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Introduction to Silicon Detectors E.G.Villani STFC Rutherford Appleton Laboratory Particle Physics Department 1.

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Presentation on theme: "Introduction to Silicon Detectors E.G.Villani STFC Rutherford Appleton Laboratory Particle Physics Department 1."— Presentation transcript:

1 Introduction to Silicon Detectors E.G.Villani STFC Rutherford Appleton Laboratory Particle Physics Department 1

2 Outlook Introduction to physics of Si and detection Examples of detectors Radiation damage Systems of detectors Conclusions 2

3 Introduction The Si detection chain 3 Sensing/ Charge creation Charge transport and collection Conversion Signal processing Data TX E Si physical propertiesSi device propertiesSi device topologies properties Detector physics: Silicon physical and electrical properties Detection principles Transport mechanisms Conversion Detector examples: Strips MAPS Detector system issues: Detection efficiency Power all the boxes of the detection chain process based upon Silicon

4  The crystalline structure is Diamond Cubic (FCC), with lattice spacing of 5.43 A Polysilicon consists of small Si crystals randomly oriented; in α-Si there is no long range order.  Apart from its abundance, the key to success of Si is related to its oxide SiO 2, an excellent insulator (BV ~ 10 7 V/cm). Micro crystals but the flexible bond angles make SiO 2 effectively an amorphous: its conductivity varies considerably (charge transport in SiO 2 via polaron hopping between non-bonding oxygen 2p orbitals) Silicon properties  After Oxygen, Silicon is the 2 nd most abundant element in Earth’s crust (>25% in mass) Si 1.48A 4

5 Silicon electrical properties 5  The appearance of Band Gap, separating CB and VB  The 6 CB minima are not located at the center of 1 st Brillouin zone, Indirect Gap CBVB-HVB-L 1 st Brillouin zone of Diamond lattice CB VB The electronic band structure can be obtained by solving 1 electron SE in periodic potential neglecting electron interactions : Bloch functions ~ a wave associated with free motion of electrons modulated by the periodic solution u n,k. The dispersion relationship E(k) is periodic in k so is specified just within the Brillouin zone.

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 E(k) is almost parabolic:  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. 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) 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: The density of states g D (E) depends on the dimension 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 CB VB *Fermi level: energy 50% occupancy 7

8 Silicon electrical properties From carrier density: Conduction of Si T = 300K: σ = q(μ n +μ p ) n i = 3.04x10 -6 mho-cm = 329 kOhm-cm  By adding atoms of dopants, which require little energy to ionize( ~10’s mEV, so thermal ambient temp is enough) we can change by many orders the carrier concentration. Doping concentration: to cm -3 In crystalline Si ~ 5*10 22 atoms cm -3 The relationship between carrier concentration and E is the same as in the intrinsic case: 8

9 Charge transport The charge transport description relies on semi-classical BTE (continuity equation in 6D phase space) Q conservation P conservation E conservation 9  Higher moments of BTE correspond to more accurate transport descriptions

10 Charge transport Under (many) simplifying assumptions the 1 st moment of BTE gives the DD model (The semiconductor equations):  DD expresses momentum conservation: it becomes invalid when sharp variation in energy of carriers occur (due to F for example: deep submicron devices, which require higher moments or alternative models) Even in low injection regime, a small F renders the drift term >> diffusion term Diffusion term Drift term 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. Most exploited concept ( ionization chambers, microstrip, hybrid pixels, CCD, MAPS…) B: Excitation: Charge or lattice (acoustic or optical phonons) some IR detectors, bolometer Bethe-Bloch formula for stopping power gives the rate of /unit length for charged particles through matter Photon interaction α MIP ~  = 3 11

12 Detection  In Si an average of 3.6 eV is required for pair creation (partly going into phonons)  The MPV of e - generated by MIP is ~77/µm, vs. an average of 110/µm The indirect BG of Si requires higher energy for charge excitation, because energy and momentum must be conserved (Phonon-assisted pair creation/recombination) Ph  Q~10 7 m -1  Q~10 10 m -1  a 12

