1. 2CERN, Geneva, Switzerland
ESIPAP School – February 2015
Workshop on the characterization of irradiated silicon sensors & Effects of Radiation on Solid State Particle Detector Performance
Marcos Fernandez Garcia1 , Christian Gallrapp2, Michael Moll1, Hannes Neugebauer1 1IFCA-CSIC, Universidad de Cantabria, Santander, Spain
2CERN, Geneva, Switzerland
Particle Detectors: LHC and LHC Detectors
Semiconductors: pn junctions
Characterization techniques of silicon sensors
Radiation damage and radiation tolerant silicon detectors
Conclusions and Outlook
OUTLINE:
Slides available at

2. How does an LHC detector look like?
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

3. First: how does the LHC look like?
Installation in existing LEP tunnel (27 Km)
 4000 MCHF (machine+experiments)
1232 dipoles B=8.3T
pp s = 14 TeV Ldesign = 1034 cm-2 s-1
Heavy ions (e.g. Pb-Pb at s ~ 1000 TeV)
First beam: Sept.2008
2012: 2 x 4 TeV
2015: 2 x 6.5 TeV p
27 Km
LHC experiments located at 4 interaction points
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

4. LHC Experiments ATLAS LHC-B CMS ALICE + LHCf +
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

5. LHC Experiments ATLAS LHC-B CMS ALICE + LHCf +
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

6. LHC example: CMS inner tracker
Outer Barrel (TOB)
Inner Barrel (TIB)
End Cap (TEC)
Inner Disks (TID)
Pixel
93 cm
30 cm
Pixel Detector
Total weight
12500 t
Diameter
15m
Length
21.6m
Magnetic field
4 T
CMS – “Currently the Most Silicon”
Micro Strip:
~ 214 m2 of silicon strip sensors, million strips
Pixel:
Inner 3 layers: silicon pixels (~ 1m2)
66 million pixels (100x150mm)
Precision: σ(rφ) ~ σ(z) ~ 15mm
Most challenging operating environments (LHC)
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

7. Micro-strip Silicon Detectors
Highly segmented silicon detectors have been used in Particle Physics experiments for nearly 30 years. They are favourite choice for Tracker and Vertex detectors (high resolution, speed, low mass, relatively low cost)
Pitch ~ 50mm
p+ in n-
Main application: detect the passage of ionizing radiation with high spatial resolution and good efficiency.
Segmentation → position
Resolution ~ 5mm
Reference: P.Allport, Sept.2010
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

8. Hybrid Pixel Detectors
Hybrid Active Pixel Sensors
segment silicon to diode matrix with high granularity ( true 2D, no reconstruction ambiguity)
readout electronic with same geometry (every cell connected to its own processing electronics)
connection by “bump bonding”
requires sophisticated readout architecture
Hybrid pixel detectors will be used in LHC experiments: ATLAS, ALICE, CMS and LHCb
Solder Bump: Pb-Sn
~ 15mm
(VTT/Finland)
Flip-chip technique
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

9. How does a silicon detector work?
Silicon Detectors
How does a silicon detector work?
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

10. How to obtain the signal?f
valence band
conduction band h e
In a pure intrinsic (undoped) semiconductor the electron density n and hole density p are equal.
For Silicon: ni  1.451010 cm-3
4.5 108 free charge carriers in this volume, but only 3.2 104 e-h pairs produced by a M.I.P.
 Reduce number of free charge carriers, i.e. deplete the detector
 Most detectors make use of reverse biased p-n junctions

11. Covalent Bonding of Pure Silicon
Energy Si
Silicon atoms share valence electrons to form insulator-like bonds.
Conduction Band (CB)
Eg =1.1eV
Valence Band (VB)
Now let us examine a common material used in the manufacture of microchips. A crystal of pure silicon is formed from individual atoms that share electrons with each other. The result of the bonding of silicon atoms is to form a crystal lattice structure of covalent bonds. A close look at the valence rings show that each silicon atom shares its valence electrons with four neighboring atoms. Thus, it would appear that the valence ring of each atom has a full complement of valence electrons. In this example, very few electrons exist except on the outer surfaces of the crystal. This may explain why silicon sometimes behaves very much like an insulator.
Thermal energy at RT: kBT40 meV
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

