The Temperature Dependence of Energy Spread and Mass Spectra of an Au73Ge27 Liquid Metal Alloy Ion Source


The Temperature Dependence of Energy Spread and Mass Spectra of an Au73Ge27 Liquid Metal Alloy Ion Source

Pilz, W.; Bischoff, L.; Mair, G. L. R.; Akhmadaliev, C.; Ganetsos, T.

Focused ion beams from liquid metal ion sources (LMISs) have become of increasing interest in recent years. A promising source is one of Ge ions, important in the semiconductor technology; e.g., local doping of SixGe1-x structures. The beam energy spread, ΔE½, of the source - which, through the chromatic aberrations of the system determines the final spot size at the target - and the peak energy deficit (PED), as well as the mass spectra of a Au73Ge27 eutectic liquid metal alloy ion source were investigated. Source temperature was the main experimental parameter. The melting point of the eutectic was determined as 366oC. ΔE½, measured with a retarding field analyser, is expressed as the full width of the distribution at half its maximum height (FWHM). Doubly-charged ions attain twice the energy of singly-charged ions, for a given voltage drop, and this has to be taken into account when determining the value of ΔE½ and the PED. Ge was found to be much more abundant in the doubly-charged state, in accordance with Brandon`s criterion [1]. From PED measurements we conclude that Ge++ as well as Au+ and Ge+ are directly field-evaporated, whereas Au++ is post-ionized from the singly-charged state. As expected for liquid metals that display a linear decrease of their surface tension coefficient with temperature, the liquid metal ion source under investigation shows an increase in emission current with increasing temperature. This is in contrast to Si containing metallic glass alloys, e.g., AuSi or AuGeSi, that have an anomalous surface tension coefficient [2]. The mass spectra, where obtained with an ExB filter. Alongside the main monomer species, cluster ions, the result of droplet disintegration, were identified. The conclusions drawn from a study of the ratio of the abundance of doubly to singly-charged monomer ions, support our deductions from PED measurements, as far as the mechanisms of creation of the monomer ions are concerned.
[1] D.G. Brandon, Surf. Sci. 3 (1964) 1.
[2] L. Bischoff and G.L.R. Mair, Recent Res. Devel. Appl. Phys. 6 (2003) 123.

Keywords: AuGe alloy liquid metal ion source; mass spectra; energy spread

  • Lecture (Conference)
    49th International Field Emission Symposium (IFES 04)July 12 - 15, 2004, Seggau Castle, Graz, Austria
  • Contribution to proceedings
    49th International Field Emmissions Symposium IFES 04, 12.-15.07.2004, Seggau, Österreich

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