AVS 52nd International Symposium
    Applied Surface Science Wednesday Sessions
       Session AS-WeM

Paper AS-WeM1
TOF-SIMS: Accurate Mass Scale Calibration

Wednesday, November 2, 2005, 8:20 am, Room 206

Session: Essential Tools for Surface Analysis
Presenter: I.S. Gilmore, National Physical Laboratory, UK
Authors: F.M. Green, National Physical Laboratory, UK
I.S. Gilmore, National Physical Laboratory, UK
M.P. Seah, National Physical Laboratory, UK
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A study is presented of the factors affecting the calibration of the mass scale for TOF-SIMS. At the present time, analysts achieve a mass accuracy of only 150 ppm for large molecules (647 amu) and for smaller fragments of < 200 amu may typically be 60 ppm. The instrumental stability is 1 ppm and better than 10 ppm is necessary for unique identification of species. In the recent interlaboratory study@footnote 1@ only 3 instruments out of 32 were within 10 ppm for smaller fragments. The experimental uncertainty can lead to unnecessary confusion where peaks are wrongly identified or are ambiguous. Here we study in detail the instrumental parameters of a popular reflection TOF-SIMS. The effect of the ion kinetic energy, emission angle and other physical and instrumental operating parameters on the measured peak position are determined. This clearly shows why molecular and atomic ions have different mass accuracy and peak shapes. These data provide the basis for a coherent procedure for optimising the instrumental settings for accurate mass calibration and rules by which inorganics and organics may be incorporated. This leads to generic sets of ions for mass calibration, which, used for re-calibration, improves the mass accuracy of the interlaboratory study data@footnote 1@ by up to a factor of 4. Now, 12 instruments out of 32 are within the 10 ppm range. The effects of extrapolation beyond the calibration range are discussed and a recommended procedure given to ensure that accurate mass of large molecules is achieved within a selected uncertainty. One bonus of this study is that the instrument can now be operated in a regime with good energy discrimination to study the fragmented energies of molecules. We shall discuss how this compares with data from G-SIMS@footnote 2@ and supports the G-SIMS concept. @FootnoteText@ @footnote 1@ I S Gilmore, M P Seah and F M Green, submitted Surf. Interface Anal.@footnote 2@ I S Gilmore and M P Seah, Appl. Surf. Sci., 161 (2000), p. 465.