1 / 101

Semiconductor detectors

Semiconductor detectors. An introduction to semiconductor detector physics as applied to particle physics. Contents. 4 lectures – can’t cover much of a huge field Introduction Fundamentals of operation The micro-strip detector Radiation hardness issues. Lecture 1 - Introduction.

chacha
Download Presentation

Semiconductor detectors

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Semiconductor detectors An introduction to semiconductor detector physics as applied to particle physics Dr R. Bates

  2. Contents 4 lectures – can’t cover much of a huge field • Introduction • Fundamentals of operation • The micro-strip detector • Radiation hardness issues Dr R. Bates

  3. Lecture 1 - Introduction • What do we want to do • Past, present and near future • Why use semiconductor detectors Dr R. Bates

  4. What we want to do - Just PPE • Track particles without disturbing them • Determined position of primary interaction vertex and secondary decays • Superb position resolution • Highly segmented  high resolution • Large signal • Small amount of energy to crate signal quanta • Thin • Close to interaction point • Low mass • Minimise multiple scattering • Detector • Readout • Cooling / support Dr R. Bates

  5. Ages of silicon - the birth • J. Kemmer • Fixed target experiment with a planar diode* • Later strip devices -1980 • Larger devices with huge ancillary components * J. Kemmer: “Fabrication of a low-noise silicon radiation detector by the planar process”, NIM A169, pp499, 1980 Dr R. Bates

  6. Ages of Silicon - vertex detectors • LEP and SLAC • ASIC’s at end of ladders • Minimise the mass inside tracking volume • Minimise the mass between interaction point and detectors • Minimise the distance between interaction point and the detectors • Enabled heavy flavour physics i.e. short lived particles Dr R. Bates

  7. ALEPH Dr R. Bates

  8. ALPEH – VDET (the upgrade) • 2 silicon layers, 40cm long, inner radius 6.3cm, outer radius 11cm • 300mm Silicon wafers giving thickness of only 0.015X0 • S/N rF = 28:1; z = 17:1 • srf = 12mm; sz = 14mm Dr R. Bates

  9. Ages of silicon - tracking paradigm • CDF/D0 & LHC • Emphasis shifted to tracking + vertexing • Only possible as increased energy of particles • Cover large area with many silicon layers • Detector modules including ASIC’s and services INSIDE the tracking volume • Module size limited by electronic noise due to fast shaping time of electronics (bunch crossing rate determined) Dr R. Bates

  10. ATLAS • A monster ! Dr R. Bates

  11. ATLAS barrel • 2112 Barrel modules mounted on 4 carbon fibre concentric Barrels, 12 in each row • 1976 End-cap modules mounted on 9 disks at each end of the barrel region Dr R. Bates

  12. What is measured • Measure space points • Deduce • Vertex location • Decay lengths • Impact parameters Dr R. Bates

  13. Signature of Heavy Flovours Dr R. Bates

  14. Decay lengths • By measuring the decay length, L, and the momentum, p, the lifetime of the particle can be determined • Need accuracy on both production and decay point E.g. B  J/Y Ks0 L Secondary vertex Primary vertex L = p/m c t Dr R. Bates

  15. Impact parameter (b) b = distance of closest approach of a reconstructed track to the true interaction point b beam Dr R. Bates

  16. Impact parameter • Error in impact parameter for 2 precision measurements at R1 and R2 measured in two detector planes: • a=f(R1 & R2) and function of intrinsic resolution of vertex detector • b due to multiple scattering in detector • c due to detector alignment and stability Dr R. Bates

  17. Impact parameter • sb = f( vertex layers, distance from main vertex, spatial resolution of each detector, material before precision measurement, alignment, stability ) • Requirements for best measurement • Close as possible to interaction point • Maximum lever arm R2 – R1 • Maximum number of space points • High spatial resolution • Smallest amount of material between interaction point and 1st layer • Good stability and alignment – continuously measured and correct for • 100% detection efficiency • Fast readout to reduce pile up in high flux environments Dr R. Bates

  18. Impact parameter* Blue = 5mm Black = 1mm (baseline) Green = 0.5 mm Red = 0.1 mm Effect of extra mass and distance from the interaction point Lower Pt *Guard Ring Width Impact on d0 Performances and Structure Simulations. A Gouldwell, C Parkes, M Rahman, R Bates, M Wemyss, G Murphy, P Turner and S Biagi. LHCb Note,LHCb-2003-034

