michael moll ( cern – ph-dt2-sd)

Radiation Tolerant Silicon Detectors

Michael Moll Michael Moll ( CERN – PH-DT2-SD)( CERN – PH-DT2-SD)

CERN – PH-DT2 – Scientific Tea meeting 13.10.2006

What is a silicon detector? – How does it work? What is radiation damage? – What are the problems? Radiation damage in future experiments: Super-LHC + (LHCb Upgrade) The CERN RD50 collaboration Strategies to obtain more radiation tolerant detectors Some examples how to obtain radiation tolerant detectors

Material Engineering Device Engineering

Summary

Outline

Michael Moll – PH-DT2 – Scientific Tea, 11 October 2006 -2 / 20-

RD50 Silicon Detector – Working principle

Take a piece of high resistivity silicon and produce two electrodes (not so easy !) Apply a voltage in order to create an internal electric field

(some hundred volts over the 0.3mm thick device) Traversing charged particles will produce electron-hole pairs The moving electrons and holes will create a signal in the electric cicuit


RD50

Resolution depends on the pitch p (distance from strip to strip)

- e.g. detection of charge in binary way (threshold discrimination) and using center of strip as measured coordinate results in:

typical pitch values are 20 m– 150 m 50 m pitch results in 14.4 m resolution

Silicon Strip Detector

Segmentation of the p+ layer into strips (Diode Strip Detector) and connection of strips to individual read-out channels gives spatial information

pitch

typical thickness: 300m (150m - 500m used)

using n-type silicon with a resistivity of

= 2 Kcm (ND ~2.2.1012cm-3)

results in a depletion voltage ~ 150 V

12

p


RD50 Example – The ATLAS module


RD50 LHCb – VELO: Silicon sensor details

42 mm8 mm

300 m thick sensors n-on-n, DOFZ wafers 42 mm radius AC coupled, double metal 2048 strips / sensor Pitch from 40 to 100 m Produced by Micron Semiconductor

-measuring sensor(radial strips with a stereo angle)

R-measuring sensor(45 degree circular segments)

[Martin van Beuzekom, STD6, September 2006]


RD50 LHCb-VELO - Module construction

Kapton hybrid

Carbon fibre

Thermal Pyrolytic Graphite (TPG)

• 4 layer kapton circuit• Heat transport with TPG• Readout with 16 Beetle chips

• 128 channels, 25 ns shaping time, analog pipeline• 0.25 m CMOS• no performance loss up to 40 Mrad• Yield > 80 %

Beetle

[Martin van Beuzekom, STD6, September 2006]


RD50 Motivation for R&D on Radiation Tolerant Detectors: Super - LHC

• LHC upgrade LHC (2007), L = 1034cm-2s-1

(r=4cm) ~ 3·1015cm-2

Super-LHC (2015 ?), L = 1035cm-2s-1

(r=4cm) ~ 1.6·1016cm-2

• LHC (Replacement of components) e.g. - LHCb Velo detectors (~2010) - ATLAS Pixel B-layer (~2012)

• Linear collider experiments (generic R&D)Deep understanding of radiation damage will be fruitful for linear collider experiments where high doses of e, will play a significant role.

5 years

2500 fb-1

10 years

500 fb-1

5

0 10 20 30 40 50 60

r [cm]

1013

5

1014

5

1015

5

1016

eq

[cm

-2]

total fluence eqtotal fluence eq

neutrons eq

pions eq

other charged

SUPER - LHC (5 years, 2500 fb-1)

hadrons eqATLAS SCT - barrelATLAS Pixel

Pixel (?) Ministrip (?)

Macropixel (?)

(microstrip detectors)

[M.Moll, simplified, scaled from ATLAS TDR]


RD50 Overview: Radiation Damage in Silicon Sensors

Two 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!)

Signal/noise ratio is the quantity to watch Sensors can fail from radiation damage !


