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

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 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? 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

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

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

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

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

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

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

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

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

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

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

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) PD07 Kobe, Japan June 27-29

Leakage Currents and Dark Rates Back side illuminated: bulk leakage current dominates: For devices thinned to 50 mm: ~10MHz @ 20oC Cooling needed: ~ 1 MHz @ 0oC PD07 Kobe, Japan June 27-29

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)

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 (@*V 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

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

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

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

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

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

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

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

Results Cross talk measured with test structures implies: ~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 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

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 300-350 nm (like MAGIC) - considerable LONS background PD07 Kobe, Japan June 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 SiPM Readout board PD07 Kobe, Japan June 27-29

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

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 device @ 0oC Gain: 105 Cross talk: 20-30 % ? High cross talk is of major concern! PD07 Kobe, Japan June 27-29