S.V. Krishnaswamy

Energy deficits in pulsed field evaporation and deficit compensated atom‐probe designs

Energy deficits of field evaporated ions which limit the mass resolution of the straight TOF atom‐probe are measured by incorporating a 90° deflecting energy discriminator. The observed dependence on the mass‐to‐charge ratio shows premature evaporation during the less than perfect subnanosecond pulse front to be the cause of the energy spread. For narrow beam apertures the mass resolution may be improved by tilting the detector plane to provide a compensating shorter path for the slower ions. For wider apertures an energy focusing combination of straight path sections and a 163° toroidal deflector, as conceived by Poschenrieder, has been adapted to our atom‐probe. Offering a mass resolution of better than 1/1000 and complete rejection of artifacts, the compensated atom‐probe FIM is now a microanalytical tool of ultimate sensitivity and high reliability.

Atom-probe FIM analysis of the interaction of the imaging gas with the surface☆

Abstract The properly calibrated atom-probe FIM can achieve a resolution Δ M M = 1 250 . This allows the identification of molecular ion compounds of the tip metal with the imaging gas in the field evaporation products. Ions of hydrogen, helium, neon and argon, the latter two also with double charges and combined with hydrogen, are seen. Identification of molecular ions is most reliable using single atomic mass tip metals such as Rh, Nb, Ta and Au, but noble gas-metal ions are also reported for W and Ir. Some molecular ions such as TaH +++ and TaNe +++ dissociate after a measurable life time. Field adsorption of the noble gases as well as the resulting reduction of the evaporation field of the metals is explained as dipole attraction due to electronic rearrangement in the surface.

Energy spectrum of field ionization at a single atomic site

Energy deficit spectra of field ions coming from above a single atomic site are measured by using an atom-probe FIM modified with a Mollenstedt energy analyzer. This device offers a resolution of 5 × 10−5 and is inherently more efficient and less noisy than a retarder. The energy spectra made up of 10 to 100 ions/sec are displayed on the screen of an assembly of two microchannel plates and are photographically recorded within a few seconds. Jason peaks for H2+ and Ne+ are confirmed, and are also found for He+. High-order multiple peaks appear when ions are taken from the flat, closely packed net planes of W and Ir field ion emitters. The results are in quantitative agreement with a resonance model similar to one by Alferieff and Duke and by Jason. Noble gas ions are also observed from the forbidden zone near the surface, and interpreted as apex-adsorbed atoms ionized by or after excitation by the electron shower coming from farther-out field ionization of other gas atoms. Energy from excited metastable apex-adsorbed atoms may account for artifact vacancies observed particularly when field evaporation is performed in neon.

Selective adsorption of 3He and 4He on clean surfaces of NaF and LiF

Abstract Atomic beam scattering was used to study the gas-surface interaction between helium and ionic crystals. In particular, the binding energies of the selectively adsorbed states were measures for two isotopes of helium on the (001) surfaces of NaF and LiF cleaved in vacuum. Consistent results were obtained by avoiding incident conditions where band structure effects invalidate the free particle approximation. The energy levels can be fit to a 9-3 model potential.

Premature field evaporation in the atom probe

The motion of ions evaporating in the subnanosecond period before a pulse has matured is invoked in the interpretation of resolution limiting energy deficits in the TOF atom‐probe. The second order, nonlinear differential equation of motion in the time and space dependent field is numerically solved by computer. Two shapes of the pulse front, in which the final voltage level is approached either from below or from an overshoot are considered. The latter case accounts for the actual observations by giving the proper magnitude and mass dependence of the energy deficits.

Debye-Waller effects in atom-surface scattering☆

The temperature sensitivity of helium atom reflection from NaF ranges from slight in the quantum regime (long wavelength, near-glancing incidence) to considerable in the classical regime (short wavelength, near-normal incidence). The latter is describable by the Debye-Waller theory only over a limited range of angles. In this range, no Beeby well-depth correction is required. The surface Debye temperature of the NaF is found to be 425 ± 20 K.

Multilayer field evaporation patterns

Abstract Multilayer field evaporation patterns of the fee metals, Ir, Pt, Rh, and the bcc metals, W, Ta, Mo, exhibit sharp networks of dark zones. These patterns are explained as ion-optical effects resulting from deviations of ion-trajectories from an exactly radial projection, as the emitter is a polyhedron rather than a sphere. Step widths on vicinals of major zone lines are retained during progressing evaporation, thereby projecting the ions into fixed directions while keeping the zone image itself dark. The bright [110] zones on fee metals reflect a focusing effect of long atom chains across the zones. Circular intensity features around the basal planes depend upon the step-wise shift of local magnification each time a residual central plane collapses. If as we surmise multilayer desorption image features are predominantly due to tip morphology, they should also be contained in the conventional field ion image, although partially hidden by brightness effects due to varying local gas supply and the large scattering disc of a single atom image. Using multilayer sequences of field ion micrographs we map the location of each atom before it evaporates as a small dot to simultate the sharpness of a single ion spot of a field desorption image. We also use another less subjective photographic superposition method. As these FIM techniques display the three essential features of the multilayer FDM image, the hypothesis of general pre-evaporation local surface migration needs not to be invoked for explaining the FDM image.

Interpretation of Pt3Co FIM images as investigated with an atom probe

Cobalt atoms in ordered Pt–Co alloys are not imaged in the field ion microscope, although in the lattice the Co atoms occupy sites equivalent to those of the Pt atoms. Using the atom‐probe FIM, we show that the invisibility of the Co atoms in Pt3Co is caused by selective field ionization of the image gas above Pt atoms only, rather than by the absence of Co atoms in the surface by preferential field evaporation.

Aiming performance of the atom probe

We evaluate the aiming performance of our artifact‐free energy focused atom‐probe FIM employing bright contrast rhenium atoms in tungsten alloys as target atoms. The aiming efficiency averaged over the entire surface is 42% with the maximum of 60% on the (011) plane limited by the efficiency of the dual channel plate detector. We conclude that aiming through a 4.7×10−4 sr angular aperture is nearly perfect, and that aiming errors due to ion trajectory deviation as seen by multilayer field desorption patterns are minimal. Aiming capability is further established by the identification of bright impurity atoms in rhodium specimens as due to 0.1% platinum. Depth profiling of dilute constituents which do not provide visibility contrast is facilitated when the base metals such as V, Fe, Ni, W, or Pt have one or more low‐abundance natural isotopes which may be employed for counting and evaluating the statistical errors. This is demonstrated with the 192Pt isotope for the case of Pt–0.1% Mo.

Highly charged ions in field evaporating 4d-transition metals

Abstract An energy-focused time-of-flight atom-probe has been employed in a search for high ionic charges obtained in field-evaporating Zr, Nb, Mo, Rh, Pd and Ag, the 4d-transition metals. Although no new higher charges than the already established ones are found for these metals the maximum abundance of the highest charge is determined in each case. An empirical rule predicting the limits of the charge of field-evaporating ions is proposed.