Prepare the cross-sectional sample by microtome or embedded in epoxy resin method. This is to prevent the layers from collapsing.

Next, thickness measurement is under FE-SEM or SEM.

Interferometric thickness measurement is quick and easy. If you routinely need to know film thickness, these instruments are very reasonable.

Dear Hui-Yen Ong, this question is repeated so many times on RG, my advise is to search within RG for them, they contain fruitfull contributions of experts.

Better will be if you follow a standard norm such as ASTM D7091-22. Please have a look at the following paper. My Regards

https://doi.org/10.1016/0040-6090(85)90273-1

Dear Hui-Yen Ong ,

The best tool for this is a reflectometer (same that Prof. James E Hanson mentions. You just need a few seconds to obtain an accurate measurement without having to make an step height or an scratch.

Hope this helps

Best regards

Paco