SCANNING PROBE TIP GEOMETRY OPTIMIZED FOR METROLOGY BY FOCUSED ION-BEAM ION MILLING

We have developed a novel technique for producing tips for scanning probe microscopy. The shape of the tip is optimized for applications where high aspect ratio surface topography is the norm, as with integrated circuit structures. The technique involves focused ion beam (FIB) milling of a tip which was previously shaped to a nominal geometry by electrochemical etching. The ion milling pattern is annular, and the ion beam is collinear with the axis of the tip. The process allows control of the ion milling dose using 20 keV Ga+ with submicron control of the annular patterns. The result is a narrow, tapered structure approximately 20-mu-m in length which ends in a point with a radius of curvature between 30 and 50 nm when grains dominate the sputter process, and radii of about 3-4 nm when there is no evidence of grain structure effects. This microstructure is 3-mu-m in diameter at the base and it protrudes from a portion of the shank of the macrostructure where the diameter is about 15-mu-m. We have sufficient control over the sputter process to yield the final tip length, taper, and radius. Cone angles between 12-degrees and 15-degrees over the first two microns from the apex can be achieved routinely, by the correct choice of annulus and ion dose. Sputter simulations predict the correct shape of the final tip profile, and show the effect of varying the ion beam focus, dose, and inner and outer annulus radii. Tips with the desired geometry have been produced in polycrystalline tungsten, iridium, and platinum-iridium. Significant improvements in scanning tip microscopy (STM) images have been consistently observed with these FIB milled tips.