Oh, I forgot to say, that the thickness of the top silicon layer is around 220nm or around 250nm.

You can use Imaging ellipsometry in a mapping mode to get the thickness of SiO2 local in 1 micron resolution.

Best regards

Christian

Thank you very much as I also learned that this works for Si, too.

i think if the si layer on top is still semitransparent than it is possible to use, yes.

The thickness of 220-250 nm is in the ideal range of the ellipsometry technique.  It will work very well for this problem. 

you could try with AFM (atomic force microscopy) covering part of the deposition to make a step from bulk and the top of the film

SEM and AFM will be destructive techniques I think?

Silicon should be quite transparent to wavelengths above 1100 nm so if your light source can do that ellipsometry should work very well.

SEM is not destructive if the film is silicon. The problem is related to the treatment of the sample. For cross-sectional SEM you need to cut a slice of Wafer, for AFM the scan area is very short (micron) so if you need a map of your thickness probably the two mentioned measurements are not so good. 

Yes when I said that the SEM would be destructive I meant that cutting a slice from a wafer would be destructive.

I think ellipsometry is the best non destructive methode :-)

See the  text book by W.R. Runyan, Semiconductor Measurement and Instrumentation, Texas Instruments. The colour charts on p. 174 are helpful for thermally grown SiO2 and sputtered films of  ZnO on  Si. He also gives a large number of alternative techniques for measuring film thickness.