Solid-liquid interdiffusion bonding for MEMS device integration

Microelectromechanical systems (MEMS) have been widely utilized in many developed applications including safety critical systems in automotive and aerospace. Several MEMS devices require controlled environment for operation and protection against volatile compounds i.e., hermetic sealing. Heretofore, this has been attained typically with anodic or glass-frit bonding methods. However, metal bonding offers several benefits compared to these, such as, possibility to combine easily hermetic sealing and creation of electrical interconnections, enhanced device performance and smaller footprint. Solid-liquid interdiffusion (SLID) bonding is one of the developed low bonding temperature methods, which is studied in this thesis by utilizing copper-tin, gold-tin and zinc-aluminum based metallurgical systems by investigating their thermodynamic-kinetic behavior as well as reliability performance. Interfacial reactions are explained based on the microstructural analysis and the mechanical reliability is evaluated with shear and tensile tests. In addition, bonds are subjected to different stresses by utilizing standard environmental tests, such as high temperature storage, thermal shock and mixed flow gas tests. This thesis presents SLID bonds having high mechanical strength and robustness in the environmental tests. The effect of voiding level in the Cu3Sn phase on the cracking propensity is reported, and the voiding level is observed to link to copper electroplating bath condition. Zinc-aluminum based alloys are examined as a cost-effective material system for metal bonding. Interfacial reactions between these alloys and common base metals are investigated and rapid intermetallic compound formation is observed in a soldering procedure that simulates a wafer bonding process. Furthermore, platinum-based contact metallization stacks are presented for Cu-Sn and Au-Sn bond metallizations. These contact metallizations are CMOS-compatible, and thus, applicable easily on e.g., MEMS device or application specific integrated circuit (ASIC) wafer in order to simplify the process integration and to increase the device performance. In high strength Au-Sn/Pt SLID bonds, the thermodynamic data is utilized to reason the rise of the re-melting temperature as a function of original platinum layer thickness. In addition, the difference in reliability performance is connected to failure mode analysis. In case of Cu-Sn/Pt bond, platinum is observed to participate into IMC formation reactions at Cu/Sn interface during soldering and to stabilize the high temperature hexagonal crystal structure of the Cu6Sn5 phase to room temperature. The thermodynamic data and reliability evaluation related to SLID bonding provided in this thesis can be directly utilized in electronics industry in 0- and higher-level integration design.