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MPE Max-Planck Institut für extraterrestrische Physik. WHI Max-Planck Institut für Physik (Werner-Heisenberg-Institut). PNSensor GmbH. Development of backside illuminated Silicon Photomultipliers at the MPI Semiconductor Laboratory. Outline: - PowerPoint PPT Presentation
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H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Development of backside illuminated Silicon
Photomultipliers at the MPI Semiconductor Laboratory
Outline:
-Motivation for a backside illuminated SiPM (BID-SiPM)-Layout of BID-SiPM-R&D Program-Results and Simulations (preliminary)-Applications in Particle Physics
on behalf of the MPI SiPM group
MPEMax-Planck Institut
für extraterrestrische Physik
WHIMax-Planck Institut für
Physik(Werner-Heisenberg-
Institut)
PNSensor GmbH
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Motivation for backside illumination
In general SiPM offer great advantages compared to photomultiplier tubes:
Simple, robust devicePhoton counting capabilityEasy calibration (counting)Insensitive to magnetic fieldsFast response (< 1 ns)Large signal (only simple amplifier needed)competitive quantum efficiency (~ 40% at 400-800 nm)No damage by accidental lightCheapLow operation voltage (40 – 70 V)
R&D goal:
Increase Quantum efficiency to the physical limit
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
QE & Fill Factor
What limits the QE?
QE = surface transmission x Geiger efficiency x geometrical fill factor
Front illuminated devices:
Large area blinded by structures– Al-contacts– Bias-resistor– Guard rings/Gap between HF implants
For 42 x 42 m2 device: 15% fill factor
Solutions: – larger pixel size (80% reached for 100 m pitch device)– back-illumination light enters through homogeneous back side, not covered by any structure
3 m light spot scanned across device
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Geiger Efficiency of electrons and holes
Avalanche Efficiency (1 m high field region)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
250000 350000 450000 550000 650000 750000
Field (V/cm)Ef
ficie
ncy
Electrons
Holes
Electrons have a higher probability to trigger an avalanche breakdown then holes
Efficiency depends on depth of photon conversion and hence on the wavelength
Solutions:-Increase overvoltage
Or: - Ensure that only electrons trigger an avalanche
holes
p-substrate
p- epip+
n+
el.
n-substrate
n- epin+
p+el.
holes
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Sensitivity at different wavelengths
light absorption in Silicon
0.001
0.01
0.1
1
10
100
1000
10000
250 450 650 850 1050
Wavelength (nm)
Ab
sorp
tio
n le
ng
th ( m
)
Thin entrance window needed
holes electrons
Holes triggeravalanche
Electrons triggeravalanche
p-substrate: photons < 450 nm: only holes contributephotons > 700 nm: lost in insensitive bulk
n-substrate: ok for short wavelengths,hole efficiency dominates for > 500 nm
Back illumination: whole thick (> 50 m) bulk absorbs photonsdesign for electron collection
Example:p-substrate
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Concept of a back illuminated SiPM
Combine SiPM and drift (photo) diode:
Each Pixel of a SiPM array is drift diode with a geiger APD as amplifying element in the center
By drift rings the electrons from photon conversions are focused into a small HF region
Homogeneous sensitivity, no dead regions
Back Illuminated Drift SiPMBID-SiPM
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Design of the avalanche cell
The HF region is created between a n+ contact at the surface and a deep p well underneath. By modulating the depth of the p-implant and/or the n-contact the HF region can be confined to a small area of a few m diameter
-> Small HF region: Low capacitance, low gain (important to fight cross talk)
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Engineering of Entrance Window
(Calculation: R. Hartmann)
Homogeneous, unprocessed thin entrance window at backside- minimal UV absorption in surface layer (important for < 350 nm)- possibility of antireflective coating
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Disadvantages
• Large volume for thermal generated currents (increased dark rate)
Maintain low leakage currentsCoolingThinning ( < 50 m instead of 450 m)
• Large volume for internal photon conversion (increases x-talk)
Lower gain (small diode capacitance helps)
Possible show stopper!
