An in situ scattered laser light measurement system, which can detect individual particles and observe their trajectories, has been produced experimentally and has been used with tungsten (W) etchback reactive ion etching (RIE) equipment. The particles which are smaller than 100 nm in size can be detected during plasma emission if the stray light that is caused by the laser light is properly suppressed. The trajectories of the particles are successfully observed in the W etchback RTE chamber by using this system under mass production conditions. Not only the appearance of the particle but also the direction of particle trajectory correlate distinctly with the specific operating state of the equipment. When the rf power was turned off, many of the particles that were observed seemed to be drawn towards the wafer. On the other hand, during injection of the N-2 purge gas to the process chamber, the few particles that were frequently observed seemed to fall down, away from the wafer.