Low-temperature Direct Bonding Of SiC Wafers Via Vacuum Ultraviolet Surface Activation

Silicon carbide(SiC),a third-generation semiconductor material with excellent properties,such as high thermal conductivity,high critical electric field and high saturation electron drift velocity,can achieve high quality and high reliability under high temperate,high frequency,high power electron and extreme conditions.However,silicon carbide itself has stable physical and chemical properties,so it is difficult to be machined,while heterogeneous integration can integrate the advantages of various materials,which provides a new approach to expand the application of silicon carbide.Among hetero-integration method of silicon carbide,wafer bonding can overcome the problems of uneven deposition film and lattice mismatch of epitaxial growth,attracting wide attention from researchers.Vacuum ultraviolet(VUV)activation bonding method is a kind of low-temperature bonding method which realizes bonding by cleaning and activating wafer surfaces under the exposure of the VUV with extremely short wavelength(110-180nm),it is low-cost,simple,environmental-friendly and only causes small damage to material surfaces.This paper adopts vacuum ultraviolet(VUV)surface activation bonding method,selects irradiation time,bonding atmosphere humidity and annealing temperature as process parameter variates,chooses effective bonding area and bonding strength as the basis to evaluate bonding quality,and obtains the optimal process parameter for the low-temperature direct bonding of silicon/SiC,silicon oxide/SiC and quartz/SiC.Finally,combining the analysis results concerning the function of VUV on the material surfaces and the characterization of bonding interface,the model of bonding mechanism was established.The results show that in the condition of VUV irradiation time(25min),bonding atmosphere humidity(15%-20%)and the stepped pressurized annealing process in which the maximum annealing temperature is 150℃,direct bonding of silicon/SiC in the non-clean room environment could be realized,with effective bonding area reaching nearly 90% and bonding strength reaching almost 2.5MPa.In the condition of VUV irradiation time(25min),bonding atmosphere humidity(5%-10%)and the stepped pressurized annealing process in which the maximum annealing temperature is 250℃,direct bonding of silicon oxide/SiC in the non-clean room environment could be realized,with effective bonding area reaching nearly 80% and bonding strength reaching almost 5MPa.In the condition of VUV irradiation time(25min),bonding atmosphere humidity(20%-30%)and the stepped pressurized annealing process in which the maximum annealing temperature is 150℃,direct bonding of quartz/SiC in the non-clean room environment could be realized,with effective bonding area reaching nearly 90% and bonding strength reaching almost 2.6MPa,and the light transmittance was extremely high.The interfaces of silicon/SiC,silicon oxide/SiC and quartz/SiC were analyzed by TEM,verifying defect-free,inter-atomic connection and c-rich transition layer(6-10 nm in thick)existing on the interfaces.Through the analysis of material surfaces before and after illumination,it is concluded that the VUV(172 nm)can effectively remove contaminations on SiC and silicon-based wafer surfaces,improve the hydrophilicity of wafer surfaces and cannot cause structural damage to material matrix.Through the analysis of XPS test,it is concluded that enriched C on the bonding interfaces is the result of collective free C formed by the breaking of Si-C bond on SiC surface after the exposure of VUV.Based on the affect of VUV on material surfaces and the experiment results,a low-temperature direct bonding model was established which applied to SiC VUV activated bonding.During room temperature bonding process,hydrophilic group on wafer surfaces connects water molecules through hydrogen bond,acting as a bridge on the bonding interfaces;during low temperature annealing process,diffused water molecules on the bonding interfaces react with material matrix,forming oxide,at the same time,substrate material expands with heat.Under external pressure,SiC with high surface hardness and water molecules’ softening function make silicon-based material surfaces’ compressive deformation occur,and contact area of bonding interfaces increases,more hydroxyl groups generate dehydration reaction,forming stable covalent bonds and thus improving bonding strength.