A serious limitation of the field ion microscope has been its inability to identify the chemical nature of the individually imaged atoms. The newly conceived atom-probe FIM is a combination probe-hole FIM and mass spectrometer having single particle sensitivity. During observation, the observer selects an atomic site of interest by placing it over a probe hole in the image screen. Pulsed field evaporation sends the chosen particle through the hole and into the spectrometer section. Preliminary results show that field evaporation of tungsten under poor vacuum conditions occurs as triply or quadruply charged WO, WN, WO2, and WN2 ions, while under better conditions doubly and possibly triply charged tungsten can be observed. Mo–Re alloys always produced doubly charged molybdenum and rhenium ions when examined in the atom-probe. Wide applications for the study of short range order in alloys, the chemical nature of precipitates and impurity atoms, and information regarding the imaging properties of various atom species, of both the substrate and adsorbed material, are foreseen.

An absolute composition depth profile of a dilute gold alloy of platinum is obtained with a true atomic layer depth resolution. At 600±20 °C, the top {001} layer Au concentration is found to be as high as 99±20% for a sample with a bulk gold concentration of 4.1±0.7%. The Au concentration decreases monotonically into the bulk, with a characteristic depth of about three atomic layers.

Analysis of solid surfaces using a pulsedlaser timeofflight atomprobe AIP Conf.

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.

Accurate identification of atomic species in the atom probe FIM requires a highly shielded tip assembly, as well as a revision of the equation formerly used to compute the m/n ratios. Voltage reflections on the unterminated pulse transmission line cause the actual evaporation pulse at the tip to be greater than the applied pulse by a ``pulse factor'' α. In addition, the recorded flight time of the ion under investigation differs from the actual flight time by a constant electronic time delay δ. Using either of two single isotope calibration techniques, both α and δ can be determined. For a shielded tip assembly either technique gives a value of α=2.00±0.05 and a value of δ=0.06±0.02 μsec. Typical absolute mass determination using these values of α and δ is within ±0.6 amu at m/n=20, and ±1.6 amu at m/n=100. When masses near a known species, like hydrogen- or helium-metal molecular ions, are to be detected, the practical resolution reaches ±0.2 amu at the middle of the mass range.