C. Piemonte piemonte@fbk.eu SiPM technology at FBK C. Piemonte piemonte@fbk.eu Fondazione Bruno Kessler, Trento, Italy

Outline FBK SiPM technology overview FBK SiPM for TOF-PET Conclusion

FBK expertise Silicon Radiation Sensors group (http://srs.fbk.eu) Simulation & design Fabrication Device testing 750m2 clean room equipped for manufacturing of silicon devices Wafer-level testing with manual and fully automatic probe stations Lab for functional testing Process and device physical simulations CAD design of the sensor

SJ-SiPM Technology Development started in 2005 in collaboration with INFN n+ p Cross section p+ subst. p epi n+ Technology characteristics: 1) Integrated polysilicon resistor 2) Very shallow junction to enhance QE at short wavelengths 2) possible ARC to optimize QE at certain wavelengths

Device Layout: example of internal structure Metal line connecting all cells in parallel (one common anode) The cathode is contacted on the rear side. Polysilicon resistor Resistor is located around the active area => no fill factor loss Field-plate to reduce electric field around the junction FF ~ 45% 40x40um2 cell ~ 55% 50x50um2 “ ~ 72% 100x100um2 “

Device Layout: examples of SiPM geometries Scale of the pictures is the same 2009 8x8 array of SiPMs 1.5x1.5mm pitch 50x50mm2 cell 2006 1x1mm2 40x40mm2 cell 2007 4x4mm2 50x50mm2 cell Produced for DaSiPM (INFN project)

Process & Device characterization at FBK Wafer level testing: Automatic IVs: forward and reverse on all devices Failure analysis in case of problems Wafer dicing Packaging of some samples Functional tests: - Dark analysis in climatic chamber - Laser response Photo-detection efficiency Energy & timing resolution with scintillators

On-wafer automatic characterization Reverse charact. - Functionality of the device - Breakdown voltage - Dark count estimate 4 elements of the array Forward charact. - Functionality of the device - Resistor value estimate 4 elements of the array

Example of faulty device Most common defect is premature breakdown Reverse IV plot Working SiPM. The current can be roughly modeled as I=q*DC*G Vbd light emission picture @16V SiPM with 1 defective cell. I=(V-Vbd)/Rq

… after packaging… Reverse IV plot Forward IV plot Temperature (C) Reverse IV plot Temperature (C) Forward IV plot (quenching resistor value is temperature dependent)

Dark analysis in climatic chamber For each bias voltage, Labview program performs following: original data, pulse identification single-cell signal (blue), mean signal (red) Dark count rate vs threshold (blue) pulse area histogram (blue), baseline (red) under construction pulse amplitude distribution vs distance pulse distance histogram

Dark analysis in climatic chamber Gain Bias Voltage (V) Breakdown & Gain Breakdown voltage (V) Temperature (C) 76mV/C Gain shift (1/K) Bias Voltage (V)

Dark analysis in climatic chamber Dark count doubles every 10C temperature increase Dark count rate (HZ/mm2) Over-Voltage (V) Quenching resistor increasing with decreasing temperature Quenching resistance (ohm) Temperature (C)

Photo-detection efficiency Measured with low level constant light PoissonLight - PoissonDark PDE = Number of incident photons 50x50um2 cell no after-pulses included

Application-oriented development Many parameters to be considered for: system performance optimization: - Photo-detection efficiency - Dark noise rate - Correlated noise (after-pulse, optical cross-talk) - Intrinsic timing capability - Signal shape - Geometry Gain and for real system implementation: Temperature dependence Operability range packaging

Example: TOF-PET Time-Of-Flight Positron Emission Tomography w/o-ToF With ToF Info form the SiPM: Energy and timing

Critical SiPM parameters for TOF-PET Photo-detection efficiency Dark noise rate Correlated noise Intrinsic timing capability Signal shape Geometry Gain Temperature dependence Operability range packaging SiPM parameters Energy resolution Timing resolution Large complicated system System requirements

