Nano-structured RC Networks - A Pathway To Artificial Skin

这项雄心勃勃的研究计划将开发一种位置敏感型触摸传感器，这种传感器在制造后可以符合柔性的不规则表面，从而可以作为人造皮肤移植的传感元件。该传感器的愿景是，其低功耗、大面积和简单的设备架构将有助于其适应医疗应用的能力。“随机阻抗网络传感器”的概念源于人工合成原子薄材料的形态，这些材料几乎完全是多晶生长的。在高端应用中，通常避免半导体中的多晶性，因为i)它们的电阻率高于它们的单晶(MC)等效物，以及ii)每对微晶(或颗粒)被一个非晶态和富含缺陷的界面(晶界)分开，这是一个额外的电阻率来源。这一规则的少数例外，如用作MOSFET栅极金属的简并掺杂多晶硅，或销售于低端消费级产品的PC-Si光伏电池，突显出PC材料在全球微电子行业中的边缘地位。第一个假设是，由于介电与环境的失配，PC薄膜固有地耗尽了自由电荷载流子。较少的载流子意味着静电屏蔽效率降低，表现为极长的屏蔽长度和较宽的俘获截面。这转化为对静电事件的长距离敏感性，例如手指的触摸。第二个假设是，颗粒和晶界的错综复杂的网络形成了一个随机取向的电阻(颗粒)和压控电容器(边界)网络，既显示了直流电阻，也显示了交流电抗。由此产生的薄膜阻抗是偏置相关的、非线性的，关键的是，还取决于位置，因为沿着材料的每个电流路径都带有特征阻抗特性。这些假设的结合使得任何静电事件，如手指触摸，都可以通过三角测量其在表面的位置来定位，使得PC薄膜成为位置敏感应用的理想衬底。要实现这种位置敏感触摸传感的新范式，需要知道颗粒-颗粒边界系统的完整电子结构，并且需要很好地了解穿过它的电荷载流子的传输机制。研究方法将包括将功能探针显微镜与宏观传输测量相结合，这将为RINS探测器的设计提供信息。最后，我们的目标是开发传感器本身，并设计一种方法，通过使用几个低功率外围探测器的读数，将其表面的静电事件映射到它们的准确位置。所提出的传感机制与目前可用的传感器之间的巨大差异为新的应用带来了令人兴奋的机会。目前，用于手机和平板设备的电容式触摸传感器由稀土材料制成的透明电极组成的正交栅组成。这限制了它们在刚性曲面或在大的预定义曲率半径上是柔性的曲面上的应用。本文提出的传感器克服了这一局限性，只使用外围电极，减少了对刚性栅格图案的需求。此外，在电流传感器中，通过两个正交电极及其附近节点之间重叠节点的电容变化来推断位置。这意味着需要按顺序寻址和读取节点，使得响应时间较长，尤其是在较大的表面上。使用少量外围探头，全部连续读取，这意味着信息处理可以快速完成，而且规模要大得多。

The ambitious research programme will see the development of a location sensitive touch sensor that can conform, after its fabrication, to flexible irregular surfaces and thus, can function as the sensing element in an artificial skin graft. The vision for the sensor is that its low power consumption, large area and simple device architecture will contribute to its ability to adapt to use in healthcare applications.The concept for the 'random impedance network sensors', or RINS, stems from the morphology of artificially synthesised atomically thin materials, that are grown almost exclusively as polycrystalline. Polycrystallinity in semiconductors is usually avoided in high-end applications because i) their resistivity is higher than their monocrystalline (MC) equivalents, and ii) each pair of crystallites (or grains) is separated by an amorphous and defect-rich interface (grain boundary) which is an additional source of resistivity. The few exceptions to the rule, such as degenerately doped polycrystalline (PC) silicon, which is used as gate metal in MOSFETs, or PC-Si photovoltaic cells marketed at the low-end of consumer-grade products, highlight the marginal position PC materials occupy in the global microelectronics industry. The first hypothesis is that PC thin films are inherently depleted of free charge carriers due to dielectric mismatch with their environment. Fewer charge carriers mean that the electrostatic screening efficiency is diminished and is manifested in extraordinarily long screening lengths and wide capture cross sections. This translates to a long-distance sensitivity to electrostatic events, such as the touch of a finger. The second hypothesis is that the intricate network of grains and grain boundaries forms a randomly oriented network of resistors (grains) and voltage-controlled capacitors (boundaries), which display both DC resistance and AC reactance. The resulting film impedance is bias-dependent, non-linear, and, crucially, position dependent, as each current pathway along the material carries a signature impedance characteristic. The combination of these hypotheses enables positioning of any electrostatic event, such as a finger touch, by triangulation of its position on the surface, making PC thin films the ideal substrates for position sensitive applications.To realise this new paradigm in location sensitive touch sensing, the full electronic structure of the grain-grain boundary system needs to be known, and the transport mechanism of traversing charge carriers across it needs to be well understood. The research methodology will include a combination of functional probe microscopy with macroscopic transport measurements, which will inform the design of the RINS detector. Finally, we aim to develop the sensor itself, and design the methodology by which electrostatic 'events' on its surface are mapped to their exact location using the reading from few low power peripheral probes.The stark difference between the proposed sensing mechanism and the sensors available today translate to exciting opportunities for new applications. Currently, capacitive touch sensors, such as those used in mobile phones and tablet devices, consist of orthogonal grid of transparent electrodes made of rare earth materials. This limits their use to applications on rigid surfaces, or surfaces that are flexible on a large, pre-defined radius of curvature. The sensor proposed here overcomes this limitations by using only peripheral electrodes, alleviating the need of rigid grid patterning. Furthermore, in current sensors location is inferred through capacitive changes at an overlap node between two orthogonal electrodes, and their nearby nodes. This means that nodes need to be sequentially addressed and read, making the response time long, especially on large surfaces. The use of few peripheral probes, all continuously read, means that processing the information can be done quickly, and on a much larger scale.