Magnification and mass resolution in local-electrode atom probes

On the field evaporation behavior of a model Ni-Al-Cr superalloy studied by picosecond pulsed-laser atom-probe tomography

The effects of varying the pulse energy of a picosecond laser used in the pulsed-laser atom-probe (PLAP) tomography of an as-quenched Ni-6.5 Al-9.5 Cr at.% alloy are assessed based on the quality of the mass spectra and the compositional accuracy of the technique. Compared to pulsed-voltage atom-probe tomography, PLAP tomography improves mass resolving power, decreases noise levels, and improves compositional accuracy. Experimental evidence suggests that Ni2+, Al2+, and Cr2+ ions are formed primarily by a thermally activated evaporation process, and not by post-ionization of the ions in the 1+ charge state. An analysis of the detected noise levels reveals that for properly chosen instrument parameters, there is no significant steady-state heating of the Ni-6.5 Al-9.5 Cr at.% tips during PLAP tomography.

Optimisation of a scanning atom probe with improved mass resolution using post deceleration

In this paper, we present calculations and experimental results obtained using post deceleration of ions in a scanning atom probe (SAP) geometry to improve the mass resolution. Various electrode geometries, tip to electrode distances in the range 50-170 microm and three different pulse shapes have been evaluated. Experimental mass resolutions of 750 FWHM and 200 FWTM have been achieved reproducibly for the 184W3+ peak without the use of a reflectron lens. 3D finite element electrostatics software has been used to simulate the ion trajectories through the instrument and thus to calculate the variations in velocities for the different electrode configurations. The observed trends are found to agree well with experimental results.

Improvement of the mass resolution of the atom probe using a dual counter-electrode

As compared to other techniques, the mass resolution of the 3D atom probe is rather poor. This low mass resolution derives from the spread in energy of field-evaporated ions. In this work, the single counter-electrode used to remove atoms from the specimen was replaced with a dual counter-electrode. A positive standing voltage V(PA) is applied on the electrode facing the specimen while the second electrode is grounded. As a result, ions experience a post-acceleration between electrodes that lowers energy deficits of ions resulting in an improvement in the mass resolution. This paper reports the study of the resulting improvement in mass resolution as a function of the post-acceleration voltage. It is also shown that, because of the evaporation pulse, ions also undergo a dynamic post-deceleration in the between electrodes. This post-deceleration contributes to the mass resolution increase. Our results show that this very simple device makes it possible to significantly improve the mass resolution of the atom probe. For a low post-acceleration voltage, the mass resolution is 800 FWHM and 200 at full-width tenth-maximum.