A measurement unit comprising a light source and a photodetector may be formed in a cavity in a substrate. The light source produces light that impinges a material layer and is reflected back to the photodetector. Through methods such as interferometry and ellipsometry, the thickness of the material layer may be calculated from the light intensity data measured by the photodetector. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.