Thin Mo Films Deposited and Analyzed Using Sub-Kev Noble Gas Ions
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Fig. 1. Sketch of the experimental setup, showing the sample positions during Ar+-IBAD film deposition of Mo (0), He+ defect decoration ((0)), and thermal He desorption ((3)).
587 Mat. Res. Soc. Symp. Proc. Vol. 396 ©1996 Materials Research Society
was found. Films with a nominal thickness d between 5 and 1000 A were investigated. A 3 cm diameter Kaufman ion source is used for Ar+ ion assistance during deposition (normal incidence). We used ion energies Ei between 25 and 250 eV, and the ion-to-atom arrival ratio 7 was varied between 0.008 and 0.083. In some cases the sample was given a thermal anneal immediately after the deposition (position 3). The second stage of the experiment involves the decoration of the point defects in the film with He as probe particles. These are brought into the material at room temperature using a mass-filtered 100 eV He' ion beam (20' off-normal incidence) and the sample, biased at -10 V, in position 2. Fluences ý were varied between 2.5 x 1013 and 1 x 1015 He÷/cm 2 . After being trapped by defects, the He probe particles are next thermally excited to dissociate from the traps. This is done by heating the sample linearly with time (position 3), using a 2.5 keV electron beam. During the temperature ramp the He desorption flux L is monitored as a function of the sample temperature T, using a quadrupole mass spectrometer (QMS). The heating rate 13was 40 K/s for all cases presented here. For optimum background suppression, an 'empty' QMS sideband-signal (mass=6) and the helium signal (mass=4) are measured altematingly (50 ms each), and the former is subtracted from the latter. The L vs. T data form the essential result of an experimental run. Such a 'thermal desorption spectrum' is a collection of peaks, each signifying the dissociation of He at a particular energy, i.e. the dissociation of He from a particular type of defect in the material. The area of a peak is a measure for the concentration of the type of defect associated with it. Immediately after the heating scan up to T = 2000 K ( 0.7 Tn), the sample undergoes free cooling to room temperature. The last 200 K take about 10 minutes. Thereafter, the sample is translated back to position 1 and a new Mo layer is deposited on top of the existing ones. It was periodically confirmed that the deposited layer was fully annealed and free of point defects after the heating run, and thus that the accumulating stack of layers kept functioning as a reproducible substrate. All data presented have been corrected for ion backscattering, secondary electron yield, ion charge exchange with neutrals, and mean He residence time in the desorption volume. RESULTS Desorption spectra are shown in figs. 2-4. Unless otherwise indicated, d = 50 A, • = 2.5 x 1014 He÷/cm 2 and, if Ar÷ ion assistance was provided, y = 0.037 and Ei = 100 eV. The 100 eV He' probing energy is so low that even in a head-on collision a Mo atom cannot receive enough energy to be displaced in the bulk lattice (34 eV). TRIM 5 calculations indicate that the mean projected range of 100
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