Delivered by Ingenta to: Rensselaer Polytechnic Institute IP : 128.113.7.137 Wed, 13 Jun 2007 13:56:06 Copyright © 2007 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 7, 2192–2196, 2007 PreferredOrientationinRuNanocolumns InducedbyResidualOxygen J. P. Singh 1 ∗ , T. Karabacak 2 , P. Morrow 2 , S. Pimanpang 2 , T.-M. Lu 2 , and G.-C. Wang 2 1 Physics Department, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India 2 Department of Physics, Applied Physics and Astronomy Rensselaer Polytechnic Institute, Troy, New York 12180-3590, USA Ru nanocolumns were grown on a native oxide covered Si(100) substrate using an oblique angle sputter deposition technique with substrate rotation at room temperature. Scanning tunneling microscopy images of conventional Ru film show the presence of straight columnar features on the film surface, which are very different from the nearly circular features observed on the nanocolumns surface. X-ray diffraction spectra confirm that these nanocolumns have (100) as the preferred ori- entation instead of the (002) orientation observed for a conventional film. The oxygen to Ru atomic ratio was determined for both the nanocolumns and the conventional film by using X-ray photo- electron spectroscopy. The nanocolumns were observed to incorporate about 6 times more oxygen than the conventional film near the surface region. We argue that the oxygen segregates onto the high-density (002) plane whereas it permeates through comparatively open planes like (100) and (101). The adsorbed oxygen atoms serve as a diffusion barrier for the landing Ru adatoms and inhibit the growth of the (002) plane. This results in the absence of the (002) plane and development of (100) and (101) planes in the nanocolumns. The oxygen plays a decisive role in determining the crystallographic orientation and the feature size/shape over the nanocolumns and conventional film surfaces. Keywords: Ruthenium, Oblique Angle Deposition, dc Magnetron Sputtering, Scanning Tunneling Microscopy, X-ray Photoelectron Spectroscopy. 1. INTRODUCTION Crystalline Ruthenium (Ru) is a noble metal with a hexag- onal close packed (hcp) structure. Among various con- tact electrodes, Ru is rather appealing for microelectronic applications because its oxide serves as a barrier to diffu- sion, but is electrically conducting. 1–2 Another application of Ru is in the capacitor electrodes. 3 An understanding of the detailed structure and surface morphology of Ru nanowires 4 is of great importance for nanoscale inter- connect technology. In general, during the growth of a polycrystalline film, the crystallographic axis is aligned along a preferred direc- tion but the orientation is random in the plane of the sub- strate. The preferred direction is normal to the substrate with the densest crystallographic planes parallel to the sub- strate, for example, the (111), (110), and (001) planes for the fcc, bcc, and hcp crystals, respectively. 5 The initial ori- entation is driven by the crystal’s tendency to approach a minimum surface energy configuration, but the final pre- ferred orientation is influenced by various factors such as ∗ Author to whom correspondence should be addressed. substrate temperature, vapor flux direction, sticking coef- ficient, re-emission, surface diffusion of landing atoms on the substrate, and residual gas in the vacuum chamber. For example, the (111) texture for sputtered Cu films has been observed to change to the (100) texture after 40 hours at room temperature. 6 In addition, oxygen induced pre- ferred orientation has been observed in many metals. 7 8 For example, the orientation of Pt film deposited on a glass substrate can be changed from the (111) to the (200) with the presence of oxygen during the deposition. 7 In this letter, we show that the Ru nanocolumns have the (100) preferred orientation instead of the (002) orien- tation observed for a conventional film. These Ru nano- columns were grown by an oblique angle deposition (OAD) technique. 9–10 The OAD allows a single step fab- rication of nanocolumns and other desired geometries. It is a physical vapor deposition in which flux arrives at a large oblique incidence angle (>80  ) from the substrate normal while the substrate is rotating. This results in the formation of isolated nanocolumns by the self-shadowing mechanisms. Our Ru nanocolumns were deposited on native oxide covered p-Si(100) (resistivity 12–25 -cm) 2192 J. Nanosci. Nanotechnol. 2007, Vol. 7, No. 6 1533-4880/2007/7/2192/005 doi:10.1166/jnn.2007.793