Recent advances in silicon single photon avalanche diodes and their applications Massimo Ghioni Politecnico di Milano, Dipartimento di Elettronica e Informazione
Outline 2 • Single photon counting: why, what and how • SPAD device technology: origin and evolution • Single element SPAD detectors � recent advances � custom SPAD vs standard CMOS technology � application cases • SPAD array detectors � application cases • Conclusions M. Ghioni Pavia, April 3, 2007
Why single photon counting? 3 For ultimate sensitivity in optical signal measurement ! � straight digital technique � overcomes limits of analog measurements (circuit noise) � photon timing with picosecond precision � measurement of ultrafast optical signals by Time Correlated Single Photon Counting (TCSPC) M. Ghioni Pavia, April 3, 2007
Why high sensitivity? 4 • Low sample concentration • Minute samples • Short exposure time • Photon losses (poor collection, absorption, etc.) • Low excitation power • Greater magnification • Ultra-weak emission (Raman scattering etc.) M. Ghioni Pavia, April 3, 2007
Photon counting/timing applications 5 detector detector primary primary quantum quantum quantum quantum bioluminescence bioluminescence calibration calibration radiometric radiometric standards standards imaging imaging scales scales single molecule single molecule quantum quantum hyper-spectral hyper-spectral detection detection cryptography cryptography imaging imaging medical medical Metrology quantum quantum imaging\ imaging\ Metrology computing computing Quantum Quantum Biotechnology Biotechnology Information Information Processing Processing lighting lighting single photon single photon sources sources photon photon Electronics Electronics displays displays Medical Medical counting count medical / non medical / non Physics Physics interactive interactive entertainment entertainment imaging imaging neutrino/ neutrino/ radioactivity radioactivity Space Space Military cherenkov/ dark cherenkov/ dark Applications Applications Meteorology Military matter detection matter detection nuclear nuclear Meteorology night vision night vision IR detectors IR detectors lidar lidar remote sensing remote sensing security security robust imaging robust imaging devices devices environmental monitoring environmental monitoring chemical – bio agent detection chemical – bio agent detection source: www.photoncount.com M. Ghioni Pavia, April 3, 2007
Available detectors 6 Vacuum Tube PMT � Currently used in photon counting/timing applications � Limited quantum efficiency Solid State APD (ordinary Avalanche PhotoDiodes) � No single photon detection Special CCD (EM-CCD, I-CCD) � Photon counting possible only at low frame rates � Limited time resolution SSPD (Superconducting Single Photon Detector) � Limited active area � Need to be operated at < 4 K SPAD (Single Photon Avalanche Diode) � Best suited for photon counting/timing applications M. Ghioni Pavia, April 3, 2007
SPAD: reverse I-V characteristic 7 No avalanche V REV [V] V BD Avalanche I REV [mA] M. Ghioni Pavia, April 3, 2007
APD vs. SPAD 8 APD SPAD ON Avalanche Quenching Reset A valanche P hoto D iode S ingle- P hoton A valanche D iode • Bias: well ABOVE breakdown • Bias: slightly BELOW breakdown • Geiger-mode: it’s a TRIGGER device!! • Linear-mode: it’s an AMPLIFIER • Gain: meaningless !! • Gain: limited < 1000 M. Ghioni Pavia, April 3, 2007
for SPAD operation… 9 mandatory • to avoid local Breakdown, i.e. edge breakdown � guard-ring feature • � uniform area, no precipitates etc. • microplasmas but for for good good SPAD performance..... SPAD performance..... but further requirements!! M. Ghioni Pavia, April 3, 2007
Earlier Diode Structures 10 Haitz’s planar diode (early 60’s) metal + n 5 µm oxide 5 µm - n guard ring p metal Avalanche physics investigation • operated at low voltage (a few tens of Volt) • limited power dissipation during the avalanche (a few hundred milliwatt) • fabricated in ordinary silicon wafer with a planar technology R.Haitz, J.Appl.Phys. 35, 1370 (1964), J.Appl.Phys. 36, 3123 (1965) M. Ghioni Pavia, April 3, 2007
Earlier Diode Structures 11 RCA reach-through diode (circa 1970) • operated at high voltage (a few hundred Volts) • high power dissipation during the avalanche (around ten watt) • proprietary non-planar technology on a ultra-pure high-resistivity silicon wafers R. McIntyre, H. Springings, P.Webb, RCA Engineer 15, 1970 M. Ghioni Pavia, April 3, 2007
