A Time-of-flight Atom-probe Field-ion Microscope for the Study of Defects in Metals

An ultra-high vacuum time-of-flight (TOF) atom-probe field-ion microscope (FIM) specifically designed for the study of defects in metals is described. Performance experiments show that this instrument can clearly resolve the seven stable isotopes of molybdenum, the five stable isotopes of tungsten, and the two stable isotopes of rhenium in a tungsten-25at.% rhenium alloy. The entire process of applying the evaporation pulse to the FIM specimen, measuring the dc and pulse voltages, and analyzing the T0F data is controlled by a Nova 1220 computer. With this automated system we can presently record and analyze 600 TOF events min -1. This note summarizes a very detailed report (1) which describes an ultra-high vacuum (UHV) TOF atom-probe FIM specifically designed for the study of defects in metals. The T0F atom-probe FIM (or more simply the atom probe) was first described by ~/ller et al. (2) and combines an FIM with a TOF mass spectrometer. With this instrument it is possible both to image the microstructural features of a metal specimen on an atomic scale and to measure the mass-to-charge ratios (m/n) of individual ions from pre-selected regions of a specimen. The atom-probe is ideally suited for the study of the interaction of both substitutional and interstitial impurity atoms with lattice defects such as vacancies, self-interstitial atoms, dislocations, grain boundaries and voids. The potential of the atom-probe for studying a wide range of materials science problems had been demonstrated by Brenner and co-workers (3,4) and by Turner, Southon and co-workers (5). A schematic d~agram illustrating thg main features of the atom probe is shown in Fig. i. A specimen with a radius of 50 to 400A is maintained at a positive potential (6-20kV) so that gas atoms surrounding the specimen are ionized over individual atomic sites and are projected radially outward to produce a visual image on the internal-image-intensification system. When a short high-voltage pulse is applied, atoms on the surface of the specimen are field evaporated in the form of ions. Those ions projected into the probe hole at the center of the internal-imageintensification system will pass down the flight tube to the ion detector. The TOFs of the ions and the voltage on the specimen are measured and the (m/n) ratios are calculated employing the equation: m/n = 2e(Vdc + aVpulse) (t-to)2/d 2 Research supported by the U.S. Energy Research and Development Administration; additional support was received from the National Science Foundation through the use of the technical facilities of the Materials Science Center at Cornell University. +Now at: Argonne National Laboratory, Argonne, Illinois 60h39.