Nanostructuring of tips for scanning probe microscopy by ion sputtering: Control of the apex ratio and the tip radius

Ion etching under well-defined conditions represents a very powerful tool to fabricate tips in a controlled and reproducible manner for Scanning Probe Microscopy which possess clean, relatively smooth, and oxide-free surfaces. The possibilities and limitations of ion etching are demonstrated thoroughly for tungsten, which is of particular interest for scanning tunneling microscopy. Iterative computer simulations and experimental studies are provided. The simulation of the etching process at the atomic level is based on the Monte Carlo program (Transport of Ions in Matter), which provides reliable values for the key input parameters such as the sputtering yield Y(θ,Eion) and the mean ion range in the material for ion energies between 1 and 6 keV. The simulation program starts with a large tip radius as a crude form and asymptotically approaches a “final form” of a very sharp tip in the course of ion etching. Our experimental results with argon ions in the energy range up to 6 keV and crude electrochemically etched polycrystalline tungsten wire agreed very reasonably with the simulated results. For example, the minimum tip radius obtained experimentally amounts to approximately 5 nm. This value corresponds closely to the ion range, which obviously sets a limit to the smallest achievable tip radius by the effect of transmission sputtering. By variation of the angle between the ion beam and the macroscopic tip, the tip angle can be controlled in the range between 12° and 35°.