Low temperature silicon wafer bonding for micromechanical applications

This paper is focused on a modified process for silicon direct bonding. Thin intermediate sodium stilicate or aluminium-phosphate layers are used to decrease the process temperatures. Oxidized silicon wafers are used for the characterization of the process. After a hydrophilic pretreatment, a diluted solution of sodium silicate or aluminium-phoshphate in water is spun onto one of the two surfaces and the two wafers are brought into contact. The attraction force ot the hydrophilic surfaces results in very close contact and the two wafers are fixed together. After a final temperature treatment in the range of 200 degree C and 350 degree C for sodium-silicate and aluminium-phosphate, respectively, surface energy of about 3 J/square m and mechanical strength in the range of 250-300 kp/square cm can be measured. In comparison, this value is obtained in conventional silicon direct bonding at temperatures above 1000 degree C. The influence of chemical and temperature treatment on the surface energies is described in detail.