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.

Publications (RSD Group)

15th Pisa Meeting on Advanced Detectors: [html]

17th Trento Workshop: [html]

VCI 2022 conference (two accepted abstracts): [pdf1] [pdf2]

39^th workshop of the RD50 group (abstract, transparencies and recording): [pdf1] [pdf2] [html]

TWEPP 2021 conference: [html]

PSD12 2021 conference (abstract, contribution and proceeding paper): [pdf1] [pdf2] [pdf3]

paper on NIM-A about the RSD1 testing as tracking detectors: [html]

paper on JINST about the machine learning algorithms applied to RSD1: [html]

38^th workshop of the RD50 group (two talks): [html1] [html2]

TIPP 2021 conference: [html]

IEEE NSS 2020 conference (summary and contribution): [pdf1] [pdf2]

IEEE NSS 2020 conference (two other talks): [html1] [html2]

36^th workshop of the RD50 group (two talks): [html1] [html2]

preprint on arXiv of the paper submitted to NIM-A about the full characterization and 4D-tracking performances of RSD1: [pdf]

paper on NIM-A about the RSD design: [pdf]

15th Trento Workshop: [html]

12^th International Hiroshima Symposium on the Development and Application of Semiconductor Tracking Detectors (two talks): [pdf1] [pdf2]

35^th workshop of the RD50 group (abstract and transparencies): [pdf1] [pdf2]

paper on IEEE-EDL about first RSD1 measurements (also available on arXiv): [pdf1] [pdf2]

VERTEX 2019, 28th International Workshop on Vertex Detectors (abstract, contribution and proceeding paper): [pdf1] [pdf2] [pdf3]

IEEE NSS 2019 conference (abstract, summary, transparencies and proceeding paper): [pdf1] [pdf2] [pdf3] [pdf4]

EPS-HEP 2019 conference (abstract, transparencies and proceeding paper): [pdf1] [pdf2] [pdf3]

34^th workshop of the RD50 group (abstract and transparencies): [pdf1] [pdf2]

IEEE NSS 2018 conference (abstract, summary and transparencies): [pdf1] [pdf2] [pdf3]

13^th Trento Workshop (abstract and transparencies): [pdf1] [pdf2]

Additional Material

presentations of the RSD project at the referee meeting of the RD_MUCOL experiment, 9-14 September 2021: [pdf1] [pdf2] [pdf3]

presentation about RSD numerical design at the AC-LGAD simulations mini-workshop, 30 March 2021: [pdf]

talk “4D tracking with Resistive AC-Coupled Silicon Detectors” at the ALICE3 Timing Layers Working Group Meeting, 13 November 2020: [pdf]

report on LGAD simulations at the PRIN meeting with INFN-PG, 9 October 2020: [pdf]

report on 4D tracking with RSD at the RD_MUCOL meeting, 25 September 2020: [pdf]

report on RSD project at the meeting with UCSC-SCIPP, Cactus Material and FBK, 11 August 2020: [pdf]

talk “Resistive AC-Coupled Silicon Detector (RSD): innovative sensors for future trackers” at the Joint FCC-ee MDI Meeting, 6 July 2020: [html]

CERN Detector Seminar “Innovative Silicon sensors for future trackers” about RSD, 5 June 2020: [html]

report on RSD project at the INFN CSN5 national meeting, 10 February 2020: [pdf]

report on RSD project at the meeting with GSI, 23 January 2020: [pdf]

report on RSD testing campaign at CERN at the meeting with UZH, 18 July 2019: [pdf]

report on RSD project at the meeting with FBK and PSI, 16 July 2019: [pdf]

Project Report at the Turin INFN Consiglio di Sezione, 9 July 2018: [pdf]

Project proposal and transparencies: [pdf1] [pdf2]

The RSD Group

Marco Mandurrino, Ph.D.