12. Electrons in N-Type Silicon with Phosphorus Dopant
Conduction Band (CB)
Valence Band (VB)
Donor atoms provide excess electrons
to form n-type silicon. Si P
40-50 meV
Phosphorus atom serves as n-type dopant
Excess electron (-)
Group 5 elements such as phosphorus have one more free electron than the silicon atom. When phosphorus is added to a crystal of silicon and then heated, phosphorus atoms become bonded with silicon atoms. The phosphorus atoms become part of the crystal lattice structure. Now, if we look closely around the phosphorus (P) atoms, we’ll see one additional electron in the lattice. These are free electrons that are available to carry current flow through the silicon crystal. Because this type of silicon crystal has an excess of negative electron charges, it is commonly referred to as n-type silicon.
In N-type Si, electrons are the majority charge carriers. They surpass the number of holes by orders of magnitude (ni~1010 cm-3, Nd= cm-3):
𝑝 𝑛 = 𝑛 𝑖 𝑁 𝑑
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

13. Holes in p-Type Silicon with Boron Dopant
Conduction Band (CB)
Valence Band (VB)
Acceptor atoms provide a deficiency
of electrons to form p-type silicon. Si B
Group 3 elements such as boron have one less free electron than the silicon atom. When boron is added to a crystal of silicon and then heated, boron atoms become bonded with silicon atoms. The boron atoms become part of the crystal lattice structure. Looking closely around the boron (B) atoms, we’ll see there are vacancies in the lattice. These vacancies are called holes. Holes have a positive charge and are also available to carry current flow through the silicon crystal. Because this type of silicon crystal has predominantly positive hole charges, it is commonly referred to as p-type silicon.
In P-type Si, holes are the majority charge carriers. They surpass the number of electrons by orders of magnitude.
+ Hole
Boron atom serves as p-type dopant
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

14. “Hole Movement in Silicon”
                  Boron is neutral, but nearby electron may jump to fill bond site.
                   Boron is now a  negative ion.
                  Only thermal energy to kick electrons from atom to atom.
The empty silicon bond sites (holes) are thought of as being  positive, since their presence makes that region positive.
                   Hole moved from 2  to 3 to 4, and will move to 5.
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, Summer 2013
-14-

15. Sketchy view of a pn-junction (I)

16. Sketchy view of a pn-junction (II)
Majority charge carriers in N-type and P-type have disappeared no current from majority charge carriers.
There is a small reverse current (leakage) coming from carriers injected by diffusion from the undepleted regions/surface (no E-field inside the undepleted bulk). Diffusion electrons (holes) enter from undepleted P (N) into depleted P SCR (N SCR). Leakage current due to minority carriers.
Leakage current increases with T, thickness and irradiation. It constitutes the “background noise” to the measurement.
In this workshop, we will measure the leakage current and capacitance characteristics of irradiated detectors

17. Detector = “reverse biased p-i-n diode”
Positive space charge, Neff =[P] (ionized Phosphorus atoms)
Poisson’s equation
neutral bulk (no electric field)
Depleted zone growth with increasing voltage ( )
Electrical charge density
Electrical field strength
Electron potential energy
Full charge collection only for fully depleted detector (VB>Vdep)
depletion voltage Vdep
detector thickness d
effective space charge density Neff
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

18. SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

19. Characterization techniques
Silicon Detectors
Characterization techniques
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

20. Testing Structures - Simple Diodes
Example: Test structure from ITE
Very simple structures in order to concentrate on the bulk features
Typical thickness: 300m
Typical active area: 0.5  0.5 cm2
Openings in front and back contact
optical experiments with lasers or LED
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

21. Characterization techniques for detectors
Detectors need to be optimized for Minimum Ionizing Particles (MIPs) detection. Study charge collection efficiency with radioactive sources (Sr90, for instance) and finally test beams (bunched beams, real scale system test).
Time resolved induced currents provide information on drift velocity, electric field configuration, trapping times.
Best measurement conditions achieved with lasers:
Reproducibility: no fluctuations in deposited energy  averaging possible -> S/N improvement.
Selectable absorption length: by varying laser wavelength. Contribution from only one type of carriers (red laser, top or bottom injection) or both types (IR laser).
Easy triggering
TCT (Transient Current Techniques) use laser pulses to induce charge carriers in the detector. Time resolved induced currents are recorded and analysed.
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

