A theory is presented which describes the initial direct wafer bonding process. The effect of surface microroughness on the bondability is studied on the basis of the theory of contact and adhesion of elastic solids. An effective bonding energy, the maximum of which is the specific surface energy of adhesion, is proposed to describe the real binding energy of the bonding interface, including the influence of the wafer surface microroughness. Both the effective bonding energy and the real area of contact between rough surfaces depend on a dimensionless surface adhesion parameter, θ. Using the adhesion parameter as a measure, three kinds of wafer contact interfaces can be identified with respect to their bondability; viz. the nonbonding regime (θ>12), the bonding regime (θ<1), and the adherence regime (1<θ<12). Experimental data are in reasonable agreement with this theory.