Characterization of thin films and intermetallic compounds in solder joint

Numerical simulation such as finite element methods have been widespread in structure designs and analyses in IC industry and researches. Only if the inputs of material properties are correct, can the outcome be referable to its applications. Thin-film materials have been widely used in many applications. The mechanical properties of thin films may be markedly distinct with different processes and processing conditions. To study the mechanical behvoir of thin films, thin films on substrates are adopted most often [1]. The most commonly used material for substrate is silicon, which has low coefficient of thermal expansion (CTE) and is a prevalent semiconductor material. The theoretical values of Young's moduli of silicon were usually applied. However, the values might be different in practice. Before further study of thin-films, the actual mechanical properties of silicon substrates should be cautiously investigated first. Intermetallic compounds (IMC) are formed when interconnections in IC packages are jointed with solder. Though IMC just comes into a small amount, it usually dominates the reliability of the interconnections because of its characteristic material properties. Individual mechanical properties of a solder/IMC/UBM layered structure are required for simulation and analyses of the interconnection reliability, but always difficult to obtain because the thickness of each layer is on micrometer order. Further, it is impossible to separate each thin layer and apply commercially available testing machines to test it. In this extended abstract, we present results and methods for obtaining mechanical properties of silicon substrates and IMC formed at the interface between lead-free solders and the copper substrate. To determine the mechanical properties of the silicon substrate, 255 um (100) and 306 um (110) thick silicon wafers with double side ground were adopted. In the original crystallographic orientation, there are three independent elastic constants for silicon, whose symmetry belongs to cubic class. After the transformation to the orientation of (100) wafers, the in-plane Young's moduli of (100) wafers calculated based on crystallographic theory are E〈100〉 = 130.0 GPa, E〈110〉 = E〈-110〉 = 168.9 GPa. The values of Young's moduli repeat every π/2 (rad). The different orientations of specimens cut form (100) and (110) wafers are shown in figure 1. Each specimen was cut into 5.5 cm × 1 cm. An ANSYS finite element model was used to examine whether the Euler beam equation is suitable for such specification with anisotropic material properties. The result shows that with proper strain gage locations, the in-plane Young's moduli could be successfully obtained with Euler beam equation, which were E〈100〉 = 101.6 GPa, E〈110〉=140.7 GPa, E〈-110〉= 140.5 GPa. The measurement agreed with the trend of theoretical values in each orientation. The results of 〈110〉 and 〈-110〉 directions only had 0.2% difference. The effect of defects became more obvious in such thin thickness of silicon wafer and made the Young's moduli smaller than the theoretical values. On the other hand, the theoretical Young's moduli of (110) wafers repeat every π (rad). The in-plane Young's moduli of (110) wafer were determined to be E〈-111〉= 170.1 GPa, E〈1-12〉=157.9 GPa with the same method, which were also smaller but closer to the theoretical values because of its thicker thickness. The theoretical and measured data with the corresponding deviation of both (100) and (110) wafers are listed in table 1. Use of the anisotropic (110) wafer aims to make characterization of material properties of the deposited thin film successful because it has different curvatures in each orthogonal directions which can offer more information to determine the thin film material properties.