Epitaxial Al and Cu films grown on CaF 2 /Si(111)
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Epitaxial Al and Cu films grown on CaF2/Si(111) Yuriy V. Shusterman, Nikolai L. Yakovlev, Katharine Dovidenko* and Leo J. Schowalter Rensselaer Polytechnic Institute, Department of Physics, Applied Physics and Astronomy, Troy, NY 12180, U.S.A. * UAlbany Institute for Materials, University at Albany SUNY, Albany, NY 12203, U.S.A. ABSTRACT The ability to grow single-crystalline Al and Cu films is of significance for several areas of materials research, such as the resistivity size effect in thin metal films, electromigration failure of interconnects, and magneto-resistance studies. Here, we explore the microstructure and resistivity of thin Al and Cu films grown on CaF2/Si(111). A three-step technique of CaF2 growth is described that permits deposition under imperfect vacuum conditions and promotes smoothness of subsequent thin metal films. Reflection high-energy electron diffraction shows that epitaxial Al(111) is obtained directly on CaF2, while epitaxial Cu(111) is obtained only by growing on a 1 nm thick Al seed layer pre-deposited on CaF2. Transmission electron microscopy reveals that 75 nm thick Al films have 150 nm wide sub-grains misoriented by less than 1 degree. For 75 nm thick Cu, the grains are only 30 nm wide and are misoriented by as much as 10 degrees. Room temperature resistivity measurements of the 10-300 nm thick Al films agree with the Fuchs-Sondheimer model in which conduction electrons scatter totally diffusely at the film interfaces. For 50-1000 nm thick Cu films, the resistivity size effect is substantially greater than the prediction of this model, which may be explained in terms of grain boundary scattering. INTRODUCTION In the next decade, the width of some metal lines inside computer chips will decrease to approximately 50 nm, which is comparable to the mean free path of conduction electrons in both Al and Cu at room temperature. At those small dimensions, electron scattering from metal surfaces will play an important role in electron transport. The well-established way to characterize surface scattering is to study the electrical properties of thin metal films. To the best of our knowledge, all previous work related to Al and Cu has been done on polycrystalline films deposited on SiO2 [1,2]. However, in such cases, the scattering at grain boundaries may be dominant. To avoid this masking effect, it is desirable to work with single-crystal films. The epitaxial CaF2 on Si is a convenient choice for a single-crystal insulating substrate [3]. Epitaxy has been reported previously for both Al and Cu on CaF2(111) [4,5], but the initial stages of their growth have not been addressed. In this work, we have studied thin Al and Cu films grown on CaF2/Si(111) by molecular beam epitaxy (MBE). The samples were characterized by reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), and transmission electron microscopy (TEM). We have also measured the resistivity dependence on film thickness for both Al and Cu, fitted these data to the Fuchs-Sondheimer size effect model, and el
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