Buried Interfaces Imaged Using Noncontact Picosecond Acoustic Microscopy
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interact primarily with other fluid molecules. Relatively few are in contact with the surface of the container. At nanometer scales, however, a much higher proportion of molecules come in contact with the confining material. This surface interaction can significantly alter fluid properties, including molecular mobility, according to the research team. As reported in the May 5 issue of Physical Review Letters (#177804; DOI: 10.1103/PhysRevLett.96.177804), the group performed molecular dynamics simulations to study the behavior of hard-sphere fluids in highly restrictive channels with different shapes and boundary interactions. They modeled changes to fluid mobility and entropy in these conditions, a critical breakthrough that will allow engineers to learn how these changes occur while avoiding the difficult task of gathering experimental data on such small scales (see Figure 1). “One way to think about how mobility relates to entropy is to think of entropy as measuring a sort of randomness at the molecular level,” Truskett said. “In a gas, where the molecules are randomly distributed, entropy is high and the gas mixes readily. In a solid, the molecules are aligned in a regular spatial pattern; there is little randomness and the solid barely mixes at all. Our discovery is that while both excess entropy and mobility of a fluid are affected by confinement, the relationship between the two quantities essentially remains the same down to very small scales.” Because scientists already have reliable methods for predicting how confinement will affect excess entropy, they can now use this information together with the group’s findings to predict how confinement will affect fluid mobility.
ders samples unsuitable for additional processing. To overcome these shortcomings, a new noncontact, nondestructive technique that involves generating and detecting ultrahigh-frequency acoustic waves directly on the sample has now been reported. S. Ramanathan (formerly at Intel Corp. and now at Harvard University) and D. Cahill of the University of Illinois at Urbana-Champaign describe this technique in the May 2006 issue of the Journal of Materials Research (p. 1204; DOI: 10.1557/JMR.2006.0141). The technique involves coating the sample with an 80-nm-thick aluminum film that acts as a transducer to generate and detect acoustic pulses. In this case, the sample consisted of a silicon wafer covered by an oxide layer, an etch-stop layer, and a second oxide layer in which an array of copper metal lines was fabricated. The copper lines were each 6 μm wide, 40 μm long, and 1 μm thick. The lines on one of these wafers were bonded to the lines on a second wafer, which was used as a handle wafer. The topmost silicon layer was then thinned to 6 μm, and an aluminum transducer film was deposited. A Ti:sapphire laser beam was directed onto the aluminum surface, where the laser pulse generated a local stress field, leading to a short-duration longitudinal acoustic pulse that travels from the aluminum film into
Buried Interfaces Imaged Using Noncontact
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