A SCATTERING RATIO METHOD FOR SIZING PARTICULATES IN A PLASMA

Particulates suspended in plasma can be sized in situ using the scatte ring ratio method, which involves measuring the ratio of the parallel and perpendicular polarizations of light scattered at 90 degrees. This method of plasma monitoring is of interest for controlling contaminat ion of silicon wafers and other thin film products during plasma etchi ng and deposition procedures. For parameters typical of plasma process ing, we report Mie scattering computations to test the method's sensit ivity to the optical design and to uncertainties in the particle param eters. A +/-20% error in the size determination can result either from an uncertainty of +/-1 either in the real or in the imaginary part of the refractive index or from a particle shape that deviates significa ntly from a sphere. A +/-5% error results from a 0.1 degrees error in aligning the scattering angle. To measure particulate diameters as sma ll as 0.05 mu m, the detector solid angle should be < 10(-5) Sr and th e extinction ratio of the polarizer must be < 10(-4). A calculation of the signal-to-noise ratio reveals that it is untenably weak for parti cle diameters smaller than about 0.04 mu m. The scattering ratio metho d is usually inapplicable for polydisperse particulates, but it will s till work in many cases for many plasma applications, in which particl es stratify in different layers according to their size.