Technology

There are many optical technologies used for measuring transparent coatings on substrates, and most work on the same principle: light is reflected from the sample surface, and each ray of light is ‘split’ at the upper surface of the coating, with some light reflecting from this upper surface and other light penetrating down to the substrate and being reflected from there. If multiple layers of material are present, then further splitting occurs at each interface, with a proportion of light being reflected each time. When all the reflected rays have left the sample, they interact with one another to form an ‘interference pattern’. This pattern is then analysed, and the characteristics of the coating can be inferred from it.

All such technologies in common use, apart from BPR, analyse the interference pattern by plotting the sample’s reflectance as a function of the light’s wavelength – as the wavelength varies relative to the thickness of the film, the light intensity changes and a pattern of bright and dark ‘fringes’ is formed. These are called spectral methods – for example, spectrophotometry, spectroscopic reflectometry or spectroscopic ellipsometry. Thick films produce many fringes – thin films produce few fringes. The amplitude of the fringes is determined by the film’s refractive index, a material property which depends on the film’s composition and is particularly strongly correlated with its density. There is, however, a problem: the refractive index is not a constant, but varies

as a function of wavelength. Hence, a metrology technique using, say, 100 wavelengths also has to deal with 100 values of the refractive index – for each material in the sample – as well as the layer thicknesses. Although the situation can be simplified by using mathematical models to represent the variation of index with wavelength (the optical dispersion of the material), the number of raw data points collected can never catch up with the number of unknown parameters, and adding extra wavelengths or widening their range does not help.

BPR solves this problem by using a single wavelength, but illuminating the sample with light at many angles simultaneously – it does this by making use of a high Numerical Aperture microscope lens (see figure 1).

Because light at different angles travels through the film for different distances before being reflected back from the substrate, a pattern of interference fringes is formed just like when a spectral method is used. However, in the case of BPR, each data point in the interference pattern corresponds to the same value of the refractive index. The optical dispersion problem goes away, and the number of raw data points comfortably exceeds the number of parameters which must be measured. Therefore, the technology can make measurements with a much higher degree of accuracy than is possible with spectral techniques, without being dependant upon the use of dispersion models and without

prior knowledge of the material properties being required. It is therefore highly suited for use on film materials whose composition may vary significantly as the manufacturing process changes – for example, drug-eluting polymer coatings on medical implants.

For coatings (such as polymers) which may be under stress, BPR offers a further advantage in its capability for measuring strain-induced birefringence. By taking cross-sections of the reflected laser beam parallel and perpendicular to the polarisation of the laser source (see figure 2) BPR is able to measure the so-called s-polarised and p-polarised components of the reflected light, independently and simultaneously. For an unstrained film, both provide the same information, but where there is strain-induced birefringence the p-polarised component varies in ways which can straightforwardly be modelled to extract the birefringence value. Not only can BPR avoid errors in the thickness measurement, which would happen if the effect of birefringence were neglected, but valuable additional information about the coating is provided for the user.

Nightingale-EOS offers this technology in the form of its unique n-Gauge product, available from 21st June 2010.

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