Department of Electrical and Computer Engineering, ‡Materials Science and Engineering Program, and §Qualcomm Institute, University of California-San Diego , La Jolla, California 92093, United States.
ACS Nano. 2014 Nov 25;8(11):11261-7. doi: 10.1021/nn503924s. Epub 2014 Oct 29.
Spontaneous attractions between free-standing nanostructures have often caused adhesion or stiction that affects a wide range of nanoscale devices, particularly nano/microelectromechanical systems. Previous understandings of the attraction mechanisms have included capillary force, van der Waals/Casimir forces, and surface polar charges. However, none of these mechanisms universally applies to simple semiconductor structures such as silicon nanowire arrays that often exhibit bunching or adhesions. Here we propose a simple capacitive force model to quantitatively study the universal spontaneous attraction that often causes stiction among semiconductor or metallic nanostructures such as vertical nanowire arrays with inevitably nonuniform size variations due to fabrication. When nanostructures are uniform in size, they share the same substrate potential. The presence of slight size differences will break the symmetry in the capacitive network formed between the nanowires, substrate, and their environment, giving rise to electrostatic attraction forces due to the relative potential difference between neighboring wires. Our model is experimentally verified using arrays of vertical silicon nanowire pairs with varied spacing, diameter, and size differences. Threshold nanowire spacing, diameter, or size difference between the nearest neighbors has been identified beyond which the nanowires start to exhibit spontaneous attraction that leads to bridging when electrostatic forces overcome elastic restoration forces. This work illustrates a universal understanding of spontaneous attraction that will impact the design, fabrication, and reliable operation of nanoscale devices and systems.
自由-standing 纳米结构之间的自发吸引力常常导致粘附或粘连，这会影响到广泛的纳米级设备，特别是纳米/微机电系统。以前对吸引力机制的理解包括毛细作用力、范德瓦尔斯/卡西米尔力和表面极性电荷。然而，这些机制都没有普遍适用于简单的半导体结构，例如硅纳米线阵列，它们经常表现出束集或粘连。在这里，我们提出了一种简单的电容力模型，用于定量研究普遍存在的自发吸引力，这种吸引力常常导致半导体或金属纳米结构之间的粘连，例如由于制造而不可避免地具有不均匀尺寸变化的垂直纳米线阵列。当纳米结构的尺寸均匀时，它们具有相同的衬底电势。存在轻微的尺寸差异会打破纳米线、衬底及其环境之间形成的电容网络的对称性，由于相邻线之间的相对电势差，会产生静电力。我们的模型通过使用具有不同间距、直径和尺寸差异的垂直硅纳米线对阵列进行了实验验证。已经确定了最小纳米线间距、直径或最近邻居之间的尺寸差异，超过这个差异，纳米线将开始表现出自发吸引力，当静电力克服弹性恢复力时，就会导致桥接。这项工作说明了对自发吸引力的普遍理解，这将影响纳米级设备和系统的设计、制造和可靠运行。
