Optical characterization of particle traps

Particles or 'dust' in etching or deposition plasmas are an important cause of product yield loss and equipment down-time. Traditional methods of particle control are only partially effective in plasma processing. This is because formation and transport of particles are strongly influenced by plasma electrical and chemical properties. Particle control in plasma processing requires understanding of these effects and their relation to aspects of tool and process design. Laser light scattering has been used to monitor the behaviour of particles for a wide range of plasma tools. This method provides information on location and transport of particles. In some variations, light scattering may also be used for particle size determination. Results have been obtained in sputter, etch and deposition tools of planar diode and magnetron-enhanced designs, Some results have also been obtained in electron cyclotron resonance tools and radiofrequency inductive tools. From this database, differences and common elements are observed for the behaviour of particles. The particle trapping phenomenon is often observed. Particle traps have an important bearing on wafer contamination. Traps cause particles to accumulate into localized regions during plasma operation, only to be suddenly released at the end of the process, thereby contaminating the wafer. Spatially resolved optical emission may be used to map the location and intensity of particle traps. This method also provides a semi-quantitative comparison with two-dimensional modelling studies. It may also be used to optimize grooved electrode design for particle contamination control.