Direct bonding - Adhesive-free assembly for extreme environments

Direct bonding technology, also known as molecular adhesion, is a very special type of assembly. Its most concise definition seems to be: "spontaneous bonding without the addition of thick liquid". The fact that no liquid is added implies, above all, that no polymeric liquid glue is used. It's a glueless assembly! The fact that it is "spontaneous" implies that the joining of the two surfaces saves energy, in order to propagate the direct bond.

The energy available to the system for bonding is called "adhesion energy". This is to be contrasted with adhesion energy (otherwise known as "bonding energy"), which represents the energy required to separate the assembled surfaces. Adhesion energy, on the other hand, is the energy that helps bring them together.

In direct bonding, to enable spontaneous bonding, the adhesion energy must be greater than the energy cost of bringing the surfaces together, i.e. greater than the elastic energy of deformation induced by the fact that the surfaces to be bonded will touch and come as close as possible, deforming as necessary. It is therefore necessary to have surfaces very close together, generally at a distance of the order of nanometers. At this scale, intermolecular forces between the two surfaces can come into play. This is why the technique is also known as "molecular adhesion". Direct bonding can be achieved by van der Waals forces, hydrogen bonds and, in some cases, capillary forces: these forces are the driving force behind direct bonding. The mechanical energy of deformation, expended in bringing the surfaces together, acts as a brake.

For simplicity's sake, the article focuses on direct bonding of fused silica. This motor and brake are described in detail, enabling us to present the mechanisms of adhesion, and the criteria to be met in terms of surface specifications, or environment, to achieve adhesion. Adhesion energy determines the mechanical strength of the interface. In the final section, we describe the techniques used to maximize adhesion energy.