Effect Of Small Size, Stress Localization And Stress Gradient On The Strength Of Silicon

该奖项支持对硅在小尺度下失效行为的研究。大多数微纳机械系统使用硅梁作为其结构部件。这些梁在操作过程中通常会受到弯曲。但硅在室温下易碎。这限制了硅器件的设计空间。然而，弯曲使靠近锚的梁表面附近的高应力局部化。此外，应力从表面向梁的中平面减小，产生应力梯度。小尺寸、应力局部化和应力梯度对硅的破坏机制的影响至今仍不明确。最近的证据表明，小规模的硅在非常高的屈服应力下可以具有延展性（即不脆）。如果是这样，那么小尺寸、应力局部化和应力梯度结合在一起可能会为硅提供延展性和高强度的优点。这种抗故障性将为硅器件的设计空间提供一个尚未开发的范例。对硅在小尺度弯曲下的失效和变形机制的详细了解将对半导体物理学和工业产生革命性的影响，并将是力学领域的根本进步。本项目旨在通过理论与实验相结合的方法，探索小硅试样在弯曲作用下的变形与破坏机理。本课题的工作假设是位错是硅在小尺度弯曲下变形的主要机制。小样品是无位错的。小尺寸和弯曲时的应力局部化提供了高的抗断裂缺陷容忍度。这是由于小应力区缺陷发生率低。在任何缺陷引起断裂之前，弯曲导致表面出现位错形核。这些位错进入块体，产生硅。但由于应力梯度的作用，屈服应力随尺寸的减小而增大。这一假设将通过三个任务来验证：(1)硅样品在弯曲下的机理建模和分子动力学模拟，(2)不同尺寸和不同温度下的微纳米单晶硅样品的弯曲实验，以及(3)在透射电子显微镜（TEM）下的原位弯曲实验，以揭示变形机制（与德国杜塞尔多夫马克斯普朗克研究所合作）。将为任务2和任务3开发一种新型微机械舞台。这项研究将与从K-12到研究生的教育和推广活动相结合。

This award supports an investigation of the failure behavior of silicon at small scales. Most micro-nano mechanical systems use silicon beams as their structural components. These beams are typically subjected to bending during operation. But silicon is brittle at room temperature. This limits the design space of silicon devices. Bending, however, localizes high stresses near the surface of the beams close to the anchors. In addition, the stresses decrease from the surface towards the middle plane of the beam, giving rise to stress gradients. The effects of small size, stress localization and stress gradient on the failure mechanisms of silicon remain elusive to date. Recent evidence suggests that silicon at small scale can be ductile (i.e., not brittle) at very high yield stresses. If so, then small size, stress localization and stress gradients together may offer the virtues of both ductility and high strength to silicon. Such failure resistance would present a yet untapped paradigm to the design space of silicon devices. A detailed understanding of the failure and deformation mechanisms of silicon at small scale under bending would be transformative for both semiconductor physics and industry, and will be a fundamental advance for the field of mechanics. The goal of this project is to explore the mechanics and mechanisms of deformation and failure in small silicon samples under bending by combining theory and experiments. The working hypothesis of the project is that dislocation is the primary mechanism of deformation in silicon under bending at small scale. Small samples are dislocation free. Small size and stress localization in bending offer high flaw tolerance against fracture. This is due to the low probability of flaw incidence in the small stressed region. Bending results in dislocation nucleation from the surface before any flaw induced fracture. These dislocations enter the bulk yielding the silicon. But the yield stress increases with decreasing size due to stress gradient. This hypothesis will be tested by undertaking three tasks: (1) mechanistic modeling and molecular dynamics simulations of silicon samples under bending, (2) bending experiments on micro-nano fabricated single crystal silicon samples with various sizes and at different temperatures, and (3) in situ bending experiments in transmission electron microscopes (TEM) to reveal the mechanisms of deformation (in collaboration with Max Planck Institute at Dusseldorf, Germany). A novel micro mechanical stage will be developed for tasks 2 and 3. The research will be integrated with education and outreach activities involving K-12 to graduate students.