Nanostructure Fabrication of Metal and Silicon by using Atomic Force Microscope Lithography

Abstract

Nano-science and nanotechnology have been rapidly developed with the demand of novel functional materials and integrated devices with dimensions in nanoscale. Since the 1990s, the scanning probe microscopy (SPM) technique has been the most powerful tool to directly characterize and modify nanoscale materials and to fabricate nanostructures. In particular, atomic force microscopy (AFM) has many advantages such as low cost, easy in-situ operation, and low environmental restriction. In addition, it has many practical applicabilities such as nanoscale lithography, high-resolution imaging, physical and chemical characterization, and atomic level manipulation. In this thesis, five topics related to AFM lithography are described; these topics pertain to the fabrication methods of metal and Si nanostructures used in nanoelectronics and masks, and high-speed lithography. First, in order to fabricate metal nanostructures without using a mask or an etching process, three electrochemical deposition methods based on AFM lithography were successfully applied. Metal was not only electrodeposited selectively on sites modified by field-induced AFM lithography, but also reduced by direct electrochemical reaction between a conductive tip and a spin-coated metal ion-containing molecular resist. In particular, in order to clearly remove the residual materials and locally define metal patterns in a direct reduction process, a highly soluble resist and an intermediate layer between a spin-coated resist and a substrate were used in the lithographic process. In the former case, the spin-coated resist mixed with copper nitrate and poly (sodium 4-styrenesulfonate) in DI water was effectively removed by DI water during development. In the latter case, the use of a TDA�HCl self-assembled monolayer (SAM) as an intermediate layer between a resist containing copper (II) acetate with dimethylformamide (DMF) and a Si substrate offered the practical advantages of effectively blocking unnecessary reactions during lithography and clearly removing residual materials during development. The well-defined nanowires were physically and electrically confirmed to be composed mainly of copper and could potentially be used as electrical electrodes or a metal etch mask. Second, in order to fabricate metal structures with a top-down approach, a selective etching process between a desired metal and mask patterns such as negative-type resist patterns or anodic oxides was successfully applied to fabricate absorber Ta or Cr/Ta structures for an extreme ultraviolet projection lithography (EUVL) mask. In particular, anodization lithography on double metal layers (Cr/Ta) was employed to form a positive-type Cr mask to deeply etch the underlying Ta in an inductively coupled-plasma (ICP) etching process. Third, in order to overcome the low KOH wet-etch resistance property of the Si oxides formed by AFM anodization lithography, the Si oxides initially eliminated on the top of the Si structures during the wet-etch process were regrown by using an electrochemical contact oxidation method. The additional oxide regrowth method, combined with AFM anodization lithography, provided a feasible solution for the fabrication of a deep Si template for a suspended CNT device. Two technical methods for improving the lithographic throughput were studied. The limit of throughput in AFM lithography compared with E-beam lithography arises from a low number of a tip and physical properties of a piezo-based scanner such as vibration and nonlinearity. These problems were effectively overcome by using a multi-tip probe with identical height on a single cantilever and a smooth driving technique that reduced the mechanical vibration of the piezo-tube scanner. A multi-tip probe system, which is different from a multiplexing system with a multi-cantilever, can be used to simply and inexpensively fabricate multiple high-density nanostructures simultaneously. The multi-tip probe lithography was used to succesfully fabricate dot array patterns. In the second method, a piezo-tube scanner operated at a sinusoidal speed instead of a steady speed remarkably reduced the image distortion generated by the mechanical vibration of a scanner. In addition, this smooth scanner driving enabled the fabrication of more linear and uniform resist nanostructures than those fabricated by using the common driving method at the world�s highest speed of approximatedly 2 cm/s. The above nanofabrication methods and speed-improving methods could be applied to mask repair, and the fabrication of high-resolution nanodevices and high-density patterned media.