Counter Electrode Design Considerations in Atom Probe Tomography Microscopes

Abstract

In the development of atom probe tomography instruments, a variety of counter electrode designs have been considered. Each design takes into account a wide variety of design criteria including; complexity of manufacturing, undesired electron emission, stage motion, cryogenic cooling, vacuum performance, voltage and laser pulse introduction, geometry, serviceability, field enhancement, signal‐to‐noise, energy spread, field of view, and cost of ownership. Examples of some designs are shown in Figure 1. The local electrode atom probe (LEAP®) puts a premium on a geometry and proximity to the specimen in order to enable faster voltage pulsing, minimization of energy spread, and the maximization of throughput with microtip geometries [1]. Such a design, with a small spot focused laser [2], maximizes field enhancement and minimizes the portion of the flight path exposed to field variations [3]. The design does however require a vibration isolated and flexible cryogenic path and a high precision stage with sophisticated alignment cameras. Prior to the proposal of a micro‐extraction electrode by Nishikawa in 1993 and until the introduction of the LEAP in 2003 [4,5], atom probe field ion microscope systems used a simple counter electrode that was simply a wire ring or a copper disc with an opening of a few millimeters. This design is simple to construct, and allows substantial flexibility in sample stage design, including the use of a goniometer stage. Especially in early atom probe design, where the field of view was as much as 100 times smaller than is achievable today, the ability to rotate and tilt the specimen towards the TOF detector was key. Alignment of the specimen with respect to the counter electrode is not critical and could be achieved by line of sight and the projection of the data to a phosphor screen or the TOF detector. The simplicity and flexibility are countered by the limitation to wire geometry specimens, a flexible and lower conductance cryogenic cooling path, low field enhancement, and degradation of mass resolving power due to the ions being exposed to varying electric fields during larger portions of the flight path. Several atom probe systems have been proposed and constructed using a flat disk counter electrode with an aperture (or a metal TEM grid) with the specimen moved aligned in close proximity to, or even protruding through the plane [6–8]. Alignment to the electrode requires wire geometry specimens, a precision stage and long range microscopes, but substantial field enhancement is possible, even while using relatively large apertures (~ 1mm) which minimizes the chance of damage to the electrode during specimen fracture events. For a ~ 1mm aperture in a disc electrode, the field enhancement is ~25% less than a local electrode and simulations show it is insensitive to specimen penetration distance (Figure 2) [9]. Achieving the highest data quality and highest throughput in an atom probe substantially complicates design requirements. A variation of the flat disk counter electrode approach removes the requirements of a precision stage, flexible connection to the cryogenic system and sophisticated alignment microscopes by doing ex‐situ alignment of a disk electrode with the specimen. Although still requiring a wire geometry specimen, such a system could have a directly couple cryogenic system and could take advantage of the substantial field enhancement, wide field of view, and relatively high data quality when compared to previous generation atom probe microscopes. This work presents our current advances in simplification of electrode geometries for atom probe and the performance associated with such designs.