A system and method is provided for determining and controlling development of a semiconductor substrate employing fluorescence spectroscopy. One aspect of the invention relates to a system and method employing fluorescence spectroscopy to facilitate control of a chemical trim etch process during development of a photoresist material layer. The chemical trim etch process comprises applying a trim compound or material to a patterned photoresist. The trim compound or material is diffusable into the sides and top of the patterned resist. The diffused regions of the resist are soluble in a developer, which facilitates creating smaller features in the patterned photoresist. The fluorescence spectroscopy system can be employed to measure isolated and dense gratings or CDs and use the evolution of the CD to determine when to terminate the chemical trim process.