CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 37 (2012) 87–93 Contents lists available at SciVerse ScienceDirect CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry journal homepage: www.elsevier.com/locate/calphad Thermodynamic assessment of Au–Ho and Au–Tm binary systems H.Q. Dong a,* , X.M. Tao b , T. Laurila a , M. Paulasto-Kröckel a a Department of Electronics, School of Electrical Engineering, Aalto University, FIN-02601 Espoo, Finland b Key Laboratory of New Processing Technology for Nonferrous Metals and Materials of Ministry of Education, College of Physical Science and Technology, Nanning 530004, PR China article info Article history: Received 2 November 2011 Received in revised form 7 February 2012 Accepted 8 February 2012 Available online 8 March 2012 Keywords: Au–Ho Au–Tm CALPHAD Thermodynamic assessment ab initio calculations abstract Phase relationships in Au–Ho and Au–Tm binary systems have been thermodynamically assessed by using the CALPHAD technique. The existing thermodynamic descriptions of the systems were improved by incorporating the ab initio calculated enthalpies of formation of the intermetallic compounds (IMCs) to the assessment. All the binary intermetallic compounds were treated as stoichiometric phases, while the solution phases, including liquid, fcc, and hcp, were treated as substitutional solution phases. Furthermore, the excess Gibbs energies were formulated with the Redlich–Kister polynomial function. As a result, two self-consist thermodynamic data sets for describing the Au–Ho and Au–Tm binary systems have been obtained. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Bonding dissimilar materials is one of the essential steps in the fabrication of modern electronic devices. Prominent examples are integrated circuit (IC) and Micro-Electro-Mechanical Systems (MEMS) devices packaging. Typically, the reliability of MEMS devices packaging at the wafer-level depends on the bonding systems to a large extent. There are two primary types of generally used bonding technologies, namely, ‘‘dielectric bonding’’ and ‘‘metallic bonding’’ [1,2]. The latter one has distinguished advantages coming from the metal layer, which can add further functionality to a final 3D IC and MEMS devices, as the metal layer provides not only mechanical but also an electrical and a thermal connection [1,3]. Therefore, it is of utmost importance to obtain a better understanding of the thermodynamics of the related metallic bonding systems. The knowledge about the formation of intermetallic compounds is particularly important. It is well established that gold, as an element, has many favor- able physical and chemical properties (i.e. good thermal and elec- trical conductivity, excellent corrosion resistance). Consequently, Au and Au metallizations are widely applied as a surface finish with traditional solder materials, as the wire or the pad metal in wire bonding and as the base metal in wafer-level bonding, examples including such systems are Au–Sn and Au–In [4–7]. Rare earth (RE) metals, which can react with almost all the elements in the periodic table, have increasingly been used in * Corresponding author. E-mail address: hongqun.dong@aalto.fi (H.Q. Dong). MEMS packaging in order to promote bonding [8–10]. For this purpose, RE metals can be used as an alloying elements in Au–Sn- based and Sn–Ag-based solders. This, however, requires that there is sufficient solubility of the RE elements to the solder base metals used. In some cases RE materials are used as one of the bonding layers between two wafers, two chips or wafer/chip, in order to provide a hermetic, conductive bond with small dimensions [11]. Hence, to utilize the full technological potential of the RE ele- ments, it is essential to have a fundamental knowledge about the Au–RE systems, such as information about the phase equilibrium and the related thermodynamic data. Unfortunately, the phase di- agrams for Au–RE have not been systematically investigated, with the notable exceptions of the thermodynamic assessment of the Au–X 1 (X 1 = Pr, La and Er) [12,13] and the experimentally inves- tigated Au–X 2 systems (X 2 = Tm, Gd, Ho, Er and Tm) [14,15]. Both Ho and Tm exhibit solid state solubility to Au (fcc) phase (about 5 at%) thus making them potential candidates as alloying elements in Au-based solders. Since there also exists an experimentally mea- sured phase diagram with Au for both of the elements [14], they were chosen for closer investigation. Thus, the thermodynamic as- sessment of the Au–Ho and the Au–Tm binary systems are carried out in this work by combining ab initio techniques [16,17] with the CALPHAD approach [18]. Two self-consistent thermodynamic data sets were obtained for the Au–Ho and the Au–Tm systems, respec- tively. The paper is organized as follows: a brief review of previous work is presented in Section 2, all the adopted methods and mod- els are described in Section 3, the results and discussions can be found in Section 4, and conclusions drawn from the simulation are listed in Section 5. 0364-5916/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.calphad.2012.02.002