Development of LGAD sensors at FBK

The High Luminosity upgrade of the Large Hadron Collider highlighted the need for a time-tagging of tracks with a precision of tens of picoseconds. This requirement motivated the development of radiation hard silicon sensors dedicated to the time-of-interaction measurement of minimum ionizing particles. Low Gain Avalanche Diodes (LGADs) are silicon sensors with internal charge multiplication and are the baseline for the timing systems of the ATLAS and CMS experiments. These sensors use their gain to improve the signal to noise ratio (SNR) of detector systems and have been engineered to withstand the harsh radiation environment of the experiments. Fondazione Bruno Kessler (FBK) developed the LGAD technology through several production runs. The improved SNR and excellent time resolution made LGADs suitable also for medical, X-ray, and space applications. A feature of LGADs is the presence of a termination structure between regions with gain that results in areas without gain between the readout channels, reducing the fill factor of the devices. Different strategies to improve the fill factor of LGADs are being developed, such as double sided LGADs, resistive AC-coupled LGADs, and trench isolated LGADs. This paper summarizes the experience acquired at FBK with the realization of more than ten sensor batches. Selected results in radiation hardness, time resolution, fill factor, and different LGAD applications will be discussed.