Low temperature wafer bonding for micromechanical applications

In this paper a new class of modified silicon direct bonding processes is presented. The new process step within the bonding procedure involves the use of thin intermediate layers to decrease the required process temperatures. The bonding process was characterized using oxidized silicon wafers. A brief description of the process: after a hydrophilic pretreatment, a diluted solution of sodium silicate, aluminum phosphate or ammonia-silica solutions in water is spun onto one of the two surfaces and the two wafers are brought into contact. The attraction force of the hydrophilic surfaces results in very close contact and the two wafers are fixed together. After a final temperature treatment in the range of 200degreeC and 350degreeC for the silicates and aluminum phosphate, respectively, the bonds obtained their maximum strengths. Surface energy of approx. 3 J/qm and mechanical strength in the range of 250 - 300 kp/qcm can be measured using sodium silicates or aluminum phosphates. In compa rison, this value is obtained in conventional silicon direct bonding at temperatures around 1000degreeC. Using ammonia-silica solutions, the surface energies achieve values of about 1.6 J/qm. The lower bonding strength is compensated by offering a full IC-compatible bonding process. The influence of chemical and temperature treatment on the surface energies is described in detail.