Surface Activated Low Temperature Direct Bonding Of LiNbO₃ To Si-based Materials And Application Research

Lithium niobate(LiNbO₃₃)is a multifunctional material that integrates piezoelectric effect,ferroelectric effect,electro-optical effect,nonlinear optical effect,photorefractive effect,acousto-optic effect and other properties.Due to its excellent electro-optical coefficient and nonlinear optical coefficient,LiNbO₃₃ is considered to be the main substrate material for the design and preparation of future photonic chips.However,the physical properties of LiNbO₃₃ such as stable crystal structure,high melting point,large thermal expansion coefficient,and high brittleness severely limit the interaction with other materials,such as silicon(Si),silicon dioxide(Si O₂),etc.Heterogeneous integration of structure and function.In addition,LiNbO₃₃ also has excellent optical properties in the mid-infrared band,but it is rarely used by people for the design and preparation of optical devices.Therefore,the development of a direct bonding technology between LiNbO₃₃ and silicon-based substrate materials(i.e.,glass,silicon dioxide,and silicon)compatible with Complementary Metal-oxide Semiconductor(CMOS)processing technology will greatly promote the development of on-chip heterogeneous integration of LiNbO₃₃ devices.First of all,this thesis conducted a study on the direct bonding between LiNbO₃₃and glass based on the CMOS process compatible ultraviolet light activation method and plasma activation method.Using atomic force microscope,surface wetting angle measuring instrument,Fourier infrared spectroscopy,Raman spectroscopy,X-ray photoelectron diffraction energy spectrum and other surface characterization methods,the interaction between vacuum ultraviolet light and plasma and LiNbO₃₃ and Glass wafers Explored separately.The experimental results show that vacuum ultraviolet light and plasma activation can greatly improve the wettability of the wafer surface to be bonded.In addition,after 15 minutes of vacuum ultraviolet light activation and 90s plasma activation,the surface roughness of LiNbO₃₃ and Glass wafers is relatively smooth and flat,and the pre-bonding effect is the best.Subsequently,the LiNbO₃₃/Glass pre-bonded wafer under this activation parameter was annealed at a low temperature under a nitrogen atmosphere at 150°C for 12 hours to further enhance the bonding strength.Through tensile testing,scanning electron microscopy,focused ion beam micro-nano processing,transmission electron microscopy and other interface characterization methods,the direct bonding strength and interface of LiNbO₃₃/Glass under vacuum ultraviolet light and plasma activation were studied respectively.The bonding strengths under vacuum ultraviolet light activation and continuous plasma activation are 2.12 MPa and 2.57 MPa,respectively.Both activation methods can effectively achieve LiNbO₃₃/Glass direct bonding,and the bonding strength is sufficient to withstand the mechanical stress,thermal stress and water stress corrosion of the bonding interface generated during machining and micro-nano processing.Observing the direct bonding interface with a transmission electron microscope,it can be found that the interface thickness under vacuum ultraviolet light activation and plasma activation are 9.6 nm and 2.2 nm,respectively.By comparing the advantages and disadvantages of the two activation methods in terms of bonding efficiency,cost,effect,etc.,the study of LiNbO₃₃/Si O₂/Si and LiNbO₃₃/Si direct bonding process based on plasma activation was established.Secondly,the direct bonding between LiNbO₃₃ and Si O₂/Si was studied using oxygen plasma activation,nitrogen plasma activation,and continuous plasma activation.Using various surface characterization methods mentioned above,the interaction between plasma and Si O₂/Si wafer was compared and analyzed.The experimental results show that after the three plasmas are activated for 90 s,the surface of the Si O₂/Si wafer becomes very hydrophilic and relatively smooth.Under the same plasma activation,the pre-bonding area is the most ideal at this time.Under the same activation time,the pre-bonding strength of LiNbO₃₃/Si O₂/Si increased sequentially under oxygen plasma activation,nitrogen plasma activation,and continuous plasma activation.According to the test results of X-ray photoelectron diffraction spectroscopy,the increase in pre-bonding strength is mainly due to the effective suspension of nitrogen-related chemical bonds on the surface of the wafer.The LiNbO₃₃/Si O₂/Si pre-bonded wafers were annealed at a low temperature under a nitrogen atmosphere at150°C for 12h.The direct bonding strengths of the three types of plasma activation were 1.85 MPa,2.13 MPa and 3.45 MPa,respectively.Observation of the direct bonding interface with a transmission electron microscope shows that the LiNbO₃₃/Si O₂/Si interface under continuous plasma activation has achieved interatomic bonding.Then,the direct bonding between LiNbO₃₃ and Si was studied by plasma activation.The experimental results found that the direct bonding strength of LiNbO₃₃/Si wafers bonded under oxygen plasma activation,nitrogen plasma activation,and continuous plasma activation,even after low-temperature annealing treatment,is not enough to withstand the stress generated during mechanical cutting.The continuous plasma activation energy assisted by water vapor greatly enhances the oxidation degree of the Si wafer surface,and enhances the degree of inter-element inter-diffusion of the LiNbO₃₃/Si direct bonding interface during the low-temperature annealing process.In addition,using the above-mentioned various surface and interface characterization methods,the effect of continuous plasma assisted by water vapor on the wafer surface and bonding interface was explored.Observation of the direct bonding interface with a transmission electron microscope shows that the LiNbO₃₃/Si interface under continuous plasma activation with water vapor assistance has achieved interatomic bonding,and the thickness of the bonding interface is 6.5 nm.Based on the above analysis results,the LiNbO₃₃/Si direct bonding mechanism and model were speculated and established.Finally,based on the excellent light transmittance of LiNbO₃₃ in the mid-infrared band,the low-temperature direct bonding process developed above was used to design and fabricate plasmon-nanofluidic devices.Through the time-coupled mode theory,the spectral response produced by the interaction between light and matter has been studied.Based on the coupling design of the loss engineering,the refraction index and vibration absorption of water are used as switches to realize the decryption and encryption of pattern information.At the same time,the preset information is encoded,and the pattern can be dynamically presented at different mid-infrared wavelengths(i.e.,2.68?m,3.16?m and 3.61?m).