge radius: ≈ 6 m

– Charge Radius determines ‘Charge Sharing’, i.e. deposition of charge on several strips.


pp p

n BUT: Unlike the face with the p-strips, nothing prevents

horizontal charge spread on back face. n-strips alone are not sufficient to isolate the charge because of an electron accumulation layer produced by the positively charged SiO2 layer on the surface.

Why not get a 2nd coordinate by measuring position of the (electron) charge collected on the opposite face? p+p+ p+

n n n

n-bulk

n n n

n-bulk

p+ p+

n n n

n-bulk

+ + +

SOLUTION: • Put p-strips in between the n-strips.OR• Put “field plates” (metal over oxide) over the n-strips and apply a potential to repel the electrons.

Double Sided Detectors


– Voltage drop between biasing ring and edge, top edge at backplane voltage.

– Typically n-type implants put around edge of the device and a proper distance maintained between p bias ring and edge ring.

– Usually one or more “guard” rings (left floating) to assure continuous potential drop over this region.

– Defects or oxide charge build-up in this region could lead to additional leakage current contributions

– If one increases the bias voltage, eventually the field is high enough to initiate avalanche multiplication. This usually occurs around 30V/m (compared to a typical operating field of <1V/m). Local defects and inhomogeneities could result in fields approaching the breakdown point.

– .

We have treated the silicon strip device as having infinite area, but it has edges. What happens at the edges? Single guard ring structure

Guard Rings and Avalanche Breakdown


Some Technicalities ……


Using a single Si crystal seed, meltthe vertically oriented rod onto the seed using RF power and “pull” the single crystal ingot

Wafer production Slicing, lapping, etching, polishing

Mono-crystalline Ingot

Single crystal silicon

Poly silicon rod

RF Heating coil

Float Zone process

Oxygen enrichment (DOFZ) Oxidation of wafer at high temperatures

Material: Float Zone Silicon (FZ)


Czochralski silicon (Cz) & Epitaxial silicon (EPI)

o Pull Si-crystal from a Si-melt contained in a silica crucible while rotating.

o Silica crucible is dissolving oxygen into the melt high concentration of O in CZ

o Material used by IC industry (cheap) o Recent developments (~2 years) made CZ

available in sufficiently high purity (resistivity) to allow for use as particle detector.

Czochralski Growth

Czochralski silicon

Epitaxial silicono Chemical-Vapor Deposition (CVD) of Silicono CZ silicon substrate used in-diffusion of oxygeno growth rate about 1m/mino excellent homogeneity of resistivityo up to 150 m thick layers producedo price depending on thickness of epi-layer but not

extending ~ 3 x price of FZ wafer


n-Si1) Start with n-doped silicon wafer, ≈ 1-10 kcm

2)

SiO2

Oxidation at 800 – 1200 0C

3) Photolithography (= mask align + photo-resist layer + developing) followed by etching to make windows in oxide

UV light

maskPhoto-resist

etch

Wafer Processing (1)


4)

5)

B

Doping by ion implantation (or by diffusion)

As

Annealing (healing of crystal lattice) at 600 0Cp+

n+

p+

6) Photolithography followed by Al metallizationover implanted strips and over backplane usually by evaporation.

Al

Simple DC-coupled silicon strip detector

Wafer Processing (2)


Bringing It All Together• Connectivity technology: some of the possibilities

– High density interconnects (HDI):industry standard and custom cables, usually flexible kapton/copper with miniature connectors.

– Soldering still standard for surface mount components, packaged chips and some cables. Conductive adhesives are often a viable low temperature alternative, especially for delicate substrates.

– Wire bonding: the standard method for connecting sensors to each other and to the front-end chips. Usually employed for all connections of the front-end chips and bare die ASICs. A “mature” technology (has been around for about 40 years).

