AVS 49th International Symposium
    Applied Surface Science Tuesday Sessions
       Session AS-TuA

Paper AS-TuA1
Using Gold Liquid Metal Ion Sources for Imaging ToF-SIMS

Tuesday, November 5, 2002, 2:00 pm, Room C-106

Session: Imaging in Surface Analysis
Presenter: N. Winograd, Pennsylvania State University
Authors: A.V. Walker, Pennsylvania State University
N. Winograd, Pennsylvania State University
Correspondent: Click to Email
Molecule-specific imaging TOF-SIMS experiments require the highest possible sensitivity in order to achieve the spatial resolution normally associated with the ion probe. For example, Ga+ ion beams can be focused to less than 100 nm in diameter. However since there are only ~104 molecules per layer and Ga+ ions have a low ionization efficiency, the maximum possible count rate per pixel is extremely small. Previous studies have suggested that sources other than Ga+ produce molecular ions and fragments with higher efficiency, e.g. In+. Gold has also been shown to be an effective source, particularly since it is possible to generate Au++, Au+ and Au2+ beams. Yield enhancements of several orders of magnitude have been reported, although the source was not used in an imaging modality.@footnote 1@ Here, we examine the prospects for routine use of Au LMIG sources for TOF-SIMS imaging. We employ Au-Si or Au-Ge eutectics to allow the metal to be melted around the tip at a temperature low enough to prevent gold evaporation. Currently, the tips have a lifetime in excess of 1500 amp-hours. Mass selection is achieved using a double blanking system which allows pulsing and mass selection. The probe size for each of the three gold species is < 300 nm. The performance of this tip will be compared against In and Ga sources for a number of standard materials including Irganox, phenylalanine and several long chain alkanethiols adsorbed on gold. In general, we find the yield of Au is enhanced by up to an order of magnitude over Ga. Of special interest is the fact that Au++ ions exhibit the largest enhancement in many cases, and that Au2+ ions do not yield significant advantage over Au+ ions. @FootnoteText@ @footnote 1@H. H. Anderson, A. Brunelle, S. Della-Negra, J. Depauw, D. Jacquet, Y. LeBeyec, J. Chaumont, H. Bernas, Phys. Rev. Letts., 80 (1998) 5433; B. Hagenhoff, R. Kersing, D. Rading, S. Kayser, and E. Niehuis, SIMS XII, (1999) 833.