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Development of a scanning atom probe for nanoscale characterisation of thin-film materials, devices and coatings

Exploitable results

A prototype scanning atom probe (SAP) has been developed which is capable of producing full 3-dimensional chemical mapping of the atomic distributions within thin film materials and devices. This instrument is an extension of the 3-dimensional atom probe (3DAP) that is now in routine use in various laboratories around the world for the study of metallic alloys. Despite the power of the 3DAP technique, its application to the study of thin films and surfaces has been limited because of the problem of fabricating the sharply curved specimen from such materials. In the SAP, the specimens are in the form of micro-tips fabricated within the surface of a planar substrate, making it simpler to fabricate specimens from thin films. A microelectrode is brought up to one of the micro-tips in order to raise the electric field sufficiently for analysis. The specimen stage developed in the project allows cooling of both specimen and counter-electrode to cryogenic temperatures. The stage also allows positioning of the electrode with a position readout up until a 10 nm precision, and with stability of better than 1 µm over many hours. The prototype SAP instrument developed in the current project uses a novel 2-terminal counter-electrode for improved mass resolution, and also incorporates a new 3-wire delay-line position sensitive detector for improved multi-hit performance. With PC-based data acquisition hardware and a specially designed 10 kHz pulser, the new detector yields improved data rates of up to 300 ions per second. Special software to allow user control of the scanning stage has been written which generates an image of the field emission currents during a raster to allow identification of micro-tip positions. The stage can subsequently be returned to any identified position under mouse control. The instrument also has a fully integrated interlocked electronics control system which makes it very user-friendly. Various specimen preparation methods were developed and tested during the course of the project. This allowed the instrument to be used for preliminary analysis of metallic multilayer films, for silicon specimens and for the study of degradation of phosphors used in display technology.