ATOM PROBE FIELD MICROSCOPE HAVING MEANS FOR SEPARATING THE IONS ACCORDING TO MASS

摘要

1,180,894. Ion beam tubes. NATIONAL RESEARCH DEVELOPMENT CORP. 18 June, 1968 [20 June, 1967], No. 28944/68. Heading HID. In a field ion microscope an ion beam corresponding to a particular atom of the specimen 12 is selected by pairs of deflection plates 18 to pass through an aperture in the image screen 16 for subsequent analysis in a mass spectrometer 25. Additional deflection plates may be provided to ensure normal incidence of the selected ion beam on the aperture. The imaging gas is preferably helium at a pressure of a few microns with the electrical field applied to the specimen 12 then slowly increased, the flux of helium ions detected at 28 suddenly ceases when the selected atom is field desorbed or evaporated. The selected. atom then leaves the specimen site as an ion which passes through the aperture in the image screen for analysis in the mass spectrometer and detection at 30 or 32. The detectors 28, 30 and 32 are adjustable in position. The helium gas may alternatively, or in addition, be recorded by virtue of the beam of neutral atoms which passes through the aperture. In a modification (Fig. 3), the detector section of the mass spectrometer comprises a venetianblind type electron multiplier and output display screen. In a further modification (Fig. 4), the deflection plates are dispensed with and the microscope section 10 (50) includes a bellows device (52) which may be used to tilt the specimen and an associated ring electrode (56) in any desired direction, the tilting motion being restrained by a spherical joint (54). The microscope is associated in this modification with a time-of-flight mass spectrometer (64). The helium supply may be interrupted before the field desorption process is initiated, e.g. by applying a voltage pulse to the specimen or to the associated ring electrode. During adjustment of the image the gas pressure may be maintained by either, closing off the pumping system or operating in a dynamic gas mode with the ring electrode associated with the specimen having a small orifice such that the gas pressure is higher in the vicinity of the specimen than in the rest of the instrument. The size of the aperture in the image screen may be adjusted by utilizing an array or revolving plate of various size holes or an iris type variable aperture. The aperture may also be of oblong shape.