CHEMICAL FREE ROOM-TEMPERATURE WAFER TO WAFER DIRECT BONDING

A limitation to the use of direct wafer bonding methods for micromachining and thin film device manufacturing has been the necessity for high temperature anneals to strengthen the bonded interface. Obviously, strong interface strength is needed to withstand backthinning processes and the rigors of device fabrication. Unfortunately, the elevated temperature exposure has a detrimental effect on implanted or diffused etch stop layers via diffusive broadening. Additionally, for many micromachined applications wafer bonding could be used as a final assembly step, replacing epoxies. However, the sensitive components of the device must be protected from thermal effects. This paper describes the use of oxygen plasmas to develop chemical free, room temperature, wafer to wafer bonding methods. The bond developed between plasma-activated silicon wafers is virtually at full strength upon contact bonding and does not require further thermal strengthening. The results for silicon dioxide bonding show that full strength material is achieved with anneals below 300 degrees C. This process has been applied to a number of wafer materials including sapphire, silicon dioxide, silicon nitride, and gallium arsenide. The data presented are the results of strength tests, interfacial defect etching, transmission electron microscopy analysis, initial interface reaction kinetics, and mechanisms studies. We also show preliminary results from a suggested model to explain the observed increases in kinetics compared to conventional aqueous solution processing of samples.