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

Northwestern University, Evanston IL

Investigators

Abstract

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.