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Description
Single-Photon Avalanche Diodes (SPAD) are playing a significant role in the development of new generation photon-detectors for High-Energy and Astroparticle experiments. The excellent spatial and timing resolution achievable make it very attractive in astronomic imaging applications for the observation of fast transient phenomena.
For several employments large photosensor arrays are required. This could be satisfied by SPAD produced in CMOS technology which would benefit of low cost and of the possibility of additional pixel circuitry integration for signal processing.
In this work we studied the noise performances of two different SPAD layouts, designed and implemented in 150-nm CMOS process, after proton irradiation.
The two structures are characterized by different junction types: the first structure is constituted by P+/Nwell junction, while the second is formed by Pwell/Niso junction.
We focused our attention on Random Telegraph Signal (RTS) mechanism, consisting in the switching of the Dark Count Rate (DCR) between two or more discrete levels. The phenomenon is strictly related to the density and distribution of defects in the semiconductor lattice and oxides.
We investigated RTS effects before and after proton irradiation exposure to a 21 MeV proton beam. RTS occurrence has been measured in more than 500 SPAD pixels and the differences addressed in two layouts are motivated and discussed. The measurements show that RTS occurrence is correlated to doping profile of the device. For some RTS pixel of two layouts we performed measurements of the RTS characteristics: DCR level as function of excess bias voltage, RTS time constants as a function of the temperature. The measurements allow to formulate a hypothesis on type of defect clusters responsible for RTS. Understanding and modelling DCR fluctuations could be very useful to find and analyse defects in standard CMOS processes and to propose new solutions to limit their noise effects by design.
