Study of particle nucleation and growth in low pressure silane plasmas

Study of particle nucleation in processing plasmas has received increased attention in recent years. On the one hand, particle generation is considered harmful (in the semiconductor industry) and on the other hand particles generated with specific size, composition and crystallinity are useful for many material and opto-electronic applications. An important claim is that solar cells demonstrate stable efficiencies if the incorporated silicon films are deposited under conditions that are at the onset of particle formation in the plasma.; To eliminate particles or to grow them with specific requirements needs a thorough knowledge of how various parameters (reactor pressure, input power, gas temperature, substrate temperatures, input gas composition) affect the growth of particles. Several experimental and numerical studies have been performed towards this end. However, explanations for a large number of fundamental issues are not yet known. To address these key issues we have developed a quasi-zero dimensional model that couples chemistry and particle growth under silane plasma conditions. The addition of diffusion losses for species and particles makes the model quasi-zero dimensional.; The following are the key results: (1) In answer to the debate regarding the pathways to cluster formation, our mechanism predicts that the main clustering pathway in silane plasmas proceeds via anion-neutral reactions that involve silyl or silylene anions reacting with silane to give larger silyl or silylene anions with hydrogen elimination. (2) The model also answers the question regarding the critical particle density for particle coagulation to begin: the positive ion density represents the critical density for coagulation. Coagulation is usually hindered due to negative charging of particles in plasmas. Particle concentrations higher than the positive ion concentration allow for significant neutral and even positive fractions that sets in coagulation. (3) Finally we answer a question that has puzzled the community for some time: an increase in the gas temperature leads to a delay in the particle nucleation signal. We have related this observation to the temperature dependence of diffusion coefficients. For particles, the increase in temperature leads to increased loss rates and slower growth rates. These effects combine to reduce the particle density drastically, leading to the observation.