Thin Film Analysis
Interferometry has been extended to measure the thickness of transparent and semi-transparent films. Thick and thin films present some potential problems during optical measurements because of the possibility of producing signal from different layers, which can confuse the measurement and sometimes produces an incorrect answer. As the effects of films coatings on interferometry measurements are now becoming more widely understood there is growing interest in film thickness measurement (thin-film surfaces and thin-film thickness).
For films with thicknesses in excess of 1-2 μm the vertical scanning measurement of the film leads to a series of fringe envelopes, each fringe envelope corresponding to a surface interface. It is relatively trivial to locate the positions of these envelope maxima and therefore determine the film thickness, assuming the refractive index is known. For thinner films the measurement leads to the formation of a single interference maxima.
In this case, it is appropriate to describe the thin-film structure in terms of optical admittances; this data that can be treated by the introduction of a new function, the ‘helical conjugate field’ (HCF) function. Researchers from Taylor Hobson have developed the HCF function to overcome the problems associated with overlap of the fringe envelopes. This function may be considered as providing a ‘signature’ of the multilayer structure measured so that through optimization, the thin-film multilayer structure may be determined on a local scale.
Thick-film analysis
The thickness and real surface roughness of films can be obtained by measuring a step or by measuring through the film to the substrate. Because of the high z resolution of the CCI technique it is an ideal technique for measuring film thickness. In CCI for thick film measurement the optics are moved vertically so that the upper and lower film surfaces pass through focus. For each pixel a set of interference fringes is obtained during the scan: the first one corresponding to the air/film interface, the last corresponding to the film/substrate interface. The CCI algorithm then calculates the maxima of the fringes envelopes with sub nanometre resolution. The film thickness is calculated from the distance between the maxima and the refractive index of the film.
Measurement of the top surface of semi-transparent surfaces can also be a problem. Normally measurement of the roughness of top surfaces of semi-transparent films is complicated by the signal coming from other surfaces. If possible the problem can be solved by setting the scan length so that only the top set of fringes are covered by the scan and the signal from the bottom set of fringes is not seen. This can be a problem if the surface is thinner than about 4 μm or if the measurement is part of an automation process. In order to overcome this problem it is possible to instruct the software to just use the signal from the top surface alone. It is also possible to measure other surfaces by specifying which of the fringe envelopes is to be studied. Care has to be taken with the analysis of the surface apart from the top surface because the measurement data can be affected by any non-homogeneity in the film.
For thinner films, the fringe envelopes may overlap. In this case, an intensity-based threshold can be applied to separate the two sets of fringes. This means that thicknesses below 2 μm can be measured with the same accuracy as the thicker films shown above. However as the fringe envelopes approach each other they will start to deform the overall shape of the waveform and therefore the thin-film measurement.
Thin-film analysis
Conventionally, thin-film structures are measured using either a spectrophotometer or an ellipsometer. For large-area vacuum-deposited thin-films (single or multi-layer) but there is now a growing requirement for measurement of thin-film structures that only exist on a local scale. It is impossible to easily study the signal from the top of the thin-film and the signal from the substrate if the film is less than about 1.5 μm because of the fringe overlap. Recent studies have shown that through the development of the HCF function, it is possible to extend CCI to include the calculation of layer thicknesses from this type of sample.
An additional benefit is that, through the knowledge of the thin-film structure, the phase-change on reflection may be compensated so that ‘true’ step-heights may be obtained. Researchers at Taylor Hobson have shown that through performing CCI experiments on a thin-film-coated substrate (single or multi-layer) together with a ‘reference’ substrate, such as the glass BK7 with a similar reflectivity to the film, it is possible to construct the HCF function. And from this procedure the film thickness can be calculated for films thicknesses down to about 20 nm.
Interfacial surface roughness
The surface roughness of a thin-film sample is often a composite roughness of the top surface and the bottom surface. This can lead to large errors in the true surface roughness of the top surface. Because the HCF can be used to obtain separate information from the different layers it can be used to analyse the surface parameters for the interfaces, including the air/top surface interface. This means that it is possible to measure the true roughness of the top surface of a thin-film sample where the film thickness is greater than about 20-30 nm. It is also possible to measure the interface roughness.
