Process, reliability test, and interfacial characterization for low temperature wafer direct bonding

Low Temperature Wafer Direct Bonding (LTWDB), as an enabling technology, can be used in preparation of MEMS material (SOI) and manufacturing of multi-layer solar cells. In the present experiment, LTWDB was achieved by surface activation method using SPM and RCA-1 solutions. The main mechanism is relevant to the high oxidizability of SPM solution and surface cleaning, activation ability of RCA-1 solution. Especially, the preparation and usage of RCA-1 were improved to greatly enhance its activation effect. For reliability tests, tensile strength tests were performed and decrease was found after thermal-cooling cycle treatment. Besides infrared system and SEM scanning system was employed to exam the interfacial characterization of the bonded wafers. Infrared system was used to observe the bonding wave when contacting two wafers under normal environment. Interface with relatively high bond rate and interfacial uniformity could also be observed under infrared transmission system. It is demonstrated to be one of the most convenient ways to test the quality of bonded pairs. Besides, SEM was used to further inspect the nano-scale intermediate layers. Two closely contacted wafers with the intermediate layers' thicknesses around 120nm could be observed. Bonded pairs with thinner intermediate layers are characterized by higher bond strength.