13 Detection A MIP forms an ionization trail of radius R when traversing Si, creating ~ 77 e - /μm MIP charge density  Low injection regime: The generated charge is too small to affect the internal electric field  Carrier dynamics does not need QM Photoelectric charge density An optical power of -60dBm (= 1nW) of 1keV photons generates ~ 6*10 6 e-/μm  High injection regime: Plasma effects  The internal electric field can be affected by the generated charge 13

14 Detection B: Excitation: Dispersion relation for phonons in Si Phonon excitation energy ~ 10 meV : much lower threshold  Can be used for sensitive bolometers 60meV EcEc EFEF EVEV EFEF ~10’s meV Eigenvalues separation in quantized structures ~ 10’s meV SiSiO 2 Poly Si 14

15 Half summary Under conditions of quasi equilibrium non small feature size low injection … The behaviour of detectors can be drastically simplified Sensing/ Charge creation Charge transport and collection Conversion E E(k): Free particle, effective mass Charge transport: Drift- diffusion model Q generation: /L 15

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

17 Signal conversion: The pn junction Homojunction: two pieces of same semiconductor materials with different doping levels: In equilibrium, the Fermi level equalizes throughout the structure thermal diffusion of charge across the junction leaves just a depleted region, with the ionized dopants : an electric potential Φ, and a field F, develops the charge concentration depends exponentially on Φ: by applying a small voltage to decrease the barrier, the charge increases exponentially by applying a voltage to increase the barrier, the depleted region Increases Unidirectionality of current characteristics In equilibrium J = 0 17

18 Signal conversion: The pn junction when charge is generated in the depleted region is swept across by the electric field sustained by the ionized dopants and the biasing.  PN junction as detector and signal converter: capacitor with a F across A device with a large depleted region 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) Large biasing voltages 18 Depletion width

19 Detectors examples Array of long silicon diodes on a high resistivity silicon substrate A strong F in the high resistivity Si region helps collect charge efficiently, mostly by drift.  The high resistivity Si is not usually used in mainstream semiconductor industry: Hybrid solution: detector (high resistivity) connected (wire/bump-bonded) to the readout electronic (low resistivity) (high res) V bias ~100’sV F ATLAS Strip detectors 80μm ≈ 300μm 19 Q transversal diffusion

20 Detectors examples ATLAS SCT 4 single-sided p-on-n sensors 2x2 sensors daisy chained Stereo angle 40 mrad Strip pitch 80 µm 768 channels/side Binary RO Bias Voltage up to 500 V Operating temperature -2 C Space point resolution: rφ 17µm / z 50 µm Power consumption: 5.6 W (initially ) to 10 W ( after 10y) Rad-hard up to 2x10^14 1-MeV n eqv/cm Strip Sensors 12 RO ASIC ATLAS SCT RO ASIC (ABCD3TA) 128 channels/ASIC RAD-HARD DMILL technology ASICS glued to hybrid 40 MHz Ck

21 Detectors examples ATLAS SCT 61 m 2 of Si 6.3 * 10^ 6 channels 50 kW total power consumption 1 barrel -4 layers modules 2 end caps -9 disks/each -988 modules 2 T solenoidal field 21

22 Detectors examples 22 ATLAS Tracker overview Pixel: (n+ on n) 1.8 m 2, 80M channels SCT: 6.3M 61 m 2 channels TRT: 0.4M channels

23 Detectors examples 23 Barrel insertion in the ATLAS Cavern ATLAS SCT Red cables : power cables

24 Detectors examples 24

25 Detectors in operation 25

26 Detectors in operation 26

27 Detectors examples RO electronic 3T ( 3MOS) MAPS structure ≈1’s  m RO electronic MAPS detectors 27 2 D array of pixels  Monolithic solution: detector and readout integrated onto the same substrate

28 Detectors examples MAPS detectors  The charge generated in the thin active region moves by diffusion mainly: ‘Long’ collection time Small signal Low radiation hardness Complex circuit topologies allow DSP on pixels for low noise N ++ (low res) P ++ (low res) P + (low-med res) V bias ~V’s Mechanical substrate 100’s μm Active region ‘s μm Electronics 0.’s μm 28

