Gas-Surface Interactions and Field-Ion Microscopy of Nonrefractory Metals

Abstract

The performance of the helium field-ion microscope depends critically upon accommodating He atoms of 0.15-eV kinetic energy to the specimen tip. The small accommodation coefficient requires the field-trapped He atom to make several hundred contacts with the cold tip surface. The hopping He atoms diffuse preferentially to tip regions where the high local field permits ionization before full accommodation is reached. Improved accommodation is achieved with the provision of an intermediate collision partner in the form of adsorbed neon or, preferably, hydrogen or deuterium. Now a high-resolution He ion image is obtained at 70% of the field used before. As the addition of hydrogen promotes field evaporation, its partial pressure must be carefully controlled to achieve image stability of the nonrefractory metals. Low-field evaporation by the hydrogen reaction permits easy conditioning of the tip surface of the nonrefractory transition metals so that artifacts caused by yielding to He evaporation field stress are no longer a problem. The field evaporation end form obtained with hydrogen added to He more closely approaches the desirable spherical shape of the emitter than does field evaporation in vacuum or in a single imaging gas. As examples, ion images of niobium, nickel, iron, and high carbon steel are shown.