Upgrade of the ATLAS Pixel Detector

The pixel detector is the innermost detector (as part of the Inner Tracker) in the ATLAS Detector located at CERN, Geneva. As the LHC gets a luminosity upgrade, called HL-LHC, the detectors have to be replaced during the LHC Phase II upgrade to handle the increased luminosity of 5 x 1035 cm-2s-1.

The new pixel detector (Fig. 1) will consists of five barrel layers and a few end-cap disks equipped with around 10,000 pixel modules covering a total area of about 14 m2. Four readout chips with approx 150,000 pixels each will be mounted on every module to measure and reconstruct the trajectories of charged particles produced in collisions.

ATLAS Inner Tracker
Fig. 1: The ATLAS Inner Tracker for the HL-LHC with the pixel detector



Our group participated in designing the flexible PCB for the prototype hybrid modules equipped with FE-I4B chips. The hybrid module (Fig. 2) consists of a flexible PCB (flex), a sensor glued to the flex, and front end chips, which are bump bonded to the sensor and wire bonded to the flex, which routes the signals and provides the HV (high voltage) to the sensor. In our labs, we built and tested modules for the main structure demonstrator, consisting of the longeron (i.e. global carbon structure), pixel quad and dual modules, a cooling system, and a radioactive source for testing purposes.

Hybrid module
Fig. 2: Hybrid module consisting of a flexible PCB, a sensor, and front end chips



As for FE-I4B demonstrator program, we designed a quad and double module flex (Fig. 3), and currently we are working on building and testing the quad modules equipped with the next generation prototype front end chips produced with a 65 nm technology, called RD53A and developed by the RD53 Collaboration. These modules will be part of a new demonstrator located at CERN, for which RD53A and RD53B chips will be used.

Siegen flex
Fig. 3: The flexible PCB from Siegen designed for four RD53A front end chips



Currently, we are producing and testing hybrid modules equipped with RD53A/B front-end chips. We are installing a new cooling system in our renewed clean room, which will be able to cool the pixel modules down to -40°C with pre-cooled nitrogen inside to prevent condensation of water at low temperatures. We produce modules for the final detector as well as for the pre-production stage. More information on the pixel module flex design, module assembly, and testing can be found here.