Fast timing silicon pixel detectors for new applications

Silicon sensors are essential in a range of fields, from cutting-edge research (e.g. particle physics, chemistry, materials science) to industry (agriculture, manufacturing), and everyday devices (cameras, security). They are the eyes of our electronic world. As we develop more precise sensors, for example cameras with smaller pixels, the potential reach of these devices increases, allowing more processes to be investigated, and with more detail.
Currently the resolution of such sensors is at the micrometre level. However, the time precision is relatively much worse, due to significant technological challenges in assigning times to the signals in the silicon. The best precision for small-pixel silicon sensors is at the nanosecond (ns) level. By comparison, light travels 300,000 micrometre per ns. Our ability to observe many processes is significantly hampered by limitations in time precision.

For fast (~1ns duration) processes, adding picosecond-level (1ps = 0.001ns) timing to micrometre-level spatial measurements effectively corresponds to the difference between still images and video, and hence has the potential to open up entire new fields of research. Such processes occur, for example, in particle and nuclear physics, chemistry, and materials science. The ultimate aim of this project is to develop sensors that for the first time simultaneously reach precision at the micrometre-level in space, and picosecond-level in time: a high-speed video camera for the smallest observable scales.

We start from a new type of sensor only developed in the past decade: Low Gain Avalanche Detectors (LGAD). By adding specially-treated semiconductor layers to the silicon, the time of signal collection is significantly reduced, making it possible to reach ~30ps precision. However, the only devices so far developed have large (mm-size) pads rather than pixels. Our programme of research will focus on ways to transform these devices into pixel sensors, by considering new geometries and doping approaches, and thin sensors. The key is to maintain as uniform an electric field as possible within the pixel, to ensure fast signal development. We have started preliminary studies, including fabrication of prototype devices, and now we are ready to push forward with an aggressive research and development phase.

Researchers from the Universities of Glasgow and Manchester will work with a commercial semiconductor manufacturer (Micron) to design and fabricate a range of new LGAD sensors, and analyse their performance using several high-tech methods ('transient current technique' - TCT and 'two photon absorption' - TPA). In parallel, we will develop realistic simulations of the detectors using TCAD models, to predict the sensor characteristics under different designs. These simulations will be validated using the TCT and TPA results from our measurements. All of our results will be published in open-access journals, taking us a step closer to the dream of '4D' precision sensors.

In parallel, we will develop a network of potential beneficiaries of these new devices, in particular for the fields of materials science and proton therapy. We have already established connections with representatives within these areas, who will help us to build the network, starting with two dedicated workshops. These will be used to build a specifications document where the required technology performances are defined. They will also enable us to reach further to identify more potential users of this new technology, in the UK and beyond.

The proposed 4D precision detectors will enable powerful new techniques in the fields of chemistry, medicine, and materials science for studying and visualising structures and processes. The following specific applications have been identified, which are described in more detail in the Pathways to Impact Statement (attached separately):
(1) Precise beam control for proton therapy, (2) X-ray correlation spectroscopy, (3) Cryo-Electron microscopy, (4) Time-of-Flight Mass Spectrometry.

Furthermore, expertise on the fabrication of pixelated LGAD will be developed during this project. This work will fortify the expertise of Micron in LGAD devices and strengthen the position of this UK company. This will allow Micron to bid for contracts to supply LGADs to the market. In HEP alone there are two potential large orders in the coming years for the CMS and ATLAS timing detectors. This proposal will improve the chance of Micron winning contracts for these large projects.