Brian C. Daly

Imaging nanostructures with coherent phonon pulses

We demonstrate submicron resolution imaging using picosecond acoustic phonon pulses. High-frequency acoustic pulses are generated by impulsive thermoelastic excitation of a patterned 15-nm-thick metal film on a crystalline substrate using ultrafast optical pulses. The spatiotemporal diffracted acoustic strain field is measured on the opposite side of the substrate, and this field is used in a time-reversal algorithm to reconstruct the object. The image resolution is characterized using lithographically defined 1-micron-period Al structures on Si. Straightforward technical improvements should lead to resolution approaching 45nm, extending the resolution of acoustic microscopy into the nanoscale regime.

Gouy Phase Shift of Single-Cycle Picosecond Acoustic Pulses

Ultrafast laser pulses are used to generate single-cycle picosecond acoustic pulses in thin metal films on silicon. For small initial excitation spot sizes, propagation of the acoustic pulses across a 485mm Si crystal leads to significant diffraction effects. The temporal reshaping of the acoustic wave form due to diffraction is investigated, and we demonstrate that the acoustic far field can be reached.

Noncontact optical metrologies for Young’s modulus measurements of nanoporous low-k dielectric thin films

Abstract. Brillouin light scattering (BLS) and picosecond laser ultrasonics (PLU) are two noncontact optical techniques that have garnered significant interest for thin film elastic constant measurements. PLU and BLS measurements were utilized to determine the elastic constants of 100 to 500 nm thick nanoporous low-k dielectric materials of significant interest for reducing capacitive delays in nanoelectronic interconnect circuits. PLU measurements with and without a metal acousto-optic transducer are described in detail and compared to previously reported BLS measurements. The values of Young’s modulus determined by both BLS and PLU were found to be in excellent agreement and consistent with nanoindentation measurements on thicker 2 micrometer films. While successful BLS measurements were achieved for films as thin as 100 nm, PLU measurements were limited to >∼200  nm thick films due to experimental constraints on observing acoustic pulses in thinner films. However, these results clearly demonstrate the capability of both BLS and PLU to determine the elastic constants of low-k dielectric materials at the desired thickness targets for future nanoelectronic interconnect technologies.

Ultrafast acoustics for imaging at the nanoscale

In this paper we present a series of experiments which show that 2-D and possibly 3-D imaging with sub-micron resolution is possible by means of ultrafast acoustic techniques. Optical pulses from a Ti:sapphire laser are used to generate picosecond acoustic pulses on one side of a ~1 mm thick Si wafer. The 1 mm distance is sufficient for the acoustic waves to diffract to the far field before they are detected by time-delayed probe pulses from the Ti:sapphire laser. The acoustic waves are either generated by a surface nanostructure or scattered from a buried nanostructure, and an image of that nanostructure is reconstructed through an analysis of the detected acoustic waves.

Nanoscale coherent acoustic phonon imaging

An ultrafast optical pump and probe technique known as picosecond ultrasonics is used to generate and detect coherent acoustic phonon pulses in nanostructured films grown on Si wafers. By detecting the phonons after they have diffracted across a millimeter thick wafer, it is possible to measure the scattered phonons in the acoustic far field. Numerical backpropagation algorithms can then be used in order to reconstruct the object which scattered the acoustic phonon pulses. We describe measurements and simulations of experiments performed on surface and sub-surface nanostructures. Results with ~500 nm image resolution are shown, and plans for improving that resolution by an order of magnitude will be described.

Nanoacoustics: propagation and imaging with THz coherent phonons

We investigate experimentally and theoretically the propagation of coherent acoustic phonon pulses generated in Si using ultrafast optical pulses. These acoustic pulses are shown to be applicable for two-dimensional nanoscale imaging. In a recent paper [1], we presented a proof-of-concept experiment which showed that it is possible to use coherent acoustic phonons for 2-D sub-micron-resolution imaging of metallic nanostructures fabricated on a semiconductor substrate. Single-cycle coherent phonon pulses are generated and detected using an ultrafast pump-and-probe technique known as picosecond ultrasonics [2,3]. The experimental arrangement is illustrated in Fig. 1. The object is located on one side of a 0.5 mm thick Si wafer and is heated with femtosecond optical pump pulses. Acoustic waves (peak frequency and bandwidth ~ 0.1 THz) launched from the object are then detected by time-delayed optical probe pulses at a number of positions on an Al transducer deposited on the opposite side of the wafer.

Picosecond ultrasonic study of surface acoustic waves on periodically patterned layered nanostructures

&NA; We have used the ultrafast pump–probe technique known as picosecond ultrasonics to generate and detect surface acoustic waves on a structure consisting of nanoscale Al lines on SiO2 on Si. We report results from ten samples with varying pitch (1000–140 nm) and SiO2 film thickness (112 nm or 60 nm), and compare our results to an isotropic elastic calculation and a coarse‐grained molecular dynamics simulation. In all cases we are able to detect and identify a Rayleigh‐like surface acoustic wave with wavelength equal to the pitch of the lines and frequency in the range of 5–24 GHz. In some samples, we are able to detect additional, higher frequency surface acoustic waves or independent modes of the Al lines with frequencies close to 50 GHz. We also describe the effects of probe beam polarization on the measurements sensitivity to the different surface modes. HighlightsAn ultrafast optical pump‐probe technique is used to generate and detect surface acoustic waves on a nanostructure.Vibrational modes up to 50 GHz are identified and compared to molecular dynamics simulations of the structure.Significant effects of probe beam polarization on the detected frequencies are also described.

Ultrafast optical measurements of coherent acoustic phonon attenuation in silicon

We report measurements of the attenuation of ∼100 GHz coherent acoustic phonons in silicon and results are compared with existing theory. The results have implications for nanoscale thermal transport models and novel acoustic imaging schemes.

Nanoscale imaging with coherent acoustic phonon pulses

We describe ultrafast coherent acoustic phonon measurements of a latent photoresist pattern with 1 mum features that is buried beneath an A1 film. Simulation results for higher resolution measurements are also described.

Coherent acoustic phonons in silicon: propagation studies and imaging applications

We generate and detect coherent acoustic phonon pulses in Si using ultrafast optics. A unified model of diffractive, dispersive, and nonlinear propagation effects is developed and verified. Imaging using diffracted phonon pulses shows submicron resolution.