An etch rate monitor apparatus has a substrate, an optical element and one or more optical detectors mounted to a common substrate with the one or more detectors sandwiched between the substrate and optical element to detect changes in optical interference signal resulting from changes in optical thickness of the optical element. The optical element is made of a material that allows transmission of light of a wavelength of interest. A reference waveform and data waveform can be collected with the apparatus and cross-correlated to determine a thickness change. This abstract is provided to comply with 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.