4 x 640 wire bonds~200 wire bonds Total ~2700 wire bondsOPAL (LEP) module


Wire Bonding• Uses ultrasonic power to vibrate needle-

like tool on top of wire. Friction welds wire to metallized substrate underneath.

• Can easily handle 80m pitch in a single row and 40m in two staggered rows (typical FE chip input pitch is 44m).

• Generally use 25m diameter aluminium wire and bond to aluminium pads (chips) or gold pads (hybrid substrates).

• Heavily used in industry (PC processors) but not with such thin wire or small pitch.

Microscope view of wire bonds connecting sensor to fan-out circuit

Electron micrograph of bond “foot”


Radiation Damage …


Radiation Damage in Silicon SensorsTwo general types of radiation damage to the detector materials:

Bulk (Crystal) damage due to Non Ionizing Energy Loss (NIEL) - displacement damage, built up of crystal defects –

I. Change of effective doping concentration (higher depletion voltage, under- depletion)

II. Increase of leakage current (increase of shot noise, thermal runaway)

III. Increase of charge carrier trapping (loss of charge)

Surface damage due to Ionizing Energy Loss (IEL) - accumulation of positive in the oxide (SiO2) and the Si/SiO2 interface – affects: interstrip capacitance (noise factor), breakdown behavior, …

Impact on detector performance and Charge Collection Efficiency (depending on detector type and geometry and readout electronics!)

Sensors can fail from radiation damage by virtue of…– Noise too high to effectively operate – Depletion voltage too high to deplete– Loss of inter-strip isolation (charge spreading)

Signal/Noise Ratio is the quantity to watch !


Run I Experience: SVX’ Signal-to-Noise

Radiation Damage limits the ultimate lifetime of the Detector-Need S/N >8 to perform online b-tagging with SVT-Need S/N >5 for offline b-tagging


Surface Damageo Surface damage generation over time:

– Ionizing radiation creates electron/hole pairs in the SiO2

– Many recombine, electrons migrate quickly away– Holes slowly migrate to Si/SiO2 interface.

Hole mobility is much lower than for electrons (20 cm2/Vs vs. 2x105 cm2/Vs)

– Some holes ‘stick’ in the boundary layero Surface damage results in

– Increased interface trapped charge (see picture)– Increase in fixed oxide charges– Surface generation centers

Metal (Al)Oxide (SiO2)

Semiconductor (Si)

-- --

--

- --

++

+++

+

+

++

+

--- -

--++ +

++

++ -

+

+

++ +

+

+

+ +

+ + + +

+

After electrontransport:

After transportof the holes:

– Electron accumulation under the oxide interface can alter the depletion voltage (depends on oxide quality and sensor geometry)

– In silicon strip sensors, surface damage effects (oxide charge) saturate at a few hundred kRad

Interface (SiOx)


Bulk Damage

O

P

Vacancy

Disordered region

Di-vacancy

Interstitial

C

Vacancy/OxygenCenter

CarbonInterstitial

CC

Phosphorous dopant

Carbon-CarbonPair

C O

Carbon-Oxygen pair

o Bulk damage is mainly from hadrons displacing primary lattice atoms (for E > 25 eV)– Results in silicon interstitial, vacancy,

and typically a large disordered region– 1 MeV neutron transfers 60-70 keV to

recoiling silicon atom, which in turn displaces ~1000 additional atoms

o Defects can recombine or migrate through the lattice to form more complex and stable defects– Annealing can be beneficial, but… – Defects can be stable or unstable

o Displacement damage is directly related to the non-ionizing energy loss (NIEL) of the interaction– Varies by incident particle type and

energy– Normalize fluence to 1 MeV n-equivalent


Microscopic defects

I

I

V

V

Distribution of vacancies created by a 50 keV Si-ion

in silicon (typical recoil energy for 1 MeV neutrons):

Schematic[Van Lint 1980]

Simulation[M.Huhtinen 2001]

Defects can be electrically active (levels in the band gap) - capture and release electrons and holes from conduction and valence band

can be charged - can be generation/recombination centers - can be trapping centers

Vacancy + Interstitial

“point defects”, mobile in silicon,can react with impurities (O,C,..)

