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Introduction to Silicon Detectors

Introduction to Silicon Detectors. G.Villani STFC Rutherford Appleton Laboratory Particle Physics Department. Outlook. Introduction to physics of Si and detection Si electronic properties, transport mechanisms, detection Examples of detectors Strips, CMOS,CCD,MOS Radiation damage

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Introduction to Silicon Detectors

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  1. Introduction to Silicon Detectors G.VillaniSTFC Rutherford Appleton LaboratoryParticle Physics Department

  2. Outlook • Introduction to physics of Si and detection • Si electronic properties, transport mechanisms, detection • Examples of detectors • Strips, CMOS,CCD,MOS • Radiation damage • Conclusions 2

  3. Introduction The Si detection chain Sensing/ Charge creation Charge transport and collection Conversion Signal processing Data TX E Si physical properties Si device properties Si device topologies properties almost all the boxes of the detection chain process based upon Silicon 3

  4. Silicon properties After Oxygen, Silicon is the 2nd most abundant element in Earth’s crust (>25% in mass) The crystalline structure is diamond cubic (FCC), 8 atoms/cell with lattice spacing of 5.43 A ~ 5x1022 cm-3 * In electronic industry all crystallographic forms are used (Single crystal, Polysilicon, α-Si) The key to success of Si is related to its abundance and oxide SiO2, an excellent insulator (BV ~ 107 V/cm). * Micro crystals but the flexible bond angles make SiO2 effectively an amorphous: its conductivity varies considerably (charge transport in SiO2 via polaron hopping between non-bonding oxygen 2p orbitals) Si 1.48A 4

  5. Silicon electrical properties Silicon Band structure The electronic band structure can be obtained within the independent electron approximation (normally 1 electron SE in periodic potential neglecting electron interactions) in terms of Bloch functions ~ a wave associated with free motion of electrons modulated by the periodic solution un,k. The energy E is periodic in k so is specified just within the 1st unit WS cell of the reciprocal lattice (the Brillouin zone). * The appearance of Band Gap, separating CB and VB * The 6 CB minima are not located at the center of 1st Brillouin zone, INDIRECT GAP VB CB 1st Brillouin zone of Diamond lattice CB VB-H VB-L 5

  6. Silicon electrical properties The detailed band structure is complicated: usually quasi-equilibrium simplifications are sufficient to study the charge transport. Assuming that the carriers reside near an extremum, the dispersion relationship E(k) is almost parabolic: (3D) * Under the assumptions of small variation of the electric field, the carrier dynamics resembles that one of a free particle, with appropriate simplifications. * The effective mass approximation takes into account the periodic potential of the crystal by introducing an effective carrier mass ( averaged over different longitudinal and transverse masses). The lower the mass, the higher mobility (µ  1/m*) * Similar approach used to calculate the E(k) for phonons. 6

  7. Silicon electrical properties • The carrier density is calculated from: • The density of states g(E), which depends on dimension; • The distribution function F(E); • Only partly filled bands can contribute to conduction: carrier density in CB and VB. • At equilibrium the carrier density is obtained by integrating the product: CB VB 3 2 1 Fermi level: energy level @ 50% occupancy In intrinsic Si a creation of e in CB leaves behind a hole in VB, that can be treated as an e with positive charge and mobility of the band where it resides 0 The density of states gD(E) depends on the dimension 7

  8. Silicon electrical properties Conduction of Si intrinsic @ T = 300K: σ = q(μn +μp) ni = 3.04x10-6mho-cm ->329kOhm-cm By adding atoms of dopants, which require little energy to ionize( ~10’s mEV, so thermal energies @ ambient temp is enough) we can change by many odg the carrier concentration. Doping concentration: 1012 to 1018 cm-3 In crystalline Si ~ 5*1022atomscm-3 In equilibrium and for non degenerate case the relationship between carrier concentration and E is the same as in the intrinsic case: 8

  9. Charge transport Near equilibrium equilibrium 0 k Charge transport: The charge transport description in semiconductors relies on semi-classical BTE (continuity equation in 6D phase space) Q conservation P conservation E conservation The distribution function f(r,k) can be approximated near equilibrium: 9

