Kortshagen The formation of ultrafine particles in processing plasmas has been a concern for some years. Contaminant particles can cause considerable product yield loss in microelectronics fabrication or they can severely impair the quality of thin films used for the production of photovoltaic cells or the coating of optical devices. While considerable progress has been made in the understanding of the transport and charging of micrometer-sized particles in plasmas, little is known about the nucleation of particles and the behavior of nanometer-sized particles. The nucleation and growth of small particles is observed in particular in plasma enhanced chemical vapor deposition systems. The particle formation in silane plasmas, which are of great importance for the deposition of amorphous silicon, has been extensively studied, however, the true nature of the particle formation mechanism has not been determined. Different and even contradicting scenarios for particle formation in silane plasmas have been proposed by various researchers. For instance, the important precursor for the particle nucleation in silane plasmas is still controversially discussed. The influence of the plasma conditions on the nucleation process are not understood. Also, experimental results on the particle nucleation phase are scarce, due to the serious difficulties in in-situ detection of nanometer-sized particles. Since the understanding of particle nucleation is a very complex problem, a combined experimental and theoretical approach will be pursued in the proposed project. The proposed research has three main objectives: 1. A nucleation model for particles in silane plasmas will be developed which takes into account the chemical aspects involved. 2. The nucleation model will be coupled to a self-consistent plasma model for a silane plasma chemical vapor deposition system in order to enable predictions about the occurrence of particle nucleation. 3. The model to be developed will be verified by a number of experimental studies on a capacitively coupled RF discharge in diluted silane. The nucleation model to be developed will consider the formation of hydrogenated silicon clusters by both anionic and neutral mechanisms. It will be coupled to an aerosol model capable of predicting particle growth, coagulation and transport. This model will be combined with a kinetic discharge model for a capacitive RF discharge. The kinetic discharge model, which will be based on already existing codes, will be used to calculate the spatial profile of the important species involved in the nucleation process. Since the nucleation rate depends sensitively on the precursor concentrations it is expected that predictions about nucleation thresholds and the spatial location of nucleation regions can be obtained. Unlike in some other approaches, the effects of a non- Maxwellian electron distribution function, which are crucial for an accurate determination of various chemical reaction rates, will be taken into account by calculating the actual distribution function by solution of the Boltzmann equation. The experimental studies will focus on the detection of small particles in order to verify the nucleation model and on the measurement of the electron distribution function using Langmuir probes in order to test the kinetic discharge model. For the particle detection a high power YAG laser will be used for evaporative particle explosion and electron detachment. Furthermore, a new diagnostics will be tested which is based on the propagation of acoustic waves coupled to the particle population in the plasma. This diagnostic should be able to reveal in-situ information about the particle size and mass even for subnanometer particles, which are extremely difficult to detect with laser methods.
