Very deep sub-micron, entirely digital, position resolution sensors.

The project introduces a novel type of silicon radiation detectors constituted by an entirely digital circuit. This is a total change of design approach with respect to the current, well established architecture of solid state sensors. These have been very successful and are the main tool for many applications in science and technology. One strength of silicon sensors is their position resolution. They have been introduced and developed for tracking charged particles in high energy physics experiment and have gone through the years to a continuous series of improvements. Nonetheless their hit position resolution has not improved for many years. A value of 1 micron was already achieved over 30 years ago, and the current best devices have resolution larger than a few microns. The main reason for the inability of improving the hit location precision on a pixel sensor is the minimum size required by the analogue circuit amplifying the signal released by the ionising radiation, making the minimum pixel dimensions of the order of a few tens of microns. The approach here proposed to realise a breakthrough for the hit resolution performance of silicon sensors consists in designing a sensor based on an entirely digital circuit. The sensing mechanism is binary, with a sensor cell changing state from one to the other of two possible values when ionising radiation is crossing a given pixel (similar to the operation of a solid state digital memory). This digital circuit is comprising a limited number of transistors (from 3 to 10), allowing for a very small pixel footprint. Depending on the feature size of the selected CMOS technology node, a single pixel could be as small as 100x100 nm2, enabling an enhancement of up to two orders of magnitude in resolution when compared to current state-of-the-art. The concept of a digital radiation sensor with the above characteristics has been validated by the proponents of the project, with successful measurements of the charge generated by a pulsed blue laser and alpha particles from a 141 Am radioactive source. The initial measurements on the very first digital sensor prototypes have also indicated the subsequent research steps to improve the detection efficiency performance (number of recorded hits over the total number of crossing ionising particles) of these devices. The results have shown that a very shallow charge collection was achieved with the prototype resulting in a reduced efficiency, limited to hits happening in correspondence of the sensitive transistor gate, rather then over the whole sensor area. This project will correct this inefficiency with dedicated design of the sensitive node and produce very precise resolution pixel sensors with high efficiency over the full ionising radiation spectrum (minimum ionising particles, charged ions, photons). The new sensors would have countless applications. In science, they would revolutionize experiments in nuclear and particle physics, allowing for a large reduction of the tracking volume, with great benefits in terms of the scope and cost of future experiments. The new sensors will also be able to track particle paths shorter than 1 micron inside a single silicon layer, allowing for directional detection of recoiling nuclei or electrons. This would enable their use for detection of elusive Weakly Interactive Massive Particle (WIMP) candidates for Dark Matter. WIMPs can interact with nuclei in the silicon lattice causing these to recoil over distances a few hundred nm. Detecting these short tracks and being able to determine the direction of the incoming particle provides a powerful handle to extract the WIMP signal from otherwise insurmountable neutrino background. These are only examples of the huge scope of these novel devices.