Wafer-Level Thermocompression Bonds

Thermocompression bonding of gold is a promising technique for achieving low temperature, wafer-level bonding without the application of an electric field or complicated pre-bond cleaning procedure. The presence of a ductile layer influences the fracture behavior of the bonds. The fabrication process was described. In addition, the effect of plasticity was explored by varying the gold bonding thickness between 0.23 to 1.4 µm. Wafers were bonded at 300°C and two different pressures: 1.25 and 7 MPa. The bond toughness of the specimens were characterized using a four-point bend delamination technique. Cohesive failure was found to be the dominant fracture mode in the thicker films. Bonds made with thin gold films failed adhesively and at lower strain energy release rates.Singapore-MIT Alliance (SMA

Gold Thermocompression Wafer Bonding

Thermocompression bonding of gold is a promising technique for the fabrication and packaging microelectronic and MEMS devices. The use of a gold interlayer and moderate temperatures and pressures results in a hermetic, electrically conductive bond. This paper documents work conducted to model the effect of patterning in causing pressure non-uniformities across the wafer and its effect on the subsequent fracture response. A finite element model was created that revealed pattern-dependent local pressure variations of more than a factor of three. This variation is consistent with experimental observations of bond quality across individual wafers A cohesive zone model was used to investigate the resulting effect of non-uniform bond quality on the fracture behavior. A good, qualitative agreement was obtained with experimental observations of the load-displacement response of bonds in fracture tests.Singapore-MIT Alliance (SMA

Fabrication and characterization of wafer-level gold thermocompression bonding

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003.Includes bibliographical references (p. 135-140).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.by Christine H. Tsau.Ph.D