Terumasa Nagano

Timing resolution improvement of MPPC for TOF-PET imaging

Large detection area and good timing resolution of Silicon Photomultiplier (SiPM) based scintillation detectors is required for TOF-PET imaging. Achieving good timing requires output pulse uniformity on each microcell to be good; this depends on minimizing the gain, the recovery time, the trace impedance variation, etc. A double metal trace, a metal resistor and a new structure were applied to produce uniform characteristics on a single Multi-Pixel Photon Counter (MPPC) channel. The resulting time resolution has been improved to 140 ps for approximately 10 photons irradiation and an overvoltage of 2.5 V on a 6×4 mnl single channel MPPC test pattern. Furthermore afterpulse probability has been dramatically suppressed to a typical value of 3% by applying new silicon wafers and new process conditions.

Improvement of Multi-Pixel Photon Counter (MPPC)

The Multi-Pixel Photon Counter (MPPC) is a solid state photon counting device using Geiger mode APDs with self-quenching resistors. Recent advancements have been low dark current, good breakdown voltage uniformity and higher sensitivity in UV region. A 3×3mm – 4×4ch. monolithic array with silicon through vias has been developed for TOF-PET in order to decrease packaging dead space. We have also studied thin metal film quenching resistors instead of poly-Si to increase QE and decrease the temperature dependency. Finally we are producing an “All in one MPPCs” which includes many of these new technologies.

Timing resolution dependence on MPPC geometry and performance

Multi-Pixel Photon Counter (MPPC) is a family of Silicon Photomultipliers (SiPMs), the feature of which are insensitivity to magnetic fields and good timing resolution making it suitable for applications such as time-of-flight positron emission tomography/magnetic resonance (TOF-PET/MR). A new series of MPPCs are available which have low after pulses, low quenching resistance variation and high photon detection efficiency (PDE). In addition a 4-side buttable package has been developed with through via technology, which makes it easy to fabricate a large area MPPC array with low dead space. In this paper, timing resolution measurements of several kinds of MPPCs are presented.

Timing resolution dependence on MPPC performance parameters

Timing resolution of the Multi-Pixel Photon Counter (MPPC) has been investigated over several years. In our previous work, differences in coincidence resolving time (CRT) depending on MPPC performance were not observed [1], probably due to other limitation such as the measurement conditions. Recently a low crosstalk (LCT) MPPC series with trenches has been developed such as the S13082-050CS, which makes it possible to apply high overvoltage and maintain good performance. In this work, CRT dependence on MPPC performance was investigated with new measurement conditions. Our results indicate that photon detection efficiency (PDE) significantly affects the absolute CRT value, while dark and crosstalk pulses only limit the operating voltage range. 100 μm LCT4 and 50 μm high fill factor (HFF) MPPCs which have high PDE showed good CRT, however energy resolution at high overvoltage was not good. 3×3 mm2 50 μm LCT5 and 75 μm LCT4 MPPCs coupled to 3×3×20 mm3 LFS (Lutetium Fine Silicate) scintillator achieved 195 ps and 180 ps CRT respectively at 5 V overvoltage.

Development of new MPPC with higher NIR sensitivity and wider dynamic range

The Multi-Pixel Photon Counter (MPPC), which is also called a silicon photomultiplier (SiPM)1,2, is one promising candidate for automotive light detection and ranging (LIDAR)3. Due to high internal gain around 106, photon counting is possible and satisfies long range measurement. Compared to photo diodes (PDs) and avalanche photo diodes (APDs), read-out circuits for MPPCs are very simple because no external amplifier is needed. Conventional MPPCs have been developed for targeting blue scintillation light around 400 nm for positron emission tomography (PET) and high energy physics experiments. In this paper we report new near-infrared (NIR)-enhanced MPPCs whose development targets include 905 nm laser light for automotive LIDAR systems. Conventional MPPCs have a p-on-n structure and show 2% photon detection efficiency (PDE) at 905 nm. Our newly developed n-on-p MPPC achieved 7% PDE without greatly changing the impurity concentration profile of the depletion layer. This n-on-p MPPC has been released as an NIRenhanced MPPC: S13720-1325CS. For further improvement of NIR sensitivity, we tried several silicon wafers and process conditions of p-n junction profiles. Even though dark noise and the voltage range have to be modified, the latest sample shows 11% PDE, suggesting potential for further sensitivity improvement.

Development of silicon hybrid SPAD 1D arrays for lidar and spectrometer applications

The MPPC, a sort of silicon photomultiplier, has good photon-counting ability and timing accuracy. Recently, a new type of MPPC that has excellent sensitivity in the green region or near-infrared, wide dynamic range, higher photon detection efficiency, and various format configurations (single channel and 1D array) was developed. To utilize its advantageous performance, dedicated readout electronics is required. For LIDAR applications, the red-enhanced MPPC can be used, and the functional for the readout system are estimating the number of photons and recording the precise time-of-arrival. For these requirements, a time-over-threshold circuit that can recognize the incoming energy down to 1 photon and a time-to-digital converter that can record time-of-arrival with 312ps resolution were integrated onto a single die. We have demonstrated that the system has the capability to measure distance with centimeter accuracy. For situations that require higher dynamic range, a high-speed comparator and counter array configuration can be provided. For weak-light-level applications like spectroscopy, a configuration consisting of a SPAD 1D array, active quenching circuit and gate function can be used. We will propose a 1D hybrid SPAD that is the optimal combination for various applications.

Silicon hybrid SPAD with high-NIR-sensitivity for TOF applications

This paper proposes a single-photon avalanche diode (SPAD) sensor array comprised of a hybrid structure which can maximize the fill factor of the active area and be compatible with the other detector layer optimized for various demands. In order to implement the hybrid structure, a 100μm pitch through silicon via (TSV) implementation method has been developed to access the back surface of the sensor layer. The achieved fill factor is up to 60%, thus, photon detection efficiency can be reached 35%. A 32×32 SPAD array and a dedicated application specific IC has been designed. We have proved the concept structure can work successfully through the characterization of the hybridized chip. On the other hand, we realized multi-event detection capability should be considered when we apply the photon counting image sensor to a time-of-flight application in high background intensity, and the new concept of a SiPM-based pixel structure has been considered. In order to prove the concept, fundamental experiments have been performed by using the new SiPMs which have extended sensitivity in the near infrared region, and a current mode front-end ROIC which can mark a time-of-arrival and distinguish a photon quantity. A walk error has been studied and found the plot of the time-of-arrival and the photon quantity can be utilized for the measurement compensation.

The reliability and reproducibility of MPPC

Multi-Pixel Photon Counter (MPPC) is solid-state photon counting device consisting of a Geiger-mode APD and quenching resistor at the most basic level. To improve the total fill factor of a MPPC array, we have adopted new technologies such as metal quenching resistors (MQR), through-silicon vias (TSV), stealth dicing (SD), and highly accurate assembly techniques. We have confirmed the reliability of these new technologies. The innovative technologies that make up the new MPPC will expand the realm of potential applications.