Jaap Velthuis (University of Bristol) 1

Radiation damage in silicon sensors• Topics:

– Damage due to protons and neutrons• Radiation induced defects• Annealing• Donor removal (type

inversion)• Alternatives to p-n

– Damage due to electrons, photons,…

In case of any questions:

Jaap.Velthuis@bristol.ac.uk

Jaap Velthuis (University of Bristol) 2

Quick review• Semiconductor detectors are used close

to primary vertex to – Limit occupancy and reduce ambiguities– Give very precise space point

• Need trick to remove free charge carriers– Use high band gap semiconductor– Cool to cryogenic temperatures– Build p-n junction and deplete detector

kT

ENNpnn GVCi 2

exp

Jaap Velthuis (University of Bristol) 3

Quick review (II)

• Energy loss described by Behte-Bloch equation– Minimum ionizing particle– Energy loss (=signal) is Landau distributed

• Particles scatter in matter, so need to have thin detectors

• MIP yields 8900 e-h pairs per 100 m Si

• If pitch ~ charge cloud, charge is shared. Need lots of strips.– Trick intermediate strip using C-charge sharing,

but non-linear charge sharing

Jaap Velthuis (University of Bristol) 4

Charged … matter• Bethe-Bloch describes

average energy loss• Collisions stochastic nature,

hence energy loss is distribution instead of number.

• First calculated for thin layers was Landau. Hence energy loss is Landau distributed.

x

ex

xL2

1exp)(0

is most probable value

Jaap Velthuis (University of Bristol) 5

-electrons• Some of generated carriers

have so much kinetic energy that they will free more charge carriers– Lots of signal– Do not know where hit was

• Responsible for tails signal distribution

• Number of ’s dependent on material: EG high-> less

-+ - -

-

++ +

Jaap Velthuis (University of Bristol) 6

Charge collection• If pitch > charge cloud all charge

collected on 1 strip

• In this case analog signal value not importantchose digital or binary readout

• To do better need to share charge over more strips need pitch20m for 300 m thick sensor

• Problem: connecting all strips to readout channel yields too many strips

12

11

0

21

0

22 dxxxdxxx

Jaap Velthuis (University of Bristol) 7

Strip pitch & Analogue vs binary

• Can improve position reconstruction by using neighbour signal. Simplest: CoG

• But the further away, the more effect on . So S/N neighbour limits resolution.

• Now choice:– use many strips to get

enough S/N on neighbour and use analogue readout

– Live with limitations, spread out strips and readout binary

i

iirecon S

xSx

2

2

22

reconix xxS

N

Jaap Velthuis (University of Bristol) 8

Strip pitch & Analogue vs binary

• If analogue, need fancy chip that consumes lots of power to process signal. – Need: shaper, pipeline, ADC

& Storage of pulse heights– Advantages: high precision,

good understanding noise– Disadvantages: high power,

lots of processing, loads of infrastructure

• ATLAS chose binary. 50m pitch thus 17m precision and easy read out

Onl

y th

is n

eede

dfo

r bi

nary

Jaap Velthuis (University of Bristol) 9

sLHC radiation dose• 5 year radiation dose

close to beam pipe ~1016 neq/cm2

– too high for state-of-the-art standard silicon sensors

• Most radiation hard material: diamond– High bandgap– Displacement energy

43eV (13-20 for Si)

Jaap Velthuis (University of Bristol) 10

Radiation with protons/neutrons

• Silicon crystal is organised in a Face-Centred-Cubic lattice (or diamond lattice)

• Remember: impurities yield lattice sites with different number of valence e- doping

• Energetic radiation knocks atoms out of lattice: similar thing

Jaap Velthuis (University of Bristol) 11

Radiation with protons/neutrons

• Energy needed to displace atom from lattice=15eV

• Damage energy dependent– ~<2keVisolated point defect– 2-12keVdefect cluster– ~>12keVmany defect clusters

• This damage is called Non-Ionizing Energy Loss (NIEL)– Results scaled to 1MeV neutrons

• Electrons and photons don’t make defects!

