Characterization of first samples of innovative backside-illuminated SiPMs

Silicon Photomultipliers (SiPMs) are widely used in High Energy Physics experiments, where they are often exposed to hostile environments like high radiation levels or cryogenic temperatures. In addition to surviving these environments, experiments demand increased sensitivity especially in the UV spectrum. This work introduces a novel solution that has the potential to mitigate radiation damage and improve overall sensitivity, through the development of back-illuminated (BSI) SiPM structures. Unlike traditional Front-Side Illuminated (FSI) devices, BSI devices allow photons to enter directly into the active region from the opposite side, reducing microcell size without affecting the fill factor and PDE, and enhancing sensitivity across a wide range of wavelengths, from vacuum ultraviolet to near infrared. The BSI technology is particularly motivated by the need for improved PDE in cryogenic environments for VUV wavelengths, such as those emitted by Argon or Xenon light, where current devices exhibit PDEs below 10%. Fully BSI analog SiPMs have not yet been fabricated, but intermediate steps in their development can already be evaluated. Preliminary measurements in a thermally regulated chamber studied the SiPM performances as a function of temperature. Additionally, the devices were tested in liquid nitrogen to assess their behavior in cryogenic conditions. The study highlights the potential advantages of BSI technology over traditional designs, particularly in terms of enhanced PDE and applicability in new generation detectors. This work represents the first characterization of SiPM devices based on innovative BSI architectures, designed and produced by Fondazione Bruno Kessler (FBK) and tested at the INFN Bologna Laboratories.