Semiconductor processing tools that use a plasma to etch polysilicon or oxides produce residue polymers that build up on the exposed surfaces of the processing chamber. These residues are generally stressed and with time can cause flaking onto wafers resulting in yield loss. Currently, residue buildup is not monitored, and chambers are cleaned at regular intervals resulting in excess downtime for the tool. In addition, knowledge of the residue buildup rate and index of refraction is useful in determining the state of health of the chamber process. We have developed a novel optical fiber-based robust sensor that allows measurement of the residue polymer buildup while not affecting the plasma process.