Evolution of the design of ultra fast silicon detector to cope with high irradiation fluences and fine segmentation

The recent development in the design of Ultra Fast Silicon Detector (UFSD), aimed at combining radiation resistance up to fluences of 10^15 neq/cm2 and fine read-out segmentation, makes these sensors suitable for high energy physics applications. UFSD is an evolution of standard silicon sensor, optimized to achieve excellent timing resolution (~30 ps), thanks to an internal low gain (~20). UFSD sensors are n in p Low Gain Avalanche Diode (LGAD) with an active thickness of ~5 um. The internal gain in LGAD is obtained by implanting an appropriate density of acceptors (of the order of ~ 10^16/cm3) close to the p-n junction, that, when depleted, locally generates an electric field high enough to activate the avalanche multiplication; this layer of acceptors is called gain layer. The two challenges in the development of UFSD for high energy physics detectors are the radiation hardness and the fine segmentation of large area sensors. Irradiation fluences of the order of 10^15 neq/cm2 have a dramatic effect on the UFSD: neutrons and charged hadrons reduce the active acceptor density forming the gain layer; this mechanism, called initial acceptor removal, causes the complete disappearance of the internal gain above fluence of 10^15 neq/cm2. For the segmentation of UFSDs, the crucial point is the electrical insulation of pads and the extension of the inactive area between pads. In this paper we present the latest results on radiation resistance of LGADs with different gain layer designs, irradiated up to 3 x 10^15 neq/cm2. Three different segmentation technologies, developed by Fondazione Bruno Kessler in Trento, will also be discussed in detail in the second part of the paper.