Jaap Velthuis (University of Bristol) 12

Radiation … neutrons

• Displacement changes band structure– Levels in middle of band gap arise– Can capture electrons/holes

(trapping)– Can donate electrons/holes– Can increase leakage current by

two-step transitions from valence to conduction band

– Can act as recombination centre

• Role dependent on neighbours and present impurities

Conduction band

+ + + + + + +

Valence band

- - - - - -

Jaap Velthuis (University of Bristol) 13

Annealing• Annealing is process in which crystal

recovers from radiation damage– Atoms occupy vacancies– Defect complexes change in different

complexes

• New defect complexes can also worsen damage (reverse annealing)

• Processes highly temperature dependent• “Not very well understood”

– Depends on which (unintentional) impurities present

– Which defects are formed– Alchemy!

Jaap Velthuis (University of Bristol) 14

Radiation damage: Leakage current

• I = Volume• Material

independent– linked to defect

clusters

• Annealing material independent

• Scales with NIEL• Temp dependence

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

kT

ETTI g

2exp2 = 3.99 0.03 x 10-17Acm-1

after 80minutes annealing at 60C

Jaap Velthuis (University of Bristol) 15

Type inversion• Dopants may be

captured into defect complexes.

• Donor removal and acceptor generation– type inversion: n p– depletion width

grows from n+ contact

• Increase in full depletion voltage

biasbi

DA

DAd VV

NqN

NNx

2

0 0.5 1 1.5 2eq [1014cm-2]

1

2

3

4

5

|Nef

f| [1

012 c

m-3

]

50

100

150

200

250

300

Vde

p [V

] (

300

m)

1.8 Kcm Wacker 1.8 Kcm Wacker 2.6 Kcm Polovodice2.6 Kcm Polovodice3.1 Kcm Wacker 3.1 Kcm Wacker 4.2 Kcm Topsil 4.2 Kcm Topsil

Neutron irradiationNeutron irradiation

cNN effeff exp0

= 0.025cm-1 measured afterbeneficial anneal

P-strips in p-bulk

Jaap Velthuis (University of Bristol) 16

Partially depleted detectors

• Undepleted region like high-ohmic resistor

• If detector partially depleted – from strip side

only charge in depleted region contributes smaller signal, similar spatial resolution

– from backplane carriers travel towards

strips, but don’t reach it signal spread over many strips poor spatial resolution

undepleted

undepleted

Jaap Velthuis (University of Bristol) 17

Example: NA60 type inversion

• After type inversion Vdep increases with dose smaller radii

• Lowering Vbias leaves larger area undepleted– Depletion from

backside layer almost dead

Jaap Velthuis (University of Bristol) 18

Example: NA60 type inversion

• After type inversion Vdep increases with dose smaller radii

• Lowering Vbias leaves larger area undepleted– Depletion from

backside layer almost dead

Jaap Velthuis (University of Bristol) 19

Example: NA60 type inversion

• After type inversion Vdep increases with dose smaller radii

• Lowering Vbias leaves larger area undepleted– Depletion from

backside layer almost dead

Jaap Velthuis (University of Bristol) 20

Example: NA60 type inversion

• After type inversion Vdep increases with dose smaller radii

• Lowering Vbias leaves larger area undepleted– Depletion from

backside layer almost dead

Jaap Velthuis (University of Bristol) 21

Example: NA60 type inversion

• After type inversion Vdep increases with dose smaller radii

• Lowering Vbias leaves larger area undepleted– Depletion from

backside layer almost dead

Jaap Velthuis (University of Bristol) 22

Example: NA60 type inversion

• After type inversion Vdep increases with dose smaller radii

• Lowering Vbias leaves larger area undepleted– Depletion from

backside layer almost dead

Jaap Velthuis (University of Bristol) 23

Example: NA60 type inversion

• After type inversion Vdep increases with dose smaller radii

• Lowering Vbias leaves larger area undepleted– Depletion from

backside layer almost dead

Jaap Velthuis (University of Bristol) 24

Thermal runaway• Problem with donor removal:

