Image contrast dependence on the field emitter in near field emission scanning electron microscopy

We have performed a comparative study of the image contrast enhancement dependence on field emitter tips using NFESEM. Effective tip emission radii were extracted from the Fowler-Nordheim behavior of a polycrystalline, (310), (111), and (lOO)-oriented W-tips. We observed no dramatic difference between the I-V values of the various tips. The effective emission radii are calculated using R. Corner´s formalism, at distances between 20nm and 60nm above the specimen. We observe a dependence on the emission radii in this range; however it is within the 20% error due to the k-factor uncertainty. The emitters that share similar emission radii values show a dependence on the emitter crystalline orientation, which was observed in the resolution of the mono-atomic steps on a W (110) substrate surface. Subsequent STM imaging of the W (110) surface in constant current (CC) mode confirm sharp step edges with single atom height. We evaluate the resolution of our images, as determined by the sharpness of the step edges, and compare it to theoretical values determined by the effective emission radii and tip-sample separation.