The Film Thickness (FT) module measures the thickness of optically transparent films and is optionally available for MT3000. FT measurements are based on the evaluation of the light spectrum of a laser beam that is reflected on the film layer boundaries. This method can measure single, double, and even triple transparent layers on non-transparent substrates.
A variable laser spot size allows for targeted measurements in the active area of integrated circuits.
The basic principle behind measuring film thickness is a comparison of spectral analysis of the reflected light intensity, once measured with and once without the layer of unknown thickness.
Non-polarized white light incident perpendicularly to the surface is applied.
The resulting spectra depend on the materials of the substrate and the film as well as on the thickness of the layer The spectral evaluation is performed fully automatically applying complex mathematical algorithms incorporated in the MT3000 software
Depending on the film thickness, two different methods can be used: either FFT or FIT.
Thick films are evaluated applying the FFT method.
Thin and multiple films are measured using FIT.
The FIT method can measure up to three layers simultaneously
Measuring thickness with the FIT method
FIT finds the best match between the measured spectrum and a theoretical model spectrum The model parameters then reveal the layer thickness. The model parameters can deal with up to three different layers.
The following figure shows a measured reflectance spectrum of a film with two layers in blue and the calculated best-fit model spectrum in red.
Blue: measured reflectance spectrum of a two layered film.
Red: the calculated best-fit model spectrum
Measuring thickness with the FFT method
FFT calculates the thickness by searching for periodic structures (minima and maxima) in the reflectance spectrum. In combination with the optical properties of the film, FFT calculates the film thickness from the frequency of the most regular and of the strongest periodic signal
In the MueTec 3000 software a user-interface window displays the frequency spectrum (highest peak corresponds to the most prominent frequency signal) and the calculated result as seen below:
The FFT frequency spectrum of a substrate reflectance spectrum
The optional MueTec FT measurement application provides the customer with the flexibility that allows reliable film thickness measurements of thickness ranges from 40 µm down to 10nm.