1. Michael Moll ( CERN – PH-DT2-SD)
CERN – PH-DT2 – Scientific Tea meeting
Radiation Tolerant Silicon Detectors
Michael Moll ( CERN – PH-DT2-SD)
Outline
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
Bonjour tout le monde.
Premièrement j'aime vous remercier de l'invitation.
Deuxièmement j'aime dire que je suis désolé pour mon Français
Si vous pensez à un moment qui mon Français est trop mauvais, je m'arrêterai et continuerai en anglais.
Cependant, je voudrais vraiment essayer de donner cette présentation en français
Alors je commence:
Aujourd'hui je voudrais parler au sujet du développement des détecteurs de semi-conducteur pour le futurs générations de collisionneurs avec de très haute luminosité
….et en particulier au sujet des développements pour le remise a niveau (upgrade) du LHC (le Grand Collisionneur Hadronique), le Super LHC.
..la plus grande partie des développements que je vais présenter
.. ont été effectuées par la collaboration CERN-RD50

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

3. 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: 300mm (150m - 500m used)
using n-type silicon with a resistivity of  = 2 KWcm (ND ~ cm-3) results in a depletion voltage ~ 150 V
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 mm– 150 mm  50 mm pitch results in 14.4 mm resolution
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

4. Example – The ATLAS module
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

5. LHCb – VELO: Silicon sensor details
R-measuring sensor
(45 degree circular segments)
300 mm thick sensors
n-on-n, DOFZ wafers
42 mm radius
AC coupled, double metal
2048 strips / sensor
Pitch from 40 to 100 mm
Produced by Micron Semiconductor
42 mm
8 mm
F-measuring sensor
(radial strips with a stereo angle)
[Martin van Beuzekom, STD6, September 2006]
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

6. LHCb-VELO - Module construction
Beetle
4 layer kapton circuit
Heat transport with TPG
Readout with 16 Beetle chips
128 channels, 25 ns shaping time,
analog pipeline
0.25 mm CMOS
no performance loss up to 40 Mrad
Yield > 80 %
Kapton hybrid
Carbon fibre
Thermal Pyrolytic Graphite (TPG)
[Martin van Beuzekom, STD6, September 2006]
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

7. Motivation for R&D on Radiation Tolerant Detectors: Super - LHC
LHC upgrade LHC (2007), L = 1034cm-2s f(r=4cm) ~ 3·1015cm-2
Super-LHC (2015 ?), L = 1035cm-2s f(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, g will play a significant role.
10 years
500 fb-1
 5
5 years
2500 fb-1
Quelle est notre motivation?
Nous avons trois raisons principales d'étudier les dommages du rayonnement et des détecteurs résistant au rayonnement
1. La première raison et le Super-LHC
Si vous regardez les champs de rayonnement dans les expériences ATLAS et CMS du Grand Collisionneur Hadronique (LHC) vous trouvez déjà des niveaux de rayonnement très élevés.
Par exemple, après 10 ans d'opération ils auront intégrés environ :
3 dix à la 15 (3 dix a la puissance 15 – 3 a la 15) particules par centimètres carrés à une distance de 4 centimètres du point d'interaction. Ceci correspond à la position de la couche intérieure des détecteurs a Pixel.
Même si on regarde le niveau du rayonnement a un point plus extérieur, par exemple à 75 cm de distance du point d’interaction on trouve un très grand nombre du particules rapides après 10 ans d’opération.
Aujourd'hui, nous sommes heureux que la technologie pour utiliser des détecteurs dans un tel environnement existe - les détecteurs survivront - mais ils souffriront fortement des dommages du rayonnement…..
Pour le Super-LHC il est prévu d’augmenter la luminosité par un facteur dix;
Après cinq ans d'opération du Super LHC, on aura un facteur cinq fois plus élevés de rayonnement comparés au LHC -- c'est un niveau de rayonnement pour lequel aucune technologie n'existe a ce jour -- pour les composants plus éloigné du point d'interaction on aura un problème - la technologie existe ; on pourrait employer par exemple des détecteurs Pixel a la place des détecteurs a micro piste. Cependant, ça serait beaucoup trop cher. donc nous avons besoin de détecteurs a bas prix qui sont résistant aux rayonnements
2. la deuxième raison est le LHC
dans les expériences du LHC sont déjà employés, des détecteurs contenant des niveaux élevés d’oxygène. Ils sont plus résistant contre le rayonnement, leurs résistances ont déjà été démontré; mais ils ont besoin d'une caractérisation plus approfondie; on a prévu de changer une partie des détecteur après quelques années .(par exemple: le LHC-b Velo détecteur)
3. La troisième raison est la recherche pour les expériences de futurs collisionneurs linéaires où les détecteurs vont souffrir du rayonnement électromagnétique;
Si on comprend les mécanismes du dommage du rayonnement électromagnétique, c’est déjà une première étape dans la compréhension des dommages du rayonnement hadronique qui sont beaucoup plus complexe.
Plus tard je vais vous prouver que le silicium avec un concentration en oxygène très élevée est un très bon matériel pour ce type d’environnement
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

8. 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 –
Change of effective doping concentration (higher depletion voltage, under- depletion)
Increase of leakage current (increase of shot noise, thermal runaway)
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 !
efficacité de collection de charge influencé par deux mécanisme
piégeage – porteurs de charge sont pièges par les défaut profond
déficit de collection de charge augmente en fonction de la fluence
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

