Computer-controlled time-of-flight atom-probe field-ion microscope for the study of defects in metals

Abstract

A time-of-flight (TOF) atom-probe field-ion microscope (FIM) specifically designed for the study of defects in metals is described. This atom probe features: (1) a variable magnification internal-image-intensification system based on a channel electron multiplier array (CEMA) for viewing the FIM image; (2) a liquid-helium-cooled goniometer stage which allows the specimen to be maintained at a temperature anywhere in the range of 13 to 450 K; (3) a low energy (less than or equal to 3 keV) gas ion gun for in-situ irradiations; (4) an ultra-high vacuum (approximately 3 . 10/sup -10/ Torr) chamber to minimize specimen contamination; (5) a high-vacuum (approximately 10/sup -6/ Torr) specimen-exchange device; (6) a Chevron ion detector; and (7) an eight-channel digital timer with +- 10 nsec resolution for measuring the TOFs of the pulse-field evaporated ions. The entire process of applying the evaporation pulse to the specimen, measuring the dc and pulse voltages, and analyzing the TOF data is controlled by a Nova 1220 computer. Data in the form of a histogram of the number of events versus the mass-to-charge ratio is displayed on a Tektronix graphics terminal. An extensive set of computer programs to test and operate the atom-probe FIM have also been developed. With this automated system we can presently record and analyze 600 TOF min/sup -1/. The instrument can clearly resolve the seven isotopes of molybdenum and the five isotopes of tungsten. Investigations of alloys have shown that the concentration of rhenium in a W-25 at. percent Re alloy; and the concentrations of titanium and zirconium in a Mo-1.0 at. percent Ti alloy and a Mo-1.0 at. percent Ti-0.08 at. percent Zr alloy can be easily measured. Investigations of a low swelling stainless steel alloy (LS1A) and a metallic glass alloy (Metglas 2826) have shown that all constituents present at a level of 0.05 at. percent or higher can be readily determined.