Plasma Activation as a Pretreatment Tool for Low-Temperature Direct Wafer Bonding Materials in Microsystems Technology

Direct bonding is a frequently used technique to join silicon or glass wafers for microsystems. For high bond strengths glass must be heated up to the softening point and silicon wafers above 800 °C. In the recent years systems and processes have been developed for activating the surfaces of glass and silicon wafer, so that the necessary temperatures can be lowered to below 200 °C [1]. For plasma activations with oxygen or nitrogen process gas it was found, that these treatments can suppress the formation of interface defects. For the activation atmospheric pressure plasmas can be used, which are characterized by moderate capital costs, small footprint, the opportunity for local or patterned surface treatment and short processing times. Appropriate systems have been commercialized by SUSS MicroTec and are distributed worldwide [2]. For the characterization of the bonding process, a method to determine the surface energie in situ during annealing was developed. This makes it possible to monitor the bonding process during the entire thermal treatment. With this method, the bonding behavior and the processes in the bond interface during the annealing can be described [3]. Experiments on various substrate materials have proved that particularly the process gas in the plasma activation affects the bonding behavior. Results for atmospheric pressure plasma activated silicon wafers annealed at 100 °C are shown in Fig. 1.