RD50

0 10 20 30 40 50 60 70 80Signal [1000 electrons]

0

100

200

300

400

500

600

Cou

nts

[M.Moll][M.Moll]

The charge signal

Collected Charge for a Minimum Ionizing Particle (MIP)

Mean energy loss dE/dx (Si) = 3.88 MeV/cm 116 keV for 300m thickness

Most probable energy loss ≈ 0.7 mean 81 keV

3.6 eV to create an e-h pair 72 e-h / m (mean) 108 e-h / m (most probable)

Most probable charge (300 m)

≈ 22500 e ≈ 3.6 fC

Mean charge

Most probable charge ≈ 0.7 mean

noise Cut (threshold)


RD50

0 10 20 30 40 50 60 70 80Signal [1000 electrons]

0

200

400

600

800

1000

1200

Cou

nts

non irradiatednon irradiated

1.1 x 1015 p/cm21.1 x 1015 p/cm2

9.3 x 1015 p/cm29.3 x 1015 p/cm2

p-type MCZ silicon p-type MCZ silicon 5x5 mm2 pad5x5 mm2 pad

90Sr - source90Sr - source

[M.Moll][M.Moll]

Signal to Noise ratio

Good hits selected by requiring NADC > noise tail If cut too high efficiency loss If cut too low noise occupancy

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

Typical values >10-15, people get nervous below 10. Radiation damage severely degrades the S/N.

Landau distribution has a low energy tail - becomes even lower by noise broadening

Noise sources: (ENC = Equivalent Noise Charge) - Capacitance - Leakage Current

- Thermal Noise (bias resistor)

dCENC

IENC

RTkENC B

less signal

more n

oise

What is signal and what is noise?


RD50 The CERN RD50 Collaboration http://www.cern.ch/rd50

Collaboration formed in November 2001 Experiment approved as RD50 by CERN in June 2002 Main objective:

Presently 261 members from 52 institutes

Development of ultra-radiation hard semiconductor detectors for the luminosity upgrade of the LHC to 1035 cm-2s-1 (“Super-LHC”).

Challenges: - Radiation hardness up to 1016 cm-2 required - Fast signal collection (Going from 25ns to 10 ns bunch crossing ?)

- Low mass (reducing multiple scattering close to interaction point)- Cost effectiveness (big surfaces have to be covered with detectors!)

RD50: Development of Radiation Hard Semiconductor Devices for High Luminosity Colliders

Belarus (Minsk), Belgium (Louvain), Canada (Montreal), Czech Republic (Prague (3x)), Finland (Helsinki, Lappeenranta), Germany (Berlin, Dortmund, Erfurt, Freiburg, Hamburg, Karlsruhe),

Israel (Tel Aviv), Italy (Bari, Bologna, Florence, Padova, Perugia, Pisa, Trento, Turin), Lithuania (Vilnius), The Netherlands (Amsterdam), Norway (Oslo (2x)), Poland (Warsaw (2x)), Romania (Bucharest (2x)), Russia

(Moscow), St.Petersburg), Slovenia (Ljubljana), Spain (Barcelona, Valencia), Switzerland (CERN, PSI), Ukraine (Kiev), United Kingdom (Exeter, Glasgow, Lancaster, Liverpool, Sheffield, University of Surrey), USA (Fermilab, Purdue University, Rochester University, SCIPP Santa Cruz, Syracuse University, BNL,

University of New Mexico)


RD50 Approaches to develop radiation harder solid state tracking detectors

Defect Engineering of Silicon

Deliberate incorporation of impurities or defects into the silicon bulk to improve radiation tolerance of detectors

Needs: Profound understanding of radiation damage• microscopic defects, macroscopic parameters• dependence on particle type and energy• defect formation kinetics and annealing

Examples:• Oxygen rich Silicon (DOFZ, Cz, MCZ, EPI)• Oxygen dimer & hydrogen enriched Si• Pre-irradiated Si• Influence of processing technology

New Materials Silicon Carbide (SiC), Gallium Nitride (GaN) Diamond (CERN RD42 Collaboration) Amorphous silicon

