Timing Resolution Improvement of MPPC for TOF-PET Imaging

T. Nagano, K. Sato, A. Ishida, T. Baba, R. Tsuchiya, K. Yamamoto

Abstract–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 6x4 mm2 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.

I. INTRODUCTION

N time-of-flight positron emission tomography (TOF-PET), TOF information provides better localization of the

annihilation event and improves the signal-to-noise ratio of the reconstructed image [1]. Timing resolution depends on the scintillation light yield, detector performance and read out noise. MPPC is a family of multi-pixel, self-quenching Geiger avalanche photodiodes [2] which are generally called Silicon Photomultipliers (SiPM) [3] [4] [5]. The features of MPPCs are low bias voltage operation, high gain, small size, and robustness. They are promising for use in TOF-PET/MRI because of their insensitivity to magnetic fields. Timing resolution required for the MPPC is less than 200 ps. Large area monolithic arrays have been developed for TOF-PET [6]. One possible candidate is the 3x3 mm2-4x4ch monolithic array. In order to obtain good timing resolution within one channel and in all channels it is necessary to improve the uniformity of output pulses.

II. LARGE AREA MPPC ARRAYS

The PET application requires a large photon detection area. Historically the first large area MPPC array, a 3x3 mm2-4x4ch discrete array was developed (Fig.1 left). It is easy to assemble a 3x3 mm2 discrete MPPCs on a substrate however a large dead space for the wire bonding pad exists between adjacent active areas, which is not ideal for scintillation light detection. The next step was development of a monolithic array which has a small dead space (Fig.1 right). The monolithic array has wire bonding pads at the edge of the chip and aluminum traces Manuscript received November 16, 2012.

T. Nagano, K. Sato, A. Ishida, T. Baba, R. Tsuchiya and K. Yamamoto are with the Solid State Division, Hamamatsu Photonics K.K., Hamamatsu-City, 435-8558 Japan

which are connected to each channel. While dead space has been reduced, trace lines to the wire bonding pads have to be long which results in degradation of the timing resolution. On the other hand a discrete array has minimal trace lines which do not degrade timing. Also it is possible to select chips which have similar characteristics. Therefore breakdown voltage variation is low and applied excess bias voltage is uniform. In the case of monolithic arrays 16 channels have to have low voltage variation, which depends on the wafer specification (quality) and the process conditions. Furthermore, in order to obtain good timing resolution from a monolithic array, it must have low trace impedance and low operating voltage variation. The trace line length varies depending on the chip size, chip shape and pad position in response to the application requirements. This article’s focus is the uniformity of characteristics within one channel to improve the timing resolution of MPPCs.

Fig.1 3x3 mm2 – 4x4ch discrete array (left) and monolithic array (right)

III. MEASUREMENT SET UP FOR TIMING RESOLUTION Fig.2 shows a schematic view of the timing resolution measurement setup. A PLP-01 pulsed laser is used as a light source of which the wavelength and the pulse width are 650 nm and 70 ps respectively. The pulse jitter is less than 10 ps and the repetition rate is 1 kHz. A photodiode sends a start signal to a time-to-amplitude converter (TAC). The MPPC input laser intensity is set to approximately 10 photons using neutral density filters. The output signal is amplified by an ORTEC VT-120B linear amplifier and connected to the stop signal of the TAC through a constant fraction discriminator (CFD). A multi-channel analyzer displays the histogram of MPPC time response. To study the timing performance of a large MPPC a 6x4 mm2 single channel test sample with 50 μm microcells was investigated. Timing resolution was measured over a range of operating voltages on several samples including the new development samples described later.

I

2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC) N22-2

978-1-4673-2030-6/12/$31.00 ©2012 IEEE 1577

Fig.2 Measurement set up for timing resolution

IV. UNIFORMITY WITHIN ONE CHANNEL In this paragraph we will discuss which parameters affect the timing resolution. Fig.3 shows a simple circuit diagram for one microcell. Each parameter is defined as follows: Cj is the junction capacitance, Cq and Rq are the quenching capacitance and the resistance, Cp and Rp are the trace capacitance and the resistance respectively. I1 is the current generated by avalanche multiplication. I2 is the current flowing into the junction capacitance. I3 is the output current through the trace line which is observed on an oscilloscope. The pulse height and the rise time of the output pulses I3 are influenced by the gain, the recovery time and the trace impedance etc. Gain is a function of Cj and the overvoltage (Eq. (1)). Overvoltage is the difference between the operating voltage and the breakdown voltage. The breakdown voltage depends on the electric field in avalanche region. The recovery time is a function of Cj and Rq (Eq. (2)). Trace impedance is a function of Rp and Cp. In order to obtain good timing resolution these parameters must be uniform for each micro cell.

