| In recent years,the feature size of integrated circuit devices is increasingly close to the physical limit of the Moore's Law,and the multifunction chip integration in electronic/optical/biomedical devices has become a key driving force for new electronic products.Bonding(also known as "joining")is an irreplaceable and important part in the integration of diversified and complicated systems.Direct wafer bonding allows the polished semiconductor wafers to be combined without the use of adhesives and plays a key role in the fields of integrated circuit fabrication,microelectro mechanical systems(MEMS)packaging and multifunctional chip integration.Due to the introduction of high temperature,thermal stress and thermal strain are inevitable for the conventional bonding method.To realize low-temperature bonding,the plasma activated bonding and ultra-high vacuum bonding methods have been developed.Both of them,however,can cause large damage to the material surface.Therefore,it is desirable to develop a novel low-temperature bonding method,which is low-cost,facile,environmental-friendly with fewer interfacial defects.In this thesis,a novel method for direct bonding of silicon-based wafers using vacuum ultraviolet(VUV)surface activation is proposed.Firstly,the VUV equipment was developed for the activation.Then,by evaluating effective bonding area and bonding strength,the optimal parameters for the bonding of silicon/quartz and quartz/quartz,including irradiation time,ambient humidity and annealing temperature,were obtained.Finally,combining the analysis results concerning the function of VUV and the characterization of bonding interface,the model of bonding mechanism was established.The results show that the VUV activated wafer bonding process has been successfully developed.The equipment enables to eliminate ozone and control the gas atmosphere.It has been successfully applied to the bonding of materials in this work.The process parameters were optimized,including VUV irradiation time(15min),bonding atmosphere relative humidity(RH=30%~40%),step annealing process profile,and the maximum annealing temperature(200?).Even in a nonclean room environment at room temperature,the effective bonding area of silicon/quartz reached up to 90% or more.After the annealing,the bonding strength of interface were higher than the bending strength of the bulk materials.The quartz/quartz bonding pair was almost completely bonded.After the annealing,the bonding strength reached 6MPa,and the light transmittance was extremely high.The interfaces of silicon/quartz and quartz/quartz were analyzed by TEM,verifying defect-free and inter-atomic connection.Therefore,the direct wafer bonding under this bonding condition can meet requirements for practical applications.Through the analysis of material surfaces before and after illumination,it is concluded that the VUV with extremely short wavelength(172 nm)can effectively remove contaminations on silicon-based wafer surfaces and enhance the surface activity of materials.At the same time,the effects of VUV assisted with water vapor was furtherly studied.The diffusion behavior of the water molecules at the interface between silicon/quartz and quartz/quartz was obtained by combining analysis results.Model of bonding mechanism was established respectively.The model consists of the following three steps: After irradiation treatment,some silicon-oxide bonds in the oxide layer on the surface of Si-based wafers are broken and the number of silicon hydroxyl groups are increased.At this time,the surface of oxide layer is connected with a large number of water molecules through hydroge n bonds to form hydration layer.During room temperature bonding(~25?),the bonded interfaces are filled with water,which is regarded as bonding "binder",and this facilitates the fulfill of prebonding.During low temperature annealing process,the dehyd ration reaction occurs at the bonding interface forming Si-O-Si covalent bonds,microgaps closing resulting in high-strength bonding between the two surfaces. |
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