Newton RCUK-CONACYT Cost-efficient and radiation-tolerant pixel detectors for ionising radiation based on thin-film technology

Thin-film technology (TFT) has many advantages over classical techniques for single crystal growth and integrated circuit design and is today used in everyday appliances like computer/TV screens and solar cells. The idea of the proposed project is to put this technology to use in a new regime - large-area cost-efficient detectors for ionising radiation - and to ensure that Mexico is enabled to develop and produce such detectors for future high-profile projects in big science.

Up to now, tracking detectors for high-energy physics have been manufactured using non-standard silicon planar technology on small wafers leading to high cost and limited availability. The LHC upgrades require large areas (100s-1000s m2) of radiation-tolerant semiconductor detectors. Cost-efficiency is the key to being able to instrument large areas and volumes with the available budget - while higher granularity of detectors is instrumental to improving the physics reach of the LHC experiments in view of the large number of pile-up interactions taking place (up to 400 at any given bunch crossing).

Thin-film based detectors might be a way to accommodate both goals as the use of established low-cost, large-area processes known from solar cells is up to 2 orders of magnitude cheaper than state-of-the-art growth method for float-zone silicon wafers. As this approach for pixel detectors is novel, it is a unique opportunity for Mexico to establish world-leading expertise and lead future pixel detector construction efforts in big science projects such as the LHC upgrade.

Beyond thin-film deposition methods to create substrates in the photovoltaic industry, thin-film technology is also used to create field-effect transistors, e.g. in TFT screens. Recent developments in thin-film transistors suggest that it might be possible to manufacture the first pre-amplifier stage necessary for a charge-collecting semiconductor detector in thin-film technology "on top" of the charge collection volume - which has been created by thin-film technology as well. The concept of in-pixel amplification has been first established in so-called HV-CMOS detectors and allows the use of very thin sensor layers (down to about 20 um), which is advantageous for material budget, production cost and radiation tolerance.

The first step of this proposal is to design, produce, irradiate and characterise passive TFT-sensors that could replace today's planar silicon pixel sensors. In a second step, electronic circuits in thin-film technology will be studied and added on top of the pixels to amplify and possibly discriminate particle hits. The ultimate goal is to obtain monolithic active pixel sensors (MAPS). Base materials under investigation will be GaAs, GaN and c-SiTF.

Particle physics is dealing with most fundamental questions, e.g. why and how do we get mass? To get answers, huge and very complex experiments are carried out, for example at the Large Hadron Collider at CERN in Geneva, where the ATLAS and CMS detectors have recently discovered the Higgs-Particle that we assume to be related to the mechanism assigning mass to all particles. To enable further studies, upgrades to the detectors are necessary, but radiation-hard particle detectors are custom-made components and thus unfortunately rather expensive - possibly too expensive to allow the optimal upgrades.

Thin-film technology is abundant in everyday life since several years now and is contained in virtually every TV set and in a sizable fraction of all solar cells. Thanks to the large markets and the need for cheap, large-area coverage, thin-film technology is very cost-efficient.

The idea of this project is to investigate whether radiation-tolerant pixel detectors could be made using thin-film technology. If successful, the sensor cost for the upgrade could be reduced by a factor 10 to 100 allowing much larger and better coverage leading to a better detector - with larger physics reach, that may be in the end make the difference in answering most fundamental questions for the origin of the universe.

Such detectors could of course not only be applied to particle physics experiments - they would be good candidates for cheap, film-less large-area x-ray detectors that could be distributed to physicians in developing countries helping to improve healthcare in rural areas.