Device Engineering (New Detector Designs) p-type silicon detectors (n-in-p) thin detectors, epitaxial detectors 3D detectors and Semi 3D detectors, Stripixels Cost effective detectors Monolithic devices

Scientific strategies:

I. Material engineering

II. Device engineering

III. Change of detectoroperational conditions

CERN-RD39“Cryogenic Tracking Detectors”

operation at 100-200K

to reduce charge loss


RD50 Silicon Materials under Investigation by RD50

Material Symbol (cm) [Oi] (cm-3)

Standard FZ (n- and p-type) FZ 1–710 3 < 51016

Diffusion oxygenated FZ (n- and p-type) DOFZ 1–710 3 ~ 1–21017

Magnetic Czochralski Si, Okmetic, Finland

(n- and p-type)

MCz ~ 110 3 ~ 51017

Czochralski Si, Sumitomo, Japan (n-type) Cz ~ 110 3 ~ 8-91017

Epitaxial layers on Cz-substrates, ITME, Poland (n- and p-type)

EPI 50 - 100 < 11017

DOFZ silicon Enriched with oxygen on wafer level, inhomogeneous distribution of oxygen

CZ silicon high Oi (oxygen) and O2i (oxygen dimer) concentration (homogeneous) formation of shallow Thermal Donors possible

Epi silicon high Oi , O2i content due to out-diffusion from the CZ substrate (inhomogeneous) thin layers: high doping possible (low starting resistivity)


RD50 Standard FZ, DOFZ, Cz and MCz Silicon

24 GeV/c proton irradiation

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 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 (verified by TCT measurements)

(verified for CZ silicon by TCT measurements, preliminary result for MCZ silicon) donor generation overcompensates acceptor generation in high fluence range

Common to all materials (after hadron irradiation): reverse current increase increase of trapping (electrons and holes) within ~ 20%


RD50 Epitaxial silicon

Layer thickness: 25, 50, 75 m (resistivity: ~ 50 cm); 150 m (resistivity: ~ 400 cm) Oxygen: [O] 91016cm-3; Oxygen dimers (detected via IO2-defect formation)

EPI Devices – Irradiation experiments

Only little change in depletion voltage

No type inversion up to ~ 1016 p/cm2 and ~ 1016 n/cm2

high electric field will stay at front electrode!reverse annealing will decreases depletion voltage!

Explanation: introduction of shallow donors is bigger than generation of deep acceptors

G.Lindström et al.,10th European Symposium on Semiconductor Detectors, 12-16 June 2005 G.Kramberger et al., Hamburg RD50 Workshop, August 2006

0 2.1015 4.1015 6.1015 8.1015 1016

eq [cm-2]

0

1014

2.1014

Nef

f(t0)

[cm

-3]

25 m, 80 oC25 m, 80 oC

50 m, 80 oC50 m, 80 oC

75 m, 80 oC75 m, 80 oC

23 GeV protons23 GeV protons

320V (75m)

105V (25m)

230V (50m)

CCE (Sr90 source, 25ns shaping): 6400 e (150 m; 2x1015 n/cm-2) 3300 e (75m; 8x1015 n/cm-2) 2300 e (50m; 8x1015 n/cm-2)

0 20 40 60 80 100eq [1014 cm-2]

0

2000

4000

6000

8000

10000

12000

Sign

al [

e]

150 m - neutron irradiated 75 m - proton irradiated 75 m - neutron irradiated 50 m - neutron irradiated 50 m - proton irradiated

[M.Moll]

[Data: G.Kramberger et al., Hamburg RD50 Workshop, August 2006]


RD50

p-on-n silicon, under-depleted:

• Charge spread – degraded resolution

• Charge loss – reduced CCE

p+on-n

Device engineeringp-in-n versus n-in-p detectors

n-on-p silicon, under-depleted:

•Limited loss in CCE

•Less degradation with under-depletion

•Collect electrons (fast)

n+on-p

n-type silicon after high fluences: p-type silicon after high fluences:

Be careful, this is a very schematic explanation,reality is more complex !


