"This is a unique combination of a scanning probe microscopy (SPM) platform, for high resolution, ultrasound waves, which are non-invasive, and holography, for phase sensitivity, that provides depth-resolved information that is otherwise not possible," Vinayak Dravid of Northwestern told nanotechweb.org. "This will help unravel embedded features and defects well below the surface without destroying the specimen."

The technique works by applying a high-frequency (~MHz) acoustic wave to the bottom of a specimen and a second acoustic wave with a slightly different frequency to a scanning probe microscope cantilever in contact with the specimen. The two waves interfere to form a surface acoustic standing wave.

If buried features are present, they change the phase and amplitude of the surface acoustic standing wave and the SPM cantilever picks up these changes. Scanning the specimen builds up a picture of the internal features that are causing perturbations to the wave.

The technique suits both hard materials, soft materials, such as polymers and biological structures, and hybrid materials. Dravid and colleagues demonstrated their technique by imaging a polymer composite containing 15 nm diameter gold nanoparticles buried about 500 nm below the surface. The nanoparticles perturb the specimen acoustic wave as they have a different elastic modulus to the polymer.

The team also used the technique to image malaria parasites inside red blood cells that had been infected for only four hours, a feat that is difficult to achieve with other noninvasive techniques such as fluorescent tagging. And SNFUH was able to image buried voids within a polymer coating on silicon-nitride-covered trenches.

According to Dravid, the technique could have applications in microelectronics – especially in measuring structures such as interconnects, vias and trenches – and in polymers, the nanoscale mechanical imaging of composites, and for looking inside cells and biological systems.

The researchers, who reported their work in Science, now plan to refine the technique for key applications