Our Clusterpartner Vortex Optical Coatings has recently published a paper about the advances of Afra Red Filters using Sputter Deposition. Infra Red Filters have been traditionally deposited using evaporation technology. In recent times the dominance of evaporation has been questioned due to the challenges of the latest instrumentation requirements and the availability of sputtering technology. Modern instruments can demand tighter specifications, higher temperature stability and a field service life of many decades often in harmful environments. Sputtered IR filters, which are more economically viable than in previous years, have been able to satisfy some of these requirements.
1. DEPOSITON BY TRADITIONAL EVAPORATION TECHNOLGY
Evaporation of IR filters has been popular because the high deposition rate means the large thicknesses required are achieved in an economically viable process time. However, the environmental properties of such films can sometimes make them unsuitable for the application intended. Two problems that have been identified are:
1.1 They can be prone to spectral drift within their working temperature range
1.2 They may not always be capable of surviving a tough environment.
2. TEMPERATURE VARIATION OF IR FILTERS
The microstructure of traditionally evaporated layers is known to contain ‘voids’ or gaps (Ref 1). When such a coating is exposed to the environment, such voids fill with moisture and the optical properties of these layers change, resulting in a filter shifting to a longer wavelength. When a filter is heated, this moisture is released to some extent and the position of the filter can shift.
Fig. 2A shows a typical performance of an evaporated filter at room temperature and when heated. Fig. 2B shows a sputtered filter during the same test. If the voids could be reduced or eliminated, these effects would also be greatly reduced. The deposition energy is much higher for sputtering and the filter has a much more densely packed structure and greatly reduced numbers of voids as a result of this, consequently the sputtered film will move less with temperature change.
Fig. 2A: Evaporated Filter Fig. 2B: Sputtered Filter
(+45 Deg. C=8 nm shift, blue Line) (+95 Deg. C= 9 nm shift, blue line)
3. ENVIRONMENTAL PERFORMANCE
In many applications, instrument designers require the instrument to operate in a challenging or aggressive environment. The key to strong environmental performance is also related to the film microstructure that deposition technologies produce. Fig. 3A below shows the structure of a traditionally evaporated film, Fig. 3B shows the sputtered equivalent (derived from ref 1).
The sputtered film contains less voids and does not allow contaminants in the environment to penetrate as easily, this means that such films are capable of surviving longer in tough conditions. This can involve reactive gases in an industrial process, exposure to high temperature and humidity or exposure to chemicals and vapors in outdoor applications.
Sputtered technology displays some definite advantages over traditionally evaporated filters and should be considered for applications where high temperature stability and robust environmental properties are fundamental requirements.
For more information visit our Clusterpartner website.