Investigating pressure induced conductive states on the nanoscale : A novel route to nano-circuitry

以低成本、高吞吐量和高分辨率传输纳米级金属图案的能力对大幅削减半导体和数据存储设备的制造成本具有巨大的影响。像纳米压印光刻这样的技术允许通过压印抗蚀剂的机械变形和后续工艺来制造这样的纳米级图案。尽管这项技术被认为是最简单的光刻方法之一，但在图案转移过程中，它仍然包括几个复杂的步骤。如果可以省去转移步骤，并且只有压印步骤可以直接导致在所选材料上印刷几纳米锐利的电路图案，那么就纳米电路的图案的产量和重复性而言，这将是一个巨大的飞跃。为了实现直接压印电路的这一理想目标，有必要首先了解材料中的物理现象，这种物理现象将允许局部压力用作工具来绘制和控制其他绝缘材料中的尖锐导电通道。至少有两种不同的机制可以在绝缘材料中引起局部压力感应导电态。具有导电域壁的铁电材料和经历金属-绝缘体相变的材料是两种主要的材料系统，在这种材料系统中，纳米级的压力可以实现材料中的受限导电态，并潜在地实现对这种界面的确定性控制。这两种体系都允许导电壁或相在整体上共存，但在充分利用效应本身之前，局部应力导致这些材料中形成导电界面或通道的机制仍有待于很好地了解。对于潜在的应用，还需要评估在压力下形成通道的简易性、稳定性和可重构性。为了解决这些问题，这项提议的主要目标是建立压力中介的局域纳米级导电态的控制，并对与这种行为相关的物理学有一个基本的理解，以便能够实现对导电界面的可靠控制，作为迈向纳米电路的第一步。通过原子力显微镜(AFM)针尖施加的压力将被用来在三种不同的材料中注入导电态：不适当的铁电材料、混合相铁电体和经历金属-绝缘体相变的材料，每种材料都代表通过压力诱导写入创建的独特类型的导电界面。在这三种情况下，都将评估尖端压力诱导的导电壁/相的可控程度，并进行实验以确定所产生的界面的物理来源和潜在的导电性机制。通过对这些材料中压力诱导导电性机制的掌握，我们的目标是能够精确地控制这些受限壁或相的形成和湮灭。将评估压力介导的传导行为控制与其他刺激的互补性，以实现传导通道和读取/擦除能力的最佳可重构性。将进行侧电极之间压力感应传导通道的概念验证演示。因此，基于原子力显微镜的方法将有助于在所讨论的材料系统中建立压致导电态的基础物理，并为开发用于直接写入纳米电路的其他纳米压印方法提供关键的见解。

The ability to transfer nanometer scale metallic patterns at low cost, high throughput and high resolution has huge implications for slashing the manufacturing costs of semiconductors and data storage devices. Techniques like nanoimprint lithography allow fabrication of such nanometer scale patterns by mechanical deformation of imprint resist and subsequent processes. Even though the technique is considered one of the simplest lithography approaches, it still comprises of several complicated steps during pattern transfer. If the transfer step could be eliminated and only the imprint step could directly result in the printing of a few nanometer sharp circuit pattern on the chosen material, that would represent a dramatic leap in terms of throughput and reproducibility of patterns for nano-circuitry. In order to achieve this visionary goal of directly imprinting circuitry, it is necessary as a first step to understand the physical phenomena in materials that would allow localised pressure to be used as a tool to sketch and control sharp conductive channels in an otherwise insulating material. There are atleast two different mechanisms that could give rise to local pressure induced conductive states in an insulating material. Ferroelectrics with conducting domain walls and materials undergoing metal-insulator phase transitions are the two primary material systems where nanoscale pressure can be used to realise confined conducting states in the material and potentially achieve deterministic control of such interfaces. Both systems allow co-existence of conducting walls or phases in the bulk but the mechanisms through which localised stress results in the formation of conductive interfaces or channels in these materials remains to be well understood before the effect itself can be fully exploited. For potential applications, it is also necessary to evaluate the ease of channel formation under pressure, their stability and reconfigurability. To address these issues, the primary goal of this proposal is to establish the pressure mediated control of localised nanoscale conductive states and develop a fundamental understanding of the physics associated with this behaviour so that reliable control of conductive interfaces can be achieved as a first step towards nano-circuitry. Pressure applied via an atomic force microscope (AFM) tip will be used to inject conductive states in three different materials : an improper ferroelectric, a mixed phase ferroelectric and a material undergoing metal-insulator phase transition, each representing a unique type of conductive interface created through pressure induced writing. In each of the three cases, the achievable degree of control of tip pressure induced conductive walls/phases will be evaluated and experiments will be performed to identify the physical origin and the mechanisms underlying the conductivity of the created interfaces. With a grasp on the mechanism of pressure induced conductivity in these materials, we aim to be able to precisely control the formation and annihilation of these confined walls or phases. The complementarity of pressure mediated control of conductive behaviour with other stimuli will be evaluated for optimal reconfigurability of conductive channels and read/erase capability. Proof-of-concept demonstration of pressure induced conductive channels between lateral electrodes will be performed. The AFM based approach developed here would thus help establish the underpinning physics of pressure induced conductive states in the discussed material systems and provide key insight for developing other nanoimprint methods for direct writing of nano-circuitry.