Abstract
The upgrade of the ATLAS tracking detector (ITk) for the High-Luminosity Large Hadron Collider at CERN requires the development of novel radiation hard silicon sensor technologies. Latest developments in CMOS sensor processing offer the possibility of combining high-resistivity substrates with on-chip high-voltage biasing to achieve a large depleted active sensor volume. We have characterised depleted monolithic active pixel sensors (DMAPS), which were produced in a novel modified imaging process implemented in the TowerJazz 180 nm CMOS process in the framework of the monolithic sensor development for the ALICE experiment. Sensors fabricated in this modified process feature full depletion of the sensitive layer, a sensor capacitance of only a few fF and radiation tolerance up to 1015 neq/cm2. This paper summarises the measurements of charge collection properties in beam tests and in the laboratory using radioactive sources and edge TCT. The results of these measurements show significantly improved radiation hardness obtained for sensors manufactured using the modified process. This has opened the way to the design of two large scale demonstrators for the ATLAS ITk. To achieve a design compatible with the requirements of the outer pixel layers of the tracker, a charge sensitive front-end taking 500 nA from a 1.8 V supply is combined with a fast digital readout architecture. The low-power front-end with a 25 ns time resolution exploits the low sensor capacitance to reduce noise and analogue power, while the implemented readout architectures minimise power by reducing the digital activity.
Highlights
Sensors produced in the modified TowerJazz 180 nm CMOS processA key target of the design is to achieve a low capacitance of the collection electrode together with full depletion of the sensitive volume
Monolithic pixel development in TowerJazz 180 nm CMOS for the outer pixel layers in the ATLAS experiment
Monolithic pixel detectors designed in the TowerJazz 180 nm CMOS technology are being proposed for the upgrade of the ATLAS Inner Tracker (ITk)
Summary
A key target of the design is to achieve a low capacitance of the collection electrode together with full depletion of the sensitive volume. To achieve full depletion of the epitaxial layer, a process modification has been developed together with the foundry [5] (see figure 1b). In this case, the sensor junction is planar and extends over the full pixel width. By applying sufficient reverse substrate bias, the depletion will extend to the n-well collection electrode and yield a small capacitance for this small electrode. The sub-matrices differ among each other in terms of a few parameters, some of which affect the shape and extension of the depleted region: pixel size, electrode size, electrode-to-deep-p-well distance. A number of Investigator chips produced in the modified process have been irradiated with neutrons at the Triga reactor in Slovenia. After irradiation (including during measurements), the chips were always kept at −15◦C or below to prevent annealing effects and maintain the leakage current at sufficiently low levels
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