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

本项目介绍了一种由全数字电路构成的新型硅辐射探测器。这是一个设计方法的完全改变，相对于目前的固态传感器，完善的架构。这些都非常成功，是许多科学和技术应用的主要工具。硅传感器的一个优势是它们的位置分辨率。它们是为了在高能物理实验中跟踪带电粒子而引入和发展起来的，并且经过多年的不断改进。尽管如此，它们的命中位置分辨率多年来一直没有提高。30多年前就已经实现了1微米的分辨率，目前最好的设备的分辨率大于几微米。像素传感器无法提高命中定位精度的主要原因是模拟电路放大电离辐射释放的信号所需的最小尺寸，使最小像素尺寸达到几十微米的数量级。本文提出的实现硅传感器命中分辨率性能突破的方法包括设计基于全数字电路的传感器。感应机制是二进制的，当电离辐射穿过给定像素时，传感器单元的状态从一个改变到另一个（类似于固态数字存储器的操作）。这个数字电路包含有限数量的晶体管（从3到10），允许一个非常小的像素足迹。根据所选CMOS技术节点的特征尺寸，单个像素可以小到100 × 100 nm2，与当前最先进的技术相比，可以将分辨率提高两个数量级。具有上述特性的数字辐射传感器的概念已得到该项目的支持者的验证，并成功测量了来自141 Am放射源的脉冲蓝色激光和α粒子产生的电荷。对第一个数字传感器原型的初步测量也表明了后续的研究步骤，以提高这些设备的检测效率性能（记录的命中次数超过交叉电离粒子的总数）。结果表明，原型实现了非常浅的电荷收集，导致效率降低，仅限于在敏感晶体管栅极的对应中发生的撞击，而不是整个传感器区域。该项目将通过专门的敏感节点设计来纠正这种低效率，并在整个电离辐射光谱（最小电离粒子，带电离子，光子）上生产非常精确的分辨率像素传感器，并具有高效率。这种新型传感器将有无数的应用。在科学方面，它们将彻底改变核物理和粒子物理的实验，使跟踪体积大大减少，在未来实验的范围和成本方面都有很大的好处。这种新型传感器还将能够追踪单个硅层内短于1微米的粒子路径，从而对反冲的原子核或电子进行定向探测。这将使它们能够用于探测难以捉摸的暗物质候选弱相互作用大质量粒子（WIMP）。WIMP可以与硅晶格中的原子核相互作用，导致它们在几百纳米的距离上反冲。探测到这些短轨道并能够确定入射粒子的方向，为从否则无法克服的中微子背景中提取WIMP信号提供了有力的处理方法。这些只是这些新设备的巨大范围的例子。

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.