Silicon Photomultipliers a new device for frontier detectors in HEP, astroparticle physics, nuclear medical and industrial applications Nepomuk Otte MPI für Physik, Munich
Outline Motivation for new photon detectors APDs in proportional and Geiger mode From single APDs in Geiger mode to Silicon Photomultipliers SiPM characteristics Current status of development PET as one example of application of SiPM
Many future experiments will use >> 100,000 photon detectors Requirements to be fulfilled by the photon detector candidate: robust and stable easy to calibrate blue sensitive low cost (+ low peripheral costs) compact low power consumption highest possible photon detection efficiency Experiments that will use this photon detector
Ground based Gamma Ray Astronomy Gamma Ray induces electromagnetic cascade relativistic particle shower in atmosphere Cherenkov light fast light flash (nanoseconds) 100 photons per m² (1 TeV Gamma Ray) MAGIC: world largest air Cherenkov telescope http://wwwmagic.mppmu.mpg.de/
Future Plans Lowering Energy threshold down to 10 GeV Improve sensitivity by factor of 10 Extend Observations into moonshine time to be achieved with Large Array of Telescopes (10 20) and High Performance Photon Detectors
Cosmic Ray Physics from Space 400 km 30 Atmospheric Sounding EECR One promising photon detector candidate Earth Fluorescence Atmosphere The Silicon Photomultiplier Čerenkov 230 km M.C.M. 02 http://www.euso-mission.org/ Highest energy cosmic rays > 10 20 ev GZKmechanism sources of CR
A look into basic operations of semiconductor photon detectors with internal amplification
Working modes of Avalanche Photodiodes log(gain) Linear mode Geiger mode Linear/Proportional Mode no gain Bias: slightly BELOW breakdown 0 Linear-mode: it s an AMPLIFIER Gain: limited < 300 (1000) High temperature/bias dependence No single photo electron resolution Reverse Bias Voltage Geiger Mode Bias: (10%-20%) ABOVE breakdown voltage Geiger-mode: it s a BINARY device!! Count rate limited Slide adapted from Cova et al. NIST 2003 Workshop on single photon detectors Gain: infinite!!
Advantages of APDs in Geiger Mode or Single Photon Avalanche Diodes (SPADs) Large standardized output signal high immunity against pickup High sensitivity for single photons Excellent timing even for single photo electrons (<<1ns) Good temperature stability Low sensitivity to bias voltage drifts Devices operate in general < 100 V Complete insensitive to magnetic fields No nuclear counter effect (due to standardized output)
The principal disadvantage for many applications: It is a binary device One knows: There was at least one electron/hole initiating the breakdown but not how many of them solved in SiPM concept
Basic unit in a SiPM is a Single Photon Avalanche Diode (SPAD) * Si Resistor V bias Al-conductor n + SiO 2 p- p+ Guardring n - Substrate p+ from B. Dolgoshein (ICFA 2001) http://www.slac.stanford.edu/pubs/icfa/ Breakdown in SPAD is quenched by individual polysilicon resistor (passive quenching)
The Silicon Photomultiplier or Geiger-APD typically 100 2000 small SPADs / mm² 1mm Bias and Output SiPM All SPADs connected in parallel Only one common signal line 30µm... V bias
SiPM output is the analog sum of all SPADs Well defined output signal per SPAD multi pixel resolution
Dynamic Range Dynamic range naturally limited by number of available SPADs working condition: Number of photo electrons < SPAD cells From probability considerations: Number of pixels fired 1000 100 10 working range 1 1 10 100 1000 10000 Number of photoelectrons 576 1024 4096 N firedcells = N available 1 e N N photon available PDE from B. Dolgoshein Light06 20% deviation from linearity if 50% of cells respond
