Nanoindention studies of DC sputtered Cu and Cu/Cr thin films
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Nanoindention studies of DC sputtered Cu and Cu/Cr thin films
G. Wei, J. Du, A. Rar and J. A. Barnard Department of Metallurgical and Materials Engineering and Center for Materials for InformationTechnology, The University of Alabama, Tuscaloosa, AL 35487-0209, U.S.A. ABSTRACT The nanoindentation behavior of DC magnetron sputtered 10 nm Cu and 10 nm Cu/2 nm Cr thin films deposited on Si (100) has been studied using a Hysitron nanomechanical system. Xray diffraction and X-ray reflectivity were used to measure the film structure and film thickness, respectively. The grain size and orientation of Cu and Cu/Cr thin films were measured by TEM. Atomic force microscopy (AFM) was used to evaluate the surface morphology and roughness. At the same load, the nanoindentaion displacement of Cu/Cr is smaller than that for Cu, i.e., the 2nm thick Cr underlayer enhances the hardness of Cu. X-ray, TEM, and AFM results show that the grain size of Cu/Cr (< 15 nm) is actually larger than Cu (~ 3 nm) indicating that the inverse Hall-Petch relationship may be operative. KEYWORDS: Nanoindention, Grain Size, Hall-Petch, Transmission Electron Microscopy INTRODUCTION Cr thin films have been widely used as underlayers (or interlayers) to improve the adhesion, microstructure, crystallographic texture, etc., of subsequently grown films [1-3]. However, relatively little is known about the effect of the Cr layer on the nanomechanical properties of the overlayer in the ultra-thin film regime. The mechanical behavior of materials with nanometersized grains is still a matter of controversy. While several studies have shown that the Hall-Petch relationship is exhibited down to the smallest grain size [4, 5], others have reported a transition from regular to abnormal Hall-Petch behavior [6, 7]. In this context, the effect of a Cr underlayer on the nanoindentation behavior of Cu thin films has been studied. The nanohardness, texture, microstructure, and topography of 10 nm Cu and 10 nm Cu/2 nm Cr thin films are compared. The inverse Hall-Petch relationship is observed in these materials. EXPERIMENTAL 10 nm Cu, 10 nm Cu/2 nm Cr and 20 nm Cr thin films were deposited on Si (100) substrates by DC magnetron sputtering. The base pressure of the sputtering system was 2.8 × 10-5 Pa (2.1 × 10-7 Torr). During sputtering, the power densities and deposition rates for Cu and Cr were 0.60 W/cm2, 0.6 nm/sec and 0.15 W/cm2, 0.1 nm/sec, respectively. The TEM (transmission electron microscopy) samples were prepared by sputtering the 10 nm Cu and 10 nm Cu/2 nm Cr thin films on SiO2 (~20 nm thick) supported TEM copper grids (from SPI supplies). The thickness of the films was measured by X-ray reflectivity. The nanoindention behavior of Cu and Cu/Cr thin films was measured at various loads (10 µN, 30 µN and 50 µN) using a Hysitron O5.7.1
nanomechanical system. The hardness of the 20 nm thick Cr reference film and the Si (100) were compared at low load (30 µN). The crystal structures of Cu and Cu/Cr thin films were measured by X-ray diffraction using a Rigaku D/Max-2BX
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