The “Resistive AC-Coupled Silicon Detectors” (RSD) is a 2-year project granted by the Commission-V of the Italian National Institute for Nuclear Physics (INFN) aiming to design, produce, test and optimize a new generation of solid-state silicon detectors intended for 4D tracking in particle physics (as high-energy experiments, like in HL-LHC accelerator at CERN, or medical applications).
Figure 1. Official logo of the RSD project.
The spatial reconstruction (3D) of tracks is reached, on the first hand, through an optimized granularity of the read-out contacts and, secondly, by using different planes of sensors. Therefore, the timing information is obtained thanks to the well-established LGAD (Low-Gain Avalanche Detectors) technology. LGAD optimized for timing measurements (i.e. Ultra-Fast Silicon Detectors, UFSD) essentially are reversely-biased n-in-p diodes where a p+ additional layer is implanted just underneath the n+ cathode. This layer, also known as multiplication or gain layer, is responsible for an high electric field at the basis of the impact ionization of charge carriers which, in turn, generates the (internal) avalanche multiplication process. The gain is kept sufficiently low in order to obtain the highest signal-to-noise ratio, as required for timing purposes (typically, timing resolution less than 30-40 ps).
Since in standard UFSD the spatial granularity is achieved by the segmentation of read-out pads, n+ cathode electrodes and gain layer, some additional implants have to be used in order to obtain the electromagnetic insulation of adjacent diodes. These implanted structures induce an efficiency loss in the inter-pixel region called dead area, which translates into a drop of the gain between pixels. High-energy physics experiments going towards an high-luminosity, high radiation fluence and high pile-up environments wants to maximize the ratio of the active area over the total sensor area (the so-called fill-factor).
The RSD project proposes to push the detection efficiency to high levels by decreasing the dead area around pixels. This result will be achieved through a simple but smart idea: indeed, RSD will be based on a slight modification of the standard UFSD design, where both the gain layer and the n+ cathode are no more segmented (while read-out pads remain properly separated). By chosing an optimized resistivity of the n-electrode, the charges generated via ionization by the particle which crosses the sensor will be frozen in the resistive layer for a discharging time long enough to be read out by the pads. Therefore, the transfer of signals to the pre-amp stage through the oxide passivation will be obtained by induction of charges due to the AC-coupling between sensor and read-out pads.
Figure 2. Basic operational principles of resistive AC-coupled read-out: (a) Electronic equivalent circuit. (b) Cross-section of a silicon n-in-p device, where the inclusion of an n+ layer (with a proper sheet resistivity) and a capacitive cup oxide allow transfering the signal to the pads by the induction of charges freezed in the resistive layer. (c) Simplified circuital model.
As a result of the RSD design, the active areas will be no more segmented. This will reflect into a rising up of the fill-factor close to the theoretical limit of 100%. Such a technological breakthrough will be beneficial in most of particle physics applications, being RSD the enabling paradigm for 4D tracking sensors working within high-luminosity environments. Indeed, an RSD sample recently achieved the milestone of concurrently measure space and time with a precision of 2.5 µm and 13.9 ps respectively: see the preprint available at arXiv:2003.04838.
The RSD project is supported by the INFN Commission-V (CSN5) and benefits from the scientific endorsement of the RD50 Collaboration at CERN and the University of California in Santa Cruz, UCSC.
The RSD Group
Marco Mandurrino, Ph.D.
Principal Investigator, researcher at INFN in Torino, expert in the field of solid-state physics, numerical simulations, particle detectors.
Anna Vignati, Ph.D.
researcher at INFN in Torino, expert in hardware and optimization of silicon detectors for medical applications in particle-therapy.
Francesco Ficorella, Ph.D.
researcher at Fondazione Bruno Kessler (FBK) in Trento, expert in measurements and testing of electronic devices and particle detectors.
Ph.D. student in Physics at Università di Torino.
Ongoing/former theses in Physics: F. Lenta (triennale, 2019).
Official RSD page at INFN Torino: [html
RSD project proposal and transparencies: [pdf1
List of RSD publications: [html