Exploring local confinement of ultrafast light to enable nondestructive acoustic metrology at the nanoscale

该项目将探索光聚焦方案，将光限制在纳米级，并开发一种新的仪器，可以检测现代电子设备中的纳米级结构缺陷。该方法具有非破坏性和非侵入性，并依赖于光波和弹性波传播的结合。声学成像方法是利用弹性波可视化固体材料内部区域的成熟方法。在半导体制造中，声成像通常用于失效分析和工艺条件评估。不幸的是，由于衍射的原因，声成像方法的空间分辨率仅限于微米尺度，这是本项目寻求解决的主要缺点。为了克服有限的分辨率，将探索光子超材料，以创建一组明亮的纳米级光学探针，用于探测高频（0.3 -1太赫兹）弹性波。在这个频率范围内的波具有几十纳米的波长，并且对纳米级缺陷（如空洞、裂纹和夹杂物）的存在极其敏感。该方案将为探测弹性波传播提供极限空间分辨率（20纳米）和时间分辨率（~ 1皮秒），并将提供并行探测能力，以促进微纳米电子结构的快速成像。此外，光学检测方法可以广泛应用于半导体成像以外的领域。这些应用包括用于医学治疗和药物开发的分子成像和生化传感。该项目将为本科生和研究生参与纳米力学和近场光学领域的多学科研究创造机会。该项目的研究成果将用于设计一个基于查询的纳米力学应用程序，用于高中物理课堂。该项目将通过开发集成等离子体超表面和皮秒激光超声的新型仪器，解决半导体电子器件传感和纳米测量中传统声学成像方法的技术障碍。超声波方法依赖于使用飞秒泵浦激光源来产生超短（带宽高达1太赫兹）弹性波脉冲。弹性脉冲将使用泵浦-探针时域光谱方法进行皮秒级时间分辨率的监测。超表面由等离子体纳米天线二聚体的二维阵列组成，通过利用纳米尺寸二聚体间隙内的电磁波共振，可以在亚波长尺度上有效地限制飞秒探测激光。每个二聚体将作为纳米级光学探针，用于检测样品表面的弹性波。为此，将处理三个具体的研究任务：(1)研究皮秒时间尺度瞬态机械变形（弹性波和振动）对等离子体纳米天线纳米约束和增强的影响；(2)设计和实现局部可寻址的纳米天线阵列，以实现在纳米尺度区域上并行探测弹性波，而无需探针扫描；(3)研究和实现高纵横比纳米结构模型电子器件的超快激光产生和弹性波检测，用于检测埋地纳米级缺陷。此外，基于时间反转技术的逆模型将用于缺陷识别、定位和尺寸确定。这些任务将促进对半导体器件中超快光和超高频（THz）弹性波的局部相互作用的现有理解。最终，这些事业将促进纳米计量学和成像方法的发展，使半导体器件的非侵入性测量成为可能，这是目前技术无法实现的。

This project will explore light focusing schemes to confine light at the nanoscale and develop a novel instrumentation that will enable detection of nanoscale structural defects in modern electronic devices. The proposed approach is nondestructive and noninvasive and relies on a combination of optical and elastic wave propagation. Acoustic imaging methods are well established methods for visualizing interior regions of a solid material using elastic waves. Acoustic imaging is commonly used for failure analysis and assessment of process conditions in semiconductor manufacturing. Unfortunately, the spatial resolution of acoustic imaging methods is limited to the micrometer scale due to diffraction, which is a major short coming that this project seeks to address. In order to overcome the limited resolution, photonic metamaterials will be explored to create an array of bright nanoscale optical probes that will be used to detect high frequency (0.3 -1 THz) elastic waves. Waves in this frequency range have wavelengths of a few tens of nanometers, and are extremely sensitive to the presence of nanoscale defects like voids, cracks, and inclusions. The proposed scheme will provide access to extreme spatial resolution ( 20 nm) and temporal resolution (~ 1 picosecond) for probing elastic wave propagation, and will provide parallel detection capabilities to facilitate rapid imaging of micro- and nano-electronic structures. Furthermore, the optical detection approach can be applied broadly beyond semiconductor imaging. These applications include molecular imaging and biochemical sensing for medical therapy and drug development. The project will create opportunities for undergraduate and graduate students to participate in multidisciplinary research in the areas of nanomechanics and near-field optics. The research outputs of the project will be used to design an inquiry based nanotechnology applet on nanomechanics for use in a high-school physics classroom.This project will address technical barriers in conventional acoustic imaging methods for sensing and nanometrology of semiconductor electronic devices through the development of novel instrumentation that integrates plasmonic metasurfaces with picosecond laser-based ultrasonics. The ultrasonic approach relies on the use of a femtosecond pump laser source for generation of ultrashort (bandwidth of up to 1 THz) elastic wave pulses. The elastic pulses will be monitored with picoseconds time-resolution using the pump-and-probe time-domain spectroscopy approach. The metasurface which is comprised of a two dimensional array of plasmonic nanoantenna dimers will enable efficient confinement of a femtosecond probe laser on a subwavelength scale, by exploiting electromagnetic wave resonances within the nanometer sized dimer gaps. Each dimer will serve as a nanoscale optical probe for detection of elastic waves on the sample surface. Towards this end, three specific research tasks will be addressed: (1) investigation of the influence of transient mechanical deformations (elastic waves and vibrations) at picoseconds timescales on the nano-confinement and enhancement in the plasmonic nanoantennas, (2) design and implementation of locally addressable arrays of nanoantennas to enable parallel detection of elastic waves on nanoscale areas without probe-scanning, and (3) investigation and implementation of ultrafast laser generation and detection of elastic waves in model electronic devices with high aspect ratio nanostructures for detection of buried nanoscale defects. Furthermore, an inverse model based on the time-reversal technique will be developed for defect identification, localization, and sizing. These tasks will advance existing understanding of the local interaction of ultrafast light and ultrahigh frequency (THz) elastic waves in semiconductor devices. Ultimately, these undertakings will facilitate the development of a nanometrology and imaging approach that permits noninvasive measurements in semiconductor devices that cannot be achieved using current technologies.