22. Transient Current Technique
Time resolved induced current can be calculated using Ramo theorem 𝑰 𝒕 =−𝒒 𝒗 𝑬 𝑾
where 𝒗 = 𝒗 (𝑬) is the drift velocity, E is the electric field and 𝑬 𝑾 the so-called weighting field
Electrons
Holes
Total
Electric field determines the charge trajectory and the velocity of the particle. It changes:
With bias voltage
Strip geometry
Irradiation of the detector
Electric field for typical detectors:
Pad diode: linear electric field (~capacitor)
Strip detector: peaked near the electrodes, linear in the center
Weighting field is the derivative of the weighting potential 𝑈 𝑊 . This potential determines how charge couples to an electrode: 𝑄=𝑞 𝑈 𝑤 2 − 𝑈 𝑤 1
(induced charge by a carrier moving from position 1 to 2 )
Weighting field for typical detectors:
Pad diode: constant
Strip detector: very asymmetric. Peaked near the collection electrode
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

23. TCT explained: diode
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

24. TCT example: diode Top TCT, h-injection (p-bulk):
Induced current maximum at front junction
Longer collection time due to smaller drift velocity
Bottom TCT, e injection:
Drift velocity increase towards the front side
Shorter collection time, and higher amplitude of pulses, both due to higher drift velocity
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

25. TCT setup
Nicola Pacifico, PhD thesis, Bari University, 2012
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

26. What is radiation damage?
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

27. Radiation Damage – Microscopic Effects
particle
SiS
Vacancy Interstitial
point defects (V-O, C-O, .. )
point defects and clusters of defects
EK>25 eV
EK > 5 keV V I
Neutrons (elastic scattering)
En > 185 eV for displacement
En > 35 keV for cluster
60Co-gammas
Compton Electrons with max. E 1 MeV (no cluster production)
Electrons
Ee > 255 keV for displacement
Ee > 8 MeV for cluster
Only point defects point defects & clusters Mainly clusters
10 MeV protons GeV/c protons MeV neutrons
Simulation:
Initial distribution of vacancies in (1m)3 after 1014 particles/cm2
[Mika Huhtinen NIMA 491(2002) 194]
Quelle est la différence entre des irradiations avec différentes particules ?
nous avons vu qu'une énergie de 25 eV est nécessaire pour produire un défaut ponctuelle et 5 kev pour former un cluster
une irradiation avec les rayons gamma ne peut pas produire le clusters parce qu’ils produisent des électrons de Compton avec une énergie maximum de 1 MeV. Les électrons avec une énergie de 1 mev peuvent seulement produire des défauts ponctuelles
…..
La différence entre les différentes particules est visualisée dans cette simulation
nous regardons un volume de 1 micromètre Cubique après des irradiations avec différentes particules d'une fluence de 10e14 p/cm2
distribution homogène des trous
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, Summer 2013

28. Impact of Defects on Detector properties
Shockley-Read-Hall statistics (standard theory)
charged defects  Neff , Vdep e.g. donors in upper and acceptors in lower half of band gap
Trapping (e and h)  CCE shallow defects do not contribute at room temperature due to fast detrapping
generation  leakage current Levels close to midgap most effective
Impact on detector properties can be calculated if all defect parameters are known: n,p : cross sections E : ionization energy Nt : concentration
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

29. Macroscopic Effects – I. Depletion Voltage
Change of Depletion Voltage Vdep (Neff)
• 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):
• “Type inversion”: Neff changes from positive to negative (Space Charge Sign Inversion)
sur l’axe x ..il diminue ..il augmente avant derrière composants court terme à long terme
région de charge spatiale
after inversion
before inversion n+ p+
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015 29

