Fabrication and characterization of wafer-level gold thermocompression bonding

Abstract

Packaging is an important aspect of microelectromechanical systems (MEMS) design. As MEMS devices traverse multiple energy domains, sometimes operating in hostile conditions, the need to maintain reliability and functionality makes packaging a challenging problem. Often, the package needs to be specially designed for each device. Given the typically low volume productions, the packaging cost can often exceed the device cost. One way to lower that cost is to package at the wafer-level. This thesis explores a low temperature wafer bonding technique: thermocompression bonding. This technique relies on the applied pressure and temperature to forge a bond. The pressure brings two surfaces into close proximity while the temperature reduces the pressure requirement to deform the surface asperities. In this work, gold thin film was used to bond two silicon substrates. The thesis discusses the fabrication process, its associated challenges, and provides guidelines to achievesuccessful bonding. Characterization of the process focused mainly on the effects of bonding temperature (260 to 300° C), pressure (1.25 to 120 MPa) and time (2 to 90 min). The resultant bond was quantified using a four-point bend-delamination technique. High bond toughness was obtained and the bond quality was found to improve with increases in the bond temperature and pressure. However, non-uniform bonding was observed. Using finite element analysis, correlation between the mask layout and non-uniform pressure distribution was found. The four-point bend-delamination technique was also evaluated for its effectiveness in measuring high toughness bonds. Non-ideality in the load-displacement behavior were observed due to the variation in the bond toughness. A cohesive zone model was used to model the fracture process. The finite element results showed qualitative agreement with experimental data. The results also indicated that the technique is not well suited for bonds with large variations in bond toughness.