Pressure-assisted fusion bonding of silicon wafers

Silicon-to-silicon bonding with an intermediate oxide layer is an important aspect of the fabrication of microsensors and actuators. In this work, we have developed a novel, two step, pressure-assisted fusion bonding process which has proved to be extremely successful in bonding two silicon wafers. Moreover, as this process does not require a very high degree of surface cleanliness and flatness, it is more suitable for practical applications. In the first step of the process, after making the two wafer surfaces hydrophillic, the wafer pair assembly is slowly heated to 100-300°C while applying pressure and voltage across them in order to ensure intimate contact. In the second step, the partially bonded wafers are heated to 1050°C. The bonds thus formed are extremely strong as shown by fracture strength measurements. The bond strength measured is of the order of 40kg/cm2. The bonded wafer pair has also been cleaved (without disturbing the bonding) to demonstrate that the bonding is indeed strong enough to withstand further processing.