SPIN COATING APPLICATIONS
Spin coating is a useful application when it is desirable to coat only one side of a part or when it is desirable to coat different sides of a part with different coatings. Spin coating is typically used for small or medium sized parts that are nearly flat and symmetrical in shape. This technique is used extensively to coat ophthalmic lenses and can be used to coat both the convex and concave sides of a lens. Spin coatings also has some limitations on parts that contain holes, protrusions, or other surface irregularities.
The basic equipment for a spin coating application consists of a spinning device, a coating delivery device, a collection tank/reservoir, a pump, and a filtering element. The spinning device holds the part securely in place, typically in an inverted position, and allows the part to be rotated at a controlled speed. The coating delivery device dispenses the coating onto the part. The collection tank/reservoir collects the excess coating (the spin-off material). The pump and filtering element circulate the coating back to the delivery device.
In general, the thickness of a spin coated film is proportional to the inverse of the square root of spin speed as in the below equation where ω is angular velocity/spin speed and hf is final film thickness.
Spin Coating Thickness Equation
This means that a film that is spun at four times the speed will be half as thick. In general, spin coating can produce uniform films from 600 to 6000 rpm, which means a maximum variation in film thickness of about 3.2. For example, a coating that has a dry film thickness of 20 nm at 6000 rpm it will be about 64 nm at 600 rpm.
The exact thickness of a film will depend upon the material concentration and solvent evaporation rate (which in turn depends upon the solvent viscosity, vapor pressure, temperature, and local humidity) and so for this reason spin thickness curves for new coatings are most commonly determined empirically. Typically, a test film is spin coated and the thickness measured either by ellipsometry or surface profilometry (Dektak). From this one or more data points, the spin thickness curve can be calculated – usually with a good degree of accuracy. The spin speed can then be adjusted to give the desired film thickness.
In a typical ophthalmic application, an inverted lens is fixed on the spinning device and lowered into the collection tank so that the surface of the lens contacts the coating solution, which is directed by a fountain onto the center of the lens. The lens is usually rotated slowly during this operation to ensure that the entire surface is properly wetted. The lens is then raised out of the coating solution and rotated at a high rate of speed. This rotation spreads the coating evenly across the surface of the part and removes any excess coating from the part. The speed of the rotation (spin rate) and the duration of the rotation can be used to control the coating thickness and can affect the dry time on the coating.
The circulation system should provide a sufficient turnover rate to adequately filter the coating solution. Ideally, the filtration system should consist of two or more filters in series, a configuration that allows for the use of coarse prefilter and finer subsequent filters. The optimum filtering configuration will depend on the coating being used and the rate of circulation through the filters. In a typical spin coating application, the circulation rate is limited by the allowable delivery rate. In general, the delivery rate should be set as high as possible, while maintaining flow control and avoiding bubble generation or splatters as the coating is sprayed on the part. The flow rate should also be adjusted to avoid creating a large pressure differential (>10 psi) across the filter elements. A high-pressure differential across the filters can cause coating gels to extrude through the filter elements. The size of the filter elements can be adjusted to reduce a high-pressure differential, although this may also affect the cleanliness of the system.
In any coating application, the amount of coating that is deposited on the part, i.e., the coating thickness is a function of the coating rheology, solvent composition, surface tension, density, and viscosity. The coating thickness can also be influenced by the coating environment. Anything that causes the coating to dry quickly, e.g., high coating temperature, high environmental temperature, low humidity, high airflow over the part, high temperature on the part, etc., can increase the coating thickness. In a spin coating application, it is generally most effective to use the coating solids and spin rate to adjust in coating thickness, while keeping the other coating properties and environmental factors constant. Good control over the coating thickness is important in achieving optimum performance of PCI VueGuard 901™ coatings. The coating thickness will affect the abrasion resistance and possibly other performance properties of a product. An optimized coating thickness can be determined empirically, according to the desired performance features of the final coated product.
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