  10. Charge transport Under (many) simplifying assumptions the 1st moment of BTE gives the DD model (The semiconductor equations): Drift term Diffusion term Transport of charge is a combination of drift and diffusion mechanism * DD expresses momentum conservation: it becomes invalid when sharp variation in energy of carriers occur (due to F for example: deep submicron devices) When feature size is 0.’sμm the DD model becomes invalid: higher momentum required 10

  11. Detection principles A: Ionization: by imparting energy to break a bond, electrons are lifted from VB to CB then made available to conduction ( ionization chambers, microstrip, hybrid pixels, CCD, MAPS…) α MIP Photon interaction Bethe formula for stopping power gives the rate of energy loss/unit length for charged particles through matter 11

  12. Detection MIP charge density Photoelectric charge density An optical power of -60dBm (= 1nW) of 1keV photons generates ~ 6*106e-/μm High injection regime: Plasma effects The internal electric field can be affected by the generated charge A MIP forms an ionization trail of radius R when traversing Si, creating ~ 80e-/μm Low injection regime: the generated charge is too small to affect the internal electric field The associated wavelength is much smaller than mean free path: Each charge is independent from each other; Carrier dynamics does not need QM 12

  13. Detection B: Excitation: Charge or lattice (acoustic or optical phonons) some IR detectors, bolometer 60meV Poly Si SiO2 Si Ec EF ~10’s meV EF EV Eigenvalues separation in quantized structures ~ 10’s meV Dispersion relation for phonons in Si Phonon excitation energy ~ 10 meV : much lower threshold 13

  14. Signal conversion: The pn junction Homojunction: consider two pieces of same semiconductor materials with different doping levels: In equilibrium, the Fermi level equalizes throughout the structure The thermal diffusion of charge across the junction leaves just the ionized dopants : an electric potential, and a field F, develops across the junction In equilibrium J = 0: using DD model Near the interface, the carrier concentration exponentially drops: a depletion region (empty of free charge) is formed. ASCE (Abrupt Space Charge Edge) approximation: A ‘positive’ voltage increases (exponentially) the charge concentration: high direct current. A ‘negative’ voltage decreases it (down to leakage): the current reduces and at the same time widens the depleted region. Unidirectionality of current characteristics 14

  15. Signal conversion: The pn junction The electric field F in the depletion region of the junction is sustained by the ionized dopants. When charge is generated is swept across by the field PN junction signal converter: A capacitor with a strong F across A device with a large depleted region W can be used to efficiently collect radiation generated charge ( Solid state ionization chamber) W • To achieve large W high field region: • Low doping (high resistivity) Silicon is needed • Large voltages Conversion: Q to V // Q to I 15

  16. Detectors examples Strip detectors Scientific applications Monolithic Active Pixel Sensors (MAPS) Imaging, consumer applications Charge Coupled Devices (CCD) Imaging, scientific and consumer applications MOS detector scientific applications RAL PPD has (is) actively involved with all these detector technologies 16

  17. Detectors examples Use of Si Strip detectors Almost all HEP experiments use Si detectors: The high density track region usually covered by pixel detectors; by strip at larger radius (cost reason) 17

  18. Detectors examples module 768 Strip Sensors RO ATLAS SCT 4 barrel layers,2 x 9 forward disks 4088 double sided modules Total Silicon surface 61.1m² Total 6.3 M channels Power consumption ~ 50kW Events rate: 40MHz Put stave pics of AUG! 18

  19. Detectors examples Strip detectors 768 Strip Sensors 80μm P++ N+ (high res) 300μm F Wires RO electronic Power supply Vbias~100’sV Array of long silicon diodes on a high resistivity silicon substrate A strong F in the high resistivity Si region helps collect charge efficiently (drift). The transversal diffusion of charge implies a spread of signal over neighbouring strips The high resistivity Si is not usually used in mainstream semiconductor industry: Hybrid solution: detectors connected (wire/bumpbonded) to the readout electronic (RO) 19

  20. Detectors examples High events rate require fast signal collection: Estimate of charge collection time in strip detector: For a detector thickness of 300um and overdepleted Vb = 50V and 10kohm resistivity tcoll(e)≈ 12ns tcoll(h)≈ 35ns The fast collection time helps the radiation hardness: The radiation damage to sensors is a crucial issue in modern HEP experiments 20