30. Radiation Damage – II. Leakage Current
Change of Leakage Current (after hadron irradiation)
80 min 60C
Damage parameter  (slope in figure) Leakage current per unit volume and particle fluence
 is constant over several orders of fluence and independent of impurity concentration in Si  can be used for fluence measurement
pente - ordres de grandeur - dosimètre
Strong temperature dependence
Consequence: Cool detectors during operation! Example: I(-10°C) ~1/16 I(20°C)
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015 30

31. Radiation Hard Silicon Detectors
How to make silicon detectors radiation harder?
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

32. The RD50 Collaboration “Radiation hard semiconductor devices for very high luminosity colliders”
RD50: 48 institutes and 261 members
38 European and Asian institutes Belarus (Minsk), Belgium (Louvain), Czech Republic (Prague (3x)), Finland (Helsinki, Lappeenranta ), Germany (Dortmund, Erfurt, Freiburg, Hamburg, Karlsruhe, Munich), Italy (Bari, Florence, Padova, Perugia, Pisa, Trento), Lithuania (Vilnius), Netherlands (NIKHEF), Norway (Oslo)), Poland (Krakow, Warsaw(2x)), Romania (Bucharest (2x)), Russia (Moscow, St.Petersburg), Slovenia (Ljubljana), Spain (Barcelona(2x), Santander, Valencia), Switzerland (CERN, PSI), Ukraine (Kiev), United Kingdom (Glasgow, Liverpool)
8 North-American institutes Canada (Montreal), USA (BNL, Fermilab, New Mexico, Purdue, Santa Cruz, Syracuse) 1 Middle East institute Israel (Tel Aviv) 1 Asian institute India (Delhi)
Details:
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

33. 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, Gallium Arsenide
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
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

34. 3D detector concept 3D PLANAR
“3D” electrodes: - narrow columns along detector thickness, - diameter: 10mm, distance: mm
Lateral depletion: - lower depletion voltage needed - thicker detectors possible - fast signal - radiation hard p+ - +
300 mm n+
50 mm 3D
PLANAR
n-columns
p-columns
wafer surface
n-type substrate
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

35. Summary – Radiation Damage
Radiation Damage in Silicon Detectors
Change of Depletion Voltage (internal electric field modifications, “type inversion”, reverse annealing, loss of active volume, …) (can be influenced by defect engineering!)
Increase of Leakage Current
Increase of Charge Trapping
Signal to Noise ratio is quantity to watch (material + geometry + electronics)
Microscopic defects
Microscopic crystal defects are the origin to detector degradation.
Approaches to obtain radiation tolerant devices:
Material Engineering: - explore and develop new silicon materials (oxygenated Si) - use of other semiconductors (Diamond)
Device Engineering: - look for other sensor geometries D, thin sensors, n-in-p, n-in-n, …
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015 35 35

36. Acknowledgements & References
Most references to particular works given on the slides
Books containing chapters about radiation damage in silicon sensors
Helmuth Spieler, “Semiconductor Detector Systems”, Oxford University Press 2005
Frank Hartmann, “Evolution of silicon sensor technology in particle physics”, Springer 2009
L.Rossi, P.Fischer, T.Rohe, N.Wermes “Pixel Detectors”, Springer, 2006
Gerhard Lutz, “Semiconductor radiation detectors”, Springer 1999
Research collaborations and web sites
CERN RD50 collaboration ( ) - Radiation Tolerant Silicon Sensors
CERN RD39 collaboration – Cryogenic operation of Silicon Sensors
CERN RD42 collaboration – Diamond detectors
Inter-Experiment Working Group on Radiation Damage in Silicon Detectors (CERN)
ATLAS IBL, ATLAS and CMS upgrade groups
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

37. …
BACKUP
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

38. The LHC Upgrade Program
LHC luminosity upgrade (Phase II) (L=5x1034 cm-2s-1) in 2022
Challenge: Build detectors that operate after 3000 fb-1
V.Boudry, CLIC Workshop, January 2015
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

39. 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
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

40. Signal degradation for LHC Silicon Sensors
Pixel sensors: max. cumulated fluence for LHC
Note: Measured partly under different conditions! Lines to guide the eye (no modeling)!
Situation in 2005
Strip sensors: max. cumulated fluence for LHC
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