29 Detectors examples Charge collection time (s) in MAPS vs. perpendicular MIP hit TPAC 1 pixel size 50x50 µm 2 Chip size ~1cm 2 Total pixels 28k >8Meg Transistors Example of MAPS detectors: 29

30 Detectors examples Example of charge collection in MAPS: simulated MIP vs.1064nm laser 2x2um 5ns pulse 30

31 Detectors example Proposed use of MAPS (50x50 μm 2 ) sensors in SuperB Vertex Tracker DNW MAPS proposed should improve collection efficiency and speed of response 31

32 Radiation damage 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/cm 3 Si ~10 13 e/h pairs  Lethal dose: 500 rad Total Body Irradiation 32

33 Radiation damage Radiation environment in LHC experiment TIDFluence 1MeV n eq. [cm years ATLAS Pixels50 Mrad1.5 x ATLAS Strips7.9 Mrad2 x CMS Pixels~24Mrad~6 x CMS Strips7.5Mrad1.6 x ALICE Pixel250krad3 x LHCb VELO-1.3 x /year All values including safety factors. 33

34 Radiation damage Microscopic effects: Bulk damage to Silicon : Displacement of lattice atoms 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 V I Vacancy + Interstitial E K >25 eV 34

35 Radiation damage Energy deposition Atoms displacement altered Lattice periodicity Band gap Spurious states  The appearance of spurious band gap states affects the electro/optical characteristics of the device: Thermal generation of carriers (increased leakage current) Reduced recombination time ( quicker charge loss, reduced signal) Charge trapping Scattering Type conversion Altered Electrical characteristics Conduction band Valence band Band gap +++ Donor levels Acceptor levels generation recombination - compensation trapping 35

36 Radiation damage Macroscopic effects:  Charge Collection Efficiency (CCE) is reduced by trapping  Noise increases because of increase leakage current  Depletion voltage increases because of type inversion MeV n-eq. 36

37 Radiation damage  To increase the Radiation Hardness of Sensors: Operating conditions (cooler – lower leakage) Material engineering ( OFZ - Diamond detectors) Device engineering (n in n/p – 3D detectors) Electrodes in the bulk – lateral collection reduces the drift distance Lower depletion voltage – less power consumption difficult to manufacture 3D DDTC similar to 3D but easier to manufacture; also Better mechanical strength. n+ p+ F 37

38 Detector systems 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 HEP experiments: large detector systems A serial powering or DC2DC approach can increase efficiency in power distribution compared to a parallel approach Alternative powering schemes: SP DC2DC 38

39 Detector systems Low power solutions are crucial for future HEP experiments: RO consists of several block, each consumes power: 10 – 1000 μW continuous Energy deposited by a particle: 0.1 – 10’s fJ Noise occupancy 1% : hit pixel fires/100sec Required energy/deposited energy >> !!! Extremely huge energy inefficiency HEP presents challenging engineering and technical issues: their solution may have important consequences in fields outside PP (biomedical, telecommunications…) 39

40 The field of semiconductor radiation detectors encompasses a number of scientific and technology fields: solid state physics, nuclear and particle physics, electrical engineering, … Some of the specific issues relevant to radiation detectors:  Development of new detection techniques, based on novel or common semiconductor material: ( phonon-based detectors, quantum detectors, compounds, low dimensional)  Integration with electronics (monolithic solution to achieve more compactness and reduce cost) and/or 3D structures  Topologies optimization (power reduction, noise reduction)  Radiation hardness Conclusions 40

41 Backup slides Quantization effects due to band bending in Si-SiO2 interface: excitation based detection Si-polySi-sub SiO 2 Q-effects I

42 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) II

43 Detection Intrinsic resolution of Si and Ge based detectors The variance in signal charge σ i associated to the ionization process is related to the phonon excitation High resolution requires smaller band gap (ε i ), direct or small phonon excitation energy Fano factor ~0.1 in Si III

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