Silicon Photomultipliers (SiPM) has become more and more important in many fields. In practical applications, its surface needs to be covered with passivation protective layer and anti-reflection coating. The usual practice is to cover the protective layer on the anti-reflection coating. However, the upper protective layer will change the anti-reflection effect. We studied the effect of SiPM surface coverage protective material on its light response characteristics. The simulation result showed that when the surface of SiPM device has SiO2 anti-reflection coating and silicon resin protective layer, the reflection light can be reduced by 27% and the photocurrent can be increased by 20% in the visible and near-infrared wavelength range. When the wavelength of light is 905 nm and the thickness of silicon resin changes in the range of 100 ~ 300 microns, the surface light reflectivity of the device can be reduced from 33% of bare silicon to 5.8% ~ 8.9%. Finally, the transmission spectrum of the double-layer medium composed of a glass slide with a main component of a SiO2 slice and a silicon resin coating was tested. The experimental results showed that the light transmittance of the double-layer coating system in the visible region increased by about 20% after the addition of silicon resin. The experimental results are basically consistent with the simulation results. This shows that the SiPM surface protective coating layer can effectively reduce the reflected light on the surface of the device and further achieve the effect of anti-reflection.
Multi-pixel photon counter (MPPC) has been widely used in the field of weak light detection due to their various advantages in recent years. However, their parasitic effects such as optical crosstalk limit their further improvement in photoelectric performance. A deep understanding of the generation and propagation mechanism of optical crosstalk in MPPC is one of the prerequisites for further improving the performance of MPPC. The mechanism of the non-uniform spatial distribution phenomenon of the prompt optical crosstalk effect (POCE) in MPPC was investigated in-depth. The experimental results show that the optical crosstalk probability (Pct) in MPPC without optical isolation trench shows obviously non-uniformity within the avalanche photo diode (APD) cell, nevertheless, for the MPPC with optical isolation trench, the uniformity of spatial distribution of Pct in APD cells is greatly improved. Therefore, the different propagation distances of crosstalk photons emitted from different positions of the source APD cell to the nearest neighbor APD cell along a straight line, are the main reason of non-uniform spatial distribution of Pct in MPPC. According to the experimental results, it can also be inferred that the emission of crosstalk photons is spatially localized during an avalanche of an APD cell, and most of the crosstalk photons are emitted during the early stage of the avalanche. The results of this paper not only have reference value for further understanding the mechanism of optical crosstalk effect in MPPC, but also be beneficial for guiding to optimize the structure and process design of MPPC, and thus improving the performance of MPPC.
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