Silicon photomultipliers (SiPM) are the cornerstones of photodetector technologies for detecting faint light in both industry and scientific research since their early development in 1990s. Their attractive performances have brought benefits into emerging applications, such as ionizing radiation detection, biomedical imaging, and light detection and ranging (LiDAR) for autonomous driving. Compared to photomultiplier tube (PMT) that is a conventional photodetector legacy technology in the radiation detection, SiPM offers several advantages, including low operation voltage, compactness, ruggedness, and relatively low cost.
Furthermore, in contrast to PMT, SiPM is insensitive to magnetic fields, which leads to its vital role as the foundation of light detection technology for the advanced medical equipment in the presence of magnetic fields, such as positron emission tomography (PET) imaging. However, the photon detection efficiency (PDE) that is defined as the ratio between the numbers of detected photons and the photons arriving at the detector, which is also one of the key measurable metrics that quantify SiPM’s performance, is still limited to about 60% and rapidly decreases from the peak as the wavelength enters into the ultraviolet range. This is due to not only the reflection losses of photons impinging on the front planar silicon surface, but also the limited fill factor (FF) caused by the dead areas (i.e., quenching resistor, isolation trench, guard ring, and contact metal) for the conventional front-illuminated structure, in addition to the recombination loss of the photo-generated primary carriers near defect centers. Therefore, a reduction in photon reflection at the surface is a prominent role in the development of high-performance SiPM devices.
In order to reduce the photon losses due to reflection, the approach of antireflection coatings (ARC) is typically used, because it can reduce the photon losses by making use of phase changes and the dependence of the reflectivity on refractive index. 
In addition to the proper refractive index and film thickness, a low extinction coefficient (κ) is also required for the ARC material to avoid a significant photon absorption by the ARC thin-film layer. The ARC materials used in conventional SiPM are thermally grown silicon dioxide (SiO2), and silicon nitride (SiNx) that can be deposited by plasma-enhanced chemical vapor deposition (PECVD) as single-layer ARC (SARC) on planar surface. In this work, we developed multi-layer ARC on textured surface with upright nano-micro pyramids to reduce the reflection, including double-layer ARC (DARC) and triple-layer ARC (TARC). For comparison purpose, the single-layer ARC as well as the bare silicon wafer without ARC on both planar and textured surfaces are also studied in this work. 
In the end, the back-illuminated SiPM integrated with the multi-layer ARC on textured surface are discussed, as a comparison to the conventional SiPM with ARC on planar surface.
- For more details, please check our journal paper published in Scientific Reports: https://www.nature.com/articles/s41598-022-18280-y.