Fabrication of nanoscale tungsten tip arrays for scanning probe microscopy-based devices

A new fabrication process for nanoscale tungsten tip arrays was developed for scanning probe microscopy-based devices. It is suitable to make a huge array on a device chip and is potentially compatible with CMOS technology. In this study, tungsten was selected as a tip material because of its hardness and conductivity. The newly developed fabrication process mainly consists of several important techniques: a combination of optical lithography and electron beam (EB) lithography to reduce the total exposure time with high resolution and chromium/tungsten/chromium (Cr/W/Cr) sandwich deposition and etching in which the first chromium layer is used as a mask and a second one is used as an etch stop. A periodic array of dots in an EB resist with a spot diameter of less than 50 nm was obtained by a combination of optical lithography and EB lithography with a positive resist (polymethylmethacrylate) in which all processing conditions were optimized carefully. A thin and uniform chromium film, deposited by ion-beam sputtering, allowed the use of thin polymethylmethacrylate (PMMA) film which led to the high resolution. The conditions of dc magnetron sputtering were also optimized in order to deposit a densely packed and low-resistivity film. The resulting tungsten tip arrays had a cylindrical shape with diameters of less than 60 nm and heights of 300 nm.