This paper demonstrates a variety of metrology methods for high selective silicon nitride etch in different process baths. Capability of measuring full matrix components is also presented. For the measurement of H 3 PO 4 and H 2 O, both NIR spectroscopy and conductivity methods work well. Si measurement in the etchant that contains an organo-silicon compound requires a new reagent development in comparison with the original reagent method developed for regular process contains inorganic Si only.