Dynamics of Ion Beam Stimulated Surface Mass Transport to Nanopores
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1020-GG02-01
Dynamics of Ion Beam Stimulated Surface Mass Transport to Nanopores David P. Hoogerheide1, and Jene A. Golovchenko2 1 Department of Physics, Harvard University, Cambridge, MA, 02138 2 Department of Physics and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, 02138 ABSTRACT We explore the ion beam-induced dynamics of the formation of large features at the edges of nanopores in freestanding silicon nitride membranes. The shape and size of these ìnanovolcanoesî, together with the rate at which the nanopores open or close, are shown to be strongly influenced by sample temperature. Volcano formation and pore closing slow and stop at low temperatures and saturate at high temperatures. Nanopore volcano size and closing rates are dependent on initial pore size. We discuss both surface diffusion and viscous flow models in the context of these observed phenomena. INTRODUCTION The advent of solid state nanopores as single-molecule detectors [1] has highlighted the importance of characterizing novel methods for fabricating nanostructures. Ion beam sculpting [2] is a robust method for making solid state nanopores, but the processes involved in the fabrication process remain poorly understood. While many potentially relevant processes, such as surface diffusion [2], sputtering [3ñ4], anisotropic deformation via thermal spikes [5ñ6], ion-enhanced viscous flow [6ñ8], radiation-induced stresses [9], and combinations of the above [10] have been studied with MeV beams or with crystalline targets, there is little data available regarding interactions of lower-energy keV beams with amorphous materials such as those found widely in silicon-based technology. In particular, it is unclear how matter transport occurs over large length scales when irradiated with low-energy beams that deposit their energy within only a few nanometers of the surface. In this paper, we discuss several previously unreported features of nanopore fabrication using keV ion beams and discuss their ramifications with respect to various extant models for the observed matter transport. EXPERIMENT Samples were fabricated from low-stress amorphous silicon nitride grown by lowpressure chemical vapor deposition (LPCVD) on a silicon substrate. The LPCVD was performed at Cornellís Nanofabrication Facility and yields silicon nitride thin films with a nominal tensile stress of 180 MPa [11]. The stoichiometry is Si3.5N4, as determined by Rutherford backscattering. Freestanding square membranes 90 µm on a side and 500 nm thick were produced by lithographic patterning and a subsequent anisotropic etch of the underlying silicon substrate in KOH.
Starting holes of the desired diameter were drilled at the center of the freestanding membrane, from the substrate side, by rastering a 10-nm diameter, 50 keV gallium ion beam produced by a FEI/Micrion 9500 focused ion beam (FIB) instrument. The resulting holes were conical in shape, opening towards the substrate side of the membrane with an apex angle of about 10 degrees, as determined by atom
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