Fig.3 A simple circuit diagram for one microcell Gain = Cj × (Vop – Vb) / q (1) Recovery time = Cj × Rq (2) q: elementary charge

Fig.4. Laser scanning for transient time distribution measurement (left) and simulation result of timing difference of output pulses depending on microcell position (right). Spot size: 1 mm, Step pitch: 330 μm

Fig.5. The transient time distribution on a 6x4 mm2 50 μm pitch test pattern relative to the pad position

The trace length from each micro cell to the wire bonding pad is different depending on its position. The trace resistance and capacitance is proportional to the length of the trace. According to the pulse simulation, the timing of the output pulses is different depending on the microcell position as indicated by A, B and C in Fig.4 right.

In order to examine the effects of the trace impedance, the transient time distribution was measured by scanning a laser across the active area (Fig.4 left). The spot size of the laser pulse was approximately 1 mm and the step pitch was 330 μm.

The Transient time distribution on a 6x4 mm2 50 μm pitch test pattern relative to the pad position is shown in Fig. 5. The transient time difference from the farthest micro cell to the wire bond compared with the nearest micro cell is approximately 300 ps. It must be reduced to obtain good timing resolution.

V. TIMING RESOLUTION OF STANDARD MPPC Timing resolution of the same sample was measured as a function of overvoltage. In this case the whole active area was irradiated by the pulsed laser. Fig.6 shows the results of the measurement. The optimum operating overvoltage is approximately 2 V and the timing resolution is approximately 250 ps. The output pulses depend on the gain and the electric field therefore it is necessary to apply a sufficient overvoltage. Ideally the timing

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resolution is expected to improve with increasing overvoltage like the dashed line shown in Fig. 6. However, the timing actually degraded at more than 2.5 V overvoltage. The output pulses in this voltage region observed on an oscilloscope indicated that higher dark pulses including higher after pulses have caused the fluctuation of the baseline and prevented improvement of the timing resolution. Based on this observation new MPPCs were developed focusing on the following three items: First, a double metal trace was applied to create a uniform and a low trace resistance. Next, a metal quenching resistor was applied instead of poly-Si to make a uniform quenching resistance. Finally a new structure and wafer process was tested to make a uniform breakdown voltage.

Fig.6. Timing resolution as a function of overvoltage on a 6x4 mm2 test pattern of standard MPPC at 10 photons irradiation

VI. IMPROVEMENT OF MPPC PERFORMANCE Trace lines of a conventional type MPPC are formed with a single aluminum layer. In this case the trace width is restricted by the process design rule and active area size. If the trace width is increased, the active area size has to be reduced. In order to increase the trace width while maintaining the current fill factor a double metal trace was applied. Trace lines were formed with AL2 on an insulator which covers AL1 as shown in Fig.7. There is no restriction of the distance between AL1 and AL2 so it is easy to increase the trace width without sacrificing the fill factor. The thickness and the width of the trace line was increased 1.5 times and 2.5 times respectively, resulting in the reduction of the trace resistance to 27% of the standard MPPC trace. The thickness of the insulator under the trace lines has also increased keeping total trace capacitance the same. As a result the transient time distribution has been reduced from 300 ps to 150 ps (Fig.8).

Fig.7. Scanning electron microscope (SEM) image of trace line using double metal trace (left) and cross-sectional focused ion beam (FIB) image along the red arrow (right)

Fig.8. Improvement of transient time distribution on a 6x4 mm2 test pattern with applied double metal trace

Poly-Si resistors are used in conventional MPPCs. This structure has grain boundaries which make it difficult to obtain good resistance uniformity within a wafer. The variation of resistance for poly-Si in a full 6-inch wafer is 19% for a 2 μm width and 37% for a 1 μm width respectively (Table 1).

