The influence of chemical pre-treatments on materials integration through wafer bonding

The integration of dissimilar materials systems through wafer bonding provides extra freedom and flexibility in the novel device design and fabrication processes. This work was focused on the understanding of the bonding chemistry and the role of the chemical pre-treatments on the direct wafer bonding of III-V materials in order to improve the bond strength and bond integrity and determine bonding requirements. Multiple internal transmission Fourier transform infrared spectroscopy, atomic force microscopy and infrared transmission imaging have been used to characterize the bonding properties including the interfacial chemistry, surface topography and bonding integrity. The bond strength was determined using the crack-opening method.; GaAs/GaAs direct wafer bonding was studied using an HCl-based pre-bonding treatment combined with ozone and oxygen plasma treatments. This HCl-based pre-treatment led to an increased bond strength and a reduced void formation, by increasing the overall covalent bonding at the interface. The removal of the initial native oxide facilitates the diffusion of water to the GaAs wafer surface where it can react to form primarily Ga-based oxides, leading to an increased bond strength. InP/GaAs direct wafer bonding was additionally investigated. The bonded pairs are prepared using either 5% HF in water or HF:ethanol (1:9) chemical pre-treatments for InP combined with HCl:H2O(1:1) rinsing for GaAs. It has been observed that the HF-based pre-treatments remove the initial native oxide. The initial room temperature bond strength is primarily determined by the strength of hydrogen bonding which, in turn, is a function of the pre-bonding treatment. For aqueous-based HF treatments, the reduction in the initial oxide thickness facilitates the reaction of the interfacial water and ethanol with the underlying InP allowing for a more rapid reaction between the interfacial water and InP, and leading to an increased bond strength. For ethanol-based HF treatments, ethanol reacts at temperatures between 200°C to 400°C, depending on the amount of H2O and OH groups initially at the interface. After annealing, the bond strength for all pre-bonding preparations can reach a high value comparable to the fracture strength of the InP.