The effect of surface roughness on direct wafer bonding

A theory is presented which describes the initial direct wafer bonding proc ess. The effect of surface microroughness on the bondability is studied on the basis of the theory of contact and adhesion of elastic solids. An effec tive bonding energy, the maximum of which is the specific surface energy of adhesion, is proposed to describe the real binding energy of the bonding i nterface, including the influence of the wafer surface microroughness. Both the effective bonding energy and the real area of contact between rough su rfaces depend on a dimensionless surface adhesion parameter, theta. Using t he adhesion parameter as a measure, three kinds of wafer contact interfaces can be identified with respect to their bondability; viz. the nonbonding r egime (theta > 12), the bonding regime (theta < 1), and the adherence regim e (1 < theta < 12). Experimental data are in reasonable agreement with this theory. (C) 1999 American Institute of Physics. [S0021-8979(99)06510-X].