 ## Thin film thickness calculator

 Web alacritas

This tool calculates the thickness of your transparent thin film from the interference peaks and valleys.
1. Take a transmission or reflection curve of your thin film.
2. Measure the positions of the peaks and valleys (need at least one of each).
3. Get a reasonable value for the refractive index of your thin film.
4. Enter this data in the form on this page.
5. Click 'calculate film thickness'
6. The thickness of your film will be calculated

 film refractive index : Wavelength units Angstroms nm microns Enter your measured wavelengths Extrema 1 : Extrema 2 : Extrema 3 : Extrema 4 : Extrema 5 : Example transmission curve ### How it works

At any extrema we know that the reflections from the front and back of the thin film are either exactly in phase or out of phase. The phase shift between the reflections from the front and back of the thin film is
phase shift = 4.Pi.n.d/wavelength
(n= film refractive index, d = film thickness). At any extrema we know that the phase shift is an integer multiple of Pi, but we don't know the value of the integer. If we take two adjacent extrema then we know the difference in the multiplying integer for each extrema is 1. We can therefore solve the phase shift equation as two simultaneous equations and extract the thin film thickness.

### Errors

In the solution of these simultaneous equations we end up using a difference between two wavelengths. So even if we know the wavelengths accurately, the errors in the differences can be significant. For example if we have a minimum at 500nm +/- 2nm and a maximum at 525nm +/- 2nm. The accuracy of the minimum measurement is 2/500 = 0.4%. But the error in the difference is +/- 4nm in 25nm, that is an accuracy of 4/25 = 16%.

So this is not a precise method of deriving thin film thickness, and to get a reasonable thickness value several extrema values should be used.