Microstructure And Mechanical Property Of The Anodic Bonding Interface Between Solid Electrolyte Glass (Ceramics) And Metals

Anodic bonding is a new way to realize the bonding between solid electrolyte glasses(ceramics)and metals through the interaction process of electricity and heat.Because of its ability to perform at lower temperature, rapidity,reliability and stability,the anodic bonding process has become a key technique applied in the manufacture of Micro electromechanical System (MEMS).Further research into the anodic bonding of various new materials for MEMS and into the micro bonding of more complex microstructures will be beneficial to development of the MEMS technique.In this paper the technological experiments of anodic bonding has been carried out respectively in the bonding of Glass/Al,Glass/Kovar alloy,and Ceramics/Al.A new technique of common anodic bonding between glass wafers and Al foils in multi-layer assemblies has been designed.By means of the scanning electron microscopy(SEM),the energy dispersive spectrometry(EDS) and transmission electron microscopy(TEM),such factors as the microstructure and chemical elements distribution of the joining interfaces,fracture appearances of the sheafing areas are analyzed.With X-ray diffraction(XRD) instrument,phase structure of the interfaces is analyzed.With electricity equivalent equation,a physical simulation is carried out for electricity characteristics in the process of the anodic bonding between glass and Al.The formation mechanism of the anodic bonding is further analyzed from such aspects as electrostatic field distribution,electrostatic adsorption change rule, ions motion,and the electrochemical,thermodynamic and dynamic characteristics of the joining interface.By means of nonlinear finite element simulation software MARC,the stress distribution in the process of cooling is analyzed for the anodic bonding samples of glass/Al and glass/Kovar alloy,and such rules are investigated as the value and form of wrap deformation,the residual stress and the strain distribution of the bonded samples at temperatures in the range of 450℃to room temperature.It can be seen from the research that successful bonding of glass/Al and glass/Kovar alloy can be realized under certain technological parameters,with the temperature in the range of 300～500℃,voltage of 400～600V,bonding time of 5～15min and pressure of 0.05～1MPa.β″-Al₂O₃ ceramics and Al can have a successful bonding with the temperature ranging from 400～500℃, voltage 150～300V,bonding time 5～15min and pressure 0.05～1MPa. Activation treatment for the surface of ZrO₂ ceramics has improved the bonding between ZrO₂ and Al,and successful bonding between them can be achieved at the voltage in the range of 150～300V and temperature of 400～500℃,which indicates ZrO₂ with the activation layer on the surface has a better bonding property.The analysis results of the joining interface microstructure shows that the joint area is made up of the three layers:glass(ceramics)—diffusion transition layer—metal.For different materials the diffusion transition layer is composed of different spinel oxides such as Al₂SiO₅,FeSiO₃,and Na₅AlO₈ respectively. The chemical elements show gradual distribution on diffusion transition layer and both sides of the interface.And there is a symmetrical gradual distribution for elements on both sides of the interface in the process of multi-layer common anodic bonding of glass and aluminum. The bonding process of solid electrolyte glass(ceramics)and metals is divided into three stages:interface polarization and electrostatic adsorption stage, the ion transferring and anodic oxidation stage,oxide solid phase diffusing or union and transition layer formation stage.Interface electrostatic force and ion diffusing or motion provide requirement for the physical chemistry reaction of the interface joining.The anodic oxidation stage and the stage of oxide solid phase diffusing or union are the main causes for the stronger bonding strength in the interface.The thermodynamic and dynamic analyses of the physical chemistry reaction in the interface indicate that the main factors and basic conditions in influencing the reaction process and the formation of the transition layer are temperature,electrostatic field strength and concentration difference of chemical elements.Such technical parameters in the anodic bonding of glass(ceramics) and metals as temperature,electrostatic field and the electrical conductivity of the materials can help the spontaneity of the boding process in the interface.In the common anodic bonding process of muti-layer sample of glass/Al/glass,there appears the symmetrical ion current and double deck of alkali metals Na⁺ ion depletion zone in the glass on both sides of anode Al of the three-layer sample,which corresponds with parallel connection of a pair of two-layer samples glass/Al.Under certain temperature and voltage,the peak current is twice more than that of the two-layer sample of glass/Al.It can be found that through the electricity equation,the physical simulation result for electricity characteristics in the anodic bonding process of both two-layer sample(glass/Al)and three-layer sample(glass/Al/glass)corresponds with experimental results.The joining strength of the bonded sample of the glass/Al/glass increase with the increasing temperature and applied voltage.Fracture occurs in the glass base near the interface with the aluminum,which shows that the joining interface has a stronger joining strength than the glass base.The successful bonding of multi-layer wafers can be realized with employment of the common anodic bonding technique,which can provide a new method and theoretical basis for the bonding of multi-layer wafers and the design of complex microelectronic equipments.The nonlinear finite element simulation software MARC analysis reflects the presence of the residual stress and strain in the process of cooling for the anodic bonded sample glass/Al(Kovar alloy).Because of the symmetry structure in the three-layer sample glass/Al/glass,the residual stress and strain near the joining interface show a symmetrical distribution,which is helpful to the slowdown of deformation.The maximum deformation(0.00418%)and equivalent strain(2.42×10^-2)are remarkably smaller than those of the two-layer sample glass/Al(0.175%and 3.47×10^-2respectively),which can make a great significance in accurate sealing for MEMS.The maximum equivalent stress and shear stress locates in the transition layer in both three-layer sample(glass/Al/glass)and two-layer sample(glass/Al), which will become weakness in the joint of anodic bonded sample.Kovar alloy has an expansion coefficient similar to glass,therefore,the residual stress and strain in the bonded sample of glass/Kovar alloy are smaller than those of the sample glass/Al,which indicates the residual stress in the joint can be reduced by using sealing materials with similar thermo physical property.