Development of backside illuminated Silicon
Photomultipliers at the MPI Semiconductor Laboratory
Outline:
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
-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
MPE
PD07
Kobe, Japan
June 27-29
Max-Planck Institut
für extraterrestrische Physik
WHI
Max-Planck Institut für
Physik
(Werner-HeisenbergInstitut)
PNSensor GmbH
Motivation for backside illumination
In general SiPM offer great advantages compared to
photomultiplier tubes:
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Simple, robust device
Photon counting capability
Easy calibration (counting)
Insensitive to magnetic fields
Fast response (< 1 ns)
Large signal (only simple amplifier needed)
competitive quantum efficiency (~ 40% at 400-800 nm)
No damage by accidental light
Cheap
Low operation voltage (40 – 70 V)
R&D goal:
Increase Quantum efficiency to the physical limit
PD07
Kobe, Japan
June 27-29
QE & Fill Factor
What limits the QE?
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
QE =
x
x
surface transmission
Geiger efficiency
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 mm2 device: 15% fill factor
PD07
Kobe, Japan
June 27-29
Solutions:
– larger pixel size
(80% reached for 100 mm pitch device)
– back-illumination
light enters through homogeneous back
side, not covered by any structure
3 mm light spot scanned across device
Geiger Efficiency of electrons and holes
Avalanche Efficiency (1 mm high field region)
Efficiency
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Electrons have a higher
probability to trigger an
avalanche breakdown
then holes
1
0.9
0.8
0.7
Efficiency depends on
depth of photon
conversion and hence
on the wavelength
Electrons
Holes
0.6
0.5
0.4
0.3
0.2
Solutions:
-Increase overvoltage
0.1
0
250000
350000
450000
550000
Or:
- Ensure that only electrons trigger an
avalanche
n+
p+
p- epi
PD07
Kobe, Japan
June 27-29
p-substrate
650000
750000
Field (V/cm)
p+
holes
el.
n+
n- epi
n-substrate
el.
holes
Sensitivity at different wavelengths
10000
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Absorption length ( m m)
1000
light absorption in Silicon
100
10
Electrons trigger
avalanche
1
0.1
Holes trigger
avalanche
holes
0.01
0.001
electrons
Thin entrance window needed
250
450
650
850
1050
Wavelength (nm)
p-substrate:
PD07
Kobe, Japan
June 27-29
photons < 450 nm: only holes contribute
photons > 700 nm: lost in insensitive bulk
n-substrate:
ok for short wavelengths,
hole efficiency dominates for l > 500 nm
Back illumination: whole thick (> 50 mm) bulk absorbs photons
design for electron collection
Example:
p-substrate
Concept of a back illuminated SiPM
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
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 SiPM
BID-SiPM
g
PD07
Kobe, Japan
June 27-29
Design of the avalanche cell
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
PD07
Kobe, Japan
June 27-29
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 ncontact the HF region can be confined to a small area of a few mm diameter
-> Small HF region:
Low capacitance, low gain (important to fight cross talk)
Engineering of Entrance Window
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
PD07
Kobe, Japan
June 27-29
Homogeneous, unprocessed thin entrance window at backside
- minimal UV absorption in surface layer (important for l < 350 nm)
- possibility of antireflective coating
(Calculation: R. Hartmann)
Disadvantages
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
•
Large volume for thermal generated currents (increased dark rate)
Maintain low leakage currents
Cooling
Thinning ( < 50 mm instead of 450 mm)
•
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
PD07
Kobe, Japan
June 27-29
Design of Devices
Hexagonal Cells
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
100-200 mm diameter
Up to 3 three drift rings
Central HF region with <8 mm diameter
Capacitance ~ 5 fF
Gain: O(105)
~ 1 mm depth
95% Geiger efficiency
@ 8V overvoltage (electrons)
Drift field extends into bulk
PD07
Kobe, Japan
June 27-29
Test structure production in 2005
-> fix parameters of avalanche cell (radius, depth, resistor values…)
-> no backside illumination yet
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Single pixel structures
Small arrays
Large arrays (20 x 25 pixel
180 mm pitch)
HF diameter: 5-25 mm
Successfully tested
6000
5000
5860
4000
e
Coincidenc
Y Axis [ mm]
5185
4510
3000
3835
3160
2000
2485
1810
1000
1135
460,0
X
is
Ax
1,940
1,935
1,930
1,925
1,920
1,915
1,910
1,905
1,900
1,895
1,890
18,885
18,890
18,895
18,900
]
m
[m
PD07
Kobe, Japan
June 27-29
0
18,850
18,855
18,860
18,865
18,870
18,875
18,880
18,905
1,885
xis
YA
]
[mm
Results: Test Structures
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
PD07
Kobe, Japan
June 27-29
Low
Medium
High
Results with light pulses from a laser (< 1 ns):
Photoelectron peaks clearly resolved up to large n(photon)
RMS of single photoelectron signal ~ 5%
Results: Test Structures
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
PD07
Kobe, Japan
June 27-29
T = 0oC
T = 10oC
T = 20oC
Gain proportional to overvoltage
Breakdown voltage in good agreement with device simulations
Test Structures
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Dark rate mainly from highly doped HF region:
For a 5 x 5 mm2 matrix with 500 pixels: ~0.2 MHz @ 20oC (8V)
