A gallium (Ga) focused ion beam (FIB) has been applied increasingly to 'site-specific' preparation of cross-sectional samples for transmission electron microscopy (TEM), scanning TEM, scanning electron microscopy and scanning ion microscopy. It is absolutely required for FIB cross-sectioning to prepare higher-quality samples in a shorter time without sacrificing the site specificity. The present paper clarifies the parameters that impose limitation on the following performances of the FIB cross-sectioning: milling rate, cross-sectioning at a right angle with respect to the sample surface, curtain structures formed on the cross sections, ion implantation and ion damage. All of these are discussed from the viewpoint of ion-sample interaction. Improvements for these performances achieved by diminishing their limiting origins or by correcting the resultants are described. Especially, the FIB scanning speed is significantly utilizable to improve the milling rate. A microsampling method, which allows the FIB incidence in a sidewards or upwards direction as well as downwards with respect to the microsample surface, is very effective to minimize the curtain structures.
We performed Monte Carlo simulation of helium (He) ion induced secondary electron (SE) emission in order to compare the secondary electron image characteristics between He and gallium (Ga) scanning ion microscopes (SIM) and scanning electron microscope (SEM). For 10-50 keV He ion bombardment SE yield increases gradually with increasing the atomic number, Z2, of the target, as well as for the electron bombardment. However, for 30 keV Ga ion bombardment, SE yield shows an opposite Z2 dependence. The calculated SE yield is much larger than that for both electron and Ga ion bombardment. The incident angle dependence of the SE yield approximately obeys the inverse cosine law even at high angles of 85 degrees and more. On the other hand, for electron bombardment, the incident angle dependences are much weaker for low energy and high Z2. These indicate that the image contrast on He-SIM is clearer than those of SEM. Among the electron excitations by incident He ions, recoiled target atoms and excited electrons, the first one having narrow excitation volume dominates the SE yield, so that the spatial image resolution in SIM using zero-diameter He beams with the energies of 10-50 keV is prospected to be smaller or better (<0.1 nm) than for 30 keV Ga ion and 1 keV electron beams.
A gallium (Ga) focused-ion-beam (FIB) has been popularly used to prepare cross-sectional samples for transmission electron microscopes (TEMs) and scanning electron or ion microscopes. However, characteristics of the FIB-prepared cross sections such as ion concentration and radiation damage have been little studied either in theory or in experiment. In the present study, cross sections prepared by 30 keV Ga FIB are modeled using a combination of analytical and Monte Carlo methods to calculate the implanted Ga concentration. It is found that the Si/W layered sample is cross sectioned at grazing angles β≈2.5° and 6° for these layers, respectively. The implanted Ga ions for the cross-sectioned Si and W layers are concentrated very near their surfaces of <10 nm to yield the Ga concentrations CGa of about 4 and 9 at % for these layers, respectively. Although there is some differences in sample materials between the calculations and the experiments, the calculated CGa values for Si and W layers roughly agree with the experimental values for the magneto-optical disk TEM sample. This agreement firmly supports the present modeling of FIB-milled cross sections.
Summary:Common and different aspects of scanning electron microscope (SEM) and scanning ion microscope (SIM) images are discussed from a viewpoint of interaction between ion or electron beams and specimens. The SIM images [mostly using 30 keV Ga focused ion beam (FIB)] are sensitive to the sample surface as well as to low-voltage SEM images. Reasons for the SIM images as follows: (1) no backscatteredelectron excitation; (2) low yields of backscattered ions; and (3) short ion ranges of 20-40nm, being of the same order of escape depth of secondary electrons (SE) [=(3-5) times the SE mean free path]. Beam charging, channeling, contamination, and surface sputtering are also commented upon.
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