Thomas F. Kelly

Local Electrode Atom Probes

The historical developments leading to the advent of Local Electrode Atom Probes (LEAP) are reviewed. An assessment of the state of the art is made, and the major advantages of LEAPs over conventional atom probes are described. The best implementations of these concepts and the remaining challenges for realization of LEAPs potential are also described. It is concluded that LEAPs should be an important tool for materials characterization at the atomic scale. Modern materials-dependent industries as diverse as steel and microelectronics should benefit from this technology.

On the many advantages of local-electrode atom probes.

Local extraction electrodes offer several crucial advantages for operation of atom probes. Because of the proximity of the local extraction electrode to the specimen, the electric field produced at the specimen apex by a given voltage is enhanced and the voltage required for field evaporation is reduced. In a voltage-pulsed atom probe, the absolute magnitude of the energy uncertainty is correspondingly reduced. High mass resolution (m/deltam > 1000) may therefore be obtained by accelerating the evaporated ions to a greater total potential after the local extraction electrode. The low extraction voltage may also be pulsed rapidly (100 ps rise time) and at high repetition rates (up to 10(5) pulses per second) using currently available solid-state pulsers. Furthermore, a local electrode and intermediate electrodes may be used as optical elements to control the image magnification. All of these benefits may be applied to any type of atom probe. Local-electrode atom probes (LEAP) should be especially advantageous for developing three-dimensional atom probes with high mass resolution and a large field of view. A sample has been developed that consists of many microtips formed on a planar sample using ion beam mask etching. Microtip samples are especially suited to LEAP. Analysis of electrically insulating samples may also be possible with microtip samples in a LEAP. This combination of features suggests flexible, high speed, high mass resolution atom probes that can work with either conventional needle-shaped specimens or the new style of planar microtip specimens.

Rapid solidification of a droplet-processed stainless steel

Individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM). Fcc is found to be the crystallization phase in powder particles larger than about 70 micron diameter, and bcc is the crystallization phase in the smaller powder particles. An important difference in partitioning behavior between these two crystal structures of this alloy is found in that solute elements are more completely trapped in the bcc structures. Massive solidification of bcc structures is found to produce supersaturated solid solutions which are retained to ambient temperatures in the smallest powder particles. Calculated liquid-to-crystal nucleation temperatures for fcc and bcc show a tendency for bcc nucleation at the large liquid supercoolings which are likely to occur in smaller droplets. The importance of small droplet sizes in rapid solidification processes is stressed.

Amorphous solidification of pure metals in submicron spheres

Abstract Submicron droplets of high purity elemental metals have been sprayed in high vacuum by electrohydrodynamic atomization. It is found that under these extreme conditions of high cooling rate and very small liquid volume, some pure metals solidify from the melt as an amorphous phase. A systematic study has been conducted for 15 metals plus silicon and germanium of the distribution of the amorphous phase with droplet size, f.c.c., b.c.c. and h.c.p. metals have been studied and it is found that the b.c.c. metals form an amorphous phase much more readily. The observed maximum diameter of amorphous spheres is less than 100 nm in all cases and is used as experimental input to an analytical simulation to determine the glass transition temperatures of the metals in each case. In general, amorphous phases are found in metals that have a reduced glass transition temperature of greater than 0.44. The dependence of glass transition temperature on other physical properties such as enthalpy of fusion and enthalpy of vaporization is also presented.

Atomic-scale analysis of CoFe/Cu and CoFe/NiFe interfaces

Internal interfaces in metallic multilayers grown on planar silicon substrates have been chemically analyzed with atomic resolution using three-dimensional atom probe microscopy. The structure studied was a NiFe/CoFe/Cu/CoFe multilayer grown with (111) texture. Atom probe measurements across the NiFe/CoFe interfaces yield widths of 1.1±0.2u200anm for NiFe grown on CoFe and 1.7±0.2u200anm for CoFe grown on NiFe. The widths of interfaces between CoFe and Cu layers vary as well, with values of 0.82±0.10u200anm for CoFe grown on Cu, but only 0.47±0.15u200anm for Cu grown on CoFe. In addition, the Fe concentration is enriched at the interface where Cu is grown on CoFe, and depleted where CoFe is grown on Cu. These results indicate that the Fe segregates to the surface during the deposition of CoFe so that the composition at the top of this layer is Fe rich.

