The enormous potential of silicon sensors in precise time measurement of ionizing particles is far from being exploited. At present, the best time resolutions in pixelated silicon is obtained by the proponents that have measured 100 and 130ps with low-noise ultra-fast SiGe HBT electronics in hybrid and monolithic implementations, respectively, without internal gain mechanism. Recently, the adoption of a gain layer in the Low-Gain Avalanche Detectors (LGAD) allowed to reach a time resolution of 30 ps, which unfortunately constitutes an unassailable physical limit for PN-junction sensor designs, that was recently understood to be produced by the Landau noise. Moreover, the location of the gain layer in the LGADs is such that a dead area of 100μm around each pixel is unavoidable; this circumstance does not permit to realise small pixels, and LGADs are produced with pads of mm² area.

New ideas are therefore needed to produce the next generation of silicon sensors able to provide simultaneously picosecond timing and excellent spatial resolution.

This project introduces a novel silicon sensor structure devised to overcome the intrinsic limits of LGAD sensors. This goal is achieved by the introduction of a fully depleted multi-junction structure. The results of the full simulation that we performed are excellent: pixels of 50×50μm2 can be achieved with few μm inter-pixel spacing together with picosecond-level time resolution in a monolithic sensor of thickness down to 25μm. These results, combined with the simplified assembly process and the reduced production cost provided by the monolithic implementation proposed here, represent the required technological breakthrough.

The multi-junction monolithic picosecond detector introduced here (that we will call MonPicoAD for brevity) represents an extraordinary enabling technology for the large spectrum of high-tech applications that will benefit of picosecond-level Time-Of-Flight (TOF) measurements of ionising radiation, like for example TOF-PET and TOF mass spectrometry. It will also offer a starting point for further progress in the field of light detection, in which a low-power, full-fill factor, picosecond pixelated detector will have a multitude of applications, among them ultra-precise lidar systems, laser communication in space and quantum computing.

Several fields in basic science also necessitate the production of thin sensors with both excellent position and picosecond time resolution for ionising radiation: the detector we propose here will offer a sustainable solution for the next generation of experiments at hadron colliders as well as for space-borne experiments in astro-particle physics, solar physics, space weather, and for interplanetary exploration.