The monolithic approach: a simplified silicon detector
Sebastian Grinstein (IFAE), 22/02/2018
The AMS H35demo chip (Image: KiT, Uni of Liverpool, Uni of Geneva and IFAE-Barcelona)
In order to assemble the necessary expertise for the ambitious programme towards the upgrade of the Large Hadron Collider (LHC) and the preparation of new experiments, the AIDA-2020 project brings together the leading European infrastructures in detector development and a number of academic institutes.
As a result of this collaboration of many international research institutions, the AIDA-2020 project is advancing the development of a new type of silicon detectors based on commercially available CMOS technologies for use in the high-luminosity upgrade of the LHC.
Silicon pixel detectors are the technology of choice for the innermost layers of the current LHC detectors. These devices offer excellent position resolution, which allows researchers to distinguish interesting collisions from the much more frequent background of ordinary events. Furthermore, they can be operated for many years in demanding radiation environments, as found close to the LHC proton collisions.
Silicon acts as a sensing medium, generating electrical signals when particles pass through it. Electronic front-end chips, attached to the silicon sensor, then amplify and process the signals. Up to now, devices which couple readout chips to sensors (hybrid silicon pixels) have been used in high-energy physics (HEP) experiments. Their performance is very good, but their cost is rather high. More recently, researchers have started to consider alternatives.
Whilst hybrid pixel detectors are specific to particle physics, a different type of silicon radiation detector is widely used outside the HEP community. For example, digital cameras feature devices fabricated using commercial CMOS microelectronics.
In CMOS-based detectors, the sensing medium and front-end electronics are part of the same silicon wafer. The technology offers a lower cost, reduced material, and a decreased level of complexity.
Considering these benefits, a research program was initiated to develop CMOS detectors for HEP. This work led to detectors which could be used for different low radiation-level applications [1] but that were not suitable for accelerator-based experiments where high level of radiation resistance was required. The performance of the devices would degrade substantially after irradiation, as the charge signal generated in the device would become too small, making it ineffective for particle detection.
However, a 2007 proposal suggested a method for overcoming this limitation [2] by making use of CMOS fabrication methods that allowed to increase the charge collection volume of the devices. The increase in the collection volume of these depleted-CMOS sensors enables a larger charge to be collected, and therefore produces a larger signal, which compensates, to some extent, the effects of irradiation.
The ATLAS collaboration, supported by AIDA-2020, is vigorously investigating depleted-CMOS devices for the detector upgrade for the high-luminosity LHC period, which is expected to start in 2026.
Already several developments look capable of meeting the challenges of the outermost pixel detector layers of the future ATLAS tracking system, however, many other challenges remain. Nonetheless, the progress already made in the large collection volume CMOS technology is being recognized in the scientific community, and the future of silicon detectors seems mostly monolithic.
[1] MIMOSA detector family.
[2] I. Peric et al., NIM A 582 (2007) 876.