Perspective: From field-ion microscopy of single atoms to atom-probe tomography: A journey: "Atom-probe tomography" [Rev. Sci. Instrum. 78, 031101 (2007)].

On October 11, 1955, Muller and Bahadur Pennsylvania State University observed individual tungsten atoms on the surface of a sharply pointed tungsten tip by cooling it to 78 K and employing helium as the imaging gas. Muller and Bahadur were the first persons to observe directly individual atoms, using a field-ion microscope FIM , which Muller had invented in 1951. This historic and seminal event occurred long before the observations of individual atoms by Z-contrast scanning-transmission electron microscopy STEM , high-resolution electron microscopy HREM , scanning tunneling microscopy STM , or atomic-force microscopy AFM . A FIM is a lensless point-projection microscope that resolves individual atoms on the surface of a sharply pointed tip at magnifications of greater than 106 times. The radius of a tip is 50 nm and is maintained at a positive potential Vdc with respect to earth. Atomic-resolution FIM images are achieved by cooling a tip into the range of 20–100 K in a high to ultrahigh vacuum system and placing it at Vdc to generate electric fields E that are between 15 and 60 V nm−1. Helium or neon gas, or a mixture of He and Ne gases, is used to image individual atoms utilizing the phenomenon of field ionization. In these large E fields He or Ne atoms are field ionized, 45 Vdc nm −1 for He and 35 Vdc nm −1 for Ne, above individual surface atoms. This is because the outermost electron of the imaging gas atoms quantum mechanically tunnels into a sharply pointed microtip at the site of an atom. This creates a small diameter cone of He+ or Ne+ ions emanating from individual surface atoms; the total current from a tip with an area of 10−14 m2 is 10−9–10−8 A, hence the current from a single atom is 10−14–10−13 A, respectively. The field-ionized He+ or Ne+ ions are then accelerated along E-field lines that are orthogonal to the equipotentials associated with a tip and they terminate on a detector, which is at earth potential. The energetic ions are ultimately converted into visible light employing a microchannel plate MCP , which is the solidstate analog of an array of photomultiplier tubes, with a gain of 106–107 for a single ion. The detection efficiency of a MCP is equal to its open area, 50% –60%, which is a weak link in the detection system and needs to be improved in the future. In the FIM mode of operation, about 104–105 surface atoms are imaged and the FIM images often exhibit the symmetry of the crystal lattice of the specimen. Thus, the physical basis of “seeing” atoms in direct space involves the quantum-mechanical process of field ionization of imaging gas atoms, which was first analyzed by Oppenheimer 1928 5 for a hydrogen atom in free space in a potential gradient of 102 Vdc m −1; he calculated the time to ionize an atom at this value of E to be 10100 s, which is a googol, whereas in a FIM the time for field ionization is 10−10 s. The E-field ionized He+ or Ne+ gas ions are the messengers that allow us to observe individual atoms on the surface of a sharply pointed specimen in direct space with excellent atomic resolution, which is a singular intellectual and scientific achievement with tremendous ramifications for solving important scientific and technological problems in materials research. Subsequently in 1956, Muller discovered the important physical phenomenon of field evaporation or field desorption , which is the sublimation of atoms as ions from a microtip radius 40 nm specimen employing a high E field that may be greater or less than the E field necessary to ionize a helium or neon atom at a given cryogenic temperature. Muller observed this phenomenon occurring by increasing the dc voltage on a tungsten specimen, and concomitantly increasing the value of E, and noticing that the atoms at the surface of a microtip are continuously “evaporating subliming ” as ions, thereby exposing the interior of a specimen, and hence the term field desorption or field evaporation. Field evaporation is sublimation with the aid of an electric field and is a material dependent property; for example, for W the evaporation field Ee is 57 V nm−1, while for Ag, Ee is 25 V nm −1 at 0 K. Field evaporation is controlled by superimposing high-voltage pulses Vpulse on top of Vdc, where the pulse fraction Vpulse /Vdc is typically between 0.1 and 0.2; the optimum value is found empirically. In this manner the internal atomic structure of a specimen is routinely observed directly and reconstructed in threedimensions 3D to reveal imperfections: that is, atomicscale defects, such as vacancies, self-interstitial atoms, chemical impurity atoms, dislocations, stacking faults, sub-boundaries, grain boundaries, clusters of atoms, precipitates, etc. A historic and seminal short course and workshop on FIM was held at the University of Florida in March 14–22, 1966. Review and original articles were presented on both the theory and application of FIM to problems in materials REVIEW OF SCIENTIFIC INSTRUMENTS 78, 030901 2007