V

I

point defects and clusters of defects

o particle SiS EK>25 eV

EK > 5 keV

Damage to the silicon crystal: Displacement of lattice atoms

80 nm


Shockley-Read-Hall statistics Shockley-Read-Hall statistics (standard theory) (standard theory)

Impact on detector properties can be calculated if all defect parameters are known:Impact on detector properties can be calculated if all defect parameters are known:n,pn,p : cross sections : cross sections E : ionization energy NE : ionization energy Ntt : concentration : concentration

Trapping (e and h) CCE

shallow defects do not contribute at room

temperature due to fast detrapping

charged defects

Neff , Vdepe.g. donors in upper

and acceptors in lower half of band

gap

generation leakage current

Levels close to midgap

most effective

enhanced generation leakage current space charge

Inter-center chargeInter-center charge transfer model transfer model

(inside clusters only)(inside clusters only)

Impact of Defects on Detector properties


Change of Depletion Voltage Vdep (Neff) …. with particle fluence:

before inversion

after inversion

n+ p+ n+

• “Type inversion”: Neff changes from positive to negative (Space Charge Sign Inversion)

10-1 100 101 102 103

eq [ 1012 cm-2 ]

1

510

50100

5001000

5000

Ude

p [V

] (d

= 3

00m

)

10-1

100

101

102

103

| Nef

f | [

1011

cm

-3 ]

600 V

1014cm-2

type inversion

n-type "p-type"

[M.Moll: Data: R. Wunstorf, PhD thesis 1992, Uni Hamburg]

• Short term: “Beneficial annealing” • Long term: “Reverse annealing” - time constant depends on temperature: ~ 500 years (-10°C) ~ 500 days ( 20°C) ~ 21 hours ( 60°C) - Consequence: Detectors must be cooled even when the experiment is not running!

…. with time (annealing):

NC

NC0

gC eq

NYNA

1 10 100 1000 10000annealing time at 60oC [min]

0

2

4

6

8

10

N

eff [

1011

cm-3

]

[M.Moll, PhD thesis 1999, Uni Hamburg]

p+

Radiation Damage: Effect on Neff


Depletion Voltage: Death of SVX Layer 0

Integrated Luminosity (fb–1)

Dep

letio

n V

olta

ge (V

)Central Prediction+1σ Prediction–1σ Prediction

100

200

300

00 2 4 6 8

Data & Extrapolation

SVXII L0 lifetime prediction based on Hamburg Model (M.Moll)

- Will SVXII L0 survive Run II ? -> Antonio’s Talk

Steve Worm, Vertex 2003


1011 1012 1013 1014 1015

eq [cm-2]10-6

10-5

10-4

10-3

10-2

10-1

I /

V

[A/c

m3 ]

n-type FZ - 7 to 25 Kcmn-type FZ - 7 Kcmn-type FZ - 4 Kcmn-type FZ - 3 Kcm

n-type FZ - 780 cmn-type FZ - 410 cmn-type FZ - 130 cmn-type FZ - 110 cmn-type CZ - 140 cm

p-type EPI - 2 and 4 Kcm

p-type EPI - 380 cm

[M.Moll PhD Thesis][M.Moll PhD Thesis]

o Damage parameter (slope in figure)

Leakage current per unit volume and particle fluence

o is constant over several orders of fluenceand independent of impurity concentration in Si can be used for fluence measurement

Radiation Damage – Leakage Current

eqVI

α

80 min 60C

Change of Leakage Current (after hadron irradiation)

…. with particle fluence:

o Leakage current decreasing in time (depending on temperature)

o Strong temperature dependence

Consequence: Cool detectors during operation! Example: I(-10°C) ~1/16 I(20°C)