  21. Detectors examples MAPS detectors ≈10’s mm RO electronic RO electronic 3T ( 3MOS) MAPS structure 2D array of ~106 pixels Monolithic solution: Detector and readout integrated onto the same substrate 21

  22. Detectors examples MAPS detectors Vbias ~V’s N++ (low res) Electronics 0.’s μm P+ (low-med res) Active region ‘s μm P++ (low res) Mechanical substrate 100’s μm The charge generated in the thin active region moves by diffusion mainly: ‘Long’ collection time Small signal Different implants arrangements for charge collection optimization Circuit topologies for low noise 22

  23. Detectors examples 10-7 Example of MAPS detectors: TPAC 1 pixel size 50x50um2 Chip size ~1cm2 Total pixels 28k >8Meg Transistors Charge collection time (s) in MAPS vs. perpendicular MIP hit 23

  24. Detectors examples CCD detectors Once the charge has been generated, it accumulates in the potential well, under the capacitor. The control circuitry shifts the accumulated charge to the end of the row, to the input of a charge amplifier. The sensor is fabricated in a optimized, dedicated process and the RO on a separate chip. Superior imaging quality but less integration and speed. Nobel Prize 2009 for Physics to inventors Boyle and Smith 24

  25. Detectors examples 5 mm Global Photogate and Transfer gate ROW 1: CCD clocks Imaging pixel ROW 2: CCD clocks 80 mm On-chip switches On-chip logic ROW 3: CCD clocks ROW 1: RSEL Global RG, RD, OD RG RD OD RSEL Column transistor In-situ Storage Image Sensor: ISIS 55Fe g source Mn(Ka) Mn(Kb) CCD in CMOS process 0.18μm Charge collection under a PG then stored under a 20 pixels storage CCD 25

  26. Signal conversion: The unipolar MOS device NMOS SiO2 Metal Oxide Semiconductor device are unipolar devices based on voltage modulation of charge. The control gate is physically separated by the active region where the charge moves by a thin (nm) layer of SiO2. N++ P++ • By applying a voltage to the G with respect to the Substrate • an electric field develops across the SiO2: a charge channel • is formed between Source and Drain. • The Ids characteristics depends on the Vgs applied. • The CMOS process refers to the minimum feature size • achievable i.e. the channel length) • Currently 45nm: the modelling of the characteristics of the • device of this size is non-trivial: • Quantization effects at the boundary; • QM tunnelling across the gate; • Hot carriers near the D/S junction; • … 26

  27. Signal conversion: The unipolar MOS device LET in SiO2 for different particles Generation rate in SiO2 vs. electric field The SiO2 is a very good insulator: a strong electric field can be applied to it and the charge generated in SiO2 by ionizing radiation efficiently collected However SiO2 is a polar material: the recombination processes are stronger than in Si. Furthermore, hole Transport is non Gaussian (low ‘mobility’) and traps form near Si interface. 27

  28. Ids(A) Pre-rad Post-rad Reprog SiO2 FG SiO2 FG Signal conversion: The unipolar MOS device Floating Gate Control Gate SiO2 Addition of a Floating Gate (FG): the electrical characteristics of the device are controlled by the charge stored in the FG. The electric field in the SiO2 due to the FG drifts charge towards/away from it. The discharge of the FG alters the device electrical characteristics Radiation sensitivity The MOS structure easily allows excitation based radiation detection Conversion: Q to I 28

  29. Radiation damage TID/Gy/yr NIEL/cm2/yr ATLAS In HEP and space applications the detectors are exposed to high level of radiation: LHC: 10’s Mrad (100kGy) over 10years of operation N.B.: 1 rad/cm3 Si ~1013e/h pairs Total Ionizing Dose (rad = 0.01Gy) Non Ionizing energy Loss (1MeV neutrons/cm2 fluence) 29

