Low-temperature Bonding Mechanism For Silicon-based Substrates Based On Glass Network Structure Control

Silicon based material is important for MEMS device fabrication in the semiconductor industry.In order to avoid the adverse effects of high-temperature process,low-temperature bonding method of silicon-based materials have attracted more and more attention for the development of new electronic packaging technology.Glass material bonding,especially the high purity optical ultraviolet silica glass bonding in the application of microfluidic devices,is a major challenge in the industry.Besides,in order to pack more components in a limited space for 3D packaging devices,low-temperature direct bonding technology of silicon substrate is highly desirable.We take two silicon-based materials including silica glass and monocrystalline silicon as the research objects to study two low-temperature bonding methods including lead-free low-melting glass frit bonding and wet chemical surface activation direct bonding,and explore their bonding mechanism based on the glass matrix and surface network structure control respectively.Through studying the lead-free Bi2O3–B2O3–Si O2 ternary glass samples,we investigated the effects of boron and bismuth content on the structure network and properties of the glass frit.Controlling the glass network structure and thermal properties by adjusting the ratio of oxides content,we prepared a low-melting glass frit with low glass transition point.Though the Bi2O3–B2O3–Si O2 glass can achieve a lower glass transition temperature,its softening temperature was still higher than that of the lead containing glass.To further reduce the glass softening and sintering temperature,we used the Bi2O3–B2O3–Zn O glass as the main component for low-melting glass frit preparation.Two kinds of oxides including silicon oxide and titanium oxide were added to tune the chemical stability and thermal expansion coefficient of these frit.The minimum sintering temperature of the prepared Bi2O3–B2O3–Zn O low-melting glass frit was found to be as low as 400?.Guided with the DSC and CTE curve of the glass,we have optimized the sintering process.The maximum bonding strength reached up to 2.5MPa under 440? sintering for 30 min.Low-temperature direct bonding of silica glass was achieved using the wet chemical activation method in a routine laboratory without an ultra-high-vacuum system and clean room.A method for the preparation of silica glass capillary via low-temperature direct bonding was also proposed.The micro behavior of the bonding interface was analyzed using EELS.This bonding method can guarantee good light transmission of the glass joints,and reduces the requirement of the bonding process on experimental conditions compared with plasma surface activated direct bonding.The bonding process for obtaining good silica glass direct bonding joints was studied,and the bonding strength can reach up to 4.5MPa which meet the micro fluidic application requirements.The surface chemical states of the silica glass before and after activation treatment were studied.The results showed that network structure near the surface of the silica glass changed after the activation,and the concentration of the surface activated hydroxyl increased.EELS analysis around the interface revealed that the glass network of the bonding layer was consistent with the glass matrix,and that a smooth and reliable bonding interface was achieved.Through the wet chemical surface activation direct bonding,silicon low-temperature direct bonding was achieved in the routine laboratory without an ultra-high-vacuum system and clean room.The p–n junction with excellent I–V characteristics can also be prepared.This method reduces the requirement of the activation bonding condition on the surface of the device,and further saves the manufacturing costs of the semiconductor device.By considering the effect of bonding temperature,bonding pressure and bonding time on the bonding quality,we found a good technological bonding process and the bonding strength can reach up to 4.2MPa.Investigation on the surface chemical states of the silicon wafer before and after activation showed that the glass network structure near the silicon oxide surface changed after the activation,and that the concentration of the surface activated hydroxyl increased.Through the EELS analysis of the interface from the cross sectional samples obtained at low and high temperature,we found that the O/Si ratio at the low-temperature direct bonding interface was lower than that of interface experienced high-temperature annealing.However,it can satisfy the requirements for electronics applications.In summary,we have developed a new bonding model for the silicon-based materials via low-temperature wet chemiacal surface activation direct bonding based on glass network structure control.The modified model mainly includes three aspects.The first is the interruption of the silicate network on the silicon oxide surface,increase of hydroxyl concentration and the formation of the hydrolyzed layer.The sencond is reorganization of glass network near the interface and initial formation of the bonding interface in the pre-bonding.But due to the geometric constraints,many nanongaps remain in the interface.The third is the formation of the reliable joint.When the pressure assisted low-temperature annealing is applied,the residual hydroxyl group is completely dehydrated and condensed,all the nanongaps are cloesed and the 3D glass network structure is reformed at the interface achieving a reliable joint.