Optical thin films for high performance telecoms and datacoms components require precision from preparation to completion. Whether the coating required is a high reflectivity dielectric mirror or a super-efficient antireflex coating the fundamental technologies are similar, of course, with an antireflection coating the layers may only be a couple of thousand atoms thick whereas a good reflector may be several microns. When chemically pure optical materials are evaporated in a vacuum they explode to fill the void travelling in straight lines to the molecules rest site. Material will then “grow” from these nucleation sites until thick layers are formed, these layers may become very stressed and spontaneous delamination may occur.

This implies that we must consider both the optical and the physical properties of the material. There is another factor that has to be addressed, the environmental stability of the finished film and; the associated attributes that affect it. The most important of these is perhaps the porosity of the film, less dense films tend to allow the ingress of moisture causing both a change in the physical dimensions and thus the optical thickness and also creating a graded interface with a material with a higher refractive index than air i.e. the effective index of the bulk film. So there are two possible solutions, post deposition processing to saturate or cure the film, or, introduction of a method to densify the film during deposition. This can be done in several ways, it is common to raise the temperature, and this is fine for lenses and windows but not for semiconductor devices.

Plasma sources come in many shapes and sizes varying from large radio frequency ball plasmas to small continuous stream ion guns. The idea for the latter stems from research into space craft propulsion where a small amount of pure gas is ejected through a tube and ionized to create an energetic plume forcing either a motile object forward or allowing a static object to bombard a close-by forming monolayer with charged atoms and thus creating a dense film. This is called Ion Assisted Deposition.

When depositing a high precision antireflection coating, that is, one with an ultra-low reflectivity it is also important that the deposition parameters do no change during the run in a way that can alter the refractive index and thus cause a bulk inhomogeneity or perhaps worse still, introduce absorption - a key issue where high laser powers are involved demanding that only the most pure deposition materials are used and the utmost care be taken in cleaning tooling, shielding and other chamber internals. Any absorption within an optical coating in a high energy laser path can lead to catastrophic optical damage and will yield films of low laser threshold damage.