T.J. Godfrey

Application of a position‐sensitive detector to atom probe microanalysis

A position‐sensitive detector system based on a wedge‐and‐strip anode has been used to build a short flight‐path atom probe which identifies both the chemical nature and position of single atoms field evaporated from the surface of a field‐ion specimen. The detector also allows digitized field‐ion images to be obtained from the region being analyzed. The prototype instrument has a lateral resolution during analysis of substantially below 1 nm, and a depth resolution of one atomic layer. Initial applications of the instrument to the analysis of nanometer‐scale precipitates in metallic alloys has shown the capability of reconstructing the three‐dimensional microstructure and microchemistry of materials.

PERFORMANCE OF AN ENERGY-COMPENSATED THREE-DIMENSIONAL ATOM PROBE

A wide acceptance angle first-order reflectron lens has been incorporated into a three-dimensional atom probe (3DAP) to provide improved mass resolution. This new 3DAP instrument is capable of resolving isotopes in the mass spectrum, with resolutions better than m/Δm=500 full width at half maximum and 250 full width at 10% maximum. However, use of a reflectron for energy compensation within an imaging system means that improvements in mass resolution result in degradation of the spatial resolution. This article addresses the detailed design of the energy compensated 3DAP, and the minimization and compensation of chromatic aberrations in the imaging performance of the instrument. Some applications of the new instrument are included to illustrate its capabilities in the atomic-scale analysis of engineering alloys.

Materials analysis with a position-sensitive atom probe

A position‐sensitive detector has been combined with time‐of‐flight mass spectrometry in the atom probe field‐ion microscope to yield a system in which both chemical identity and spatial information are obtained for individual ions field‐evaporated from the specimen surface. This allows the variations in composition originally present in the sample to be reconstructed in 3‐D with sub‐nanometre resolution. The prototype position‐sensitive atom probe is being used to study phase chemistry in a number of metallurgical alloys, including accurate composition determination of 1–2 nm Cu‐rich precipitates formed in Fe–1.3% Cu–1.4%Ni aged to peak hardness. Other applications of the position‐sensitive atom probe (POSAP) include the analysis of surface layers on superconductors and atom probe studies of semiconductor multiple quantum wells. These initial applications of the instrument are reported, and the limitations and intended improvements to the instrument are discussed.

Improvements in three-dimensional atom probe design

Abstract An improved position-sensitive atom probe has been designed which uses a combination of a parallel timing system and a silicon photodiode array camera. The use of two separate data acquisition systems allows the two functions of accurate positioning and flight time determination to be divorced, thus removing the compromises which must be made when these functions are carried out with only a single detector. The resulting instrument is able to determine flight times and positions of impacts straightforwardly, even when multiple ions are evaporated on a single pulse, and should be capable of operating at evaporation rates close to that of a conventional probe-hole atom probe.

Improvements in the mass resolution of the three-dimensional atom probe

Abstract A new three-dimensional atom probe has been developed, in which energy compensation has been included in order to improve the mass resolution of the technique. A first order reflectron is used as its energy compensating device. The incorporation of a reflectron in a three-dimensional atom probe is less straightforward than when used in a conventional atom probe, and special care has to be taken in the design to ensure satisfactory performance, both in terms of mass resolution and spatial resolution. Mass resolutions achieved with this instrument are better than m Δm = 500 full width at half maximum and 250 full width at 10% maximum.

Design of a scanning atom probe with improved mass resolution

A design for a high mass resolution scanning atom probe is described, which utilizes a two-conductor microelectrode held at 10–100 μm from the specimen. Field evaporation pulses are applied to the part of the counter-electrode closest to the specimen, while the output is maintained at ground. If the gap between the two conductors is small, field evaporated ions pass through the microelectrode while the pulse voltage is essentially constant, and thus the resultant spread in ion energies is small and the mass resolution in time-of-flight mass spectrometry is correspondingly improved. Initial results indicate improvements of 4–5 times over the mass resolution obtained with a simple counter electrode.

Surface analysis with a position-sensitive atom probe

A position-sensitive detector has been combined with time-of-flight mass spectrometry in a short-flight-path atom probe to yield an instrument in which both the chemical identity and surface position are obtained for individual atoms removed by field evaporation from the surface of a field-ion specimen. This enables thin films and surface layers to be analysed, not only with atomic-layer depth resolution, but also with sub-nanometre lateral resolution. The new instrument is described, and examples are presented showing the application of the technique to the study of the oxidation of metal surfaces, and surface segregation in an yttrium barium cuprate ceramic-oxide superconductor.

Design of a Scanning Atom Probe with Improved Mass Resolution using Post Deceleration.

Calculations and experimental results obtained using post deceleration of ions to improve the mass resolution are presented. Various extraction and counter electrode geometries, tip to extraction electrode distances and 3 different pulse shapes have been evaluated. Experimental mass resolutions of 767 FWHM and 219 FWTM have been achieved reproducibly for the 184W3+ peak without the use of a reflection lens. 3D finite element electrostatics software has been used to simulate the ion trajectories through the instrument and thus calculate the expected mass resolution for the different electrode configurations. The observed trends are found to agree well with experimental results

A FIM atom probe study of vanadium oxidation

Vanadium specimens have been prepared and imaged for the first time in the field ion microscope. The composition of oxide films formed on vanadium at temperatures between 290 and 473 K has been determined by atom probe microanalysis. It is shown that the outer oxide film has a composition close to V2O3 in this temperature range, and that an intermediate layer of suboxide of composition approximately V9O forms between the outer oxide and the metallic substrate. Appreciable solubility of oxygen in the vanadium metal matrix is also observed.