41. Signal degradation for LHC Silicon Sensors
Pixel sensors: max. cumulated fluence for LHC and LHC upgrade
Note: Measured partly under different conditions! Lines to guide the eye (no modeling)!
LHC upgrade will need more radiation tolerant tracking detector concepts! Boundary conditions & other challenges: Granularity, Powering, Cooling, Connectivity, Triggering, Low mass, Low cost!
Strip sensors: max. cumulated fluence for LHC and LHC upgrade
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

42. Reverse biased abrupt p+-n junction
w = depletion depth d = detector thickness U = voltage
Neff = effective doping concentration
Poisson’s equation
with
Maintenant un transparent pour ceux qui ne sont pas des experts. Il explique ce que c'est la tension de désertion totale.
Tout commence par l'équation de Poisson; ce qui fait un lien entre le potentiel électrique et la concentration effective de dopage. Dans le détecteur non irradie le concentration effective de dopage est équivalent à la concentration en phosphore.
Si vous résolvez l'équation de Poisson vous obtenez la distribution du potentiel électrique et la distribution du champ électrique. Si vous appliquez une certaine tension vous obtenez une certaine profondeur de désertion. Il y a une région avec le champ électrique et une région sans champ électrique. la région avec le champ électrique est déserte des porteurs libres de charge, mais la région sans champ électrique n'est pas déserte des porteurs libres de charge. Si un MIP (particule au minimum de ionisation) croise le détecteur il crée des paires électron - trou. Dans la région déserte le électron et le trou sont séparées par le champ électrique et ils donnent un signal en dehors du détecteur. Dans la région non déserte le électron et le trou sont séparées par le champ électrique et ils donnent pas un signal en dehors du détecteur. Donc, pour obtenir un signal de tous des paires électron - trou vous en a besoin de déserte tout le volume de détecteur. Alors, vous devez appliquer une tension qui est assez haut pour avoir un champ électrique dans tout le détecteur. cette tension est la tension de tension de désertion totale. Il est directement proportionnel au concentration effective de dopage et il est proportionnel à l'épaisseur de détecteur carrée
région de charge spatiale
depletion voltage
effective space charge density
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

43. NIEL - Hypothesis
How to normalize radiation damage arising from different particles?
NIEL – Non Ionizing Energy Loss scaling using hardness factor k of a radiation field (or monoenergetic particle) with respect to 1 MeV neutrons
E energy of particle
D(E) displacement damage cross section for a certain particle at energy E
D(1MeV neutrons) = 95 MeV·mb
f(E) energy spectrum of the radiation field
The integrals are evaluated for the interval [EMIN,EMAX], being EMIN and EMAX the minimum and maximum cut-off energy values, respectively, and covering all particle types present in the radiation field
M.Moll, Bethe Forum on Particle Detectors, Bonn – April 2014

44. NIEL – Non Ionizing Energy Loss Displacement damage functions
1 MeV neutron equivalent damage 1
1 MeV
Φ 𝑒𝑞 = 𝜅 𝑥 Φ 𝑥
𝜅 𝑝 =0.62 (24 GeV/c protons)
𝜅 𝑝 = 1.85 (26 MeV protons)
𝜅 𝜋 = 1.14 (300 MeV pions)
𝜅 𝑛 = 0.92 (reactor neutrons >100 keV)
Hypothesis: Damage parameters scale with the NIEL
Be careful, does not hold for all particles & damage parameters (see later)
M.Moll, Bethe Forum on Particle Detectors, Bonn – April 2014