Haitz’s planar diode 12 • Deep diffused guard ring � causes the photon detection efficiency (PDE) to be non uniform in the active zone PDE = QE x η - QE = quantum efficiency - η = avalanche triggering probability M. Ghioni Pavia, April 3, 2007
Haitz’s planar diode 13 - Haitz’s structure has drawbacks in applications requiring high-resolution photon-timing - Long diffusion tail - Multi-exponential tail makes deconvolution more difficult G. Ripamonti and S. Cova, Solid State Electron. 28, 925 (1985) T.A.Louis et al, Rev.Sci.Instrum. 59, 1148 (1988). M. Ghioni Pavia, April 3, 2007
Epitaxial SPAD structure 14 5 10 4 10 Counts 3 10 2 10 1 10 0 10 0 2 1 3 4 5 Time (ns) - Shorter tail duration - p+ implantation for V BD control - Fully isolated devices on wafer - Guard Ring still employed � non-uniform PDE, non-exponential tail M.Ghioni, S.Cova, A.Lacaita, G.Ripamonti, Electron. Lett. 24, 1476 (1988) M. Ghioni Pavia, April 3, 2007
Double-epitaxial SPAD structure 15 5 10 4 10 Counts 3 10 2 10 1 10 0 10 2 0 3 5 1 4 Time (ns) • Short diffusion tail with clean exponential shape • Active area defined by p+ implantation • No guard-ring (uniform PDE) neutral p layer thickness w tail lifetime τ = w 2 / π 2 D n • Adjustable V BD and E-field SUITABLE for array fabrication • A.Lacaita, M.Ghioni, S.Cova, Electron.Lett. 25, 841 (1989) M. Ghioni Pavia, April 3, 2007
Double-junction SPAD structure 16 FWHM = 35ps FWHM = 35ps h ν + n FW(1/100)M = 125ps FW(1/100)M = 125ps + p FW(1/1000)M = 214ps FW(1/1000)M = 214ps p-epi p ++ p ++ n-substrate • Patterned p++ buried layer • No Tail (no carrier collection from neutral layer) • Suitable for small area devices ( Φ ~ 10µm) A.Spinelli, M.Ghioni, S.Cova and L.M.Davis, IEEE J. Quantum Electron. QE-34, 817 (1998) M. Ghioni Pavia, April 3, 2007
Device technology: prospect 17 • Two different approaches � standard CMOS technology � custom SPAD technology have to face most requested improvements: � higher photon detection efficiency (especially in the red region) � larger active area (~ 100 µm) � shorter diffusion tail M. Ghioni Pavia, April 3, 2007
Custom SPAD technology 18 • Full process flexibility makes it possible to address the most demanding requirements 0.7 Excess Bias Voltage 0.6 Photon Detection Efficiency 10 V 7 V 0.5 5 V h ν + n 0.4 + p 0.3 p + p 0.2 n 0.1 0 400 500 600 700 800 900 1000 Wavelength (nm) → Top epi-layer thickess/doping adjusted to increase PDE M. Ghioni Pavia, April 3, 2007
Custom SPAD technology 19 4 10 FWHM = 35 ps h ν 3 + n 10 Counts + FW1/100M = 370 ps p p 2 10 + p n 1 10 0 10 0 400 800 1200 1600 2000 Time (ps) → Bottom epi-layer thickess adjusted to achieve short diffusion tail M. Ghioni Pavia, April 3, 2007
Custom SPAD technology 20 h ν + n � heavy phosphorus diffusion + p p + p � p/p+ segregation gettering n → Specific designed gettering processes for removing transition metal impurities responsible for: - thermal carrier generation (dark count rate - DCR) - carrier trapping (afterpulsing effect) M. Ghioni Pavia, April 3, 2007
21 Dark Count Rate (primary noise) • Thermally generated carriers trigger avalanche pulses • Shot noise, equivalent to dark current in PINs / APDs Field-Enhanced Generation Thermal Generation via GR centers M. Ghioni Pavia, April 3, 2007
Field-enhanced generation 22 Coulombic well Dirac well • Poole-frenkel effect • Phonon-assisted tunneling � barrier height lowered � barrier width decreased � Phonon process is thermally activated � Tunneling is temperature independent � Overall temperature dependence is a function of electric field M. Ghioni Pavia, April 3, 2007
Custom SPAD technology 23 10000 h ν + n 1000 SPAD with "standard" electric Counts (c/s) + 100 p p + p 10 n SPAD with "engineered" electric field 1 0.1 -80 -60 -40 -20 0 20 Temperature (°C) → Electric field engineered to avoid band-to band tunneling � Field-enhanced generation less intense � DCR strongly reduces with temperature M. Ghioni Pavia, April 3, 2007
Large area SPADs: dark count rate 24 Dark Count Rate (DCR) • Avalanche pulses triggered by 100000 thermally generated carriers • Equivalent to the dark current in 25 10000 PINs and APDs 200 µm 1000 Counts (c/s) 100 100 100 µm Practical Exploitation of DCR vs T 10 Peltier cooling to -20°C 50 µm 1 � is simple / cheap / rugged 0.1 � reduces DCR by a factor 25 – 100 -50 -40 -30 -20 -10 0 10 20 Temperature (°C) Typical performance @5V excess bias voltage M. Ghioni Pavia, April 3, 2007
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