| Very-high-frequency(VHF)capacitively coupled plasmas(CCPs)are widely used in the material process,such as etching and film deposition in the integrated circuit industry,due to their high plasma density and low ion bombardment energy.In industrial production and scientific research,argon,as one of the most commonly-used feedstock gases,is often mixed with fluorocarbon gas,oxygen,nitrogen,etc.Studying the evolutions of important parameters such as metastable atom density,neutral gas temperature,and plasma density in argon discharges is of great significance for understanding the physical mechanism behind it,and making better use of argon in industrial production.In this thesis,a variety of experimental diagnostics are used in a capacitively coupled argon discharge driven by VHF,to study several hot physical issues such as the argon metastable atom,neutral gas heating effect,and standing wave effects.In Chapter 1,the development of low-temperature plasma sources and some commonly-used experimental diagnostics are firstly introduced.Then,a new type of diagnostic method--Fiber Bragg Grating(FBG)sensor is introduced in detail.At last,the research progress and some existing problems on several hot physical issues,such as argon metastable atoms,neutral gas heating effect,and electromagnetic effects in CCP,are reviewed.In Chapter 2,we briefly introduce two sets of VHF CCP devices with small-and large-area electrodes.Subsequently,several experimental diagnostic systems used in the experiment are presented,including tunable diode laser absorption spectroscopy,microwave resonance probe,FBG sensor,and floating double probe.In Chapter 3,the tunable diode laser absorption spectroscopy is used to systematically investigate the effect of high-frequency power,low-frequency power,pressure,and oxygen content on the densities of the two metastable atoms(1s5 and 1s3)of argon in a VHF dual-frequency CCP.The primary generation and loss mechanisms of metastable atoms are discussed and analyzed by combining a simplified theoretical model.The results show that the density of argon 1s5 state is one order of magnitude higher than that of 1s3 state due to their difference in degeneracy,while the changes in the densities of the two metastable states with different discharge parameters are very similar because they are at similar energy levels.With the increase of high-frequency power,the metastable atom densities in pure argon discharge increase rapidly at first and then tend to saturate.In the argon discharge mixed with 5%or 10%oxygen,the metastable atom densities reach a saturated value at higher high-frequency power.In dual-frequency driven plasmas,low-frequency power has a weak effect on the density of metastable atoms.With the increase of the gas pressure,the metastable atom densities increase quickly at first and then decrease slowly after reaching a peak under certain gas pressure,and at higher concentrations of oxygen,the maximum metastable atom density occurs at lower gas pressures.We also find that the addition of a small amount of oxygen leads to a significant reduction in the density of metastable atoms,because in a mixed gas discharge,the process of quenching loss of metastable atoms due to collisions with oxygen molecules plays a dominant role,and this kind of quenching effect is more pronounced at higher gas pressures.By combining a simplified theoretical model,it is known that metastable atoms are mainly produced from processes of high-energy electron collisions with ground state argon atoms,and are lost through diffusion,low-energy electron collision quenching,and neutral molecular collision quenching.While in the mixed gas discharge with oxygen,the effect of quenching loss of oxygen molecules on the density of argon metastable atom is very significant.In Chapter 4,the FBG sensor is first applied to CCP for the measurement of neutral gas temperature in an argon discharge,and the heating effect of neutral gas is discussed and analyzed.The temperature measurement with the FBG sensor is based on the thermal balance between the sensor and the neutral gas molecules.Compared with the spectral diagnostic methods,the FBG sensor has advantages of lower cost,more accurate temperature measurement,and its immunity to interference from electromagnetic fields encountered in plasma environments.It is found that as the gas pressure increases,the heat conduction between the FBG sensor and the neutral gas is enhanced,and the characteristic time of thermal equilibrium decreases.With the increase of high-frequency power,the temperature of the neutral gas increases.Unlike the default“room temperature”and“uniform distribution”of some previous studies,it is found that under certain experimental conditions,the temperature of the neutral gas is 10 to 100? higher than the room temperature,and shows a large uneven distribution in space.Besides,the spatial distribution of the neutral gas temperature and ion density behave similarly with the change of pressure,which verifies the recent simulation results.Furthermore,this indicates that the heating of the neutral gas mainly originates from the elastic collisions of the high-energy ions traversing in the sheath region and the subsequent heat conduction process.In addition,the experimental results also confirm that the FBG sensor can be used to determine the gas temperature in CCP accurately.In Chapter 5,we focus on large-area CCP driven by VHF through two methods,the microwave resonance probe and FBG sensor,to study the standing wave effect and plasma radial uniformity in argon discharges.(1)The radial distribution of plasma density is measured by using a microwave resonance probe.It is found that there is a very pronounced standing wave effect at low pressure and VHF discharge,characterized by a "center-high" radial distribution of plasma density.With the increase of the rf power,the standing wave effect is enhanced.A "multi-node" radial distribution is observed due to the existence of higher-order harmonics.As the gas pressure increases,the standing wave effect becomes weaker,and the"cut-off band" phenomenon about electromagnetic wave propagation appears,which validates the previous simulation results.In addition,in order to improve the plasma uniformity,a low-frequency source was introduced into the VHF drive discharge.It is found that the edge effect generated by the low-frequency source can be used to balance the standing wave effect under appropriate experimental conditions,and plasma with better uniformity can be obtained.(2)The FBG sensor is used to measure the radial distribution of the neutral gas temperature,and the influence of standing wave effect on neutral gas heating under different gas pressures and RF powers is systematically studied.Results have shown that at low-pressure VHF(100 MHz)discharges,the neutral gas temperature exhibits a "center-high" radial distribution,which indicates the existence of the strong standing wave effect.As the RF power increases,this"center-high" distribution becomes more pronounced,indicating that the standing wave effect is enhanced.As the gas pressure increases,this "center peak" weakens,while a "cut-off band"effect appears.In contrast,at a low-frequency(27 MHz),the neutral gas temperature exhibits an"edge-high" radial distribution at all gas pressures,indicating strong edge effects while no standing wave effect exists.In addition,the results show that the radial distribution of the neutral gas temperature is basically consistent with the plasma density under the same experimental conditions.In an electropositive argon discharge,the temperature distribution of the neutral gas can reflect the plasma density distribution,and indirectly characterize the physical phenomena such as the standing wave effect in a VHF CCP.In Chapter 6,the main conclusions,innovations,and future research plans of this paper are presented. |
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