Optical Contact Bonding
When two transparent solid pieces with highly flat and clean surfaces are put into close contact, they will form a solid connection (bond), where they are firmly held together by intermolecular forces such as Van der Waals forces and hydrogen bonds. It may not be necessary to apply a high pressure, as the pieces naturally join. If the pieces are made of the same material, e.g. of some optical glass, they may even join such that one obtains a single piece with no apparent internal interface. It will then generally be close to impossible to separate the parts again; attempts to do so will usually lead to irregular fracture. Therefore, correct positioning needs to be found before the parts are joined.
Light propagating through the contact region may not be affected at all by the previous interface, propagating through the device just as through a single piece of a fully homogeneous medium, e.g. without acquiring any additional wavefront distortions and without causing reflections. If the materials are dissimilar, they will in general differ in refractive index, and that results in some Fresnel reflections as described by Fresnel equations, but without any significant additional effects such as wavefront distortions.
Common practical experience is that solid parts do not strongly stick together when brought into contact. This is because their surfaces are normally not sufficiently flat and clean, and the mentioned attractive forces, although strong in principle, have a very short range, which may easily be exceeded. For example, even microscopic roughness and/or a very thin layer of grease or dust particles on the surfaces will prevent the formation of a contact with the described mechanical and optical properties. (That has been recognized by Lord Rayleigh already in 1937 .)
However, there are methods of optical contact bonding, which essentially involve particularly careful polishing and cleaning of the surfaces, reaching a surface flatness on nanometer dimensions. Optical contact bonding methods do not involve the use of a contact agent such as an optical glue; they are adhesive-free. Details of the applied methods depend on what materials are involved and on how large the surfaces to be bonded are. Depending on the case, various processing steps may be involved after high precision surface preparation:
- Simply pressing the parts together with moderate pressure may be enough, but higher pressure is applied in some cases.
- A high temperature treatment (annealing) may help, particularly when joining dissimilar materials, but causing some level of interdiffusion.
- Some interdiffusion is sometimes also achieved by applying a high voltage; this is called anodic bonding, field-assisted bonding or electrostatic sealing. It may be applied, for example, for joining silicon parts with metals in the context of infrared optics, but it does not necessarily lead to optical bonding.
- Some additional process may be applied after surface polishing and cleaning, e.g. some chemical activation process for increasing the strength of the formed bond (chemically activated direct bonding).
Generally, this kind of bonding is more labor-intensive and costly than adhesive-based techniques due to the extreme precision requirements.
In some cases, the term optical contact bonding is understood in a more general sense as methods for joining solid pieces while achieving an optical contact. The term can then include methods based on applying an optical adhesive. However, the more commonly assumed stricter sense of the term includes only adhesive-free methods, and these usually have various advantages such as perfect optical quality, maximum resistance to high optical intensities, highest durability and no side effects due to additional substances.
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