Depth Dependence and Chemical Effects in Ion Mixing of Ni on SiO 2
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DEPTH DEPENDENCE AND CHEMICAL EFFECTS IN ION MIXING OF Ni ON SiO2 T. C. BANWELL AND M.-A. NICOLET California Institute of Technology,
Pasadena,
California
91125
ABSTRACT We report on our studies of Ni transport induced by 290 keV Xe irradiation of thin Ni films eygporfged on 2 thermally grown SiO2 at Xe fluences of 10 -10 cm and at temperatures of 77-750 K during irradiation. A simple etching technique was used to remove the free Ni leaving the SiO2 layer with incorporated Xe and Ni, whose profiles are 2 + directly measured using 2 MeV He backscattering spectrometry. Marker experiments are used to verify the selectivity of the etching procedure. An apparent discontinuity in the Ni concentration across the Ni-SiO2 interface may produce the high selectivity observed with2n our etching process. Features associated with both cascade mixing and recoil implantation are readily discernible in the residual metal profiles. An exponential tail is evident beyond u 500 A of the SiO and is insensitive to temperature. Within 500 0 2 A of the surface, the Ni profile demonstrates a strong temperature dependence which affects both the cascade mixing and recoil implantation processes. These profiles show favorable agreement with theory for samples implanted at temperatures of < 300 K, while deviations in the high temperature behavior suggest a way chemical effects may alter the collision processes. INTRODUCTION Atomic transport induced by heavy ion irradiation is generally attributed to two principle mechanisms; cascade mixing (and enhanced diffusion), hereafter called diffusional mixing, is associated with numerous short range displacements that may be influenced by thermally activated processes, and recoil implantation which produces long range displacements that are insensitive to t mperature [11. Diffusional mixing has a characteristic parabolic ( O-- -) incident ion fluence dependence [2,3]. Recoil implantation has a characteristic linear (q) fluence dependence and is significant in bilayer samples where large concentration gradients exist. Theoretical investigations indicate that the profile of primary recoils from a surface layer can be described far from the interface by MBe x, where x is the normal distance from the bilayer interface and B,4 are constants (4,51. Chemical effects may influence these processes, particularly when large gradients in chemical potential exist. We study Ni transport induced by 290 keV Xe irradiation of thin Ni films on SiO . We are interested in the role of the Ni-SiO2 interface and 2 chemistry on the ion induced mixing. EXPERIMENTAL Substrates with a u 6500 A SiO layer were prepared by 11001C steam 0 oxidation of polished Si wafers.2 250 A Ni layers were deposited on the SýO by e-beam evaporation in an oil-free vacuum system at pressures of 4 < 10 orr. Xenon implantations were made at sample temperatures of 77, 300 and 750-770 K withl?90 kaV Xe (R = 390 A, AR = 140 A in Ni) [61 to a fluenje i of 1-15 x 10 cm . The gamples were Eubsequently examined by 2 MeV He+ ion backscattering spectrometry
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