1 10 100 1000 10000annealing time at 60oC [minutes]

0

1

2

3

4

5

6

(t)

[10-1

7 A/c

m]

1

2

3

4

5

6

oxygen enriched silicon [O] = 2.1017 cm-3

parameterisation for standard silicon [M.Moll PhD Thesis]

80 min 60C

…. with time (annealing):

Tk

EIB

g2exp


Recent Bias Current (Re-) Analysis

P.Dong et. al, upcoming CDF note

L0


Deterioration of Charge Collection Efficiency (CCE) by trapping

tQtQ

heeffhehe

,,0,

1exp)(

0 2.1014 4.1014 6.1014 8.1014 1015

particle fluence - eq [cm-2]

0

0.1

0.2

0.3

0.4

0.5

Inve

rse

trapp

ing

time

1/

[ns-1

]

data for electronsdata for electronsdata for holesdata for holes

24 GeV/c proton irradiation24 GeV/c proton irradiation

[M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund][M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund]

Increase of inverse trapping time (1/) with fluence

Trapping is characterized by an effective trapping time eff for electrons and holes:

….. and change with time (annealing):

5 101 5 102 5 103

annealing time at 60oC [min]

0.1

0.15

0.2

0.25

Inve

rse

trapp

ing

time

1/

[ns-1

]

data for holesdata for holesdata for electronsdata for electrons

24 GeV/c proton irradiation24 GeV/c proton irradiationeq = 4.5.1014 cm-2 eq = 4.5.1014 cm-2

[M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund][M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund]

where defectsheeff

N,

1

Radiation Damage – Trapping


o Two basic mechanisms reduce collectable charge:– trapping of electrons and holes (depending on drift and shaping time !)– under-depletion (depending on detector design and geometry !)

o Example: ATLAS microstrip detectors + fast electronics (25ns)

o n-in-n versus p-in-n - same material, ~ same fluence- over-depletion needed

0 100 200 300 400 500 600bias [volts]

0.20

0.40

0.60

0.80

1.00

CC

E (a

rb. u

nits

)n-in-n (7.1014 23 GeV p/cm2)

n-in-n

p-in-n (6.1014 23 GeV p/cm2)

p-in-n

Laser (1064nm) measurements

[M.Moll: Data: P.Allport et al. NIMA 513 (2003) 84]

o p-in-n : oxygenated versus standard FZ- beta source- 20% charge loss after 5x1014 p/cm2 (23 GeV)

0 1 2 3 4 5p [1014 cm-2]

0

20

40

60

80

100

Q/Q

0 [%

]

oxygenatedstandard

max collected charge (overdepletion)

collected at depletion voltage

M.Moll [Data: P.Allport et all, NIMA 501 (2003) 146]

Decrease of Charge Collection Efficiency


Oxygenation Benefits

Michael Moll, IWORID Glasgow 2004• oxygenation increases radiation hardness

• sometimes, standard FZ exhibits similar radiation hardness - reasons unclear

• Concentrate R&D on CZ and EPI silicon


Summary• Silicon strip detectors built on simple pn junction principle have become a ‘mature’ technology over 25 years.

• Provide reliable tracking in high density/high rate environment

• Widespread use thanks to cost drop and advances in microelectronic industry

• Silicon Radiation hardness to a few 1015 p/cm2 - radiation hardness frontier > 1016 p/cm2 (SLHC inner pixel layer) - CZ, EPI, new materials/structures?

• Silicon People are fun to work with, outgoing and (usually) in a good mood (eh …)

•

Have fun in California - You deserve it!


