Real-time control of chemical-mechanical polishing processing by monitoring ionization current

A method and apparatus are described for detecting an endpoint of a film removal process in which a target film overlying a stopping film is removed. A chemical reaction product is generated from at least one of the target film and the stopping film; this chemical reaction product is converted to a separate product. The separate product is exposed to ionizing radiation. The ionization current generated by the radiation is monitored as the target film is removed. A change in the ionization current corresponds to a change in concentration of the separate product, thereby indicating the endpoint of the film removal process. In the particular case of removal of a silicon dioxide film overlying a silicon nitride film by chemical-mechanical polishing, the reaction product is ammonia extracted from the polishing slurry. The ammonia is converted to ammonium chloride by a reaction with hydrogen chloride vapor. As the ammonium chloride, entrained in a carrier gas, is pumped through a detection unit, a change in the concentration thereof is detected by monitoring a change in the ionization current generated by a source of alpha particles.