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|Low temperature Si-Si, SiO2-SiO2 covalent bonding with thin siloxane layer|
|Keywords: direct bonding, siloxane layer, oxygen plasma enhancement|
|We present new Si to Si, SiO2 to SiO2 bonding methods for low temperature applications (<300oC). Direct bonding process between Si (or SiO2) substrates makes strong bonding force and has the bonded body non-conductive compared with metallic bonding processes. However high temperature over 1000oC is needed for the reliable Si to Si and SiO2 to SiO2 direct bonding processes. To reduce the bonding temperature, thin siloxane layer and low-powered oxygen plasma enhancement was used in this study. We used dimethyl siloxane layer having siloxane chains (-Si-O-)n and methyl ends. Siloxane layer is able to be bonded strongly with Si-based substrates at low temperature (<300oC) when oxygen plasma is treated on it. However polymerized siloxane layer such as PDMS has much higher coefficient of thermal expansion (CTE) of 300ppm/oC than Si of 2.6ppm/oC. When the bonded structure is cooled or heated, the interfaces is possibly distorted and cracked by the high stress between siloxane layer and Si substrate. To solve these above problems, we invented new fabrications to reduce the thickness of siloxane layer down to 3~4nm, monomer layer levels. Extremely thin thickness of siloxane layer prevented the distortions by above CTE differences. Because the siloxane layer is made extremely thin and the Si to Si bonding structure with and siloxane layer has very strong adhesion properties, the bonded body keeps reliable bonding force even when it is heated to high temperature (>900oC ). The feasibility of wafer-level bonding process was demonstrated in this study. We investigated the siloxane layer thickness by TEM images. The bonding strength was measured over 30MPa and confirmed by dicing method by 5mm. We also expended this new development to SiO2 to SiO2 bonding structures. The bonding between non-Si based substrates such as GaN was also possible using amorphous Si thin film coated on these substrates|
Samsung Advanced Institute of Technology