Limits of ion-beam depth-profiling as used in diffusion studies of oxidation-sensitive materials

Ion-beam depth-profiling of an oxidation-sensitive amorphous Zr-Ti-Cu-Ni-Be alloy in comparison to pure crystalline Co is studied using a radiotracer-technique employing the isotopes Co-57 and Co-60. Samples covered by a surface monolayer of both radiotracers were prepared and serial-sectioned by sputtering under different vacuum conditions to determine the depth resolution function of Co. In the case of the amorphous alloy the integral sputtering rate and the depth resolution function depend strongly on the base pressure. At high base pressure, broadening of the resolution function and a local minimum at medium depths is observed. The broadening can be explained by an increasing sputtering rate due to elimination of reactive residual gas during sputtering and by segragation of oxidation-sensitive constituents towards the sample surface. In the case of less oxidation-sensitive Co the influence of the base pressure can be neglected. For both materials a strong mass dependence of the resolution function due to preferential sputtering of the lighter isotope is observed. In conclusion, for precise measurements of diffusion in oxidation-sensitive materials by means of ion-beam depth profiling an average penetration depth of more than 100 nm and a base pressure below 10(-7) mbar are desirable.