9. noise The charge signal Most probable charge ≈ 0.7 mean Mean charge
Collected Charge for a Minimum Ionizing Particle (MIP)
Mean energy loss dE/dx (Si) = 3.88 MeV/cm  116 keV for 300m 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) ≈ e ≈ 3.6 fC
Mean charge
Most probable charge ≈ 0.7 mean
Cut (threshold)
efficacité de collection de charge influencé par deux mécanisme
piégeage – porteurs de charge sont pièges par les défaut profond
déficit de collection de charge augmente en fonction de la fluence
noise
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

10. What is signal and what is noise?
Signal to Noise ratio
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)
What is signal and what is noise?
less signal
more noise
efficacité de collection de charge influencé par deux mécanisme
piégeage – porteurs de charge sont pièges par les défaut profond
déficit de collection de charge augmente en fonction de la fluence
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 Radiation damage severely degrades the S/N.
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

11. The CERN RD50 Collaboration http://www.cern.ch/rd50
RD50: Development of Radiation Hard Semiconductor Devices for High Luminosity Colliders
Collaboration formed in November 2001
Experiment approved as RD50 by CERN in June 2002
Main objective:
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!)
Presently 261 members from 52 institutes
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)
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

12. 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:
Material engineering
Device engineering
Change of detector operational conditions
CERN-RD39 “Cryogenic Tracking Detectors” operation at K to reduce charge loss
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

13. Silicon Materials under Investigation by RD50
Symbol
 (cm)
[Oi] (cm-3)
Standard FZ (n- and p-type) FZ
1–710 3
< 51016
Diffusion oxygenated FZ (n- and p-type)
DOFZ
~ 1–21017
Magnetic Czochralski Si, Okmetic, Finland (n- and p-type)
MCz
~ 110 3
~ 51017
Czochralski Si, Sumitomo, Japan (n-type) Cz
~ 8-91017
Epitaxial layers on Cz-substrates, ITME, Poland (n- and p-type)
EPI
< 11017
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)
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

14. Standard FZ, DOFZ, Cz and MCz Silicon
24 GeV/c proton irradiation
Standard FZ silicon
type inversion at ~ 21013 p/cm2
strong Neff increase at high fluence
Oxygenated FZ (DOFZ)
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%
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

15. EPI Devices – Irradiation experiments
Epitaxial silicon
Layer thickness: 25, 50, 75 m (resistivity: ~ 50 cm); 150 m (resistivity: ~ 400 cm)
Oxygen: [O]  91016cm-3; Oxygen dimers (detected via IO2-defect formation)
G.Lindström et al.,10th European Symposium on Semiconductor Detectors, June G.Kramberger et al., Hamburg RD50 Workshop, August 2006
105V (25mm)
230V (50mm)
320V (75mm)
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
CCE (Sr90 source, 25ns shaping):  6400 e (150 mm; 2x1015 n/cm-2)  3300 e (75mm; 8x1015 n/cm-2)  2300 e (50mm; 8x1015 n/cm-2)
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

16. Device engineering p-in-n versus n-in-p detectors
n-type silicon after high fluences:
p-type silicon after high fluences:
p+on-n
n+on-p
p-on-n silicon, under-depleted:
Charge spread – degraded resolution
Charge loss – reduced CCE
n-on-p silicon, under-depleted:
Limited loss in CCE
Less degradation with under-depletion
Collect electrons (fast)
Be careful, this is a very schematic explanation, reality is more complex !
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

17. n-in-p microstrip detectors
n-in-p: - no type inversion, high electric field stays on structured side collection of electrons
n-in-p microstrip detectors (280mm) on p-type FZ silicon
Detectors read-out with 40MHz
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 !
CCE ~ 6500 e (30%) after p cm-2 at 900V
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

18. Introduced by: S.I. Parker et al., NIMA 395 (1997) 328
3D detector - concepts
Introduced by: S.I. Parker et al., NIMA 395 (1997) 328
“3D” electrodes: - narrow columns along detector thickness, - diameter: 10mm, distance: mm
Lateral depletion: - lower depletion voltage needed - thicker detectors possible - fast signal - radiation hard
ionizing particle
carriers collected
at the same time
n-columns
p-columns
wafer surface
n-type substrate
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

19. 3D detector - concepts Simplified 3D architecture
Introduced by: S.I. Parker et al., NIMA 395 (1997) 328
“3D” electrodes: - narrow columns along detector thickness, - diameter: 10mm, distance: mm
Lateral depletion: - lower depletion voltage needed - thicker detectors possible - fast signal - radiation hard
n-columns
p-columns
wafer surface
n-type substrate
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
metal strip
hole
[C. Piemonte et al., NIM A541 (2005) 441]
hole
Hole depth mm
Hole diameter ~10mm
C.Piemonte et al., STD06, September 2006
First CCE tests under way
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-

20. Further information: http://cern.ch/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; Feq= 41015 cm-2, 300mm, 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 Feq 8x1015cm-2, 75mm EPI, …. thicker layers (150 mm presently under test)
3D detectors : drawback: very difficult technology ….. steady progress within RD50
Further information:
Michael Moll – PH-DT2 – Scientific Tea, 11 October / 20-