  19. Why Silicon • Semiconductor with moderate bandgap (1.12eV) • Thermal energy = 1/40eV • Little cooling required • Energy to create e/h pair (signal quanta)= 3.6eV c.f Argon gas = 15eV • High carrier yield  better stats and lower Poisson stats noise • Better energy resolution and high signal  no gain stage required Dr R. Bates

  20. Why silicon • High density and atomic number • Higher specific energy loss • Thinner detectors • Reduced range of secondary particles • Better spatial resolution • High carrier mobility  Fast! • Less than 30ns to collect entire signal • Industrial fabrication techniques • Advanced simulation packages • Processing developments • Optimisation of geometry • Limiting high voltage breakdown • Understanding radiation damage Dr R. Bates

  21. Disadvantages • Cost  Area covered • Detector material could be cheap – standard Si • Most cost in readout channels • Material budget • Radiation length can be significant • Effects calorimeters • Tracking due to multiple scattering • Radiation damage • Replace often or design very well – see lecture 4 Dr R. Bates

  22. Radiation length X0 • High-energy electrons predominantly lose energy in matter by bremsstrahlung • High-energy photons by e+e-pair production • The characteristic amount of matter traversed for these related interactions is called the radiation length X0, usually measured in g cm-2. • It is both: • the mean distance over which a high-energy electron loses all but 1=e of its energy by bremsstrahlung • the mean free path for pair production by a high-energy photon Dr R. Bates

  23. Lecture 2 – lots of details • Simple diode theory • Fabrication • Energy deposition • Signal formation Dr R. Bates

  24. Near intrinsic bulk Highly doped contacts Apply bias (-ve on p+ contact) Deplete bulk High electric field Radiation creates carriers signal quanta Carriers swept out by field Induce current in external circuit  signal Detector = p-i-n diode n+ contact ND=1018cm-3 ND~1012cm-3 p+ contact NA=1018cm-3 Dr R. Bates

  25. Why a diode? • Signal from MIP = 23k e/h pairs for 300mm device • Intrinsic carrier concentration • ni = 1.5 x 1010cm-3 • Si area = 1cm2, thickness=300mm  4.5x108 electrons • 4 orders > signal • Need to deplete device of free carriers • Want large thickness (300mm) and low bias But no current! • Use v.v. low doped material • p+ rectifying (blocking) contact Dr R. Bates

  26. p-n junction Carrier density p+ n (5) (1) (2) Electric field (6) (3) Dopant concentration Electric potential (4) Space charge density (7) Dr R. Bates

  27. p-n junction • take your samples – these are neutral but doped samples: p+ and n- • bring together – free carriers move • two forces drift and diffusion • In stable state Jdiffusion (concentration density) = Jdrift (e-field) • p+ area has higher doping concentration (in this case) than the n region Dr R. Bates

  28. p-n junction • Fixed charge region • Depleted of free carriers • Called space charge region or depletion region • Total charge in p side = charge in n side • Due to different doping levels physical depth of space charge region larger in n side than p side • Use n- (near intrinsic)  very asymmetric junction • Electric field due to fixed charge • Potential difference across device • Constant in neutral regions. Dr R. Bates

  29. Resistivity and mobility • Carrier DRIFT velocity and E-field: • mn = 1350cm2V-1s-1 : mp = 480cm2V-1s-1 • Resistivity • p-type material • n-type material Dr R. Bates

  30. Depletion width • Depletion Width depends upon Doping Density: • For a given thickness, Full Depletion Voltage is: • W = 300mm, ND 5x1012cm-3: Vfd = 100V Dr R. Bates

  31. Reverse Current • Diffusion current • From generation at edge of depletion region • Negligible for a fully depleted detector • Generation current • From generation in the depletion region • Reduced by using material pure and defect free • high lifetime • Must keep temperature low & controlled Dr R. Bates

  32. Capacitance • Capacitance is due to movement of charge in the junction • Fully depleted detector capacitance defined by geometric capacitance • Strip detector more complex • Inter-strip capacitance dominates Dr R. Bates

  33. Noise • Depends upon detector capacitance and reverse current • Depends upon electronics design • Function of signal shaping time • Lower capacitance  lower noise • Faster electronics  noise contribution from reverse current less significant Dr R. Bates