Photon Detection Efficiency (PDE) or Effective Quantum Efficiency Most important parameter of a photon detector!! limiting factors: Intrinsic quantum efficiency Fraction of sensitive area (20% - 80%) Surface reflection losses Probability for Geiger breakdown (depends on electric field) SPAD recovery time (passive quenching Active volume / absorption length W.Oldham, P.Samuelson, P.Antognetti, IEEE Trans. ED (1972) In total: Currently claimed best PDE values are ~40% >60% seem feasible
Problems: Optical Crosstalk High Dark Count Rate
Optical Crosstalk SPADs not only detect photons they also emit photons during breakdown Hot-Carrier Luminescence 10 5 avalanche carriers 3 emitted photon e.g A. Lacaita et al, IEEE TED (1993) Emission microscopy picture of a prototype SiPM
Photons can trigger additional cells Sketch from Cova et al. NIST 2003 Workshop on single photon detectors Optical crosstalk Artificial increase in signal Excess Noise Factor of SiPM can be quite significant
How to suppress Optical Crosstalk? Possible counter measures: Lowering bias voltage decrease in breakdown probability (Price to pay: lower PDE) Lowering SPAD cell capacity Optical insulation between SPAD cells
Blocking Photons with Grooves 0 10000 SiPM Z-type. U-U bd =8V. k opt =1,85. t gate =80ns. 1 QDC LeCroy 2249A. Noise. 1000 Counts 100 10 1 200 400 600 800 1000 QDC channel Gain: 3 10 6 ; No grooves Suppression of crosstalk seems possible Excess Noise Factor ~1 10000 1000 events 100 Gain: 3 10 7 ; with grooves 10 from B. Dolgoshein MEPhI 1 0 100 200 300 400 500 600 channel
Dark Count Rate It is a Complex Topic; here only the very basics: Two main contributions: Free Carrier Generation: Depends on temperature (Can be cooled away) Tunneling: Depends on operation voltage Influenced by design of the device
Dark Count Rate Silicon photomultipliers are sensitive to every single electron high single electron dark rate (10 5 10 6 1/sec*mm² at room temp.) But: In most applications trigger threshold at several photoelectrons accidental trigger rate << single electron dark rate In addition: Strong reduction of noise by lowering operation temperature (Factor two every 8 C) Y. Musienko
Let s go shopping Various very intense developments ongoing in Industry (>4) and Research Institutes: Center of Perspective Technology and Apparatus CPTA, Moscow MEPhI/Pulsar Enterprise, Moscow JINR(Dubna)/Micron Enterprise HAMAMATSU RMD (Abstract 218) SensL, Ireland Max-Planck Semiconductor Lab, Munich In general devices are still in prototype stage
MEPhI/Pulsar/MPI In collaboration with MPI for Physics (Munich) Intended application: Current Air Cherenkov device parameters Telescopes @ 56V: (MAGIC) Cosmic Dark rate: Ray 500kHz space at missions -60 C (e. g. EUSO) Gain: 10 7 Development aimed at: PDE: (see next slide) sensor area 10x10 mm² Photon Detection Efficiency >60% Largest existing SiPM 5x5 mm² 2500 APD cells
5x5mm² SiPM: Photon Detection Efficiency 90 80 70 No antireflection coating of SiPM T (70 nm SiO 2 ) Efficiency ε, % 60 50 40 30 20 10 0 300 350 400 450 500 550 600 650 700 Wavelength λ, nm SiPM (T = -60 0 C) PMT XP2020Q limiting above 400nm 0,00573 0,0145 0,0974 0,312 1,38 1,72 3,3 4,43 PDE Absorption length x 0, µm = T SiO ε packing ε Geiger QE ε 2 packing = 0.5 ε Geiger 1 B.Dolgoshein,LIGHT06
Hamamatsu: Digital Pixel Photon Detector T. Takeshita Snowmass 05 Device from early 2005
