AFM tip-based mechanical nanomachining of 3D micro and nano-structures via the control of the scratching trajectory

This paper presents a novel mechanical material removal method to produce nanostructures with a precise control of their three dimensional (3D) surface topography. The method employs the tip of an atomic force microscope (AFM) probe as the cutting tool and a closed-loop high precision stage to control the machining path of the tip. In this approach, the tip only describes vertical motions while the stage is actuated along lateral directions in a raster scan strategy. The machining of features with 3D nanoscale topography in this way is the combined result of the tip applying a constant normal load on the sample while varying the distance (i.e. the feed) between two parallel lines of cut. More specifically, an increased feed leads to a reduced machining depth and vice-versa. Thus, the main difference with mechanical milling or turning at such small scale is that this method relies on the control of the feed to determine the machined depth. To support the interpretation of the process outcomes, an analytical model is developed. This model expresses the relationship between the feed and the machined depth as a function of the contact area between the tip and the material. The critical achievable slope of produced nanostructures was derived from this model and validated using experimental tests. This parameter corresponds to the maximum inclination of the surface of a nanostructure that can be machined with the proposed method. From the knowledge of the critical slope, the machining of periodic nanostructures was demonstrated on a single crystal copper workpiece. In principle, the method reported here could be implemented to any instrument with micro- and nano-indentation capabilities by exploiting their load-control feedback mechanism.