INVESTIGATION OF EMITTER TIPS FOR SCANNING TUNNELING MICROSCOPE-BASED MICROPROBE SYSTEMS

This paper reports the preparation and characterization of field emitter tips for use in a scanning tunneling microscope aligned field emission (SAFE) microprobe system. With the tip being at close proximity to the extraction electrode, new demands are imposed on the emitter tips: (1) a low extraction voltage, (2) a well-defined emission pattern, preferably a single lobe emission, and (3) a high angular emission density. A combined field ion-field electron emission microscope equipped with a special stage for mounting a small aperture in close proximity to the emitter tip, which was used to simulate the first element of the electro-optical system of the SAFE microprobe, was used to analyze different tip preparation techniques. A low-temperature field-assisted thermal annealing process has been developed to routinely produce sharp W <111> tips well suited for SAFE operation. Tips having an effective tip radius of less than 500 angstrom, an emission half cone angle of less than 10-degrees, and a peak angular emission density of 7-mu-A/sr at a total emission current of 1-mu-A were successfully prepared and used in the SAFE microprobe system.