– Need higher voltages to deplete detector

– Higher voltage higher leakage current

– Higher leakage current more power dissipated in detector

– More power heating– heating more leakage current

• “Solution”: cool detectors– ATLAS will operate at –7oC

Jaap Velthuis (University of Bristol) 25

Solutions radiation damage• Start with n+ strips in n-type detector

– After inversion substrate p type depletion now from strip side (LHC-b)

– Build p-type detectors with n-strips• Different crystal orientation

– Less dangling bonds at Si-SiO2 interface

• Material Engineering• Operate very cold • Use different material (e.g. CVD

diamond)• 3D-structures

Jaap Velthuis (University of Bristol) 26

N+-on-n detectors• Standard: p-strips on n-bulk• Problem: n-bulk becomes p-type

– pn-junction moves from strip-bulk to

bias contact-bulk interface– Only good spatial resolution when

fully depleted

• Solution: make n+-strips in n-bulk– After radiation: n+-strips and p-bulk– Disadvantages:

• strips not well isolated before radiation. Need p-strips (or spray) between n-strips.

• Need guard rings at bottom (expensive)

n+-strips N-type bulkbecomes p-typeAfter radiation

n+ biascontact

Jaap Velthuis (University of Bristol) 27

P-type detectors• P-type bulk with n-

type strips– Collect electrons

instead of holes• Electron mobility ~3>

hole mobility less trapping

– Depletion starts from strip side

• Even at partial depletion good spatial resolution

– No need for guard rings on backside cheaper than n+-on-n

1E15cm-2 10 yearsDose for ATLAS strips

Jaap Velthuis (University of Bristol) 28

Material engineering

• Diffusing oxygen suppresses V2O formation – V + O VO

– V + VO V2O

– V2O reverse annealing

• Still alchemy…

St = 0.0154

[O] = 0.0044 0.0053

[C] = 0.0437

0

1E+12

2E+12

3E+12

4E+12

5E+12

6E+12

7E+12

8E+12

9E+12

1E+13

0 1E+14 2E+14 3E+14 4E+14 5E+14

Proton fluence (24 GeV/c ) [cm-2]

|Nef

f| [c

m-3

]

0

100

200

300

400

500

VF

D f

or 3

00

m t

hic

k d

etec

tor

[V]

Standard (P51)O-diffusion 24 hours (P52)O-diffusion 48 hours (P54)O-diffusion 72 hours (P56)Carbon-enriched (P503)

Jaap Velthuis (University of Bristol) 29

Lazarus Effect• Remember:

• Radiation induces (more) traps. Capture and emission very temperature dependent.

• Cooling makes sure traps stay filled no trapping

• Can actually operate forward biased• Downside: need to keep detector in

cryostat

kT

ENNpnn GVCi 2

exp

Jaap Velthuis (University of Bristol) 30

CVD Diamond• Remember:

• Limit background charge by using large bandgap material– Si: EG=1.12 eV 1.5E10 free carriers/cm3

– Diamond: EG=5.47 eV 6E-28 free carriers/cm3 no need for pn-junction

– Diamond lattice very strong very radiation hard

• Note: large EG only few eh pairs produced (3600 vs 8900 per 100 m), but also lower noise

kT

ENNpnn GVCi 2

exp

Jaap Velthuis (University of Bristol) 31

Diamond growth• Trap- and recombination

centers limit charge collection

• Trapped charge at grain boundaries builds up a polarization field superimposed on the biasing field

• Substrate and growth side must be ground and polished for good quality

Jaap Velthuis (University of Bristol) 32

ATLAS CVD diamond pixel module• Sensor:

– Active area: 61x16.5mm2

– Thickness 800µm– Pixel size 400(600)x50µm2

– 46k pixels• ATLAS frontend chip FE-I3

– 0.25µm IBM– Radiation tolerant >50MRad– Designed for Si sensors– Binary/low res. analog readout

