A high-resolution pulsed-laser atom-probe field-ion microscope

The atom-probe field-ion microscope is a unique instrument for the analysis of materials on an atom-by-atom and atomic layer-by-atomic layer basis. It combines a point projection microscope, which allows one to observe individual atoms in direct lattice space on the surface of a sharply pointed tip under a high positive voltage, with a time-of-flight mass spectrometer that can measure mass-to-charge ratios of individual atoms field-evaporated from the surface of a tip during a few nanosecond wide high-voltage or laser pulse. The ability to distinguish atomic isotopes and molecular clusters of one species from the other is crucial in the atom-probe analysis of multi-component materials. One of the main sources of errors in such an analysis is energy deficits of field-evaporated ions inherent to the pulsed high-voltage evaporation process. Therefore, in this study the task of designing and fabricating an atom-probe field-ion microscope capable of compensating these energy deficits, and versatile enough to be able to analyze a wide spectrum of engineering materials, was undertaken. The instrument has the following features: (a) ultrahigh vacuum (UHV); (b) a specimen exchange device that will store up to 10 specimens and allows transferring specimens directly from atmospheric pressure to UHV conditions; (c) a closed cycle helium refrigerator with a temperature regulator capable of cooling the specimen to 10 K; (d) a 3D goniometer with ±30° tilt in the horizontal and vertical planes; (e) a variable distance, high gain MCP assembly for field-ion microscopy; (f) a single stage reflectron lens; (g) a 1500 Hz solid-state high voltage pulser; (h) a 20 Hz, 300 μJ/pulse, 4 ns pulsed nitrogen laser analysis capability. The mass resolution of this instrument is $Dmnmn=1800$ at full-width half-maximum (FWHM) with high-voltage pulsing and $Dmnmn=1900$ at FWHM with laser pulsing.