1. Fields of application

Direct bonding is based on bringing two surfaces into contact without the use of glue or additional material. The two surfaces, perfectly polished and clean, establish intermolecular bonds between themselves (Van der Waals, hydrogen or covalent bonds, etc.): these bonds are responsible for bonding the surfaces together.

In the field of terrestrial optics, the first patent for direct bonding dates back to 1928. It was filed by W.E. Williams, on two interferometers featuring blade assemblies made by direct bonding. More recently, in 1963, this technology appeared in a Philips patent for a He-Ne gas laser, whose mirrors, positioned at either end of the cavity, were attached to the cavity by direct bonding.

The process was then little used until the mid-1980s, when the first large-scale applications of direct bonding appeared in the fields of...