Solidification structures in submicron spheres of iron-nickel: Analytical evaluation☆

Abstract Kinetic modeling of solidification in small droplets is developed using classical nucleation theory. Both primary and alternative crystallization phases of iron-nickel alloys are considered. Calculated continuous-cooling-nucleation (CCN) curves and nucleation temperatures for different compositions and particle sizes are used to predict the tendency of phase formation. The calculations are compared with experimental results presented in a companion paper [Acta metall.36, 2525 (1988)] on submicron droplets and it is found that formation of the crystalline phases can be edicted semiquantitatively using classical nucleation theory. Experimental results show that an amorphous phase predominates in particle sizes less than about 30 nm diameter. These data are used to simulate the viscosity function of the liquid alloys and to determine a kinetic glass transition temperature as a function of composition for each phase. Using an isothermal treatment of transient nucleation, transient effects are found to be important close to the kinetic glass transition temperature, however, a non-isothermal treatment of transient nucleation is needed for this application.

Atom probe analysis of planar multilayer structures

Atom probe field ion microscopy has been used to analyze a planar-deposited layered structure in plan view. The specimens were prepared with a newly developed method that involves a combination of photolithography and focused ion-beam milling. A multilayer structure consisting of {Ta/CoFe/(Cu/CoFe)15/Ru/(CoFe/Ru)5/Ru/NiFe} was sputter deposited for use as a test stack. The corresponding thicknesses of these layers were 7/13(3/3)/50/(3/1)/50/150 nm. The nanometer-scale periodicity of the Cu/CoFe stack is readily apparent in transmission electron microscopy images of a field ion specimen fabricated from this material, suggesting that the specimen preparation procedure does not lead to destruction of the multilayer structure. Atom probe analysis of the bulk NiFe layer and the Ru/NiFe interface revealed the distribution of impurity atoms in the film, and these may affect the magnetic properties of the multilayers. Whereas a uniform distribution of C, N and Ar was observed, segregation of O was observed in the...

Magnification and mass resolution in local-electrode atom probes

Abstract The mass resolution of a local-electrode atom probe can be improved by accelerating the evaporated ions to a higher energy in order to reduce the energy-deficit-related dispersion. A simple model of the instrument is developed and used to estimate the effects of this secondary acceleration on image magnification and mass resolution. Effects of non-instantaneous secondary acceleration, variations in the secondary acceleration field distribution and electrode length are evaluated using the model. The addition of an acceleration electrode after the extraction electrode is shown to improve the performance of local-electrode atom probes.

The Microstructure and Phase Relationships in Rapidly Solidified Type 304 Stainless Steel Powders

The microstructure and relative amounts of fcc and bcc phases have been studied for rapidly solidified Type 304 stainless steel powders produced by vacuum gas atomization (VGA) and centrifugal atomization (CA). The VGA powder solidifies with a cellular microstructure while the CA powder has a dendritic microstructure. The volume fraction of fcc phase in the CA powder is found to increase from 40 Pct to 97 Pct with increasing particle size from 30 to 125 µm. In the VGA powder, the volume fraction of fcc phase is found to decrease from about 90 Pct to 77 Pct over the same range of particle sizes. The origins of the fcc and bcc phases in each powder are considered. It is concluded that bcc is present as both a primary crystallization phase in the smaller CA particles (<75 µm) and as compositionally stabilized eutectic ferrite at the cell walls of particles of both CA and VGA powders in which fcc was the primary crystallization phase.

Atom probe analysis of roughness and chemical intermixing in CoFe/Cu films (invited)

Three-dimensional atom probe analyses of the interfaces between CoFe and Cu layers has shown that both roughness and chemical intermixing can occur independently. Interfaces formed by the deposition of Cu onto CoFe mimic the roughness present in previously deposited interfaces, but have a very small amount of interfacial mixing. In contrast, interfaces formed by the deposition of CoFe onto Cu are less rough, but more chemically intermixed. The region of chemical intermixing formed when CoFe is deposited onto Cu (0.7–1.0 nm) is approximately two times larger than that when Cu is deposited onto CoFe (0.3–0.5 nm).