Physical bounds of metallic nanofingers obtained by mechano-chemical atomic force microscope nanolithography

To obtain metallic nanofingers applicable in surface acoustic wave (SAW) sensors, a mechano-chemical atomic force microscope (AFM) nanolithography on a metallic thin film (50 nm in thickness)/piezoelectric substrate covered by a spin-coated polymeric mask layer (50-60 nm in thickness) was implemented. The effective shape of cross-section of the before and after etching grooves have been determined by using the AFM tip deconvolution surface analysis, structure factor, and power spectral density analyses. The wet-etching process improved the shape and aspect ratio (height/width) of the grooves and also smoothed the surface within them. We have shown that the relaxed surface tension of the polymeric mask layer resulted in a down limitation in width and length of the lithographed nanofingers. The surface tension of the mask layer can be changed by altering the initial concentration of the polymer in the deposition process. As the surface tension reduced, the down limitation decreased. In fact, an extrapolation of the analyzed statistical data has indicated that by decreasing the surface tension from 39 to 10 nN/nm, the minimum obtainable width and length of the metallic nanofingers was changed from about 55 nm and 2 μm to 15 nm and 0.44 μm, respectively. Using the extrapolation's results, we have shown that the future SAW sensors buildable by this nanolithography method possess a practical bound in their synchronous frequency (∼58 GHz), mass sensitivity (∼6125 MHz-mm2/ng), and the limit of mass resolution (∼4.88 x 10-10 ng/mm2).