Material Contrast of Scanning Electron and Ion Microscope Images of Metals

Abstract

IntroductionThe rapid technical development of FIM (Focused Ion Beam) technology has spawned an increase in spatial resolution capability in scanning ion microscopy (SIM) technology 1 . Furthermore, FIM has been used for preparation of thin specimens in transmission electron microscopy 2 and micro-fabrication of electronic devices in the semiconductor industry 3 . Recently, a scanning ion microscope with a helium field ion source has been developed 4 . Thus, the contrast formation of emission electron images in scanning ion microscopy has been the object of study for analyzing images of materials specimens, similar to the theory behind scanning electron microscope (SEM) contrast formation 5,6,7 . Furthermore, whether the electron emission yield γ induced by ion impact is periodic or non-periodic as a function of Z 2 (the atomic number of the target) has not been well studied in the low energy region from several keV to the several tens of keV values used in SIM. Thus, in the present article, comprehensive experiments on γ as a function of atomic number Z 2 and the electronic structures of target metals have been performed for a number of metals for incident beams of 10 keV electron, 3 keV Ar ExperimentOne of the tools used in this experiment is a scanning Auger electron microscope (JAMP-7800F) with a back pressure of 7x10 -8 Pa installed with a hemi-spherical electron energy analyzer, a secondary electron detector system, consisting of a scintillator and a photo-multiplier tube, and an Ar + -ion gun with a beam diameter that is variable from 0.1 to several mm. The other tool used is a scanning Ga + -ion microscope (Micron-9000) with a detector system composed of an annular micro-channel plate ,which is mounted coaxially with the incident beam and can be biased for detecting either positive or negative particles. Secondary electron yields are, in general, far larger than those of the secondary negative ions, so negative particle imaging is essentially electron imaging. The energies of the electron and Ar + ion beams of the JAMP-7800F were 10 keV and 3 keV, respectively, and the energy of Ga + -ion beam of the Micron-9000 was 30 keV.Secondary electron yields were measured from the integrated intensities of the secondary electron spectra in an energy range from 0 to 30 eV for the JAMP-7800 and from the output signals of the micro-channel plate for the Micron-9000. The secondary electron yields obtained in this study are therefore relative values in each experiment, because the experimental arrangements of the secondary electron detectors differ. Experimental resultsStrips of several metal films of 1µm thickness and 0.1~1 mm width were deposited on a Si(100)2×1 clean surface in the order of Z 2 . The target films were Al, Cu, Ag, and Au metals with a similar electronic structure. Fig.1(a) shows the secondary electron image obtained by the JAMP-7800F using a 10 keV electron beam. Brightness increases with Z 2 . Whereas Fig.1(b) shows the secondary electron image obtained by the Micron-9000 using a Ga + -ion beam ...