Study On Low-Temperature Direct Bonding And Failure Mechanism Of Si-Based Wafers Via Plasma Activation

Silicon and quartz are the two most common materials in micro-nano fabrication,which have a wide use in fabricating micro/nano fluidic chip.Wafer direct bonding is a key technology to realize 3D heterogeneous integration without intermediate layer.Among these bonding methods,low temperature plasma activated bonding has attracted much attention because it can reduce the thermal stress caused by thermal mismatch between materials with different coefficients of thermal expansion?CTE?,in recent years.However,the surface of the materials is easily damaged and the interlayer is prone to defects under traditional capacitively coupled plasma treatment.The inductively coupled oxygen plasma can reduce surface damage of materials compared with the capacitively coupled plasma,as a result,it was used in this thesis to activate the surface of silicon and quartz in order to overcome the above problems.The changes of surface physical morphology,chemical state and wettability under different treatment conditions of O₂ plasma were analyzed.The results show that the plasma can clean the surface of materials efficiently and result in a large number of the-OH suspension bonds on the surface.At the same time,the three-dimensional morphology of the wafer surface is flattened and the physical structure is refined,which increases the contact area between the upper and lower surfaces during the bonding process.On this basis,this thesis explores and optimizes the low temperature heterogeneous/homogeneous bonding process of silicon/quartz,quartz/quartz.The results showed that silicon/quartz,quartz/quartz can achieve good room temperature bonding after 90 seconds of plasma treatment.Silicon/quartz with the step-type post-bonding thermal preservation at 150°C can reach more than 95%effective bonding area and the bonding strength reached 4.67 MPa.Quartz/Quartz after 150°C linear post-bonding thermal processing can reach 95%or more effective bonding area and the bonding strength can reach up to 5.52 MPa.During the tensile tests,the fractures occur at the bulk materials.In order to reduce the bias error of tensile test results,a stress correction model is proposed.Through the observation of the bonding interface,it is found that the interface is complete,continuous,defect-free and exhibits good transmittance.In addition,quartz/quartz bonding pairs can guarantee the same dielectric properties as the quartz matrix.In this study,an inductive plasma activated silicon/quartz,quartz/quartz direct bonding model is proposed,which consists of three stages:the first stage is through the plasma treatment material surface,providing a large number of hydrophilic Si-OH groups.The second stage is to achieve room temperature connection between the two surfaces by means of intermolecular forces such as Van der Waals force,capillary force,hydrogen bond.In the third stage the dehydration reactions between Si-OH on two bonding surface are realized by post-bonding low temperature reinforcement.In this process,the interaction between molecules in the bonding interface is transformed into covalent bonds to obtain higher bonding strength.The failure mechanism of bonding interface in various service environments is further studied in the thesis.With analyzing the failure behavior of direct bonding interface of silicon-based wafers activated by plasma,the interfacial failure mechanism based on covalent bond,atomic corrosion fracture,interfacial crack initiation expansion and interfacial bridge growth of three molecular chains is proposed.By using this mechanism,an improved crack propagation method is developed,which can be used to realize the recovery and reuse of quartz chips in the field of micro-flow control.