  30. Radiation damage Radiation environment in LHC experiment TIDFluence 1MeV n eq. [cm-2] @ 10 years ATLAS Pixels 50 Mrad 1.5 x 1015 ATLAS Strips 7.9 Mrad 2 x 1014 CMS Pixels ~24Mrad ~6 x 1014 CMS Strips 7.5 Mrad 1.6 x 1014 ALICE Pixel 250 krad 3 x 1012 LHCb VELO - 1.3 x 1014/year All values including safety factors. 30

  31. V I Vacancy + Interstitial EK>25 eV Radiation damage Microscopic effects: Bulk damage to Silicon : Displacement of lattice atoms (~ Kinetic Energy Released) Atoms scattered by incoming particles leave behind vacancies or atoms in interstitial positions (Frenkel pairs). Low energy particle ~ point defects High energy particles ~ cluster defects 31

  32. compensation - Radiation damage Energy deposition Atoms displacement altered Lattice periodicity Band gap Spurious states Altered Electrical characteristics Conduction band trapping recombination generation Donor levels +++ Band gap Acceptor levels Valence band • The appearance of spurious band gap states affects the electro/optical characteristics of the device: • Thermal generation of carriers (increased leakage current @ same T) • Reduced recombination time ( quicker charge loss , reduced signal) • Charge trapping • Scattering • Type conversion 32

  33. Radiation damage • Detrimental Macroscopic effects: • Noise increases because of increase leakage current • Charge Collection Efficiency (CCE) is reduced by trapping • Depletion voltage increases because of type inversion 1015 1MeV n-eq. 33

  34. Radiation damage • To increase the Radiation Hardness of Sensors: • Operating conditions (cooler – lower leakage) • Material engineering ( OFZ - Diamond detectors) • Device engineering (n in n – 3D detectors) • Electrodes in the bulk – lateral collection • The device achieve full depletion • Low depletion voltage • short collection time • claim reduction in signal 33% after 8.8X1015 1Mevn • difficult to manufacture • 3D DDTC similar to 3D but easier to manufacture; also • better mechanical strength. • * Radiation damage affects also the RO electronics, • but modern process can address the problem efficiently • ( guard rings, sub micron devices) 34

  35. Addendum - Detector systems HEP experiments: large detector systems Challenging engineering issues The ATLAS SCT (semiconductor tracker) detector. The thick red cables on show feed the detector with half of its power – adding more will take up even more space 35

  36. Addendum - Detector systems Alternative powering schemes: SP ATLAS SCT Barrel 3 at CERN. Half of the 384 cables are visible; the rest enters the other end of the detector. DC2DC A serial powering or DC2DC approach can increase efficiency in power distribution compared to a parallel approach 36

  37. Conclusions • The field of semiconductor detectors encompasses different scientific and technology • fields: solid state physics, nuclear and particle physics, electrical engineering, … • Some of the issues relevant to radiation detectors: • Radiation hardness • Topologies optimization (power reduction, noise reduction) • Development of new detection techniques based on novel and well established semiconductor material: ( phonon-based detectors, compounds, low dimensional) • Integration with electronics (monolithic solution to achieve more compactness and reduce cost),3D structures 37

  38. Backup - Detector systems Power reduction at detector level = At pixel level, power consumption could be optimized by using a non linear approach: The positive feedback structure is biased near threshold (variable) A small signal triggers the structure I

  39. Backup - Detection The variance in signal charge σiassociated to the ionization process is related to the phonon excitation Fano factor ~0.1 in Si High resolution requires smaller band gap (εi ), direct or small phonon excitation energy Intrinsic resolution of Si and Ge based detectors II

  40. Backup -Detection Ph: DQ~107 m-1 DQ~1010 m-1 p/a The indirect BG of Si requires higher energy for charge excitation, because energy and momentum must be conserved (Phonon-assisted pair creation/recombination) In Si an average of 3.6 eV is required for pair creation Put values of photon momentum typ. III

  41. Backup Quantization effects due to band bending in Si-SiO2 interface: excitation based detection SiO2 Si-sub Si-poly Q-effects IV

  42. Backup - The bipolar transistor device • A bipolar transistor can be thought of as a two diode system, connected in anti series; • One is forward biased; • The other is reverse biased • The bipolar transistor can be (and it is) used as a high gain detector • Main limitations arising from speed: the minority carriers diffuse through the base ( relatively low speed) V

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