Gallium clusters from a liquid metal ion source

ABSTRACT Gallium ion clusters Ga+ n with n ranging up to 30 have been measured from liquid gallium ion sources using a time‐of‐flight spectrometric technique. The observations were made as a function of total ion emission current, angle of emission, and temperature of the liquid metal. Under all conditions Ga+ is the overwhelmingly dominate species emitted. The cluster data reveal several remarkable features. First, emission of clusters tends to be very low below 2 μA emission current, to rise steeply with current to 10 or 20 μA and decrease for higher current. Second, within the range examined, clusters with n=8 and 15 have particularly low abundances. Third, the energy distributions of the resolvable clusters are bimodal with characteristic peaks at approximately 30 and 120 eV below the source potential. Fourth, the angular distributions of the clusters are more nearly axial than that of the primary species. The temperature of the liquid metal has no noticeable effect on cluster emission over the range examined. A possible model for cluster formation will be discussed.

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    ABSTRACT: The charge distribution of microdroplets emitted from liquid metal ion sources (LMIS’s) has been investigated as a function of total beam current. For microdroplets with charges of more than 10 000 electronic units, the distribution changes rapidly with total beam current for currents of more than 60 μA. The charge distribution has also been examined when the extractor of the LMIS is driven with an external ac voltage. The total emission of highly charged microdroplets shows a resonance at ∼4 kHz. This resonant behavior may be related to surface wave instabilities that have been suggested as a primary mechanism of droplet formation.
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    ABSTRACT: Recently, a mass spectrometer for liquid metal ion sources (LMIS) has been built and set into operation. This device uses an E × B-filter as mass dispersive element and provides sufficient resolution to analyse the emission of clusters from LMIS to much higher mass ranges (>2000 amu) than commercially available mass filters for focused ion beam systems. It has also been shown that for small masses the composition of clusters from different isotopes can be resolved. Furthermore, a rather high fluence of monodisperse clusters in the range of 10(6)-10(7) clusters/s can be achieved with this setup. This makes it a promising tool for the preparation of mass selected clusters. In this contribution, theoretical considerations as well as technical details and the results of first measurements are presented.
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    ABSTRACT: By using an isotopically pure liquid metal ion source (LMIS) of gallium on a tungsten needle it has proven possible to determine the full width at half‐maximum (FWHM) energy spread of Ga clusters up to Ga+4 as a function of both emission current and emission angle. A surprising result was that the FWHM of the dominant Ga+ peak remained constant with emission angle, while that of the cluster ions declined markedly. This parallels the behavior of the current associated with each Ga cluster species—the decline of each cluster ion species with angle is much faster than for either of the two monomeric species, Ga+ or Ga++, indicating a common mechanism for the formation of the latter two species. It is tempting to attribute the rapid falloff with emission angle of the cluster ions amounts and FWHM to a droplet origin for these species. Droplet emission is know to be strongly concentrated in the axial region of the LMIS.
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