• Electron drift increases time jitter
Small pixels, Increased mobility at low temperature<2 ns possible
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Design of Devices
Hexagonal Cells
100-200 m diameterUp to 3 three drift rings
Central HF region with <8 m diameter
Capacitance ~ 5 fFGain: O(105)~ 1 m depth
95% Geiger efficiency @ 8V overvoltage (electrons)
Drift field extends into bulk
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Test structure production in 2005
-> fix parameters of avalanche cell (radius, depth, resistor values…)-> no backside illumination yet
Single pixel structuresSmall arraysLarge arrays (20 x 25 pixel180 m pitch)HF diameter: 5-25 m
Successfully tested
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Results: Test Structures
LowMediumHigh
Results with light pulses from a laser (< 1 ns):Photoelectron peaks clearly resolved up to large n(photon) RMS of single photoelectron signal ~ 5%
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Results: Test Structures
T = 0oC T = 10oC T = 20oC
Gain proportional to overvoltageBreakdown voltage in good agreement with device simulations
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Test Structures
Dark rate mainly from highly doped HF region: For a 5 x 5 mm2 matrix with 500 pixels: ~0.2 MHz @ 20oC (8V)
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Leakage Currents and Dark Rates
Back side illuminated: bulk leakage current dominates:
For devices thinned to 50 m: ~10MHz @ 20oCCooling needed: ~ 1 MHz @ 0oC
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Top Wafer
a) oxidation and back side implant of top wafer
b) wafer bonding and grinding/polishing of top wafer
c) process passivation
open backside passivation
d) deep etching opens "windows" in handle wafer
Processing thin detectors (50 m)
Successfully tested with MOS diodes (keep low leakage current ~ 100 pA/cm2)
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Cross Talk Studies
Dark spectrum of 25 m arrays
x-talk heavily suppressed due to small HF region and large pitchBackground due to pile up (suppressed by cooling to -20 C)
2pe signal clearly visible:
Probability for x-talk ~ 10-4 (@*V U)
For backside illumination: Bulk is sensitive to cross-talk photons
Use MC to extrapolated to full structure
1 pe~ 2x106
2 pe< 200
V
en
trie
s
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Monte Carlo Simulation of cross talk
0
0.05
0.1
0.15
0.2
0.25
1 2 3 4 5 6 7 8 9 10 11
n(photon)
p(n
)
2.9
Generate photons
Propagate through device
Photon Converts1. In pixel of origin2. In neighbour pixels
1. Active region2. Apply geiger efficiency
e e
drift drift
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Monte Carlo Simulation of cross talk
Generate photons
Propagate through device
Photon Converts1. In pixel of origin2. In neighbour pixels
1. Active region2. Apply geiger efficiency
Number of photons poisson distributedLacaita, IEEE TED, 40 (1993): 2.9 photons/105 e- (E > 1.14 eV)
Use black body spectrum with T=4300K
-However: el/holes not in thermal equilibrium
-Band structure not taken into account(2.9 ph > 1.14 eV => 8.5 ph total)
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Monte Carlo Simulation of cross talk
Generate photons
Propagate through device
Photon Converts1. In pixel of origin2. In neighbour pixels
1. Active region2. Apply geiger efficiency
Calculate photon absorption lengthFrom Photon energy
1 mm
100 m
e e
Surface reflection given by n=3.57
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Monte Carlo Simulation of cross talk
Generate photons
Propagate through device
Photon Converts1. In pixel of origin2. In neighbour pixels
1. Active region2. Apply geiger efficiency
drift drift
Electrons: drift in HF region (if bulk depleted)Apply local Geiger efficiency (as function of overvoltage)
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Monte Carlo Simulation of cross talk
Generate photons
Propagate through device
Photon Converts1. In pixel of origin2. In neighbour pixels
1. Active region2. Apply geiger efficiency
Cross talk spectrum
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Contribution from photons with range from O(pitch) – O(mm)
Cross talk spectrum
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
larger pitch -> x-talk spectrum narrowLarger pitch -> less x-talk
Dependence on Pitch
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Results
Cross talk measured with test structures implies:
~54 photons (E>1.14 eV) per avalanche (@*V U)
For a backside illuminated device with 100 m pitch:
cross-talk probability: 99.99%
Due to large capacitance (47 fF of HF region + coupling capacitances), the gain is very high: ~4 x 106
Scaling to the expected gain of 105:
Cross talk 20-30 %
Still high but could be manageable
Extrapolation has large systematic error!
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Further Program
2nd step: production of fully functional backside illuminated SiPMs:
-> including drift rings-> double sided processing, deplete bulk
Finished: End 2007
Various test structures(single pixel, small arrays)
Arrays:30 x 31 pixel Diameter HF region: < 8 mPitch: 100, 120, 150, 200 mArea: 3x3 mm2 – 6x6 mm2
In addition: some front illuminated arrays
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Applications in (Astro-) Particle Physics
Main advantages: Drawbacks:High QE Dark rateGood sensitivity for Cross talk 300nm < < 1000nm Costs
(double sided processing)
Can be used where QE and spectral response are important but high dark rate can be tolerated
•Air cerenkov telescopes - peak wavelength 300-350 nm (like MAGIC) - considerable LONS background
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Applications
• Cerenkov detectors in particle physics detectors
• Compact high density calorimeters
low light yield,
direct coupling to blue scintillator
• In colliding beam experiments: dark rate can be suppressed
using coincidence with beam
no self trigger needed (cross
talk!)
• Large scale applications (big calorimeters) probably excluded (costs)
Practical advantage:
Direct coupling of entrance window to scintillator (no wire bonds)
Scintillator
SiPMReadout board
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Smart SiPMs
clock
x, y, t
SiPM
BiCMOS analogue
CMOS digital
Connect ASIC chip to back-illuminated SIPMFor each pixel: signal detection & active quenching
-Fast timing (no stray capacitances)-Low threshold -> low gain-Active quenching -> low gain-Minimal cross-talk-Single pixel position resolution-Veto of noisy pixels
Could be made using
•Bump bonding•3D integration techniques
H-G MoserMax-Planck-Institute for
PhysicsSemiconductor
Lab
PD07Kobe, JapanJune 27-29
Summary
Backside illuminated Silicon Photomultipliers are developed at the MPI Semiconductor LaboratoryApplication: Upgrade of Magic Camera
Aim for highest quantum efficiency in a large spectral range (only limited be quality of entrance window)
First test structures (not yet back illuminated) produced and tested
Production of full devices ongoing
Design goals:
QE: 80% for 300 – 950 nm, peaking at > 90%Dark rate: <1MHz for 0.25 cm2 device @ 0oCGain: 105
Cross talk: 20-30 % ?
High cross talk is of major concern!
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