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

Over the last forty years, advances in semiconductor processing techniques have led to continued dimensional scaling of individual components in integrated circuits, leading to vast improvements in performance of electronic devices. Today, integrated circuits contain billions of electronic transistors and interconnects, each having minimum dimensions in the tens of nanometers. At this scale, the extreme sensitivity of electronic properties to small changes in dimension, structure, roughness, defect content, and physical properties, suggests that precise metrology would be needed to predict the functional performance and reliability of electronic components and devices. At present, new metrology techniques are critically needed for detection of buried defects such as electromigration voids in copper interconnects and other defects in multi-layered integrated circuit components. Existing methods with high spatial resolution rely on scanning probe technologies, which are destructive and have limited penetration depth. Nondestructive methods like acoustic or optical microscopy provide low spatial resolution due to diffraction effects. The objective of this project is to explore the combination of the nanoscale spatial resolution of near-field scanning optical microscopy (NSOM) and picosecond time-resolved sensitivity of acoustic waves generated by an ultrafast laser, in order to address the metrology need. By combining these techniques, ultrahigh frequency (>300GHz) acoustic waves corresponding to wavelengths below 100nm can be generated. Furthermore, local perturbations in the acoustic wave field resulting from interactions with buried submicron structures can be measured with nanoscale spatial resolution. Specifically, this project would, (1) explore a plasmonic nanoantenna approach through which light can be confined on a submicron scale for ultrafast laser generation and detection of acoustic waves, (2) study the sensitivity of the acoustic waves to buried structures depending on their dimension and mechanical properties, and (3) explore time reversal acoustic techniques for acoustic imaging below the classic acoustic diffraction limit by studying the effects of multiple scattering from buried sub-acoustic wavelength structures.