Speaker
Description
A monolithic silicon photo-multiplier, including 1024 micro-cells with a 30 $\mu$m pitch and a fill-factor of 50% and covering an overall area of about 1 mm$^2$, has been designed in a 110 nm CMOS image sensor technology. The SiPM, targeting applications with light detection in dual-readout (DR) calorimetry, takes advantage of an asynchronous, parallel counter to compute the number of hit pixels. It provides the count in digital form with a worst-case (slow-slow process corner, full scale signal) latency time of 15~ns, which can compete with conventional analog processing-based solutions. Time-of-arrival of the first photon and duration of the signal are measured by means of two 12-bit time to digital converters with a sub-100 ps resolution. The careful design of a fast NAND/NOR tree (NNt), which preserves the symmetry of the signal path as much as possible, minimizes fixed pattern jitter, thus ensuring an uncertainty in the arrival time of the first photon of less than 30 ps (after parasitic extraction). Noise mitigation is accomplished both by individually disabling hot micro-cells and by selecting a suitable thresholding mode from three available options. The first one relies on the measurement of the NNt output signal duration to detect signals exceeding one single photon. The second option enables data readout when at least two photons are read out in coincidence from any two adjacent double-rows (each double-row including 64 cells). The third one, similar to the second option, is based on the coincidence readout of at least three photons from any three adjacent double-rows. The prototype of the proposed digital SiPM is also instrumented for a full characterization of the SiPM from the standpoint of uncorrelated (dark count rate) and correlated (after-pulsing, cross-talk) noise. The conference paper will present a detailed discussion of the main features of the detector, together with results from post-layout circuit simulations, highlighting its advantages with respect to established analog SiPM solutions.
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