Coating Materials News Vol 7 Issue 4

Introduction:
A large industry has grown up around the coating of ophthalmic lenses to reduce surface reflections. AR coated eyeglasses have been historically more popular in Asia and Europe than in the US by 4 to 1, but the trend is changing as people begin to appreciate the benefits of reduced glare.

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.

• Surface cleaning / preparation
• Low temperature deposition
• Repeatability
• Low reflection and consistent residual color
• Adhesion and scratch resistance of the coating
• Environmental durability

Adhesion to Polymer Surfaces:
The first problem that required solution was adherence to the polymer surface. A typical coating load of 20 lens pair imposes a surface area of nearly 5000 cm² from which water and other volatiles can degas and either contaminate or reduce surface adhering forces. Since polymers absorb water and cannot be heated to temperatures sufficient to drive off absorbed or adsorbed water, the water must be either pumped away or incorporated in the coating in a benign combination. Lenses are generally heated above room temperature before loading into the vacuum chamber. Then they are pumped in high vacuum for a period of time to remove the weakly bond water. Often a cold surface is included in the chamber onto which water will condense. Following this partial dehydration, a thin adhesion layer, generally <50å chromium,="" is="" deposited.="" the="" cr="" reacts="" with="" residual="" water="" and="" oxygen="" to="" oxidize="" to="" a="" non-absorbing="">_xlayer to which subsequently condensing oxide compound layers bond. This step is crucial in preparing the polymer surface for the TiO₂ layer.

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:
Sputter deposited coating of lenses in place of evaporated deposition has the advantage of minimizing the temperature of the lenses, thereby reducing thermal coefficient difference induced stress levels. Further, these depositions possess higher packing densities and are therefore more stable to wet exposure including erosion by skin salts and oils. However, sputter deposited layers often have higher intrinsic stress, leading to haziness due to microscopic crazing, especially if cohesive and adhesive strengths are marginal. This is one of the reasons evaporation is more widely used in the ophthalmic industry than sputtering (another is cost vs. volume throughput).

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.

Deposition Parameters:
Specifications for eyewear include low reflection and high transmission as well as good durability. It is important therefore that the coating layers have low absorption in addition to good adherence and hardness. Low absorption is not a problem when coating glass where the temperature of the substrate can be increased to near 200º C to insure complete oxidation and high packing density. The conditions for coating polymer surfaces are more demanding since the benefits of high temperature are not available.

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.

Coating Durability:
Resistance to scratching is a requirement for eyeglasses. The customer often does not exercise care in cleaning, and succeeds in scratching the coating. The AR coating, in spite of its composition based on oxides, is not as durable as the bulk materials or as coatings deposited on glass where the temperature can be increased. Coatings on plastic substrates are soft because they possess lower packing density (with the exception of sputtered films). The chemical hard coat mentioned previously increases the scratch resistance to a certain level. Layers of silica have been applied directly to CR-39 to increase scratch resistance using plasma enhanced CVD. At 3µm minimum thickness they have proven very effective. The cost penalty for this process is not always justified.

Typical AR Design:
The following is an illustration of a 4-layer silica / titania design on CR-39, and its computed performance. The residual reflection is placed in the green-blue region for cosmetic appeal. The thin layers require accuracies in thickness and refractive index of <5%. material="" properties="" and="" evaporation="" process="" control="" must="" be="" very="" repeatable="" to="" ensure="" high="" yield="" production.="">

 Computed performance of the design showing a green residual reflection. Average R over the visible region is ~0.5%.

Tinting of Eyewear:
Apart from the residual color from the AR coating, lenses are sometimes tinted to produce a fashionable color. As mentioned above, one method of increasing the durability of lenses is to deposit a thick layer of silica directly on the CR-39. If a few percent by weight of metals such as gold, silver, copper chromium, etc. is added to the silica, selective absorption of the metal ion will produce coloration.

Projected Advances:
Scratch resistance improvement of polymer lenses is one of the major areas needing attention. This will come about through advances in materials and deposition processes. A factor driving the eyeglass market is the added expense of coating, which is a large fraction of the manufacturing cost of the lens itself. Batch coating helps the problem, but technology exists for increasing batch sizes at low risk. Stripping the coating off in the event of errors is possible, but costly.

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.

Conclusion:
This summary article serves to aquatint the reader with the current status of ophthalmic coating, and with some of the challenges yet to be conquered.

Samuel Pellicori, Principal Contributor
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922

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