Silicon wafer bonding: chemistry, elasto-mechanics, and manufacturing

Summary form only given. Three subjects closely associated with the silicon wafer bonding process are discussed: (i) the chemistry, (ii) the elastomechanics, and (iii) manufacturing of bubble or void-free wafer bonding in a non-cleanroom environment. The surfaces of the silicon wafers to be bonded must be hydrophilic. Silicon-direct-bonding (SDB) for power device applications or bonding via an intentionally introduced oxide initially always occurs via water adsorbed on the native or intentionally introduced oxide layers. The amount of adsorbed water has been changed by performing the contacting of the wafers at temperatures ranging from room temperature up to about 350°C for different wafer combinations. The speed of the bonding contact wave decreases with increasing temperature. Bonding ceases to operate between about 100 and 320°C depending on the specific wafer combination. Since silicon wafer bonding requires flat wafers, a flatness criterion for bonding is derived. The flatness requirements on commercially available prime grade wafers are stringent enough to fulfil the criterion. On the other hand, the criterion is much harder to fulfil for buried structures even if the height of localized steps are in the 10-1000 Å range. Manufacturing of void-free bonded wafers requires not only the use of sufficiently flat wafers but also the elimination of dust particles between the wafers. A method by which the wafers to be bonded are kept close together during cleaning and drying is discussed. This method permits void-free wafer bonding even outside of a cleanroom in a laboratory