1. 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
MPE
Max-Planck Institut
für extraterrestrische Physik
WHI
Max-Planck Institut für Physik
(Werner-Heisenberg-Institut)
PNSensor GmbH
PD07
Kobe, Japan
June 27-29

2. Motivation for backside illumination
In general SiPM offer great advantages compared to photomultiplier tubes:
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 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

3. 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 mm2 device: 15% fill factor
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
PD07
Kobe, Japan
June 27-29

4. Geiger Efficiency of electrons and 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 n+ p+
holes
el. p+ n+
el.
p- epi
n- epi
holes
p-substrate
n-substrate
PD07
Kobe, Japan
June 27-29

5. Sensitivity at different wavelengths
Thin entrance window needed
holes
electrons
Holes trigger
avalanche
Electrons trigger
Example:
p-substrate
p-substrate: 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
PD07
Kobe, Japan
June 27-29

6. 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 SiPM
BID-SiPM g
PD07
Kobe, Japan
June 27-29

7. 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 mm diameter
-> Small HF region:
Low capacitance, low gain (important to fight cross talk)
PD07
Kobe, Japan
June 27-29

8. Engineering of Entrance Window
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)
PD07
Kobe, Japan
June 27-29

9. Disadvantages
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

10. Design of Devices Hexagonal Cells 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

11. Test structure production in 2005
-> fix parameters of avalanche cell (radius, depth, resistor values…)
-> no backside illumination yet
Single pixel structures
Small arrays
Large arrays (20 x 25 pixel
180 mm pitch)
HF diameter: 5-25 mm
Successfully tested
PD07
Kobe, Japan
June 27-29

12. Results: Test Structures
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%
PD07
Kobe, Japan
June 27-29

13. Results: Test Structures
T = 0oC
T = 10oC
T = 20oC
Gain proportional to overvoltage
Breakdown voltage in good agreement with device simulations
PD07
Kobe, Japan
June 27-29

14. Test Structures Dark rate mainly from highly doped HF region:
For a 5 x 5 mm2 matrix with 500 pixels: ~0.2 20oC (8V)
PD07
Kobe, Japan
June 27-29

15. Leakage Currents and Dark Rates
Back side illuminated: bulk leakage current dominates:
For devices thinned to 50 mm: 20oC
Cooling needed: ~ 1 0oC
PD07
Kobe, Japan
June 27-29

16. Processing thin detectors (50 mm)
a) oxidation and back side implant of top wafer
c) process  passivation
Top Wafer
open backside passivation
b) wafer bonding and grinding/polishing of top wafer
d) deep etching opens "windows" in handle wafer
PD07
Kobe, Japan
June 27-29
Successfully tested with MOS diodes (keep low leakage current ~ 100 pA/cm2)

17. Cross Talk Studies Dark spectrum of 25 mm arrays
entries
Dark spectrum of 25 mm arrays
x-talk heavily suppressed due to small HF region and large pitch
Background due to pile up (suppressed by cooling to -20 C)
2pe signal clearly visible:
Probability for x-talk ~ 10-4 DU)
For backside illumination:
Bulk is sensitive to cross-talk photons
Use MC to extrapolated to full structure
1 pe
~ 2x106
2 pe
< 200 V
PD07
Kobe, Japan
June 27-29

18. Monte Carlo Simulation of cross talk
Generate photons
Propagate through device e
Photon Converts
In pixel of origin
In neighbour pixels
Active region
Apply geiger efficiency
drift
PD07
Kobe, Japan
June 27-29

19. Monte Carlo Simulation of cross talk
Generate photons
Number of photons poisson distributed
Lacaita, IEEE TED, 40 (1993):
2.9 photons/105 e- (E > 1.14 eV)
Propagate through device
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)
Photon Converts
In pixel of origin
In neighbour pixels
Active region
Apply geiger efficiency
PD07
Kobe, Japan
June 27-29

20. Monte Carlo Simulation of cross talk
1 mm
Generate photons
100 mm
Propagate through device
Calculate photon absorption length
From Photon energy e
Photon Converts
In pixel of origin
In neighbour pixels
Active region
Apply geiger efficiency
PD07
Kobe, Japan
June 27-29
Surface reflection given by n=3.57

21. Monte Carlo Simulation of cross talk
drift
Generate photons
Propagate through device
Photon Converts
In pixel of origin
In neighbour pixels
Active region
Apply geiger efficiency
Electrons: drift in HF region (if bulk depleted)
Apply local Geiger efficiency (as function of overvoltage)
PD07
Kobe, Japan
June 27-29

22. Monte Carlo Simulation of cross talk
Generate photons
Propagate through device
Photon Converts
In pixel of origin
In neighbour pixels
Active region
Apply geiger efficiency
Cross talk spectrum
PD07
Kobe, Japan
June 27-29

23. Cross talk spectrum
Contribution from photons with range from O(pitch) – O(mm)
PD07
Kobe, Japan
June 27-29

24. Dependence on Pitch larger pitch -> x-talk spectrum narrow
Larger pitch -> less x-talk
PD07
Kobe, Japan
June 27-29

25. Results Cross talk measured with test structures implies:
~54 photons (E>1.14 eV) per avalanche 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 %
Still high but could be manageable
Extrapolation has large systematic error!
PD07
Kobe, Japan
June 27-29

26. 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 mm
Pitch: 100, 120, 150, 200 mm
Area: 3x3 mm2 – 6x6 mm2
In addition: some front illuminated arrays
PD07
Kobe, Japan
June 27-29

27. Applications in (Astro-) Particle Physics
Main advantages: Drawbacks:
High QE Dark rate
Good sensitivity for Cross talk
300nm < l < 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 nm
(like MAGIC) - considerable LONS background
PD07
Kobe, Japan
June 27-29

28. 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
SiPM
Readout board
PD07
Kobe, Japan
June 27-29

29. Smart SiPMs g Connect ASIC chip to back-illuminated SIPM
For 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 g
clock
x, y, t
Could be made using
Bump bonding
3D integration techniques
SiPM
BiCMOS analogue
CMOS digital
PD07
Kobe, Japan
June 27-29

30. Summary 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 0oC
Gain: 105
Cross talk: % ?
High cross talk is of major concern!
PD07
Kobe, Japan
June 27-29