Study Of Active Bonding Mechanism Between Sn3.5Ag4Ti(Ce,Ga) And Non-metallic Substrates At Low Temperature

Wafer bonding technology has important application in the new-type SiP, 3D packages and MEMS fabrication. Traditional wafer bonding technology is usually conducted under the condition of high temperature which leads to the thermal stress problems that can cause devices to work unsteadily and with low reliability. Meanwhile, excessive high temperature still cause the functional component of wafer materials to diffuse again which makes the electrical characteristic become worse. As a consequence, low temperature bonding technology is increasingly valued and has become a key technology for fabricaiton of the new packages. Based on the combination of theory and experiment, this thesis systematically studied the soldering mechanism of wafer and ceramic substrates using Sn3.5Ag4 Ti alloy filler at low temperature. The wetting mechanism of solid-liquid interface has been investigated by thermodynamic analysis. Active bonding dynamic processes at low temperature have been explored and the corresponding interface models are established. The shear strength of the joints has been tested based on the chip testing standards and the fracture mechanism has been analyzed. The results achieved in the thesis can provide theoretical and experimental basis for low temperature active bonding to be applied in chip attachment or MEMS fabrication.The microstructure of Sn3.5Ag4Ti（Ce, Ga） active solder and its formation mechanism have been analyzed. Results show that the stronger binding energy between Ti and Sn atoms is the important factor of Sn-Ti clusters in the active solder. The effect of Ti on the wettability of Sn3.5Ag4Ti（Ce, Ga） on wafer and ceramic substrate, the diffusion mechanism of Ti, and the thermodynamics analysis of solid-liquid interface based on the possibility of active adsorption at the interface have been investigated. Combining with the obvious segregation of Ti at the interface of the wafer and ceramic substrate in experiments, it can be speculated that the adsorption of Ti at the interface may be one of the important driving force that improves the wettability of solder on the wafer and ceramic surface.The mechanism of active bonding between Sn3.5Ag4Ti（Ce,Ga） and silicon wafer at low temperature has been studied. Experiment results show that Ti obviously segregates at the silicon/solder interface with soldering time of 30 min, and TiSi₂ phase is formed discontinuously along the interface. The mechanism of solid-liquid interface reaction and compound formation in the bonding process has been analyzed based on the active adsorption and reaction thermodynamics theories. In order to better understand the active bonding mechanism, an interfacial model between Si and Sn3.5Ag4Ti（Ce,Ga） active solder is presented. Theoretical analysis results suggest that there might be two bonding mechanisms coexisting in the soldering process: the chemical adsorption of Ti on the silicon interface and the interfacial reaction between Ti and Si. Two mechanisms coexist and play a critical role together to obtain reliable bond between silicon and active solder. The bonding shear strength increases with the increase in soldering time and is measured to be 22.25 MPa, 23.32 MPa, 25.34 MPa and 27.56 MPa with soldering time of 1min, 15 min, 30 min, and 60 min respectively, and meets the requirements of Si die bonding application.The mechanism of active bonding between Sn3.5Ag4Ti（Ce,Ga） and silicon dioxide at low temperature has been investigated. The microstructure of the bonding interface, the distribution of Ti at the interface and the formation of the interfacial reaction products have been explored. Experiment results show that there are some similarities but also obvious differences compared with the Si/Si substrate bonding. In addition to the obvious segregation of Ti at the SiO₂/solder interface, the Ti distribution layer is thick and continuous, and TiSi and TiO₂ phases are formed along the interface. The mechanism of active bonding between Sn3.5Ag4Ti（Ce,Ga） and silicon dioxide has been analyzed based on the active adsorption and reaction thermodynamics theories. In order to better understand the bonding process, a bonding dynamic process model is established. Both theoretical and experimental results suggest that the chemical adsorption of Ti on the silicon dioxide interface is still one of the important driving force that improves the wettability of Sn3.5Ag4Ti（Ce,Ga） on the surface of SiO₂. The interface reaction between Ti and Si O₂ to generate more reactants are the main mechanism of bonding. The shear strength of SiO₂/SiO₂ joints is measured to be 11.15 MPa, 14.10 MPa, 16.37 MPa, and 17.91 MPa with soldering time of 1min, 15 min, 30 min, and 1h respectively, and meets the requirements of SiO₂ or glass substrate bonding application.The active bonding mechanism between Sn3.5Ag4Ti（Ce,Ga） and alumina at low temperature has been studied, and results present that the active element Ti also plays an important role in the soldering process. Maybe the titanium atoms at the interface and oxygen atoms in alumina exchange electron to form ion bonds or covalent bonds and complete the chemical adsorption, so as to make the reliable bonding. No reaction products could be detected at the solder-alumina interface by SEM and XRD. These observations indicate that there may be no chemical reaction occurred in the process of bonding, which is in accordance with the analysis results of reaction thermodynamics and bonding kinetics theories. The shear strength of Al₂O₃/Al₂O₃ joints increases with the increase in soldering time and is measured to be 12.31 MPa, 15.46 MPa, 16.15 MPa, and 17.39 MPa with soldering time of 1min, 15 min, 30 min, and 1h respectively.Furthermore, effects of isothermal aging on interfacial morphological evolution and bond strength of Sn3.5Ag4Ti（Ce,Ga）/Si joints have been investigated. The effects of isothermal aging on the distribution of Ti at the interface and the formation of the interfacial reaction products have been explored as well. Experiment results show that Ti segregates more obviously at the interface of Sn3.5Ag4Ti（Ce,Ga）/Si joint with soldering time of 30 min after aged at 190℃ for 720 h, and the Ti distribution layer is thick and continuous. Maybe the active element Ti continues to diffuse to the silicon surface during the isothermal aging. Besides, it is found that new silicide TiSi phase is formed at the interface. The reason may be that Si and Ti atoms slowly diffuse in the process of isothermal aging, and both of them meet at the interfaces and chemical reaction happens, so as to form the new substance. However, the shear test results illustrate that the shear strength has no obvious change compared with that of the as-bonded substrates. The possible reason is that the new compound formed in the process of aging may strengthen the strength of the bonding interface, but may decreases the bonding strength due to brittleness.Effects of isothermal aging on interfacial morphological evolution and bond strength of Sn3.5Ag4Ti（Ce,Ga）/Si O₂ joints have been studied. Results present that the thickness of interface reaction products TiSi and TiO₂ is increased, and the Ti distribution layer is thicker than that of aging before. Maybe the active element Ti continues to diffuse to the silicon dioxide surface during the isothermal aging, and chemical reaction continuous to happen, so as to form more substances. The shear test results illustrate that the shear strength decreases obviously compared with that of the as-bonded substrates. The reason may be that both the TiSi and TiO₂ are brittle compounds, and the increase of the brittle materials in the process of aging is more prone to brittle fracture, which affects the strength of bonding.