Framework of the development HYPERimage http://www.hybrid-pet-mr.eu Development of a hybrid TOF-PET/MR test system with improved effective sensitivity First clinical whole body PET/MR investigations of breast cancer

Challenges in Simultaneous ToF-PET/MR Ultra compact, stable and reliable ToF Solid State PET Detector Modification of MRI to enable PET integration Interference reduction of PET with MR Stable and high accurate MR attenuation and motion correction techniques Identification of key applications Philips is committed to the clinical development of the PET/MR applications Whole body support in both PET & MR Full clinical support to drive application development Core components are the PET from Gemini-TF and 3T MR Achieva XR The PET detector here is equipped with mu-metal to shield the fringe field of the MR Fully integrated WB PET/MR currently addressed in Philips Research and within an EU program. Here, the Achieva XR system has to be modified in order to make room for the PET detector. However, the most challenging component in this system is the development of a new PET detector that is able to handle the large B0, GC and RF fields plus the secondary effects like vibrations and temperature. At the moment it has not been decided whether or not such a system will enter the development process within Philips.

HYPERimageToF-PET The Stack Pre-clinical system The module Why SiPMs? PMT APD SiPM MR compliant no yes ToF compliant WB system

2008 FBK SiPM technology g g 3x3x15mm3 LYSO crystal 3x3mm2 SiPM 50x50mm2 cell Energy spectrum Na22 Measurements by Philips Aachen dE/E~14% FWHM @511keV Energy (a.u.) Coincidence time resolution unfolded timing resol. ~305ps FWHM

HYPERimage SiPM geometry

HYPERimage SiPM - production Wafer view 2x2 array of ~4x4mm2 SiPMs 3600 cells per element Lot production time: ~3-4 months

Data analysis (1) - yield white = OK red = premature breakdown green/blue = problems after breakdown

Data analysis (2) Breakdown voltage Polysilicon resistor x105

Solution for Scintillator coupling bonding pad epoxy epoxy (120mm) Si

Light transmittance vs wavel. Epoxy layer properties Light transmittance vs wavel. layer thickness bonding pad epoxy LSO/LYSO: OK LaBr: epoxy can be removed from active area leaving only a support frame around the device

The SiPM tile 1300 working arrays delivered for the preclinical system 32.7mm Fill factor ~ 84% (not including SiPM FF) Flat surface for crystal mounting 32.7mm 500mm 1300 working arrays delivered for the preclinical system PCB design and mounting at Uni. Heidelberg and Philips

The stack The module First very preliminary tests: SiPM tile ASIC tile Mounting and measurements at Uni. Heidelberg and Philips First very preliminary tests: Energy resolution ~16% FWHM Coincidence res. time ~670ps FWHM => The stack works! Now, need to optimize the set up.

PET performance improvement g g 4x4x22mm3 LYSO crystal 4x4mm2 SiPM 67x67mm2 cell Na22 511keV/1.2MeV With 2008 technology: dE/E ~ 14% and CRT ~ 460ps Optimizing geometry and technology: dE/E ~ 11% FWHM (corrected for non linear.) CRT ~ 380ps FWHM (timing res. of 270ps) Energy spectrum CRT vs threshold Measurement @ Philips

Conclusion SiPM technology is improving quickly, meeting requirements of the most challenging applications FBK has a competitive technology for TOF-PET systems: - excellent performance; - efficient package solution to cover large areas; - production capability; - effective test methodology; Work in progress: - new technology for higher PDE; - new package solution.

Acknowledgment Many thanks to all who contributed to this work. In particular: FBK SRS group: Gabriele Giacomini Alberto Gola Elisabetta Mazzucca Mirko Melchiorri Alessandro Piazza Nicola Serra Alessandro Tarolli Nicola Zorzi HyperImage: in particular Philips Research Aachen University of Heidelberg DaSiPM & MEMS projects