Modeling discharge and surface processes in capacitively coupled reactors
Yang, Yang (2010) Modeling discharge and surface processes in capacitively coupled reactors. PhD thesis, Iowa State University.
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Plasmas are ideal for producing reactive species (radicals, ions) for modifying surface properties to achieve desired mechanical or chemical functionality. Two of the most technologically (and commercially) important applications of plasmas are etching/deposition for microelectronic fabrication and functionalization of polymers. Among these applications, capacitively coupled plasma (CCP) sources are widely used. In this work, different types of capacitively coupled plasma sources and fluorination of polypropylene in a large-area CCP source are modeled using a 2-d hybrid plasma equipment model. As improvements to the model, algorithms such as a full-wave Maxwell solver, fully implicit electron drift-diffusion transport, and fully implicit electron momentum transport were developed and integrated into the model. In this thesis, we looked at the following problems: Magnetically enhanced, capacitively coupled radio frequency plasma sources are finding continued use for etching of materials for microelectronics fabrication. MERIE (magnetically enhanced reactive ion etching) sources typically use magnetic fields of tens to hundreds of Gauss parallel to the substrate. Multi-frequency sources are used to separately control the magnitude of the ion and radical fluxes (typically with a high frequency source) and the ion energy distributions (typically with a low frequency) to the substrate. The properties of a dual frequency MERIE reactor are discussed using results from a computational investigation. There is a gradual convergence of the ion flux to the wafer from being nearly uniform to center peaked with increasing strength of radial magnetic field from 0 G to 200 G. There are peaks in electron temperature at both electrodes and a local minimum in the bulk plasma for a radial magnetic field of 150 G due to local sheathing heating from decreased cross field mobility. Dual frequency, capacitively coupled plasma (DF-CCP) tools for etching and deposition for microelectronics fabrication typically use a high frequency (HF, tens to hundreds of MHz) to sustain the plasma and a low frequency (LF, a few to 10 MHz) for ion acceleration into the wafer. With an increase in both the high frequency and wafer size, electromagnetic wave effects (i.e., propagation, constructive and destructive interference) can affect the spatial distribution of power deposition and reactive fluxes to the wafer. Results from a two-dimensional computational investigation of a DF-CCP reactor, incorporating a full-wave solution of Maxwell’s equations, are discussed. As in single frequency CCPs, the electron density transitions from edge high to center high with increasing HF. This transition is analyzed by correlating the spatial variation of the phase, magnitude and wavelength of the HF electric field to the spatial variation of the electron energy distributions (EEDs) and ionization sources. This transition is sensitive to the gas mixture, particularly those containing electronegative gases due to the accompany change in conductivity. Process parameters, such as pressure, gas mixture, and LF and HF power deposition are important to determining the uniformity of the plasma and properties of ions incident on the wafer. The consequences of process parameters, i. e., pressure, gas mixture and LF and HF power on uniformity and ion energy distributions to the wafer are also investigated. Due to the coupling of finite wavelength, electromagnetic skin, electrostatic edge and electronegative effects, there are no simple scaling laws for plasma uniformity. The plasma uniformity is ultimately a function of conductivity and energy relaxation distance of electrons accelerated by electric fields in and near the sheath. There is a strong second-order effect on uniformity due to feedback from the electron energy distributions (EEDs) to ionization sources. The trends are correlated to the spatial variation of the HF electric field, to the total power deposition and to the spatial variation of EEDs and ionization sources. Another application of CCP sources is polymer surface modification. The surface energy and adhesion properties of commodity polymers such as polypropylene (PP) can be controlled by functionalization of the surface layers in plasmas. We developed a surface reaction mechanism for fluorination of PP in fluorine containing CCP plasmas which includes a hierarchy of reactions beginning with H abstraction by F atoms and followed by passivation by F and F2, and cross-linking, ion (sputtering, scission) and photon (H2 abstraction, scission) activated processes. Predicted surface compositions show good agreement with experiment results. The lack of total fluorination with long plasma exposure is found to be likely caused by cross-linking, which creates Carbon–Carbon (C-C) bonds that might otherwise be passivated by F atoms. Increasing steric hindrances as fluorination proceeds also contribute to lower F/C ratios.
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