Acoustic microscopy has been used to study a variety of microstructures, material defects, and biological tissues. One of the key limitations in pulsed acoustic microscopy is the duration of sound pulses that can be generated. The goal of this ongoing project is to develop an acoustic microscope that uses ultrashort laser pulses, with durations on the order of 100 fs, to generate short sound pulses, with durations on the order of 10 ps. These short sound pulses will then be used to form images of the sample being studied analogous to a traditional acoustic microscope. This technique is referred to as scanning opto-acoustic microscopy (SOAM). <br/><br/><br/> As a first step towards the construction of a SOAM instrument, a system which contains all of the key components except for the acoustic lens and the sample scanning stages was constructed. The acoustic pulses generated by the ultrafast optical source, instead of being focused by an acoustic lens, were launched into the coupling fluid directly. This technique is referred to as planar opto-acoustic microscopy (POAM). The first samples examined were periodic groups of nanostructures. With a repeat distance in the submicron regime, the acoustic pulse was probing anywhere from 20 - 100 nanostructures at once. By comparing experimental results with simulations, the average features of groups of nanostructures, such as height and spacing, could be measured with nanometer sensitivity. The relative sizes of the echoes in the POAM data were used to infer information about the average critical dimensions and profile of groups of structures. For structures with channels that were less than 60 nm wide, the amplitude and arrival time of the echo from the bottom of the channels were very sensitive to the channel width and depth. POAM was shown to provide a way of measuring the dimensions of these features non-destructively. The dimensions inferred from the POAM data were compared to destructive measurements of the sample profiles using scanning electron microscopy. For narrow channels, the presence of slip at the water-side wall interface would have a measurable effect on the echo from the bottoms of the channels. POAM data was used to place an upper limit of 5 nm on the slip length at the water-silicon nitride interfaces.<br/><br/><br/>
Grimsley, Thomas James,
"Planar Opto-acoustic Microscopy Applied to the Metrology of Periodic Nanostructures"
(2012).
Physics Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z0B56H1T
