Modeling Pulsed Power Plasmas and Applications to In-situ Nanoparticle Growth

Abstract

Low temperature plasmas (LTPs) have been an essential tool for semiconductor device fabrication. LTPs have been increasingly used in other fields, such as for the synthesis of nanometer sized particles. The prevalence of LTPs is largely due to the unique plasma environment – energy is coupled into the plasma electrons, resulting in unique chemistry far from thermal equilibrium. As such, the specifics of how power is coupled into the plasma system plays a primary role in how chemical reactivity can be utilized. This has led to techniques such as using pulsed power to increase control of the plasma, however, this technique has not been fully characterized particularly from a computational modeling perspective. Modeling of pulsed inductively coupled plasmas was done using the Hybrid Plasma Equipment Model (HPEM), a 2-dimensional reactor scale plasma multi-fluid simulator, and found that pulsing in electronegative plasmas requires resolving both the electromagnetic and electrostatic effects from the antenna. These systems are a sensitive function of the matching network used for power delivery, with good agreement from experiments. LTPs have been used to synthesize high quality nanoparticles (NPs) that would be difficult to create using traditional methods, which may sinter or fail to anneal. Particles in plasmas generally charge negative, leading to mutual coulomb repulsion that results in monodisperse particle size distributions. However, a comprehensive growth mechanism of NPs in plasma is not well understood. Measuring even the most basic plasma parameters in these reactors proves challenging, leaving large gaps in knowledge about their fundamental operation. In this thesis, a 3-dimensional kinetic simulation for particle growth and trajectories in LTPs, the Dust Transport Simulator (DTS), was developed to interface with the HPEM for self-consistent reactor scale modeling of plasma-based nanoparticle synthesis. Electrostatic trapping of particles in the plasmas was found to be a major component of the nanoparticle growth mechanism – a result contrary to what was previously believed in the field, and further confirmed by multiple experiments and models from several research groups. The updated DTS was used to develop scaling laws for nanoparticle growth rates as functions of plasma operating conditions, a necessary tool for the future design of NPs. Finally, the idea of using pulsed power was theorized as a viable tool to customize particle size distributions by manipulating the nanoparticles trapped in the plasma. The main contributions to the field of low temperature plasmas and nanoparticle forming plasmas are as follows. First, pulsing the power in inductively coupled plasma (in many cases) requires transitioning from primarily electrostatic to electromagnetic power deposition, which is a sensitive function of the matching network. Second, electrostatic trapping of nanometer sized particles growing in the plasma occurs under typical operating conditions and may be the predominant factor for predicting particle growth. Third, trapped particles can be manipulated, for example by pulsing the power, to customize the nanoparticles grown in the plasma. Finally, the updated capabilities of the DTS may be used by others to investigate particle growth in low temperature plasmas.