The Use of 2-Mev He + Microbeams for Ion-Beam Mixing of Bi-Layer Systems
- PDF / 370,541 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 95 Downloads / 156 Views
THE USE OF 2-MEV HE+ MICROBEAMS FOR ION-BEAM MIXING OF BI-LAYER SYSTEMS
JOHN B. DAVIS AND R.E. BENENSON Dept. of Physics, SUNY at Albany,
Albany,
NY 12222
ABSTRACT We have observed ion-beam mixing in three bi-layer targets, Sb/Si, Cu/Si and Cu/Al, using 2-MeV He+ beams of 4 - 9 um width. Detailed investigation of Cu/Al revealed that 1) the depth of the mixed layer exhibits a linear dependence on dose, 2 equivalent to 80 A for every 1018 He* ions/cm ; 2) there is evidence of lateral diffusion of the mixed atoms away from the line of application of the beam; and 3) the sputtering coefficient for 2.0-MeV He* on Cu is 0.07 sputtered atoms per ion. INTRODUCTION The chief advantage of using helium for ion-beam mixing is that the same beam can be employed simultaneously for both mixing and RBS analysis. Also, sputtering is significantly less than for ions of heavier noble gases, although the ratio of mixing to sputtering may not be any better. In order to compensate for the low mixing efficiency of helium, a microbeam system 1 can be used to focus the ion beam to a width of less than 10 um. With typical beam conditions, ' a current of 45 2 pA and a beam-width of 5-um, a dose of 10 ions/cm can be delivered to the target in about two hours. Despite the high dose, the power absorbed by the target at this flux is only 90 uWatts, and we have not observed any evidence of beam-heating of the samples. The three bi-layer systems we examined are known to mix using beams of heavier ions than helium, or have potential technical applications. We selected the Cu/Al system for detailed study because of its ease of preparation, the fact that these metals readily form alloys, and the importance of such alloys for interconnects in silicon technology. EXPERIMENTAL METHOD All targets were prepared by evaporation of a metallic film on a thick substrate. In the case of Sb/Si and Cu/Si, the substrate was cut from a silicon wafer, then chemically etched with a 10% solution of HF acid. For the Cu/Al samples, the substrate was ordinary aluminum foil. The mean range for 2-MeV He+ in these samples is between 4.5 and 6.5 um. Hence, we abandoned the practice customarily used with heavy ions, in which the surface film thickness is made comparable to the mean ion range. However, in order to minimize the relative thickness loss due to sputtering, the surface films were kept as thick as possible, within the limits of maintaining adequate separation between the RBS signals of the substrate and overlayer. The film thicknesses were 5000 A Mat. Res. Soc. Symp. Proc. Vol. 201. c 1991 Materials Research Society
296
Cu on Al, 3500 A Sb on Si and 1900 A Cu on Si. The investigation proceeded in three stages, always using a 2-MeV He+ ion microbeam for irradiation and RBS analysis. First, Sb/Si and Cu/Si targets were irradiated to doses between 0.5 and 1.5 x 10 ions/cm , respectively, and their RBS spectra, taken in situ with the same beam, were recorded. Representative spectra are shown for Sb/Si and Cu/Si in Fig. 1, both before and after receiving the doses
Data Loading...
