I have measured a reduction in thickness for a thin film using the ellipsometry technique and I need to quantitatively show that the measured value is significant.
As mentioned by Uwe, the best way to validate the thickness results from ellipsometry is to confirm them using another method. For given materials and thickness ranges (and for a given optical model), you use the other method to get the validation for a few samples, and then you can trust that ellipsometry is giving you the right thickness within a certain confidence limit.
However, if the thickness of the film is very small, the refractive index and the thickness parameters in the model will likely be correlated, which significantly lower the confidence on the calculated thickness values. It becomes even more important to use the right value of refractive index (and it is not rare that it changes with thickness for very thin films), which makes it even more useful to use a second method of thickness determination (it can help finding the right value of refractive index, as well as validating the thickness value).
As mentioned by Uwe, the best way to validate the thickness results from ellipsometry is to confirm them using another method. For given materials and thickness ranges (and for a given optical model), you use the other method to get the validation for a few samples, and then you can trust that ellipsometry is giving you the right thickness within a certain confidence limit.
However, if the thickness of the film is very small, the refractive index and the thickness parameters in the model will likely be correlated, which significantly lower the confidence on the calculated thickness values. It becomes even more important to use the right value of refractive index (and it is not rare that it changes with thickness for very thin films), which makes it even more useful to use a second method of thickness determination (it can help finding the right value of refractive index, as well as validating the thickness value).
Dear Uwe & Dear Daniel,
Many thanks for your help. I measured the thickness change of a thermo-sensitive polymer (poly-N-isopropylacrylamide) deposited on a Si wafer substrate by iCVD upon heating. The initial thickness is about 100 nm. Upon heating, ellipsometry results shows a 4 nm decrease in thickness. Now I am trying to understand whether this 4 nm is significant?
Since it should be calculated upon heating, I am not sure if electron microscopy, for example, can help. Is there any possibility that I can deduce something purely based on the ellipsometric parameters e.g the magnitude of delta (phase difference) and the Psi (amplitude difference ) about the significance of the measured thickness change?
Regards,
Sirous
Does the ellipsometric analysis also show a variation of the refractive index? What about a potential oxide layer on silicon, that could grow after the heating treatment? Ellipsometry is quite sensitive to small changes in a sample, but extracting accurate information can be difficult. For a 100-nm film on Si, I feel that a good selection of angles and wavelengths, and the use of a good model, should lead to a reliable estimation of the thickness (assuming a variable angle spectroellipsometric instrument).
What about X-ray techniques like x-ray reflectivity - this usually allows a very precise thickness determination (although this is not an ellipsometry technique ;-D)! Kind regards, Dirk
You could also do X-ray fluorescence analysis, where already the intensity of the lines belonging to your thin film gives an indication of the thickness change. If properly calibrated, you can also quantifiy a mass deposition ( density * thickness ) for your thin layers.
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