This project seeks to develop a novel photoacoustic (PA) microscopy system that would enable the mapping of deeply buried subsurface defects in optically opaque materials. Imaging of subsurface objects with sub-100 nm dimensions in opaque materials remains a major concern to integrated circuit reliability and performance. PA techniques combine the benefits of the large penetration depth of ultrasound with the lateral spatial resolution of an optical probe. However, the spatial resolution of PA techniques is limited by diffraction. To overcome these challenges, a novel approach is proposed for sub-wavelength PA technology by creating highly localized optical and ultrasound fields using: a) surface plasmonc nano-focusing for sub-100 nm confinement of the optical probe for high sensitivity detection of laser generated high frequency (0.5 - 10 GHz) ultrasound, and b) time reversal acoustics for spatial focusing of the scattered ultrasound field from nanoscale sub-surface objects. This PA system would provide an unprecedented spatial resolution and penetration depth for imaging sub-surface defects in microelectronic devices.
The educational and out-reach programs proposed are deigned to excite high school level students to pursue careers in STEM and provide unique multidisciplinary training opportunities for graduate and undergraduate students in the area of photoacoustic imaging.
" has enabled the integration of near-field optics with photoacoustic microscopy technique for nondestructive materials characterization and imaging of subsurface defects in microelectronic components. Acoustic methods provide a nondestructive approach for the characterization of the dynamic elastic behavior of solids and for probing their interior structure. The velocity of the acoustic wave depends on the elastic properties of the solid medium. Defects in a solid such as grain boundaries, dislocations, voids, etc, impede the propagation of acoustic waves leading to wave scattering and attenuation. Measurement of the wave velocity and amplitude can be used for the characterization of the elastic properties and imaging the microstructure. Photoacoustic techniques are particularly suitable for characterization of local properties because they use laser sources for generation and detection of high frequency (GHz) acoustic waves. The technique is suitable for the characterization of elastic properties of small scale structures and for microscale defect metrology applications. The amount of spatial information from a sample microstructure that can be obtained using a photoacoustic measurement is fundamentally limited by size of the probe laser and the acoustic wavelength. Typically, a light source cannot be focused to a spot size that is smaller than half its wavelength, using conventional optical components. Using a probe light source in the visible range of the electromagnetic spectrum, the probe spot size is close to 1 micrometer. Alternative techniques with higher spatial resolution, like atomic force microscopy, lack the bandwidth needed for the detection of high frequency acoustic waves. The latter is important since the spatial sensitivity of acoustic waves to small microstructural features increases with frequency (or wavelength). This project seeks to enable the nondestructive mapping of deeply buried microstrucutal objects like voids and defects with sub-100 nm dimensions in opaque materials, which remains a formidable challenge to integrated circuit reliability and performance. In this project, a new microscopy approach, called "Ultrasonic Near-Field Optical Microscopy" is realized. The technique relies on two fundamental building blocks. First, a hybrid plasmonic nanofocusing probe is used for adiabatic conversion of surface Plasmon polaritons (coupled oscillation of free carriers in metal, and photons) to localized plasmons at the apex of a tapered metallic probe. When the probe-tip is positioned close to a sample surface, a strong electromagnetic cavity is formed, which enhances the light scattered from nanoscopic objects on the sample. The probe allows for local measurement of optical properties on a sample with high spatial resolution. Second, the photoacoustic approach is explored for laser generation of high frequency (up to 200MHz) acoustic waves. The acoustic waves are detected locally on the sample surface by monitoring the dynamic perturbation of the scattered light in the far-field due to the change in the cavity length between the probe-tip and sample. Access to subsurface information is obtained by recording the lateral variations in the acoustic wave amplitude at the sample surface with nanoscale spatial resolution. We demonstrated that the plasmonic probe provides a local displacement sensitivity of 0.3 x 10-12m/ Hz 1/2 and a lateral spatial resolution of less than 40nm. We also demonstrated the broadband detection capability of the technique by measuring the transient ring-down vibrations of an impulsively actuated nanomechanical resonators with nanosecond time resolution and broadband acoustic waves. Unlike conventional scanning probe techniques that have limited bandwidth due to the sensing atomic force microscope cantilever, the bandwidth of the plasmonic nanofocusing probe is only limited by the detection electronics. Furthermore, we showed that the probe is sensitive to both out-of-plane and in-plane sample displacements. We envisage that integrating the plasmonic probe with an ultrafast pump and probe laser system has the potential to provide unprecedented access to extreme length scales (nanometers) and time scales (picoseconds). This unique tool can facilitate the study of, coupled dynamics of plasmons and phonons in single and periodic nanostructures, single protein binding events, dynamic phase changes in low-dimensional nanostructures, stochastic vibration of nanomechanical structures, and ultrafast acoustic phonon propagation in nanoelectronics for nondestructive photoacoustic metrology applications.
