| Dual-frequency capacitively coupled plasmas (DF-CCPs) have been widely used in the dielectric etching in the semiconductor manufacturing industry due to its independent control of the flux and energy ions bombarding the substrate surface, which have greatly mitigated the contradiction between the etching rate and device damage. Essential plasma parameters, such as electron density and ion energy, are important for understanding the plasma properties and optimizing the etching process. So far, the studies on the DF-CCPs focus on the argon discharge, and yet few works have been conducted on the electronegative gases widely used in the actual industrial processes, such as O2, CF4, c-C4F8. Although a few numerical studies have been done on the reactive gas discharges, such as CF4, experimental studies on them are very scarce. Therefore, the present thesis experimentally investigates the dependences of electron density and ion energy on the controllable parameters (e.g., high frequency (HF) power, low frequency (LF) power and frequency, gas pressure) in the O2, Ar/O2 and Ar/O2/CF4 discharges. In addition, PIC/MC (Particle-In-Cell and Monte Calro) and fluid models are also applied to validate the experimental results.The oxidizing or corrosive gas discharges have little effect on the microwave hairpin probe and quadrupole mass spectrometer used in the dissertation in comparison with the traditional diagnostic methods (e.g., Langmuir probe). The electron density and ion energy distribution can be obtained by the hairpin probe and mass spectrometer respectively.In Chapter 1, applications of low-temperature plasmas in the integrated circuit industry, low-temperature radio-frequency (rf) plasma sources are introduced. And then the recent advances and hot topics of the DF-CCPs are reviewed.In Chapter 2, the experimental setup (i.e., dual-frequency capacitively coupled discharge device) is simply introduced. And then the principle, structure and usage of the diagnostic methods (i.e., microwave hairpin probe, optical probe, quadrupole mass spectrometer, Octiv PolyTM Precision Power, Impedance & VI Sensor) used in the dissertation are detailly described. Finally, the numerical models including PIC/MC and fluid models are simply described.The dependences of electron density on the HF power, LF power and gas pressure in the O2 and Ar/O2 discharges are systematically investigated by utilizing the microwave hairpin probe in Chapter 3. In experiment the electron density is mainly determined by the HF power and linearly increases with increasing HF power. At relative high HF powers (e.g.,120 W in experiment) the electron density decreases with increasing LF power, and yet when the HF power is low (e.g.,30 W in experiment) the electron density increases. With increasing gas pressure, the electron density first increases to a maximum value at a middle gas pressure and then decreases slowly. The addition of argon gas results in the increased electron density and has no effect on the relation between electron density and controllable parameters. Results obtained by the PIC/MC model are compared with measured ones and both are in good agreements.The dependences of ion energy distribution on the LF power, LF frequency and gas pressure in the Ar/O2 discharge are systematically investigated by utilizing the energy resolved quadrupole mass spectrometer in Chapter 4. The ion energy distributions are mainly influenced by the LF frequency and LF power, which can lead to the opposite changes in the energy width and two-peak position. With increasing LF power the high-energy peak shifts towards the high-energy region and the energy width increases. However, with increasing LF frequency the high-energy peak shifts towards the low-energy region and the energy width decreases. Ion energy distributions are also affected by the gas pressure from two aspects. On the one hand, the increasing gas pressure results in decreasing averaged ion energy and increasing low-energy ions because of the intensive resonant charge exchange collision. On the other hand the sheath potential and thickness change with the gas pressure, which indirectly affects the energy width and peak position. In addition, some results from the PIC/MC model are given and general agreements between experiment and simulation can be obtained. However, there are some discrepancies between them, and finally some reasons are analyzed.In Chapter 5, the effects of the HF power, LF power and gas pressure on the electron density and ion energy distribution in the Ar/CF4 and Ar/O2/CF4 discharges are detailly investigated by utilizing the microwave hairpin probe and quadrupole mass spectrometer, respectively. The dependences almost keep unchanged in spite of the additive oxygen. The electron density is determined by the HF power and hardly affected by the LF power. In the Ar/O2/CF4 discharges the ion energy distribution is mainly determined by the LF power and LF frequency and the HF power plays a small role. In addition, the axial distributions of charge particle densities from the fluid model are illustrated, which are good for understanding physical characteristics of the CF4 discharge.In Chapter 6, the effects of driving frequency on the electron density in the oxygen discharge are invesitaged by utilizing the microwave hairpin probe and Impedans voltage-current sensor. The power absorbed by the plasma is studied and it is found that the power lost in the matching network can reaches to 50% at 100 MHz. Under constant absorbed power condition, the electron density increases with driving frequency increasing from 13.56 to 40.68 MHz. when the driving frequency further increases from 60 to 100 MHz the electron density slightly changes depending on the gas pressure. The phenomenon can be understood as influences of standing wave effect at 100 MHz and the electron series resonance effect at 40.68 MHz. Under constant discharge voltage condition, the electron density increases when the driving frequency increases from 13.56 to 40.68 MHz, and then decreases with the frequency further increasing from 60 to 100 MHz. The results are attributed to the facts that the electron series resonance occurs at 40.68 MHz and that the power absorbed by the plasma decreases with the driving frequency increasing from 60 to 100 MHz.In Chapter 7, Major results and innovation in the dissertation are given and future plans are also proposed. |
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