  In this paper we present a complete characterization of the first batch of
Resistive AC-Coupled Silicon Detectors, called RSD1, designed at INFN Torino
and manufactured by Fondazione Bruno Kessler (FBK) in Trento. With their 100%
fill-factor, RSD represent the new enabling technology for high-precision
4D-tracking. Indeed, being based on the well-known charge multiplication
mechanism of Low-Gain Avalanche Detectors (LGAD), they benefit from the very
good timing performances of such technology together with an unprecedented
resolution of the spatial tracking, which allows to reach the micron-level
scale in the track reconstruction. This is essentially due to the absence of
any segmentation structure between pads (100% fill-factor) and to other two
innovative key-features: the first one is a properly doped n+ resistive layer,
slowing down the charges just after being multiplied, and the second one is a
dielectric layer grown on Silicon, inducing a capacitive coupling on the metal
pads deposited on top of the detector. The very good spatial resolution
(micron-level) we measured experimentally - higher than the nominal pad pitch -
comes from the analogical nature of the readout of signals, whose amplitude
attenuates from the pad center to its periphery, while the outstanding results
in terms of timing (less than 14 ps, even better than standard LGAD) are due to
a combination of very-fine pitch, analogical response and charge
multiplication.
