Description
The Belle II experiment currently records data at the SuperKEKB $e^+e^-$
collider, which holds the world luminosity record of $5.2\times10^{34}$
$\textrm{cm}^{-2}\textrm{s}^{-1}$ and plans to push up to
$6\times10^{35}\textrm{cm}^{-2}\textrm{s}^{-1}$, after an upgrade
of its interaction region. To cope with the increased backgrounds, a new
fully pixelated vertex detector is under design. Its 5 layers will be
instrumented with the same DMAPS, OBELIX, designed in the Tower 180 nm
technology, and recently submitted for fabrication.
The OBELIX pixel matrix is inhereted from the TJ-Monopix2 sensor,
whose characterization has focused on its performance after irradiation
with integrated fluences up to the expected $5\times10^{14}\textrm{MeV} \textrm{n}_\textrm{eq}\textrm{cm}^{-2}$. The new
sensor features a 33 µm pitch with a 7-bit Time-Over-Threshold
digitization. The new read-out architecture has been designed to support
triggered operation and additional features needed for improving
time-stamping and contributing to Belle II track triggering. The
specifications require the detector to sustain a maximum average hit
rate of 120 $\textrm{MHz}\textrm{cm}^{-2}$, while keeping an
overall material budget lower than 3\% X0. A light
mechanical structure is foreseen, with two different approaches.
The ladders of the 2 innermost layers are cut out directly from the
processed wafers and connected with a post-process redistributed metal
layer. Passive cooling using a thin layer of high-conductance material
(TPG) beneath the sensors and connected to actively cooled blocks has
been chosen. The post-irradiation performance of TPG was recently
validated. The 3 outermost layers, instead, are composed of staggered
ladders, realized with a light carbon fibre structure supporting a
liquid-cooled plate in contact with the sensors connected to a flex
printed cable. This contribution will review the status of the
development of the sensor and the detection modules.