• Noise same as bare FE-chip– Noise ≈137e-

– Threshold ≈1454e-

– Threshold spread ≈25e-

Jaap Velthuis (University of Bristol) 33

CVD diamond radiation hardness

• Still S/N≈18-25 after 1.8x1016 p/cm2 (~500 Mrad) depending on field

• No problem operating in sLHC conditions– Noise limited by

electronics, NOT by sensor capacitance or leakage current

E=2V/µm

E=1V/µm

Jaap Velthuis (University of Bristol) 34

Single crystal diamond

• Largest single crystal diamond 14x14mm– No grain boundaries → no trapping

• Produced 400 µm thick single chip sensor using ATLAS FE chip

Jaap Velthuis (University of Bristol) 35

Diamond signal spectrum

S/N≈99N=136.8 e-

Jaap Velthuis (University of Bristol) 36

CVD diamond as radiation monitor

• Babar uses diamond radiation monitors for almost 2 years

• Response is fast and material doesn’t die

Jaap Velthuis (University of Bristol) 37

Czochralski silicon (Cz)

Czochralski Growth

• Pull Si-crystal from a Si-melt contained

in a silica crucible while rotating.• Silica crucible is dissolving oxygen into

the melt high concentration of O in CZ

• Material used by IC industry (cheap) • Recent developments (~2 years)

made • CZ available in sufficiently high purity

(resistivity) to allow for use as particle detector.

Jaap Velthuis (University of Bristol) 38

0 2 4 6 8 10proton fluence [1014 cm-2]

0

200

400

600

800

Vde

p [V

]

0

2

4

6

8

10

12

Nef

f [10

12 c

m-3

]

CZ <100>, TD killedCZ <100>, TD killedMCZ <100>, HelsinkiMCZ <100>, HelsinkiSTFZ <111>STFZ <111>DOFZ <111>, 72 h 11500CDOFZ <111>, 72 h 11500C

Czochralski silicon (Cz)• Standard FZ silicon

• type inversion at ~ 21013 p/cm2

• strong Neff increase at high fluence

• Oxygenated FZ (DOFZ)• type inversion at ~ 21013

p/cm2

• reduced Neff increase at high fluence

• CZ silicon and MCZ silicon – no type inversion in the

overall fluence range donor generation overcompensates acceptor generation in high fluence range

24 GeV/c proton irradiation

Jaap Velthuis (University of Bristol) 39

3D detectors

• Problems standard sensors: – After radiation high voltage needed to fully

deplete high currents high noise & thermal runaway

– Need guard ring structures lot of wasted space

• Make “strips” vertical inside bulk

3D standard

Jaap Velthuis (University of Bristol) 40

3D detectors

• Sideways depletion: smaller distance between electrode and strips lower depletion voltage

• Sideways charge collection – Edgeless (dead edge < 5 m)– Still use full 300 m thickness– Rapid charge collection (~2 ns)– Radiation hardness

3D standard

P=50, 100 or 200 m

Physicaledge

Jaap Velthuis (University of Bristol) 41

3D detectors• S/N source test=13 with 121 m

thick detector

Jaap Velthuis (University of Bristol) 42

3D detectors• Efficiency loss underneath electrodes

– P-type loss 66%– N-type loss 43%

• Signal: 10-90% <5µm !– Standard sensors need 5-6mm

• Used for TOTEM

electrodes

Jaap Velthuis (University of Bristol) 43

Summary radiation hardness

• Radiation damage in sensors mainly bulk damage– Atoms knocked out of their lattice

position extra levels in band gap • Effectively donor removal (type inversion)• High leakage currents

– High noise– Thermal runaway

• Problems to get full depletion

Jaap Velthuis (University of Bristol) 44

Summary radiation hardness (II)

• Solutions:– n+-on-n or even better n-on-p detectors– Material engineering (oxygenated Si/CZ)– Cool

• ATLAS at –7oC• cryogenic temperatures (Lazarus effect)

– Use different materials like diamond– Use different detector type like 3D