Optical contact bonding

Optical contact bonding is a glueless process whereby two closely conformal surfaces are joined together, being held purely by intermolecular forces

History

Isaac Newton has been credited with the first description of totally conformal interaction observed through the interference phenonoma known as Newtons rings, though it was S.D. Poisson in 1823 who first described the optical characteristics of two identical surfaces in contact. It was not until the 19th Century that objects of such precision were first made that the phenonoma was observed. This clinging together was described as "wringing together", or as "ansprengen" in german. By 1900 optical prisms were being made using optical contact bonding, and the following century saw further research into the phenonoma at the same time as ideas of interatom interactions where first being studied.

Explanation

Intermolecular forces such as Van der Waals forces, hydrogen bonds, and dipole dipole interactions are not typically sufficiently strong to hold two apparently conformal rigid bodies together, since the forces drop of rapily with distance, and the actual area in contact between the two bodies is small due to surface roughness, and minor imperfections.

However if the bodies are conformal to an accuracy of better than 10 angstroms, then a sufficient surface area is is close enough contact for the intermolecular interactions to have an observable real world physical manifestation ie the two objects stick together.

Such a condition requires a high degree of accuracy and surface smoothness, which is typically found in optical components such as prisms.

Production of an optical contact bond

In addition to both surfaces being practically conformal (in practice often completely flat), the surfaces must also be extremely clean and free from any small contamination that would prevent or weaken the bond - including grease films, or specks of dust. For bonding to occur the surfaces only need to be brought together, the intermolecular forces draw the bodies into the lowest energy conformation and no pressure is needed to be applied.

Advantages

Since the method requires no binder, balsam or glue the physical properties of the bound object are the same as the objects joined. Typically glues and binders are more heat sensitive, or have undesirable properties compared to the actual bodies being joined. The use of optical contact binding allows the production of a final product with properties as good as the bulk solid.

Uses

Template:Sectionstub Orginally the process was confined to optical equipment such as prisms - the earliest examples being made around 1900, later the scope of use was expanded to microelectronics, and other miniturised devices.