Design Optimization of the UFSD Inter-Pad Region

This paper reports on a measurement campaign to characterize the inter-pad region of Ultra-Fast Silicon Detectors (UFSDs) manufactured by Fondazione Bruno Kessler. The devices under test are either pixel or strip arrays, featuring a large number of different inter-pad layouts; both pre-irradiation and irradiated sensors have been measured. The aim of the study is to link the design parameters of the inter-pad region to the operation of the sensors, providing insights into the design of UFSD arrays with narrow inter-pad gaps. We concluded that, in the UFSD design, the doping level and the area of the p-stop should be kept low, in order to avoid the early breakdown of the device and the micro-discharges effect; UFSDs with such characteristics proved also rather insensitive to floating pads and irradiation. Thanks to these findings, it was possible to design a UFSD array that yields the expected performance with an inter-pad width as small as 25 μm, significantly improving its fill factor with respect to standard designs. Two innovative experimental techniques are presented in this work: the first one is based on a TCT setup, the second makes use of an ultra-low light CCD camera.