Sapphire, due to its unique physical properties, can be used for manufacturing of optical components that work at extreme conditions – high temperature, pressure, mechanical load, aggressive environment and radiation. The quality of polished surface is of great importance for successful application of optical products.
Polishing technology that existed earlier and that was characterized by the use of strong etching reagents at chemical-mechanical treatment led to high roughness grade (Ra about 10A), but did not provide high precise surface accuracy, even for plane windows. Products produced by such technology find application in watch industry, electronics, etc., but are not good for precise optical devices.
The main difficulty of mechanical and chemical treatment combination is in the fact that most suspensions, used as etching reagents, at the same time corrode material of polishing instrument.
As a result of investigations worked up in our company, the technology of chemical-mechanical polishing of sapphire, that provides minimum roughness and, at the same time, high surface quality and surface accuracy, has been developed.
Surface Roughness during the polishing process was tested by power-atomic microscope Solver Pro.
Figures below demonstrate profilograph pictures of samples polished by our technology at various stages of surface preparation. As it is clear from the pictures, both, roughness and a surface structure, are being changed eventually – roughness becomes smaller and a surface structure becomes more homogeneous.
2D and 3D profiles for the time of treatment 15 minutes (Ra=6.046 nm)
2D and 3D profiles for the time of treatment 30 minutes (Ra=4.218 íì)
2D and 3D profiles for the time of treatment 60 minutes (Ra=2.001 íì)
2D and 3D profiles for the time of treatment 90 minutes (Ra=1.31 íì)
2D and 3D profiles for the time of treatment 240 minutes (Ra=0.969 íì)
The achieved roughness Ra of high precise polished surface is 9À. It is equal to the value that other manufacturers get at surfaces with low surface accuracy requirements. Such a small roughness with no «diamond sleeks» almost exclude light scattering that is very important, especially for laser application.
According to our technology high values of surface accuracy are also achieved at the same time – see below.
A complicating factor in precision machining of sapphire is the anisotropy of its mechanical and other physical properties. Least of all, it affects on flat surfaces, so there it is possible to achieve the highest results. And the most difficult of the spherical surfaces are hemispherical domes and microlenses. The technology was developed for hemispherical domes and microlenses with size from 3 mm to 180 mm and a wall thickness, respectively, from 0.25 mm to 6 mm.
According to the developed technology we manufacture windows with diameter up to 250 mm, surface accuracy (TWD or Flatness) up to 0.1 lambda (lambda 633 nm) and surface quality up to 20/10 MIL.
Inteferogram of transmittance wave front for sapphire window with parameters:
- Diameter: 120 mm;
- Thickness: 15 mm;
- Surface Quality: 20/10 MIL.
The technology of polishing has been developed and domes with diameter from 10 mm to 200 mm and wall thickness from 2 mm are being manufactured with average parameters as follows:
- Thickness variation over the whole part: 0.02 mm;
- Surface accuracy: in the central zone – 0.5 lambda (lambda 633 nm), in edge zones 1-1.5 lambda (lambda 633 nm);
- Surface Quality: 60/40 MIL.
Interferogram for the inside surface of optical sapphire dome, R=79.995 (central zone D50 mm)
Interferogram for the inside surface of optical sapphire dome, R=79.995 (edge zone D50 mm)
Interferogram for the optical sapphire dome with, R=85.015 (central zone D50 mm)
Interferogram for the optical sapphire dome with, R=85.015 (edge zone D50 mm)
Photo of sapphire dome D170 mm and domes D48 mm and D20 mm
- Diameter – 5-250 mm
Surface accuracy – up to 0.1 lambda
Surface quality: 20/10 MIL
Interferogram for the surface of sapphire lens