Study of sputtering on thin films due to ionic implantations

1. Fig. 01: SRIM Simulation of implantion of Xe into Ti + Si Fig. 03: The physical phenomena of Sputtering with Xe beam into Ti + Si Fig. 02: RUMP spectrum for Ti over Si and the prediction of Ar and Xe ions on the surface Fig-04: The RBS Analysis Method Study of sputtering on thin films due to ionic implantations F. C. Ceoni, M. A. Rizzutto, M. H. Tabacniks, N. Added, M. A. P. Carmignotto, C.C.P. Nunes, W.G.P.Engel, A. O. Delgado, A. R Lima IFUSP Instituto de Física da Universidade de São Paulo Research`s goal: The Sputtering is one of the possible interactions due to incident beam on the targetrelated to the removal of atoms from the superficial layers of a given material. In the present work we are investigating the sputtering process studying its effects on thin films irradiated with low energies (10-30 keV) ion beams. Sample preparation Samples were made of monoelementar metallic films as Ti, Cu, Ag, Au over a Si or C substrate. The thickness of the films were planned using RUMP and SRIM simulations (fig 1–2). The simulations check all possible interactions of incident beam (range, ion position on the surface, etc.). The final thickness of these films were measured using RBS (Rutherford Back-Scattering) and compared with the values expected in the process of evaporation. Ion implantation process Usually Ar+, Ne+, Xe+ beams were used for implantation in our lab.For this analysis Xe ion beam were used at with 20 kV for the accelerating potential. The samples were placed into the sample holder and were irradiated in the same conditions used in the simulations. When the beam reaches the target, atoms of the film are ejected from the sample reducing the mass and consequently the superficial density of the film. Using RBS technique after the implantation is possible to determine the sputtering values. Rutherford Backscattering Spectroscopy (RBS) measurements Before and after the implantation, the film superficial density was measured using Rutherford Backscattering Spectroscopy (RBS), at LAMFI-IFUSP with 2.2 MeV He+ ion beam. RBS is a method for analysis and characterization of materials. Due to the low energy of the incident beam the only interaction possible is the elastic scattering. The detection of a small percentage of backscattered ions is done with a barrier detector obtaining a energy spectrum for these particles. Using the kinetic factor for each elements present in the target, it was possible to calculate the superficial density of all elements. In the RBS analysis is also possible to get the thickness of film and the depth profile for each element. Sputtering The first experiment to measure sputtering rate was performed using Si/Ti films (~2500Å) with 20KeV Xe+ ion beam for a total dose of 2.0x1015 ions/cm2 (fig. 05). After implantation the target were analyzed again by RBS method to obtain the sputtering rate value. The RBS spectra before and after implantation are shown in fig. 06. The data results from this second RBS analysis show the peak of the Xe ions in the surface. The Ti peak show the same area as can be seen in the overlapping spectra. It was not possible to determine the sputtering rate due to experimental uncertainties. Another experiment was carried out using a similar Si/Ti film as shown in fig 07 but now modifying experiment procedures. In the implantation process a mask was used to cover half of the sample. In this case the RBS analysis could be done in sequence for the implanted and non-implanted surface avoiding problems of variation of the analysis conditions. Fig. 05: Ti/Si Target before implantation Fig-06: Overlapping of RBS Spectrabefore (black) and after (red) Xe+ implantation Results Fig. 08 shows the RBS spectra for the implanted and non-implanted samples. In both analysis we can see that the Ti peak area is about the same and was not possible to measure the expecting reduction of superficial thickness for this element. The uncertainties due to the statistics and charge measurements do not allow quantify of this very small sputtering rate. In the next experiment we will implant samples with a higher dose of Xe (~ 2.0x1016 ions/cm2) to increase Ti sputtered atoms. Probably this will reduce significantly Ti superficial density (increasing the difference in the peak areas in RBS spectra). Fig. 07: Ti/Si Target with half area implantated Fig. 08: RBS Spectraoverlapping the measurement before (black) and after (red) Xe+ implantation