Basic question about measurement of thin film by means of spectral reflectance

[pyrometer]

Hallo alle users,

as the title indicates, I have a question about measurement of thin film thickness by means of spectral reflectance.

Given, the to be measured thin film has thickness less than 600 nm, but the measuring light has only wavelengh larger than 600 nm. Does it mean, that the measurement results with this kind of light is not reliable for the wavelengh of the light is larger than the thickness of the to be measured film. In other words, the wavelength of measuring light should be smaller than the thickness of the to be measure film, shouldn't it?

Thanks a lot in advance!

 

[technician]

Light will be reflected with a maximum for the first time when the film is λ/4 thick.
This gives a λ/2 path difference through the film. There is a λ/2 phase shift at the air/film interface giving a total of λ phase shift and therefore a maximum.
So the important distance is λ/4 !

 

[pyrometer]

Thank you technician!