Characterization of silicon sensors after irradiation with fast particles Alison G. Bates and Michael Moll CERN - Geneva - Switzerland CERN, Summer Student Workshop 26 th to 28 th July 2005

Workshop outline Presentation on silicon sensors: - Operation of silicon detectors - Introduction to radiation damage and annealing Exercise - Calculate a C-V-curve - Calculate the defect concentration in silicon Experiment - Measure CV and IV curves - Annealing experiment Data Analysis and Conclusions for detector operation in the LHC ……Outlook: New detector concepts

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop m 2.4 m End Cap (TEC) Example from LHC: The CMS tracker Outer Barrel (TOB) Inner Barrel (TIB) Pixel Inner Disks (TID) CMS - Currently the Most Silicon  Micro Strip:  ~ 214 m 2 of silicon strip sensors  11.4 million strips  Pixel:  Inner 3 layers: silicon pixels (~ 1m 2 )  66 million pixels (100x150  m)  Precision: σ(rφ) ~ σ(z) ~ 15  m  Most challenging operating environments (LHC) 93 cm 30 cm Inner Tracker CMS Pixel Detector

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Tracking detectors – Radiation levels  Detectors and electronics will be harshly irradiated !  ATLAS - Inner Detector:  eq up to 3  cm -2 per operational year ATLAS - Inner Detector  What is the impact on silicon detectors ? CMS Tracker  200 m 2 silicon sensors

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 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

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Si Silicon atoms share valence electrons to form insulator-like bonds. Covalent Bonding of Pure Silicon Eg =1.1eV Conduction Band (CB) Valence Band (VB) Energy

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Donor atoms provide excess electrons to form n-type silicon. Si P P P Electrons in N-Type Silicon with Phosphorus Dopant Phosphorus atom serves as n-type dopant Excess electron (-) Conduction Band (CB) Valence Band (VB)

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Conduction in n-Type Silicon Free electrons flow toward positive terminal. Positive terminal from power supply Electron Flow Negative terminal from power supply

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Acceptor atoms provide a deficiency of electrons to form p-type silicon. + Hole Boron atom serves as p-type dopant Si B B B Holes in p-Type Silicon with Boron Dopant Conduction Band (CB) Valence Band (VB)

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 “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. Hole moved from 2 to 3 to 4, and will move to 5. The empty silicon bond sites (holes) are thought of as being positive, since their presence makes that region positive.

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 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

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 p-n-junction

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Poisson’s equation Reverse biased abrupt p + -n junction Electrical charge density Electrical field strength Electron potential energy effective space charge density depletion voltage Full charge collection only for V B >V dep ! Positive space charge, N eff =[P] (ionized Phosphorus atoms)

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Poisson’s equation Reverse biased abrupt p + -n junction effective space charge density depletion voltage with w = depletion depth d = detector thickness U = voltage N eff = effective doping concentration

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Testing Structures - Simple Diodes  Very simple structures in order to concentrate on the bulk features  Typical thickness: 300  m  Typical active area: 0.5  0.5 cm 2  Openings in front and back contact  optical experiments with lasers or LED Example: Test structure from ITE

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 The Charge signal Mean charge Most probable charge ≈ 0.7  mean 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

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Radiation Damage – Microscopic Effects particle Si S Vacancy + Interstitial point defects (V-O, C-O,.. ) point defects and clusters of defects E K >25 eV E K > 5 keV V I I I V V  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

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Radiation Damage – Microscopic Effects particle Si S Vacancy + Interstitial point defects (V-O, C-O,.. ) point defects and clusters of defects E K >25 eV E K > 5 keV V I  Neutrons (elastic scattering)  E n > 185 eV for displacement  E n > 35 keV for cluster  60 Co-gammas  Compton Electrons with max. E   1 MeV (no cluster production)  Electrons  E e > 255 keV for displacement  E e > 8 MeV for cluster Only point defects point defects & clusters Mainly clusters 10 MeV protons 24 GeV/c protons 1 MeV neutrons Simulation: Initial distribution of vacancies in (1  m) 3 after particles/cm 2 [Mika Huhtinen NIMA 491(2002) 194]

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Point Defects  Intrinsic defects:  The Vacancy (denoted V): an atom is removed.  The Self-interstitial (denoted I ): a host atom sits in a normally unoccupied site or interstice (various sites: bond centres, tetrahedral sites, interstitial + displaced regular atom).  Extrinsic defects: due to an impurity. These can be:  Substitutional, such as carbon substitutional (denoted C s )  Interstitial (such as the carbon interstitial (denoted C i ).

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Point Defects - Lattice strain Ge, Sn CsCs C i, O i Link: Vacancy - Hydrogen DefectVacancy - Hydrogen Defect

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Impact of Defects on Detector properties Shockley-Read-Hall statistics (standard theory) Impact on detector properties can be calculated if all defect parameters are known:  n,p : cross sections  E : ionization energy N t : concentration Trapping (e and h)  CCE shallow defects do not contribute at room temperature due to fast detrapping charged defects  N eff, V dep e.g. donors in upper and acceptors in lower half of band gap generation  leakage current Levels close to midgap most effective

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Macroscopic Effects – I. Depletion Voltage Change of V dep (N eff ) before inversion after inversion n+n+ p+p+ n+n+ Annealing  Short term: “Beneficial annealing”  Long term: “Reverse annealing” time constant : ~ 500 years (-10°C) ~ 500 days ( 20°C) ~ 21 hours ( 60°C)  Type inversion: SCSI – Space Charge Sign Inversion

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Macroscopic Effects – II. Leakage Current Hadron irradiation Annealing  Damage parameter  (slope)  independent of  eq and impurities  used for fluence calibration (NIEL-Hypothesis)  Oxygen enriched and standard silicon show same annealing  Same curve after proton and neutron irradiation 80 min 60  C

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Macroscopic effects - III. CCE Deterioration of the Charge Collection Efficiency ATLAS microstrip + RO electronics  Two mechanisms reduce collectable charge:  Trapping (electrons and holes)  Underdepletion (detector design and geometry) Data: Gianluigi Casse; 1 st Workshop on Radiation Hard Semiconductor Devices for High Luminosity Colliders; CERN; November 2002  Oxygenation has no influence on trapping.  After 5·10 14 p/cm 2 (24GeV/c) - 80% of charge collected (25ns) - overdepletion needed !