45. Radiation Damage – Microscopic EffectsV
Spatial distribution of vacancies created by a 50 keV Si-ion in silicon (typical recoil energy for 1 MeV neutrons)
van Lint 1980
M.Huhtinen 2001
Sur ce transparent une figure simplifiée des dommages microscopiques du rayonnement est montrée.
la figure représente une partie du cristal de silicium  environ 600 Angstroem par 600 Angstroem
Ici, une particule rapide (par exemple un neutron avec de une énergie de 1 MeV) frappe un atome de silicium de cristal de silicium.
les simulations nous indiquent que l'énergie cinétique moyenne donnée à l'atome de silicium est de 50 kev
l'atome, qui a été frappé, ensuite traverse  le cristal et produit d'autres dommages (d'autres déplacements d’atomes)
Un atome qui est déplacé de son endroit de mailles s'appelle un interstitiel (I)
le trou qui rester dans le mailles s'appelle un vacancy (V) 
Ce sont des défaut ponctuelles
si les ions de silicium ont une énergie plus grande que 5 kev ils peuvent également produire des clusters
ceci se produit souvent à l'extrémité de la voie
un cluster est un espace où beaucoup d'atomes sont déplacés dans un petit volume
il y a beaucoup de dommages dans ce petit volume du cristal
le vacancy  et l’interstitiel, tout les deux sont mobiles dans le mailles du cristal,
ils migrent jusqu'à ce qu'ils trouvent des impuretés et forment des défauts (par exemple une vacancy et un atome d'oxygène forment un défaut de Vo
il y a une bonne concordance entre la figure simplifiée ici et une simulation représentée a droite
particle
SiS
Vacancy Interstitial
point defects (V-O, C-O, .. )
point defects and clusters of defects
EK>25 eV
EK > 5 keV V I
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

46. Radiation Damage – III. CCE (Trapping)
Deterioration of Charge Collection Efficiency (CCE) by trapping
Trapping is characterized by an effective trapping time eff for electrons and holes:
where
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
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015 46

47. Silicon Growth Processes
Floating Zone Silicon (FZ)
Czochralski Silicon (CZ)
The growth method used by the IC industry.
Difficult to produce very high resistivity
[Oi ] ~ 5  1017 cm-3
Poly silicon
RF Heating coil
Single crystal silicon
Czochralski Growth
Float Zone Growth
Epitaxial Silicon (EPI)
Basically all silicon tracking detectors made out of FZ silicon [Oi ]< 5  1016 cm-3
Some pixel sensors: Diffusion Oxygenated FZ (DOFZ)silicon [Oi ]~ 1-2  1017 cm-3
Chemical-Vapor Deposition (CVD) of Si
up to 150 mm thick layers produced
growth rate about 1mm/min
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

48. Standard FZ, DOFZ, MCz and Cz 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 (for experts: there is no “real” type inversion, a more clear understanding of the observed effects is obtained by investigating directly the internal electric field; look for: TCT, MCZ, double junction)
Common to all materials (after hadron irradiation, not after  irradiation):
reverse current increase
increase of trapping (electrons and holes) within ~ 20%
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

49. Silicon materials for Tracking Sensors
Signal comparison for p-type silicon sensors
Note: Measured partly under different conditions! Lines to guide the eye (no modeling)!
LHC
SLHC
n-in-p technology should be sufficient for Super-LHC at radii presently (LHC) occupied by strip sensors
highest fluence for strip detectors in LHC: The used p-in-n technology is sufficient
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

50. Solid State Detectors – Why Silicon?
Some characteristics of Silicon crystals
Small band gap Eg = 1.12 eV  E(e-h pair) = 3.6 eV ( 30 eV for gas detectors)
High specific density 2.33 g/cm3 ; dE/dx (M.I.P.)  3.8 MeV/cm  106 e-h/m (average)
High carrier mobility e =1450 cm2/Vs, h = 450 cm2/Vs  fast charge collection (<10 ns)
Very pure < 1ppm impurities and < 0.1ppb electrical active impurities
Rigidity of silicon allows thin self supporting structures
Detector production by microelectronic techniques  well known industrial technology, relatively low price, small structures easily possible
Alternative semiconductors
Diamond
Gallium arsenide (GaAs)
Silicon Carbide (SiC)
Germanium (Ge)

51. Principle of operation
Goal: precise charged particle position measurement
Use ionization signal (dE/dx) from charged particle passage (In a semiconductor, ionization produces electron hole (e-h) pairs
Problems: - In pure intrinsic (undoped) silicon there are more free charge carriers than those produced by a charged particle - electron – hole pairs quickly re-combine
Solution: - Deplete the free charge carriers and collect electrons or holes quickly by exploiting the properties of a p-n junction (diode) - electric field is used to drift electrons and holes to oppositely charged electrodes
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

