Study On Low-temperature Wafer Direct Bonding Technology

With rapid development for recent 20 years, wafer bonding has become the basic technology in MEMS (Micro-Electro-Mechanical System) packaging and an important tool in MEMS engineering field. WLP (Wafer Level Package), 3-D Chip Stacking and SOI (Silicon-on-insulator) are the three impetuses for the development of wafer bonding technology. This thesis throws a sight on low-temperature wafer direct bonding which possesses many unique advantages compared with other bonding technology, such as avoiding of the defects which brought by high-temperature annealing processing. The technology of micro-fabrication on wafer substrate through bonding the interfaces with cavities has a bright future on applications. The main contents of this thesis include three aspects: bonding technology process, application study on bonding with cavities and quality analysis.The first part of this thesis gives a detailed analysis on the mechanism of wafer direct bonding through cleaning and activation methods and surface physical and chemical characteristics. Bearing ratio is introduced to interpret the effect of surface roughness on bonding quality. The bonding qualities applying different cleaning methods are compared with the help of AFM (Atomic Force Microscope) which is used to measure the exact surface roughness before and after each cleaning step.In the second part, the low-temperature direct bonding technology is applied on wafer bonding with cavities. Three different types of cavities are fabricated on wafers using lithography technology and RIE (Reactive Ion Etching) technology. Three-dimensional profiles of the cavities are measured and RF-images after bonding are compared to analyze the effects of different cavity structures on bonding quality and the forming reasons of bonding defects.The last part of the thesis pays attention to tensile strength which is one of the most important evaluation parameters of bonding quality. Temperature-humidity combined test is conducted and the variation of the samples'tensile strength before and after the test is analyzed. Moreover, the effect of UV activation on bonding strength is assessed.