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Fabricating conductive wires with nanometer-scale width has been one of the most challenging issues in the current semiconductor technology. Due to rapid scale-down of semiconductor devices, such thin wires have been highly demanded as leads and electrodes. Based on the lithography utilizing atomic force microscopy (AFM), we developed a new method to draw a metal wire whose width is less than 10 nm, which utilizes a metal-tip cantilever originally developed for non-contact AFM. The final goal we are pursuing is drawing wires to make electrical contacts to nano-materials, such as molecules, to measure their conductance. Since the method is based on AFM, we can observe molecules to directly confirm their positions and the number of the molecules before drawing wires, which is difficult and thus often argued in conductance measurements with the break-junction method.
Conventional AFM lithography uses a metal-coated tip in tapping-mode AFM. By the field evaporation of metal atoms from the metal-coated tip to a substrate, induced by a voltage pulse applied between the tip and sample, a metal dot can be formed, and by repetitive pulses a metal wire can be drawn. So far, the width of the narrowest Au wire drawn in this method is 22 nm. There are several plausible factors to limit the wire width, and one of the important factors is a curvature radius of the tip apex. We fabricated a cantilever with a sharp Au tip by attaching a thin Au wire to a regular Si cantilever using a precisely controllable micromanipulator and milling it with focused ion beam technique [1]. The curvature radius of the fabricated Au-tip cantilever is ~10 nm, much less than that of the normal coated tip by a factor of 5 to 10.
Using the Au-tip cantilever in non-contact AFM, which has a better force and distance regulation than the tapping-mode AFM, we formed nano-sized Au dots on a Si wafer substrate. By continuously depositing the Au dots, we successfully drew Au wires narrower than the one attained by conventional AFM lithography. The dependence of the nano-sized Au dot formation on the force, the tip to sample distance, the applied voltage and the radius of tip apex will be discussed in the presentation.
[1] K. Akiyama, et al., Rev. Sci. Instrum., 76, 033705 (2005).
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