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Jaap Velthuis (University of Bristol)1 Radiation damage in silicon sensors Topics: –Damage due to protons and neutrons Radiation induced defects Annealing.

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Presentation on theme: "Jaap Velthuis (University of Bristol)1 Radiation damage in silicon sensors Topics: –Damage due to protons and neutrons Radiation induced defects Annealing."— Presentation transcript:

1 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

2 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

3 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

4 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. is most probable value

5 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: E G high-> less - + - - - + + +

6 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 important chose digital or binary readout To do better need to share charge over more strips need pitch 20 m for 300 m thick sensor Problem: connecting all strips to readout channel yields too many strips

7 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

8 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. 50 m pitch thus 17 m precision and easy read out Only this needed for binary

9 Jaap Velthuis (University of Bristol)9 sLHC radiation dose 5 year radiation dose close to beam pipe ~10 16 n eq /cm 2 –too high for state- of-the-art standard silicon sensors Most radiation hard material: diamond –High bandgap –Displacement energy 43eV (13-20 for Si)

10 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

11 Jaap Velthuis (University of Bristol)11 Radiation with protons/neutrons Energy needed to displace atom from lattice=15eV Damage energy dependent –~<2keV isolated point defect –2-12keV defect cluster –~>12keV many defect clusters This damage is called Non- Ionizing Energy Loss (NIEL) –Results scaled to 1MeV neutrons Electrons and photons dont make defects!

12 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 ------

13 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!

14 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 = 3.99 0.03 x 10 -17 Acm -1 after 80minutes annealing at 60 C

15 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 = 0.025cm -1 measured after beneficial anneal P-strips in p-bulk

16 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 dont reach it signal spread over many strips poor spatial resolution undepleted

17 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

18 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

19 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

20 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

21 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

22 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

23 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

24 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 –7 o C

25 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-SiO 2 interface Material Engineering Operate very cold Use different material (e.g. CVD diamond) 3D-structures

26 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 bulk becomes p-type After radiation n + bias contact

27 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 years Dose for ATLAS strips

28 Jaap Velthuis (University of Bristol)28 Material engineering Diffusing oxygen suppresses V 2 O formation –V + O VO –V + VO V 2 O –V 2 O reverse annealing Still alchemy…

29 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

30 Jaap Velthuis (University of Bristol)30 CVD Diamond Remember: Limit background charge by using large bandgap material –Si: E G =1.12 eV 1.5E10 free carriers/cm 3 –Diamond: E G =5.47 eV 6E-28 free carriers/cm 3 no need for pn-junction –Diamond lattice very strong very radiation hard Note: large E G only few eh pairs produced (3600 vs 8900 per 100 m), but also lower noise

31 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

32 Jaap Velthuis (University of Bristol)32 ATLAS CVD diamond pixel module Sensor: –Active area: 61x16.5mm 2 –Thickness 800µm –Pixel size 400(600)x50µm 2 –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 -

33 Jaap Velthuis (University of Bristol)33 CVD diamond radiation hardness Still S/N18-25 after 1.8x10 16 p/cm 2 (~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

34 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

35 Jaap Velthuis (University of Bristol)35 Diamond signal spectrum S/N99 N=136.8 e -

36 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 doesnt die

37 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.

38 Jaap Velthuis (University of Bristol)38 Czochralski silicon (Cz) Standard FZ silicon type inversion at ~ 2 10 13 p/cm 2 strong N eff increase at high fluence Oxygenated FZ (DOFZ) type inversion at ~ 2 10 13 p/cm 2 reduced N eff 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

39 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

40 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 Physical edge

41 Jaap Velthuis (University of Bristol)41 3D detectors S/N source test=13 with 121 m thick detector

42 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

43 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

44 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 –7 o C cryogenic temperatures (Lazarus effect) –Use different materials like diamond –Use different detector type like 3D


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