RD50

0 2 4 6 8 10fluence [1015cm-2]

0

5

10

15

20

25

CC

E (

103 e

lect

rons

)

24 GeV/c p irradiation24 GeV/c p irradiation

[M.Moll][M.Moll]

[Data: G.Casse et al., NIMA535(2004) 362][Data: G.Casse et al., NIMA535(2004) 362]

n-in-p microstrip detectors

n-in-p microstrip detectors (280m) on p-type FZ silicon Detectors read-out with 40MHz

CCE ~ 6500 e (30%) after 7.5 1015 p cm-2 at

900V

n-in-p: - no type inversion, high electric field stays on structured side - collection of electrons

no reverse annealing visible in the CCE measurement ! e.g. for 7.5 1015 p/cm2 increase of Vdep from

Vdep~ 2800V to Vdep > 12000V is expected !

0 100 200 300 400 500time at 80oC[min]

0 500 1000 1500 2000 2500time [days at 20oC]

02468

101214161820

CC

E (

103 e

lect

rons

)

800 V800 V

1.1 x 1015cm-2 1.1 x 1015cm-2 500 V500 V

3.5 x 1015cm-2 (500 V)3.5 x 1015cm-2 (500 V)

7.5 x 1015cm-2 (700 V)7.5 x 1015cm-2 (700 V)

M.MollM.Moll

[Data: G.Casse et al., to be published in NIMA][Data: G.Casse et al., to be published in NIMA]


RD50 “3D” electrodes: - narrow columns along detector thickness,

- diameter: 10m, distance: 50 - 100m Lateral depletion: - lower depletion voltage needed

- thicker detectors possible- fast signal- radiation hard

3D detector - concepts

n-columns p-columnswafer surface

n-type substrate

ionizing particle

carriers collectedat the same time

Introduced by: S.I. Parker et al., NIMA 395 (1997) 328


RD50 “3D” electrodes: - narrow columns along detector thickness,

- diameter: 10m, distance: 50 - 100m Lateral depletion: - lower depletion voltage needed

- thicker detectors possible- fast signal- radiation hard

3D detector - concepts

n-columns p-columns wafer surface

n-type substrate

Introduced by: S.I. Parker et al., NIMA 395 (1997) 328

Simplified 3D architecture n+ columns in p-type substrate, p+ backplane operation similar to standard 3D detector

Simplified process hole etching and doping only done once no wafer bonding technology needed

Simulations performed Fabrication:

IRST(Italy), CNM Barcelona

[C. Piemonte et al., NIM A541 (2005) 441]

hole

hole metal strip

C.Piemonte et al., STD06, September 2006

Hole depth 120-150mHole diameter ~10m

First CCE tests under way


RD50 Conclusion New Materials like SiC and GaN have been characterized (not shown in this talk) .

CCE tests show that these materials are not radiation harder than silicon Silicon (operated at e.g. -30°C) seems presently to be the best choice

At fluences up to 1015cm-2 (Outer layers of SLHC detector) the depletion voltage change and the large area to be covered is major problem:

MCZ silicon detectors could be a cost-effective radiation hard solution

p-type (FZ and MCZ) silicon microstrip detectors show good results: CCE 6500 e; eq

= 41015 cm-2, 300m, collection of electrons,

no reverse annealing observed in CCE measurement!

At the fluence of 1016cm-2 (Innermost layer of a SLHC detector) the active thickness of any silicon material is significantly reduced due to trapping. New options:

Thin/EPI detectors : drawback: radiation hard electronics for low signals needed e.g. 3300e at eq 8x1015cm-2, 75m EPI, …. thicker layers (150 m presently under test)

3D detectors : drawback: very difficult technology ….. steady progress within RD50

Further information: http://cern.ch/rd50/

michael moll ( cern – ph-dt2-sd)

Documents

strips diode strip detector

bulk crystal damage

detector performance

center of strip

performance loss

detector materials

pitch results

lhcb velo detectors