Analyzing Thin Films with a Specular Reflectance Accessory

The role of thin films in solar cells and electronics is significant. They are also applied over eyeglasses to minimize glare (anit-reflection (AR) coatings) or to improve light transmission through the lenses. Since AR coatings are gaining traction among many consumers, it is necessary to measure and control these coatings.

The reflectance spectra of ophthalmic lenses are shown in Figure 1. The red line represents the reflectance spectrum of a lens applied with a hard coating, whereas the black line represents a lens consisting of an AR coating in addition to a hard coating.

Figure 1. (a) Ophthalmic lens with AR coating and hard coating; (b) Reflectance spectra of ophthalmic lenses.

Coating Thickness Measurement

Determining the thickness of the applied coating is a key QC process for industries using thin films as coatings. In ophthalmics, AR and the thickness of hard coatings are measured using spectral reflectance. In the ophthalmic industry, multiple coatings are applied in different combinations to manufacture superior products. Shimadzu’s Film Thickness software allows the analysis of a single film on a lens.

The wavelength between valleys and peaks in interference patterns is involved in the calculation of film thickness (d) expressed in the following equation as long as the angle of incidence and the coating’s refractive index are known:

Where, λ₁ and λ₂ are wavelengths of two noticeable peaks (or valleys); Δm = Number of peaks between the wavelengths, λ₁ and λ₂; n = Refractive index of the film; and θ = Angle of incidence.

The capability of analyzing multiple coatings is crucial to determine the area of interest in ophthalmics. Determining the thickness of individual coatings is also equally important before making a final product. This article discusses the application of the Shimadzu UV-2600 to analyze a single protective coating applied over an acrylic substrate using film thickness software and 5 degree relative specular reflectance accessory.

Experimental Procedure

The following scan conditions were used to acquire the reflectance spectra of an acrylic lens containing a single protective coating using the UV-2600 equipped with 5 degree relative specular reflectance accessory:

The mirror assemblies were mounted on the sample to obtain a background scan. In this setup, the mirrors were faced down in the reference stages to collect a baseline scan. Using a blank un-coated lens in place of the sample-side reference mirror provides the ideal background to measure lenses and acrylic materials using the 5 degree relative specular reflectance accessory. In this experiment, an aluminum mirror was employed as a sample mirror to collect a baseline scan. The acrylic lens then replaced the sample mirror to collect a reflectance spectrum (Figure 2).

Figure 2. (a) 5 degree relative specular reflectance accessory and external view of light beam path with accessory in place; (b) 5 degree relative specular reflectance accessory sitting in UV-2600 sample compartment with acrylic lens in place.

A small layer of the protective coating was removed and suspended in solutions of different refractive indices to estimate the coating’s refractive index by means of the Becke line test. The estimated refractive index of the protective coating was between 1.393 and 1.440.

Experimental Results

Figure 3 presents the reflectance spectra of the acrylic lens applied with protective coating. The protective coating’s thickness can be estimated with the help of the Shimadzu Film Thickness software if the values of incident angle and the coating’s refractive index are known.

Figure 3. Reflectance spectra of acrylic lens with hardcoat layer.

The Becke line test results are presented in Figure 4, showing the images captured for the samples and the estimated refractive index values.

Figure 4. Becke line test of coating particles suspended in solutions with refractive indices of 1.393 and 1.440.

After estimating the refractive index of the protective coating, the film thickness was derived for the 5° incident angle. The estimated value was between 3.638 and 3.761µm (Figure 5).

Figure 5. Summary of film thickness calculations by the film thickness software.

Conclusion

From the results, it is evident that the Shimadzu Film Thickness software can measure the coating thickness if the values of incident angle and refractive index are already known. The thickness of many different coatings can be easily and rapidly calculated using the Shimadzu Film Thickness software.

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.