52. Conduction in n-Type Silicon
Free electrons flow toward positive terminal.
Positive terminal from power supply
Electron Flow
Negative terminal from power supply
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

53. Conduction in p-Type Silicon
Electron Flow
Hole Flow
Positive terminal from voltage supply
Negative terminal from voltage supply
+Holes flow toward negative terminal
-Electrons flow toward positive terminal
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

54. Doping, resistivity and p-n junction
Doping: n-type silicon
add elements from Vth group  donors (P, As,..)
electrons are majority carriers
Doping: p-type silicon
add elements from IIIrd group  acceptors (B,..)
holes are majority carriers Si P
e.g. Phosphorus
resistivity r
carrier concentration n, p
carrier mobility mn, mp
p-n junction There must be a single Fermi level!  band structure deformation  potential difference  depleted zone
detector grade
electronics grade
doping
 1012 cm-3
 1017 cm-3
resistivity 
 5 k·cm
1 ·cm

55. SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

56. SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

57. Reverse biased p-n junction
Positive space charge, Neff =[P] (ionized Phosphorus atoms)
Poisson’s equation
neutral bulk (no electric field)
Depleted zone growth with increasing voltage ( )
Electrical charge density
Electrical field strength
Electron potential energy
Full charge collection only for fully depleted detector (VB>Vdep)
depletion voltage Vdep
detector thickness d
effective space charge density Neff

58. SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015

59. Single Sided Strip Detector
Segmentation of the p+ layer into strips (Diode Strip Detector) and connection of strips to individual read-out channels gives spatial information
typical thickness: 300mm (150m - 500m used)
pitch
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

60. Signal to noise ratio becomes even lower by noise broadening
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)
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 !
Noise
Signal
Cut (threshold)

61. Detector Module
Detector Modules “Basic building block of silicon based tracking detectors”  Silicon Sensors  Mechanical support (cooling)  Front end electronics and signal routing (connectivity)
Example: ATLAS SCT Barrel Module
SCT = SemiConductor Tracker ASICS = Application Specific Integrated CircuitS TPG = Thermal Pyrolytic Graphite
128 mm
 Silicon sensors (x4) x 64 mm p-in-n, single sided - AC-coupled strips - 80m pitch/12mm width
 ASICS (x12) ABCD chip (binary readout) DMILL technology channels
 Wire bonds (~3500) mm Al wires
 Mechanical support - TPG baseboard - BeO facings
ATLAS – SCT microstrip sensors barrel modules forward modules m2 silicon, strips
 Hybrid (x1) - flexible 4 layer copper/kapton hybrid mounted directly over two of the four silicon sensors - carrying front end electronics, pitch adapter, signal routing, connector
s(rf) ~ 16 mm, s(z) ~ 850mm [NIMA538 (2005) 384]

62. Pixel Detectors HAPS – Hybrid Active Pixel Sensors Solder Bump: Pb-Sn
segment silicon to diode matrix with high granularity ( true 2D, no reconstruction ambiguity)
readout electronic with same geometry (every cell connected to its own processing electronics)
connection by “bump bonding”
requires sophisticated readout architecture
Hybrid pixel detectors are used in all LHC experiments: ATLAS, ALICE, CMS and LHCb
Solder Bump: Pb-Sn
~ 15mm
Flip-chip technique

63. Example: The CMS Silicon Tracker
Inner Tracker
Outer Barrel (TOB 6-layer)
Inner Barrel (TIB – 4 layer)
5.4 m
2.4 m
End Cap (TEC)
Inner Disks (TID)
CMS – Compact Muon Solenoid
Micro Strip:
~ 214 m2 of silicon strip sensors
11.4 million strips
Pixel:
Inner 3 layers: silicon pixels (~ 1m2)
66 million pixels (100x150mm)
Precision: σ(rφ) ~ σ(z) ~ 15mm
Most challenging operating environments (LHC)
Pixel
Pixel Detector
93 cm
30 cm

64. TCT example: diode G. Kramberger,PhD thesis, Un. Ljubljana, 2001
SSD team (Fernandez Garcia, Gallrapp, Moll,Neugebauer) CERN, February 2015