PD07
Kobe, Japan
June 27-29
Leakage Currents and Dark Rates
Back side illuminated: bulk leakage current dominates:
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
PD07
Kobe, Japan
June 27-29
For devices thinned to 50 mm:
Cooling needed:
~10MHz @ 20oC
~ 1 MHz @ 0oC
Processing thin detectors (50 mm)
a) oxidation and back side implant of
top wafer
c) process  passivation
Top Wafer
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
open backside passivation
b) wafer bonding and
grinding/polishing of top wafer
PD07
Kobe, Japan
June 27-29
d) deep etching opens "windows" in
handle wafer
Successfully tested with MOS diodes (keep low leakage current ~ 100 pA/cm2)
Cross Talk Studies
x-talk heavily suppressed due to
small HF region and large pitch
Background due to pile up
(suppressed by cooling to -20 C)
entries
Dark spectrum of 25 mm arrays
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
1 pe
~ 2x106
2 pe
< 200
2pe signal clearly visible:
Probability for x-talk ~ 10-4 (@*V DU)
V
For backside illumination:
Bulk is sensitive to cross-talk
photons
Use MC to extrapolated to full
structure
PD07
Kobe, Japan
June 27-29
Monte Carlo Simulation of cross talk
0.25
2.9
p(n)
0.2
0.15
0.1
0.05
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Generate photons
0
1
2
3
5
6
7
n(photon)
Propagate through device
e
PD07
Kobe, Japan
June 27-29
4
Photon Converts
1. In pixel of origin
2. In neighbour pixels
1. Active region
2. Apply geiger efficiency
drift
e
drift
8
9
10
11
Monte Carlo Simulation of cross talk
0.25
2.9
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Generate photons
p(n)
0.2
0.15
0.1
0.05
0
1
2
3
4
5
6
7
8
9
10
11
n(photon)
Propagate through device
Number of photons poisson distributed
Lacaita, 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
PD07
Kobe, Japan
June 27-29
Photon Converts
1. In pixel of origin
2. In neighbour pixels
1. Active region
2. Apply geiger efficiency
-Band structure not taken into
account
(2.9 ph > 1.14 eV => 8.5 ph total)
Monte Carlo Simulation of cross talk
1 mm
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Generate photons
100 mm
Propagate through device
Calculate photon absorption length
From Photon energy
PD07
Kobe, Japan
June 27-29
Photon Converts
1. In pixel of origin
2. In neighbour pixels
1. Active region
2. Apply geiger efficiency
e
e
Surface reflection given by n=3.57
Monte Carlo Simulation of cross talk
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Generate photons
drift
drift
Propagate through device
PD07
Kobe, Japan
June 27-29
Photon Converts
1. In pixel of origin
2. In neighbour pixels
1. Active region
2. Apply geiger efficiency
Electrons: drift in HF region (if bulk
depleted)
Apply local Geiger efficiency (as
function of overvoltage)
Monte Carlo Simulation of cross talk
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Generate photons
Propagate through device
PD07
Kobe, Japan
June 27-29
Photon Converts
1. In pixel of origin
2. In neighbour pixels
1. Active region
2. Apply geiger efficiency
Cross talk spectrum
Cross talk spectrum
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
PD07
Kobe, Japan
June 27-29
Contribution from photons with range from O(pitch) – O(mm)
Dependence on Pitch
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
PD07
Kobe, Japan
June 27-29
larger pitch -> x-talk spectrum narrow
Larger pitch -> less x-talk
Results
Cross talk measured with test structures implies:
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
~54 photons (E>1.14 eV) per avalanche (@*V DU)
For a backside illuminated device with 100 mm 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!
PD07
Kobe, Japan
June 27-29
Further Program
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
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 mm
Pitch: 100, 120, 150, 200 mm
Area: 3x3 mm2 – 6x6 mm2
PD07
Kobe, Japan
June 27-29
In addition: some front illuminated arrays
Applications in (Astro-) Particle Physics
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Main advantages:
High QE
Good sensitivity for
300nm < l < 1000nm
(double sided processing)
Can be used where QE and spectral response are important but high dark
rate can be tolerated
•Air cerenkov telescopes
(like MAGIC)
PD07
Kobe, Japan
June 27-29
Drawbacks:
Dark rate
Cross talk
Costs
- peak wavelength 300-350 nm
- considerable LONS background
Applications
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
•
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
SiPM
Readout board
PD07
Kobe, Japan
June 27-29
Smart SiPMs
Connect ASIC chip to back-illuminated SIPM
For each pixel: signal detection & active quenching
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
-Fast timing (no stray capacitances)
-Low threshold -> low gain
-Active quenching -> low gain
-Minimal cross-talk
-Single pixel position resolution
-Veto of noisy pixels
g
clock
x, y, t
Could be made using
SiPM
•Bump bonding
•3D integration techniques BiCMOS analogue
PD07
Kobe, Japan
June 27-29
CMOS digital
Summary
H-G Moser
Max-PlanckInstitute for
Physics
Semiconductor
Lab
Backside illuminated Silicon Photomultipliers are developed
at the MPI Semiconductor Laboratory
Application: 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 @ 0oC
Gain: 105
Cross talk: 20-30 % ?
High cross talk is of major concern!
PD07
Kobe, Japan
June 27-29