Hamamatsu 0-100-1.5 (100 pixels), U=48.9V, T=22.6C 30 25 PDE [%] 20 15 10 5 0 350 400 450 500 550 600 650 700 750 800 Wavelength [nm] Latest devices achieve ~40% PDE @ 450nm (D. Renker) D. Renker (2005) Gain: 10 7 Dark noise: 550kHz @ room temperature Crosstalk: 30%
Metal Resistive layer Semiconductor (MRS) ~100% Geometrical occupancy PDE limited by semitransparent metal electrode 10,000 cells/mm² are possible with this technology from K. Voloshin NIM A 539 (2005) See results on PET later
MRS: PDE 25 20 Photon detection efficiency (Room temperature) XP2020 PMT INR/JINR APD CPTA APD PDE [%] 15 10 5 0 350 400 450 500 550 600 650 700 750 800 Wavelength [nm] Y.Musienko (2005)
Ongoing Development: SiPM exploiting Backillumination By the Semiconductor Laboratory affiliated to the MPIs for Physics and Extraterrestrial Physics depleted bulk path of the photo electron photon avalanche regions Si 50µm 450µm Blow up of one cell output predicted characteristics: PDE > 80% Single photo electron time jitter ~ nsec Cooling is mandatory
shallow p+ drift path of the photo electron photon drift rings p+ n type depleted bulk deep n 50 µm... 450 µm avalanche region quenching resistor 100 µm output line test structures of novel avalanche structure will be finished next month After successful evaluation prototypes end 2007 Crosstalk problem can be a showstopper!! will be evaluated by dedicated structures small cell capacitance is of advantage
Possible Applications of SiPM The SiPM opens up a great variety of possible applications Calorimeter readout in magnetic fields (CALICE, ILC, ) Space applications (EUSO, ) Astroparticle experiments (MAGIC, ) Medical imaging (PET) Fast timing applications (<1nsec) time resolved X-Ray correlation spectroscopy Fiber trackers Large pixilated photon detectors In some applications the SiPM is already superior to PMT s or APD s Some examples
SiPMs in PET Advantage: very compact, no sophisticated amplifier needed, direct coupling of SiPM to crystal no cooling Factor 4 area miss match between SiPM and crystal Energy resolution 22% FWHM on 22 Na coincidence spectrum Time resolution 1.5 nsec FWHM Otte, et al. NIM A 545 (2005) Things have quite improved since then
First result of measurments with MW-3 (3x3 mm 2 ) Geiger- mode APDs from Dubna (Z. Sadygov) + LYSO crystals (2x2x10 mm 3 ) 22 Na + LSO (2x2x10 mm 3 ; reflector = teflon) MW-3 (3x3 mm 2, n.1): RT, U = 138.0V, I = 1.05µA Energy Resolution: 12% FWHM 2000 2000 Counts 1500 1000 1500 1000 500 0 150 200 250 Time Resolution: 540ps (limited by crystal) 500 511 kev : A/A = 12.7% (FWHM) 1275 kev : A/A = 7.7% A 1275 / A 511 = 2.60 MRS diode used 0 0 200 400 600 Amplitude (pc) Alexey Stoykov, Dieter Renker (PSI)
High granularity needed CAlorimeter for the LInear Collider Experiment see also: Gerald Eigen Abstract 211 SiPM is equivalent to PMTs and APD (not shown) Calice collaboration
Things not discussed 30 minutes are by far not enough to give an overview on SiPM Cell recovery Quenching mechanisms Importance of parasitic capacitances Afterpulsing
Summary The silicon photomultiplier is a real breakthrough in photon detection!! High photon detection efficiency (>60%) Offers high internal amplification (>10 5 ) Fast timing (<nsec) Low power consumption (1 100µW/mm²) It can not be damaged by exposure to strong source of light No aging CMOS like technology prospects for cheap mass production <10$ per mm²
Summary High dark count rate not a showstopper for most applications Optical crosstalk is a problem but solvable Current parameters of available prototypes: Detector area: 5x5 mm² Photon detection efficiency: ~40% Dark rate at room temperature: 10 5-10 6 counts/sec/mm²