  34. Fabrication • Use very pure material • High resistivity • Low bias to deplete device • Easy of operation, away from breakdown, charge spreading for better position resolution • Low defect concentration • No extra current sources • No trapping of charge carriers • Planar fabrication techniques • Make p-i-n diode • pattern of implants define type of detector (pixel/strip) • extra guard rings used to control surface leakage currents • metallisation structure effects E-field mag  limits max bias Dr R. Bates

  35. Starting material Usually n- Phosphorous diffusion P doped poly n+ Si Stages dopants to create p- & n-type regions passivation to end surface dangling bonds and protect semiconductor surface metallisation to make electrical contact Fabrication stages n- Si Dr R. Bates

  36. Fabrication stages • Deposit SiO2 • Grow thermal oxide on top layer • Photolithography + etching of SiO2 • Define eventual electrode pattern Dr R. Bates

  37. Fabrication stages • Form p+ implants • Boron doping • thermal anneal/Activation • Removal of back SiO2 • Al metallisation + patterning to form contacts Dr R. Bates

  38. Fabrication • Tricks for low leakage currents • low temperature processing • simple, cheap • marginal activation of implants, can’t use IC tech • gettering • very effective at removal of contaminants • complex Dr R. Bates

  39. Energy Deposition • Charge particles • Bethe-Bloch • Bragg Peak • Not covered • Neutrons • Gamma Rays • Rayleigh scattering, Photo-electric effect, Compton scattering, Pair production Dr R. Bates

  40. Charge particles- concentrating on electrons • At   3 dE/dx minimum independent of absorber (mip) • Electrons • mip @ 1 MeV • E>50 MeV radiative energy loss dominates Momentum transferred to a free electron at rest when a charged particle passes at its closest distance, d. integrate over all possible values of d Dr R. Bates

  41. at end of range specific energy loss increases particle slows down deposit even more energy per unit distance Well defined range Bragg Peak E = 5 MeV in Si: (increasing charge) R (m) p 220  25 16O 4.3 Useful when estimating properties of a device Dr R. Bates

  42. Energy Fluctuation • Electron range of individual particle has large fluctuation • Energy loss can vary greatly - Landau distribution • Close collisions (with bound electrons) • rare • energy transfer large • ejected electron initiates secondary ionisation • Delta rays - large spatial extent beyond particle track • Enhanced cross-section for K-, L- shells • Distance collisions • common • M shell electrons - free electron gas Dr R. Bates

  43. e/h pair creation • Create electron density oscillation - plasmon • requires 17 eV in Si • De-excite almost 100% to electron hole pair creation • Hot carriers • thermal scattering • optical phonon scattering • ionisation scattering (if E > 3/4 eV) • Mean energy to create an e/h pair (W) is 3.6 eV in Si (Eg = 1.12 eV  3 x Eg) • W depends on Eg therefore temperature dependent Dr R. Bates

  44. Delta rays • Proability of ejecting an electron with E  T as a function of T b) Range of electron as a function of energy in silicon Dr R. Bates

  45. Displacement from d-electrons • Estimate the error • Assume 20k e/h from track • 50keV d-electron produced perpendicular to track • Range 16mm, produces 14k e/h • Assume ALL charge created locally 8mm from particle’s track Dr R. Bates

  46. Centroid displacement Resolution as function of pulse height Consequences of d-electrons Dr R. Bates

  47. Consequence of d-electrons 45º 45º 15 m E.g. CCD 300 m Most probable E loss = 3.6keV 10% proby of 5keV  pulls track up by 4 m E.g. Microstrip Most probable E loss = 72keV 10% proby of 100keV  pulls track up by 87 m Dr R. Bates

  48. Signal due to the motion of charge carriers inside the detector volume & the carriers crossing the electrode Displacement current due to change in electrostatics (c.f. Maxwell’s equations) Material polarised due to charge introduction Induced current due to motion of the charge carriers See a signal as soon as carriers move Signal formation Dr R. Bates

  49. Signal • Simple diode • Signal generated equally from movement through entire thickness • Strip/pixel detector • Almost all signal due to carrier movement near the sense electrode (strips/pixels) • Make sure device is depleted under strips/pixels If not: • Signal small • Spread over many strips Dr R. Bates

  50. Lecture 3 – Microstrip detector • Description of device • Carrier diffusion • Why is it (sometimes) good • Charge sharing • Cap coupling • Floating strips • Off line analysis • Performance in magnetic field • Details • AC coupling • Bias resistors • Double sides devices Dr R. Bates

More Related