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Tungsten tips for scanning tunneling microscopy (STM) measurements under ultrahigh vacuum conditions are fabricated routinely according well tested standard etching procedures with results satisfying most purposes. Higher requirements on the curvature of tip apex appear in case of imaging surface structures - islands or mounds - with characteristic dimensions in scale of tenth of nanometers. Using STM for in-vivo imaging of growth during deposition, when screening effect of the tip plays a crucial role, increases need of an ultra-sharp tip as well. Modification of a standard procedure regularly provides tungsten tips with an apex radius of 25 nm, in some cases results are even one order better. Transmission electron (TEM) and scanning electron microscopes were used for studying influence of various tip treatment. Autoemission current was used for testing tip quality and cleaning the tip. Measurements of tip emission in dependence on real apex radius (obtained by TEM) were performed for a set of various fabricated tungsten tips and resulted in experimental relation for estimation of the apex radius. A destruction of a tip by means of the emission current with respect of the apex radius was investigated. Cleaning the tip by electron bombardment was found a rather risky procedure due to the non-reproducibility originated in strong dependence of an effect on very tip geometry. More reliable and reproducible results were reached by heating a tip by means of an electric contact between a tungsten tip body and another attached tungsten wire. The heating up to the temperature between 1050 and 1150 K removes tungsten oxide without decreasing autoemission current. |