Recently metal quenching resistors have been developed. This technology was already reported at NSS in Valencia 2011 [7]. Metal resistors have higher sheet resistance and lower variation compared with poly-Si resistors. The variation of resistance is approximately 10% in both widths shown in Table 1.

Additionally, the temperature coefficient of resistance is less than 1/5 compared with poly-Si quenching resistors (Fig.9, Table 2). This means that temperature fluctuations do not influence the quenching resistance of metal resistors as much. It is also effective for low temperature application such as liquid nitrogen cooling etc.

Table 1 Uniformity of resistances compared with poly-Si and metal quenching resistor at 10 kΩ/square sheet resistance in full 6-inch wafer

Fig.9. Temperature dependence of resistance compared with poly-Si and metal quenching resistor at 310 kΩ resistance

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Table 2 Temperature coefficient of resistance compared with poly-Si and metal quenching resistor at 310 kΩ resistance The final topic is about optimization of the p/n junction profile of MPPC. Several different silicon wafers and process parameters were tested to improve MPPC performance. As a result a new MPPC was developed which has very low after pulse probability. Fig.10 shows the afterpulse probability as a function of overvoltage. The dark blue dots and the pink dots represent the after pulse probability of the conventional MPPC and new structure MPPC respectively. The amount of impurities in the avalanche region has decreased for the new structure MPPC by employing pure silicon wafers and new process conditions. As a result the after pulse probability of the new structure type has been dramatically reduced to typically 1% at 1.5 V and 3% at 3 V overvoltage.

Fig.10. Afterpulse probability compared with conventional MPPC and new structure MPPC

VII. RESULTS Fig.11 shows the timing resolution of a 6x4 mm2 MPPC test

pattern compared with the standard MPPC and the new MPPC which includes double metal trace, metal resistor and new structure. Due to the dramatic suppression of after pulses higher overvoltage operation is possible without degradation of the timing resolution. At an overvoltage of 2.5 V a sharp leading edge in the output pulses and a stable baseline has been observed on an oscilloscope.

Furthermore, the timing resolution has improved from approximately 250 ps to 140 ps at optimum overvoltage because of the reduction of transient time distribution.

However, at a 4 V overvoltage the base line fluctuation was observed which indicates dark noise pulses prevent further improvement of the timing resolution. It is expected that further reduction of the dark noise pulses will make it possible to obtain less than 130 ps timing resolution at a higher overvoltage range.

Fig.11. Timing resolution compared with the standard MPPC and the new MPPC which includes double metal trace, metal resistor and new structure.

Table 3 Comparison between specifications of the standard type and the new type MPPC

VIII. SUMMARY The timing resolution of large size MPPC was investigated. Uniformity of gain, recovery time and trace impedance on each microcell is important to achieve good timing resolution. A double metal trace, a metal quenching resistor and a new structure were applied to reduce trace impedance, resistance variation and afterpulse probability. As a result MPPC performance has been dramatically improved. Comparison between specifications of the standard type and the new type MPPC are shown in table 3.

REFERENCES [1] Ruud Vinke, “Time-of-flight PET with SiPM sensors on monolithic

scintillation crystals”, 2011, 155p, ISBN 978-90-367-4738-7 [2] K. Yamamoto et al., Newly developed semiconductor detectors by

Hamamatsu, International Workshop on New Photon-detectors PD07, Kobe University, Kobe, Japan, 27-29 June 2007, PoS(PD07) 004.

[3] V. Golovin, V. Saveliev, Nucl. Instr. and Meth. A 518 (2004) 560. [4] V. Saveliev, Nucl. Instr. and Meth. A 535 (2004) 528. [5] B. Dolgoshein et al., ICFA Instrumentation Bulletin. [6] K. Sato et al., Application Oriented Development Multi-Pixel Photon

Counter (MPPC), 2010 IEEE Nuclear Science Symposium Conference Record

[7] T. Nagano et al., Improvement of Multi-Pixel Photon Counter (MPPC), 2011 IEEE Nuclear Science Symposium Conference Record

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