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Radiation Damage  I. Change of Depletion Voltage  Under-depletion  Type inversion (segmented detector side not in the high field region any more)  Reverse annealing: keep detectors cold even if experiment is not running  II. Increase of Leakage Current  Noise, power dissipation, thermal runaway  Cooling of detectors during operation needed  III. Degradation of Charge Collection Efficiency  Loss of signal due to trapping  Loss of signal due to under-depletion

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Our experiment  Measure Capacitance-Voltage and Current-Voltage curves for: a) non irradiated detectors b) irradiated detectors (1e13 p/cm 2 and 1e14 p/cm 2 )  Perform an annealing experiment with the irradiated detectors: Isochronal annealing (iso-chronus = same time) - annealing steps of 10 minutes at 50, 60, 70, 80, 90, 100, … °C  Analysis of the results

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Damage Projection - ATLAS Pixel B-layer  eq (year) = 3.5  cm -2 (full luminosity) > 85% charged hadrons  Radiation level:  eq (year) = 3.5  cm -2 (full luminosity) > 85% charged hadrons 1 year = 100 days beam (-7  C) 30 days maintenance (20  C) 235 days no beam (-7  C)  LHC-scenario: 1 year = 100 days beam (-7  C) 30 days maintenance (20  C) 235 days no beam (-7  C)  New: Std. Silicon:Std. Silicon: rad.harder than predicted by RD48 DOFZ:DOFZ: reverse annealing delayed and saturating with high fluences

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Annealing mechanisms  Migration and complex formation  Defects become mobile at certain temperature and migrate through the silicon lattice e.g. Vacancies (V) between 70 and 200 K (depending on their charge state).  Migrating defects are gettered at sinks, recombine with their counterparts or form new defects (complex) by association with identical or other types of defects e.g. V + O i  VO i.  Dissociation  A complex dissociates into its components if the lattice vibrational energy is sufficient to overcome the binding energy. At least one of the constituents migrates through the lattice until it forms another defect or disappears into a sink e.g. at  350°C : VO i  V + O i.  All mechanisms need to overcome a certain energetic barrier E A E m, E F, E B = activation energies (E A )

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Reverse Annealing - Temperature dependence  Measurement of N Y (t) at different temperatures Extraction of  Y (T) Arrhenius plot    activation energy : E Aa = (1.33  0.03) eV  frequency factor : k 0Y =1.5  {4  34  } s -1  interpretation: decay of defects (k 0 close to most abundant phonon frequency)  prediction: time constants for other temperatures

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Rate of Reaction - Example I  Defect dissociation (e.g. at  350°C : VO i  V + O i )  Simple description as 1 st order process (like radioactive decay) N X = defect concentration k = rate constant  Rate constant k is given by the Arrhenius relation k 0 = frequency factor E A = activation energy k B = Boltzmann constant (8.6 x eV/K)  Frequency factor k 0 lies in the order of the most abundant phonon frequency  k B T/ h = 2.1·10 10 x T[K] s -1  s -1 (at 300K) Ref.: [Corbett 1966] “ attempt-to-escape frequency”

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Rate of Reaction - Example II  Diffusion limited processes  Diffusion limited reaction of two defects X and Y N X = concentration defect X Ny = concentration defect Y D = Diffusivity R = capture radius  Diffusion constant D 0 is given by the Arrhenius relation D 0 = diffusion constant E A = activation energy k B = Boltzmann constant (8.6 x eV/K)  Special case: N X << N Y e.g. V + O i  VO i with [V]<<[O i ]  similar kinetics as for simple 1 st order process (Example I)  4  R D 0 N Y  k 0

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Outlook

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005 Influence of Carbon and Oxygen concentration 24 GeV/c proton irradiation Compared to standard silicon:   High Carbon  less radiation tolerant   High Oxygen  more radiation tolerant

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005  Electrodes:  narrow columns along detector thickness-“3D”  diameter: 10  m distance:  m  Lateral depletion:  lower depletion voltage needed  thicker detectors possible  fast signal Device Engineering: 3D detectors n n p p n n n n Present size up to ~1cm 2 proposed by Sherwood Parker

CERN DT2/SSD Michael Moll and Alison G. Bates – Summer Student Workshop 2005  Electrodes:  narrow columns along detector thickness-“3D”  diameter: 10  m distance:  m  Lateral depletion:  lower depletion voltage needed  thicker detectors possible  fast signal  Hole processing :  Dry etching, Laser drilling, Photo Electro Chemical  Present aspect ratio (RD50) 13:1, Target: 30:1  Electrode material  Doped Polysilicon (Si)  Schottky (GaAs) Device Engineering: 3D detectors n n p p n n n n Present size up to ~1cm 2 proposed by Sherwood Parker