Optimization of High Brightness Cs Ion Source and Ion Optics for UHV-IMMA

Abstract

An ultra - high vacuum ion microprobe mass analyser (UHV-IMMA) aimed at hydrogen detection and microanalysis of semiconductor devices has been developed [1] at ULVAC. A schematic diagram of the UHV-IMMA is shown in Fig. 1. A cesium ion beam extracted from the surface ionization type ion source is focused by the stigmatic focusing sector type mass analyser onto the image aperture. The image aperture (300 μm in diam.) determines the initial spot size and acts as an orifice for effective differential pumping. The ion beam passing through the image aperture is focused onto the sample surface by both of the symmetrical einzel lens (the 1st lens) and the asymmetrical lens (the 2nd lens). Two octupole deflectors used for raster scanning are located between these lenses to minimize the deflection-induced aberration and are operated so as to make the primary ion beam always pass through the center of the second lens. The secondary ions ejected from the sample surface travel in the spherical electrostatic ion energy analyser (IEA) and the quadrupole mass spectrometer (QMS). The number of secondary ions is counted by a high rate pulse counting system.