High Efficiency, Ultra high-density Silicon Photomultipliers

Silicon photomultipliers (SiPMs) are arrays of many single-photon avalanche diodes (SPADs), all connected in parallel. Each SPAD is sensitive to a single photon. The SiPM output is proportional to the number of detected photons. These sensors are becoming more and more popular in different applications and they have been significantly improved over last years, decreasing the noise, increasing the cell fill-factor (FF) and thus achieving very high photon-detection efficiency (PDE). In FBK (Trento, Italy), we developed several different SiPM technologies and recently, pushing the fabrication process limits, we produced the new “ultra-high-density” (UHD) technology. This technology features deep-trenches between cells, for electrical and optical isolation, and is characterized by a very small cell pitch, from 12.5 μm down to 5 μm. Such very small cell dimensions pose several issues both in terms of design and fabrication of the SPADs. TCAD simulations have been used to redesign the cell structure, minimizing their border region. As an example, the SiPM with 5 μm cell-pitch has about 40000 SPADs per squared millimeter, thus a very high dynamic range. These very-small-cells SiPM have a lower correlated noise (including lower afterpulsing probability), faster recharge time and very good detection efficiency (despite the small SPAD dimension).