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The new technique relies on the prin- object. These changes in turn alter the y modifying a standard scanning ciple that when two sets of vibrations in- deflection of the cantilever. “The result probe microscope to send acoustic is a point-by-point pictorial representatersect with each other, they create a waves of slightly different frequencies tion of the acoustic wave, which prothrough a sample and probe tip simulta- standing wave pattern. The two sets of vides a quantitative account of the invibrations are high-frequency acoustic neously, Vinayak Dravid and Gajendra ternal features of the specimen being waves—one propagated through the Shekhawat at Northwestern University sample and the other from the cantilever scanned,” explains Dravid. have developed a nondestructive techWith 15-nm-diam gold nanoparticles of a scanning probe microscope. These nique for imaging subsurface nanoscale layered on a poly(2-vinylpyridine)-coattwo sets of acoustic waves, at ~2.1 MHz objects (Science 2005, 310, 89–92). ed silicon wafer and covered with a 500from the bottom of the sample and at Dubbed scanning near-field ultrasound nm-thick polymer film, Dravid and Shekholography (SNFUH), the new method ~2.3 MHz from the cantilever, establish hawat were able to produce a provides depth information and (a) (b) high-definition image that respatial resolution at the 10–100SNFUH AFM vealed the buried gold nanopartinm scale. cles. In contrast, an atomic force Dravid and Shekhawat used their microscopy (AFM) image of the modified microscope to image masame system was featureless. laria-infected red blood cells withThe researchers then imaged a out any extra labeling steps. The second test system consisting of holographs revealed what appear to 400-nm-wide and 1-µm-deep be malaria parasites within the red trenches between 500-nm-wide blood cells as well as discrete struc1 µm 1 µm features on a polymer-coated silitural features of the cells. Image con nitride substrate. Whereas the contrast, consistent with malaria (a) AFM image shows surface features of red blood cells; standard AFM image displayed the parasite colonization of red blood (b) SNFUH image demonstrates contrast from parasites intrenches, the SNFUH image recells, was visible as early as 4 h after side the cells. (Adapted with permission. Copyright 2005 vealed the presence of voids buried infection. “The resolution seen in American Association for the Advancement of Science.) within the polymer-coated silicon the red blood cell images is better nitride layer, as well as hardening of the a standing acoustic wave on the surface than the best optical microscope,” says polymer along the walls of the trench of the sample. An acoustic antenna, in Craig Prater at Veeco. “This is a very that occurred during polymer curing. the form of a tip mounted on the canpromising technology for imaging obThe next step for Dravid’s group is to tilever and connected to SNFUH cirjects, both inorganic and biological, beuse the phase information obtained durcuitry, monitors the standing wave. cause it blends the subsurface imaging The phase and amplitude of the stand- ing scanning to create 3-D tomographic capabilities of acoustic microscopy with ing wave depend on the elastic properties images. “That would allow us to resolve the nanometer-scale resolution of the overlapping features and to determine of the sample beneath the cantilever. A [atomic force microscope].” the relative depth of subsurface features,” homogeneous sample will have uniform Dravid’s goal in constructing the says Dravid. elastic properties from the top surface to SNFUH device was to develop a means Diebold believes that once other labthe bottom, but a sample with subsurface of imaging nanoscale features buried oratories start working with this new features or buried objects within it will within devices such as semiconductor system, the field of near-field subsurface not be homogenously elastic. chips. “This is a very promising and poimaging will take off. “This work has As the microscope scans a sample, tentially important first step to 3-D, any local differences in material elasticity the potential to be very inspiring, with nondestructive imaging, which would applications far beyond the semiconducproduce corresponding changes in the be a development critical to the semitor community.” a phase and amplitude of the standing conductor industry,” says Alain Diebold —Joe Alper wave immediately above the subsurface at SEMATECH. © 2006 AMERICAN CHEMICAL SOCIETY
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