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Generation and control of chemically reactive RF plasmas for self assembly of nanomaterials
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Type
Thesis
Author
Ren, Yuping
Supervisor
Xu, Shuyan
Lee, Paul Choon Keat
Abstract
This thesis focuses on the development, modeling and application of a novel mid frequency inductively coupled plasma (ICP) for deterministic synthesis of nano structured material. As compared with the low frequency ICP, the experimental requirement of the new system is significantly simplified.
Being one of the widely used precursors in plasma processing of advanced materials pure argon plasma is first studied. Kinetic energy loss of electrons and ions makes the biggest impact on the energy balance in chemically active plasma. We found that the total kinetic energy loss is regulated by the following three mechanisms. First, the electron energy distribution function (EEDF) is Duryvesteyn-like and strongly dependent on the operating conditions of the discharge. Second, only the electrons with energy exceeding the plasma potential are able to escape from bulk plasma to the wall. Finally, the kinetic energy loss per electron and ion pair is indirectly controlled by the plasma potential. The collisional energy loss is found to consist of ionization, momentum transfer, and radiation energy loss. The radiation rate coefficients are obtained by combining the emission cross section and the measured EEDF. Most of the radiation energy loss is found to attribute to two UV lines (104.8 and 106.7 nm). About one third of the collisional energy loss is responsible for ionization. Most of the collisional energy loss is due to radiation and only a few percent collisional energy loss are attributed to elastic collision.
A total of 190 chemical reactions involving 48 species have been considered in our model. Combining the measured EEDF and known electron impact cross section, the rate constants of electron impact ionization and/or dissociation reactions have been obtained. In this model 46 particle balance equations for every species, the energy balance equation and the charge balance equation have been formulated. The species densities change, which strongly depend on the input power and CH4 flow rate, have been calculated for Ar/CH4 and Ar/CH4/H2 plasmas. A good agreement between simulation and experimental measurements of electron temperatures for Ar/CH4 and Ar/CH4/H2 plasmas proves the validity of the global model. Furthermore, density change of the plasma species with varying input power agrees well with optical emission spectroscopy (OES) and quadrupole mass spectrometer (MQS) measurements.
Nano structured carbon thin film has been synthesized using the mid frequency ICP source in Ar/CH4 and Ar/CH4/H2 discharges on Co and Ni catalyzed substrates with varying plasma parameters and growth conditions. XRD is used to identify the structure and crystallization of the obtained thin film. The 3 sp fraction in the carbon thin film is estimated from the ratio of GIDI )(/)( and G peak position of Raman spectroscopy by a three-stage model. The sp3 concentration is further investigated independently by XPS. For both Ar/CH4 and Ar/CH4/H2 discharges, it is found that the sp3 / sp2 ratio increases from ~5% to ~20% in the nano structured thin film when the input power increases from 400 W to 2000 W. The sp3 fraction in the films drops from ~20% to ~10% as CH4 flow rate is increased from 8 sccm to 32 sccm at 1600 W RF power. In addition, bias voltage applied to the substrate holder is seen to dramatically change the deposition rate of the thin film. But it has little effect on the sp3 ratio in the films. Finally, a high growth temperature shows a significant enhancement of the sp3 concentration in the films (from ~10% at 300 to ~22% at 500 oC)
Being one of the widely used precursors in plasma processing of advanced materials pure argon plasma is first studied. Kinetic energy loss of electrons and ions makes the biggest impact on the energy balance in chemically active plasma. We found that the total kinetic energy loss is regulated by the following three mechanisms. First, the electron energy distribution function (EEDF) is Duryvesteyn-like and strongly dependent on the operating conditions of the discharge. Second, only the electrons with energy exceeding the plasma potential are able to escape from bulk plasma to the wall. Finally, the kinetic energy loss per electron and ion pair is indirectly controlled by the plasma potential. The collisional energy loss is found to consist of ionization, momentum transfer, and radiation energy loss. The radiation rate coefficients are obtained by combining the emission cross section and the measured EEDF. Most of the radiation energy loss is found to attribute to two UV lines (104.8 and 106.7 nm). About one third of the collisional energy loss is responsible for ionization. Most of the collisional energy loss is due to radiation and only a few percent collisional energy loss are attributed to elastic collision.
A total of 190 chemical reactions involving 48 species have been considered in our model. Combining the measured EEDF and known electron impact cross section, the rate constants of electron impact ionization and/or dissociation reactions have been obtained. In this model 46 particle balance equations for every species, the energy balance equation and the charge balance equation have been formulated. The species densities change, which strongly depend on the input power and CH4 flow rate, have been calculated for Ar/CH4 and Ar/CH4/H2 plasmas. A good agreement between simulation and experimental measurements of electron temperatures for Ar/CH4 and Ar/CH4/H2 plasmas proves the validity of the global model. Furthermore, density change of the plasma species with varying input power agrees well with optical emission spectroscopy (OES) and quadrupole mass spectrometer (MQS) measurements.
Nano structured carbon thin film has been synthesized using the mid frequency ICP source in Ar/CH4 and Ar/CH4/H2 discharges on Co and Ni catalyzed substrates with varying plasma parameters and growth conditions. XRD is used to identify the structure and crystallization of the obtained thin film. The 3 sp fraction in the carbon thin film is estimated from the ratio of GIDI )(/)( and G peak position of Raman spectroscopy by a three-stage model. The sp3 concentration is further investigated independently by XPS. For both Ar/CH4 and Ar/CH4/H2 discharges, it is found that the sp3 / sp2 ratio increases from ~5% to ~20% in the nano structured thin film when the input power increases from 400 W to 2000 W. The sp3 fraction in the films drops from ~20% to ~10% as CH4 flow rate is increased from 8 sccm to 32 sccm at 1600 W RF power. In addition, bias voltage applied to the substrate holder is seen to dramatically change the deposition rate of the thin film. But it has little effect on the sp3 ratio in the films. Finally, a high growth temperature shows a significant enhancement of the sp3 concentration in the films (from ~10% at 300 to ~22% at 500 oC)
Date Issued
2009
Call Number
TA418.9.N35 Ren
Date Submitted
2009