Coating Materials News Vol 7 Issue 4
|October - December, 1997
Coating Ophthalmic Lenses
The majority of lenses in use today are of polymer composition rather than glass. The low temperature tolerance and mechanical softness of polymers presented the coater with a new set of problems. This article reviews some of the technology that has been in place for many years in this industry.
The two most common polymers used in eyeglasses are injection molded polycarbonate and thermoset cast CR-39. Each has its advantages. The impact resistance of polycarbonate is well known, but it is less scratch-resistant than CR-39. Polycarbonate has a higher refractive index than CR-39, 1.586 vs. 1.503 respectively at 586 nm, so AR coating is easier. The higher reflection of polycarbonate is reason enough to apply an AR coating.Some of the technology required to bring AR coating of polymer lenses to a routine production process include the following:
Adhesion to Polymer Surfaces:
An oxygen plasma can be introduced to promote adhesion through the formation of surface polymer-metal bonds. Pure argon plasmas do not promote nucleation, but adding at least 10% oxygen is effective in making the plasma reactive. Some metals nucleate well with nitrogen vs. oxygen plasmas. When used in excess, this technique can backfire by creating deeper damage and preventing strong bonding. Generally a minute of 5kV discharge at a partial pressure of ~10 µm is sufficient unless the surface is contaminated. This "glow discharge" cleaning step can also drive water and hydrocarbons off the surface. Both the plasma cleaning and the metal adhesor layer are often used to insure good nucleation, growth, and adhesion.
Another technique that aids adhesion and final durability is to apply a chemical hard coating to the surface before AR coating. The material is an acrylic, and is applied by dipping or spinning, then cured through reaction with UV. The new surface layer is up to 7 µm thick. Formulations are available for the variety of polymers used in the optical industry.
Deposition Techniques in Use:
As discussed in previous CMN issues however, sputter deposition lends itself to easier automation and thereby more consistent repetition of optical performance than electron-beam evaporation. In the latter, irregular erosion of the source material affects the deposition distribution pattern, and might thwart the randomizing ability of the rotation pattern to produce sufficiently uniform and repeatable thicknesses.
It is important, therefore, that the source material in the e-beam hearth be properly prepared and remain consistent in form as well as composition. Uniform evaporation of the low index material, SiO2 , is particularly difficult to maintain. The starting silica material is in the form of small irregular pieces, and evaporation proceeds only from the surfaces where the e-beam is fusing the material. The e-beam must be swept at a high rate to create a source larger than a point in order to maintain a high deposition rate without drilling. Without consistent sweep, a tunnel will be drilled from which the distribution pattern will leave in a narrow beam.
All of the titanium oxide compounds available for vacuum deposition have been investigated in the search to provide a material form and oxidation state that will produce low absorption films at deposition rates appropriate with volume production. The meltable compounds Ti2O3 and Ti3O5 are favored starting compounds for forming TiO2. The evaporation rate of titania layers is easier to control because the material melts into a pool. Mixtures of titania with zircionia and tantala have been developed as substitutes for pure titanium oxides. They produce a more consistent composition and crystal state, thus leading to homogeneous films.
Typical deposition parameters for growing the silica layer are oxygen partial pressure near 2 x 10-5 Torr, evaporation rate 2-5 Å/s. The parameters will vary with coating chamber geometry and possibly with time as water is degassed. A shift in color and average reflectance will be experienced upon venting into the microstructure of the film layers. Processes that increase packing density, such as ion assist or sputtering, will decrease the stabilization time. The designer must anticipate and accommodate the shift in the production process.
Typical AR Design:
Computed performance of the design showing a green residual reflection. Average R over the visible region is ~0.5%.
Tinting of Eyewear:
Another durability issue is chemical resistance, where the user often does not or cannot clean his or her glasses with the care the deserve. Partial solubility of the coating, made worse by the presence of pinholes, is a concern because the coatings are not fully dense and impermeable. Again, economical techniques and materials need development. Sputter AR coatings provide advantages in durability, but at a greater expense. Harder oxides such as ZrO2 and Nb2O5 are being explored as substitutes for TiO2. But increased stress experienced, which reduces durability.
Dr. Ervin Colton, Editor
P.O. Box 1178 | Milwaukee, WI 53201
Phone: 414-289-9800 | FAX: 414-289-9805
Samuel Pellicori, Principal Contributor
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