IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 21 (2009) 314016 (7pp) doi:10.1088/0953-8984/21/31/314016 Palladium diffusion into bulk copper via the (100) surface E Bussmann 1 , J Sun 2,3 , K Pohl 2 and G L Kellogg 1 1 Sandia National Laboratories, Albuquerque, NM 87185, USA 2 Department of Physics and Materials Science Program, University of New Hampshire, Durham, NH 03824, USA Received 31 December 2008, in final form 2 April 2009 Published 7 July 2009 Online at stacks.iop.org/JPhysCM/21/314016 Abstract Using low-energy electron microscopy, we measure the diffusion of Pd into bulk Cu at the Cu(100) surface. Interdiffusion is tracked by measuring the dissolution of the Cu(100)–c(2 × 2)-Pd surface alloy during annealing (T > 240 ◦ C). The activation barrier for Pd diffusion from the surface alloy into the bulk is determined to be (1.8 ± 0.6) eV. During annealing, we observe the growth of a new layer of Cu near step edges. Under this new Cu layer, dilute Pd remaining near the surface develops a layered structure similar to the Cu 3 Pd L 1 2 bulk alloy phase. (Some figures in this article are in colour only in the electronic version) 1. Introduction Binary alloy thin films of Pd and Cu have attractive properties for a variety of technological applications [1, 2]. Owing to Pd’s ability to dissociatively adsorb H 2 , Pd–Cu alloy thin films are useful in catalysis, e.g. to promote water–gas shift reactions as in hydrogen purification [2]. In microelectronics, Pd–Cu alloys may be used to make components, e.g. interconnects, less susceptible to electromigration damage [3–5]. In such applications, ultrathin Pd–Cu alloy films are of interest, e.g. as electromigration-resistant surface coatings or gas-permeable membranes. At the Cu(100) surface, submonolayer coverages of Pd form surface or interfacial alloys, involving only the two or three outermost atomic planes [1, 6–13]. Besides providing a model system to study the growth of alloy thin films, we have found that the surface alloy slows the surface diffusion process [5], which limits the rate of electromigration in sub-micrometer-wide Cu wires [3]. Since Pd and Cu are bulk miscible, the surface alloy is unstable at temperatures sufficient for Pd interdiffusion into bulk Cu. Previous studies of the interdiffusion of Pd into Cu have explored the process on macroscopic length scales with ‘cook-and-look’ techniques [14–17]. To our knowledge, previous studies have not examined the stability of the surface alloy and the early stages of the interdiffusion process. In this work, we use low-energy electron microscopy (LEEM) to characterize the 3 Present address: IBM Research Division, T J Watson Research Center, Yorktown Heights, NY 10598, USA. temperature-dependent dissolution of the surface alloy into the Cu bulk. The LEEM enables us to observe the interdiffusion and the evolution of the near-surface structure at the nanometer scale in real-time. In the temperature range of our experiments (T > 150 ◦ C), submonolayer coverages of Pd form a buried surface alloy at the Cu(100) surface [1, 6–13]. The structure and growth of the buried surface alloy are well understood [6–13]. On terraces, figure 1(a), the buried alloy consists of a c(2 × 2)- ordered Pd–Cu underlayer covered by a monolayer of nearly pure Cu [11–13]. Near step edges, Hannon et al found that some Pd is also present in the third atomic layer, as shown in figure 1(b). Hannon et al explained that this structure originates from step flow during the growth of the alloy [12]. As Pd adsorbed onto the terrace is incorporated into the second atomic layer, Cu is displaced to the surface. The displaced Cu migrates to nearby steps, causing the steps to advance. The advancing steps grow over the buried alloy on the terrace, so that some Pd then resides in the third atomic layer. On the upper side of the step, arriving Pd continues to be incorporated into the second layer as well, leading to Pd in both the second and third layers. The buried surface alloy is intrinsically thermally metastable; increasing Pd–Cu coordination lowers the configurational energy of the system [12, 13] and mixing of Pd into the Cu bulk is favored entropically. In this work, we use LEEM to characterize this inherent thermal instability by directly imaging the dissolution of the buried surface alloy 0953-8984/09/314016+07$30.00 © 2009 IOP Publishing Ltd Printed in the UK 1