Backup


Property Diamond GaN 4H SiC Si Eg [eV] 5.5 3.39 3.26 1.12 Ebreakdown [V/cm] 107 4·106 2.2·106 3·105 e [cm2/Vs] 1800 1000 800 1450 h [cm2/Vs] 1200 30 115 450 vsat [cm/s] 2.2·107 - 2·107 0.8·107 Z 6 31/7 14/6 14 r 5.7 9.6 9.7 11.9 e-h energy [eV] 13 8.9 7.6-8.4 3.6 Density [g/cm3] 3.515 6.15 3.22 2.33 Displacem. [eV] 43 15 25 13-20

o Wide bandgap (3.3eV) lower leakage current

than silicon

o Signal:Diamond 36 e/mSiC 51 e/mSi 89 e/m

more charge than diamond

o Higher displacement threshold than silicon

radiation harder than silicon (?)

R&D on diamond detectors:RD42 – Collaboration

http://cern.ch/rd42/

Recent review: P.J.Sellin and J.Vaitkus on behalf of RD50 “New materials for radiation hard semiconductor detectors”, submitted to NIMA

Sensor Materials: Diamond, SiC and GaN


Microscopic defects

I

I

V

V

Distribution of vacancies created by a 50 keV Si-ion

in silicon (typical recoil energy for 1 MeV neutrons):

Schematic[Van Lint 1980]

Simulation[M.Huhtinen 2001]

Defects can be electrically active (levels in the band gap) - capture and release electrons and holes from conduction and valence band

can be charged - can be generation/recombination centers - can be trapping centers

Vacancy + Interstitial

“point defects”, mobile in silicon,can react with impurities (O,C,..)

V

I

point defects and clusters of defects

o particle SiS EK>25 eV

EK > 5 keV

Damage to the silicon crystal: Displacement of lattice atoms

80 nm


Radiation Damage in Silicon

o Two general types of radiation damage– “Bulk” damage due to physical impact within the crystal– “Surface” damage in the oxide or Si/SiO2 interface

o Cumulative effects– Increased leakage current (increased Shot noise)– Silicon bulk type inversion (n-type to p-type)– Increased depletion voltage– Increased capacitance

o Sensors can fail from radiation damage by virtue of…– Noise too high to effectively operate – Depletion voltage too high to deplete– Loss of inter-strip isolation (charge spreading)

o Ratio of signal/noise is the important quantity to watch

Close proximity to the interaction region means the sensors are subject tohigh doses of radiation


Depletion voltage is often parameterized in three parts (Hamburg model): Neff(T,t,) = NA + NC + NYo Short term annealing (NA)

NA = eq iga,iexp(-ka,i(T)t)– Reduces NY (beneficial)– Time constant is a few days at 20 C

o Stable component (Nc)Nc = Nc0(1-exp(-ceq))+gceq– Does not anneal (does not depend on time

or temperature)– Partial donor removal (exponential or

limited exponential)– Creation of acceptor sites (linear)

o Long term reverse annealing (NY)NY = NY,∞[1-1/(1+ NY,∞kY(T)t)], NY,∞= gYeq – Strong temperature dependence– 1 year at T=20 C is the same as <1 day at

T=60 C or ~100 years at T= -7 C (ATLAS)– Can be significant long term; must cool Si

Figure 9: Dependence of Neff on the accumulated 1 MeV neutron equivalent fluence for standard and oxygen enriched FZ silicon irradiated with reactor

neutrons (Ljubljana), 23 GeV protons (CERN PS) and 192 MeV pions (PSI).

0 0.5 1 1.5 2 2.5 3 3.5eq [1014cm-2]

1

2

3

4

5

6

7

|Nef

f| [1

012 c

m-3

]

100

200

300

400

Vde

p [V

] (3

00m

)

neutronsneutrons

neutronsneutronspionspionsprotonsprotons

pionspionsprotonsprotons

oxygen rich FZ

standard FZstandard FZ

Bulk Damage – Depletion Voltage

1 10 100 1000 10000annealing time at 60oC [min]

0

2

4

6

8

10

N

eff [

1011

cm-3

]

NY, = gY eq

NC

NC0

gC eqgC eq

NA = ga eqNA = ga eq

Fig.13: Annealing behaviour of the radiation induced change in the effective doping concentration Neff at 60C.


