October 09, 2014
Most optical coatings are used to enhance reflection or transmission properties of a substrate material within an optical system. They usually consist of one or more thin layers of various materials in order to achieve the desired reflection/transmission ratio. These layers are deposited on an optical component such as a lens or mirror. The performance of an optical coating is dependent on the number of layers, the thickness of the individual layers and the refractive index difference at the layer interfaces. The optical coatings used on precision optics fall into a number of categories such as anti-reflection coatings, high-reflection coatings, beamsplitter coatings, filter coatings, extreme ultraviolet coatings and transparent coatings.
A number of metrology tools have been employed to measure film thickness. Coherence Scanning.Interferometry (CSI) is becoming a popular technique because of its high lateral resolution and speed. However, one of the limitations of traditional interferometry is the thickness of the coating that can be measured. Typically it needs to be larger than 1.5 μm to obtain accurate data. It is now possible to measure thicknesses down to 50 nm or less using Coherence Correlation Interferometry (CCI)1 together with HCF (Helical Complex Field)3 techniques.
Film thickness analysis – the solution
A new solution to this problem (HCF)3 has been developed to extract the film information. Through the application of the HCF function, Coherence Correlation Interferometry (CCI) has become the ideal method to obtain film thickness information. HCF can be used for thickness measurement with better than 1% accuracy within the range of ~ 5 μm to ~ 300 nm. Film thicknesses down to 50 nm have been measured; however, care needs to be taken with these very thin films as the accuracy depends on the optical properties of the material.
Conclusions
The development of the Helical Complex Field (HCF) functioning together with Coherence Correlation Interferometry provides us with the ideal metrology tool to perform fast and accurate measurements of coated optical surfaces.
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