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~ 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, interracial 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.
In order to determine whether low temperature wafer bonding is thermodynamically prohibited or simply a slow kinetic reaction, a systematic evaluation of the interface bond kinetics of silicon direct wafer bonding was performed over the temperature range of 2O0-1000~ for annealing times ranging from 15 rain to 45 days. The tensile, shear, and torsion tests were developed to monitor the strength kinetics of silicon and silicon dioxide bonded wafers. The strength evolution is found to obey an Arrhenius relationship over the temperature range of 200-1O00~Tensile strength of St-St bonded substrates varies from a minimum of 0.08 MPa at contact to a maximum of 4.25 MPa after high temperature thermal annealing. The failure of low temperature annealed samples to develop strong bonding in short periods of time has been correlated to microvoid formation at the interface as determined by TEM examination. Geometrical studies indicate that the microvoid formation is the result of trapped gases and can be reduced by appropriate choices of bonded geometries.
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