Bulk Damage – Leakage Currento Defects created by bulk damage provide intermediate states within

the band gap– intermediate states act as ‘stepping stones’ of thermal generation of

electron/hole pairs– Some of these states anneal away; the bulk current reduces with time

(and temperature) after irradiation o Annealing function (t)

– Parameterized by the sum of several exponentials iexp(-t/i)– Full annealing (for the example below) reached after ~1 year at 20ºC– At low temperatures, annealing effectively stops


Bulk Damage Effects (Simple View)o Leakage Current: I(t)V

– Current depends on (t) (annealing function), V (volume), and (fluence).

– Annealing reduces the current– Independent of particle type

o Depletion Voltage:Vdep = q|Neff|d2/20– Depends on effective dopant

concentration (Neff = Ndonors – Nacceptors), sensor thickness (d), permitivity (0).

– Depletion voltage is often parameterized in three parts:

• Short term annealing (Na)• A stable component (Nc)• Long term reverse annealing (NY)

P N

P N

Conc

ent ra

tion

(log

scal

e)Co

ncen

t ratio

n(lo

g sc

ale)

Small difference: small depletion V

Large area: large current

Large difference: Large depletion V

Small area: small current

Before Irradiation:

After Irradiation:


Bulk Damage – Leakage Resultso Measured values of (t)

– Typically one quotes measured values of (t) after complete annealing at T=20ºC: ∞ = (t=∞)

– Some typical ‘world averages’ for ∞ are • 2.2 x 10-17 A/cm3 for protons, pions• 2.9 x 10-17 A/cm3 for neutrons

– Recent results show (t=80min,T=60ºC) = 4.0 x 10-17 A/cm3 for all types of silicon, levels of impurities, and incident particle types (NIM A426 (1999)86).

1011 1012 1013 1014 1015

eq [cm-2]10-6

10-5

10-4

10-3

10-2

10-1

I /

V

[A/c

m3 ]

n-type FZ - 7 to 25 Kcmn-type FZ - 7 to 25 Kcmn-type FZ - 7 Kcmn-type FZ - 7 Kcmn-type FZ - 4 Kcmn-type FZ - 4 Kcmn-type FZ - 3 Kcmn-type FZ - 3 Kcm

n-type FZ - 780 cmn-type FZ - 780 cmn-type FZ - 410 cmn-type FZ - 410 cmn-type FZ - 130 cmn-type FZ - 130 cmn-type FZ - 110 cmn-type FZ - 110 cmn-type CZ - 140 cmn-type CZ - 140 cm

p-type EPI - 2 and 4 Kcmp-type EPI - 2 and 4 Kcm

p-type EPI - 380 cmp-type EPI - 380 cm

100 5 101 5 102 5 103 5 104 5 105

annealing time at 60oC [minutes]

0

2.10-17

4.10-17

6.10-17

(t)

0

2.10-17

4.10-17

6.10-17

oxygen enriched silicon [O] = 2.1017 cm-3oxygen enriched silicon [O] = 2.1017 cm-3

parameterisation for standard silicon parameterisation for standard silicon

Fig. 7.: Fluence dependence of leakage current for detectors produced by various process technologies from different silicon materials. The current was measured after a heat treatment for

80 min at 60C [14].

Fig.8: Current related damage rate as function of cumulated annealing time at 60C. Comparison between data obtained for

oxygen diffused silicon and parameterisation given in Ref. [14].


Rainer Wallny

Silicon Detector Workshop at UCSBMay 11th, 2006

Slides ruthlessly stolen from: Paula Collins, CERN Alan Honma, CERN Christian Joram, CERN Michael Moll, CERN Steve Worm, RAL

Silicon Detectors – How They Work

rainer wallny silicon detector workshop at ucsb may 11th, 2006

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