3D Nanostructures in Hydrogen Silsesquionxane Achieved by Proton Beam Writing
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laser spot, and the information from scans of the sample is later converted to a two-dimensional image. One problem inherent in large-NA systems is a reduction in the depth of focus, causing a deterioration of image quality in the out-of-focus region. The synthetic-aperture focusing technique (SAFT), which employs a point detector such as a needle hydrophone, was developed to reduce this phenomenon. In the February 15 issue of Optics Letters (p. 474), M.L. Li of Texas A&M University and co-researchers have introduced a virtual detector for PAM that allows the SAFT to be applied to the large-NA transducer without losing depth of focus and eliminates the need for the point detector. In this system, the focal point of the transducer is considered to be a virtual point detector. Li and colleagues said, “Photoacoustic (PA) waves that are generated within a certain solid angle are assumed to be detected by the virtual detector.” Linear scanning of both the laser and the transducer creates a superposition of the PA waves produced by the biological tissue. This superposition, coupled with a coherence factor, facilitates synthetic-aperture focusing and is the basis for improved lateral resolution and signal-to-noise ratio. The researchers offered a brief description of the PAM system, but focused largely on advances made using the new virtual point detector. First, a proof-of-principle study of a 6-μm carbon fiber immersed in 1% Intralipid solution was performed. In this study, “depth-independent lateral resolution of the carbon fiber” was achieved, “without affecting the axial resolution.” Second, in vivo experiments were carried out on the scalps of rats. According to the researchers, the technique provides a much clearer vascular distribution that is attributed to improved lateral resolution and signal-to-noise ratio. KEVIN P. HERLIHY
3D Nanostructures in Hydrogen Silsesquioxane Achieved by Proton Beam Writing The most commonly used technique for three-dimensional (3D) nanolithography is e-beam writing. Therefore, different resists have been developed to define high-resolution features. Among them is (HSiO3/2)8, a hydrogen silsesquioxane (HSQ) from Dow Corning. In HSQ, resolution below 20 nm has been reported and single lines down to 7 nm wide have been observed. Recently, it has been shown that HSQ can also be used as an extreme ultraviolet (EUV) resist using 13.4-nm wavelength, and 26-nm wide lines have been demonstrated. In the March 8 issue of Nano Letters (DOI: 10.1021/nl052478c; p. 579), J.A. van Kan, A.A. Bettiol, and F. Watt from the Centre for Ion Beam Applications at the National University of Singapore have presented their results on highenergy proton-beam writing (p-beam writing) in HSQ resist at the 20-nm level. Their method employs a focused megaelectronvolt proton beam scanned in a predetermined pattern over a suitable resist that is subsequently chemically developed. The researchers said that in both electron-beam (e-beam) and p-beam writing, the energy loss of the primary beam is dominated by energy tra
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