Experimental Investigation Of Discharage Modes In Radio-frequency Capacitively Coupled Plasmas

Radio-frequency capacitively coupled plasmas(rf-CCPs)have been widely used in etching and deposition processes in microelectronic manufacture,due to their simple structures and the abilities of generating large area and homogeneous plasma,as well as separately controlling plasma density and ion energy by using dual-frequency sources.There are three typical discharge modes in rf-CCP,i.e..a mode,? mode and DA mode.The first two modes can be found in most gas discharges,e.g.,the electropositive gases such as Ar and Ne,and the electronegative gases such as CF4 and O2,while the DA mode can generally only be found in the electronegative gas discharges.The energy deposition process and the plasma parameters,e.g.,electron density,electron energy probability function(EEPF)and the excitation/ionization rate distribution,etc,can be significantly different when the discharge is sustained in different modes.However,the changes of these plasma parameters will have a very important influence on plasma etching and deposition processes in the practical industrial applications.Therefore,for the purpose of optimizing the structure of the chamber and the discharge parameters,it is of great significance to identify the plasma properties under different discharge modes and systematically understand the effects of changing external conditions on discharge mode.The main motivation of this thesis is to study the characteristics of each of the three discharge modes mentioned above,and the mode transitions caused by changing the gas pressure,the driving frequency,the rf voltage and the electrode gap,etc.In the experiments,we measured the electron density and the spatio-temporal distribution of excitation rate induced by electron collision at various discharge parameters by using hairpin probe and phase resolved optical emission spectroscopy(PROES)method,respectively.In addion,some of the experimental results are verified by using PIC/MCC(Partical-in cell and Monte Calro collision)simulation.In chapter 1,firstly,the applications of plasma in microelectronics industry and the development of CCP source are briefly introduced.And then,several typical discharge modes in CCP are introduced in detail.In chapter 2,we first introduce the experimental setup and PIC/MCC model.And then,we review the principle and structures of the hairpin probe and the PROES method.In chapter 3,the discharge mode transitions in capacitively coupled CF4 plasmas are studied by synergistically using two diagnostic methods in combination with PIC/MCC simulations.And very consistent results are obtained in the experiments and simulations.Under the conditions covered in this chapter the influence of secondary electrons can be neglected,thus only the a and DA mode can be observed in the experiment.By increasing the power at a given driving frequency and pressure,the discharge mode experiences a transition from DA to a mode.This is ascribed to the fact that at high powers,the sheath heating is enhanced,leading to a decrease in electronegativity and an increase in electron density.At higher pressures,the electrons are more likely to be attached due to more frequent collisions,which leads to a reduction in electron density and an increase in electronegativity.Therefore,by increasing the pressure at a given driving frequency and power,a discharge mode transition from a mode to DA mode is induced.Furthermore,we conclude that as the driving frequency is increased,the pressure,at which the mode transition occurs,is increased,while the power,at which the mode transition occurs,is decreased.In chapter 4,we systematically study the effects of secondary electron emission(SEE)in SF-CCP.In neon discharge,the emission intensity increases almost linearly with the rf voltage at all driving frequencies covered here,while the variation of the electron density with the rf voltage behaves differently at different driving frequencies.In particular,the electron density increases slowly at the low-voltage side and grows rapidly when the rf voltage is higher than a certain value,indicating a transition from a to ? mode at at low driving frequencies(<10 MHz),while at high driving frequencies the electron density increases linearly with the rf voltage.Furthermore,the rf voltage,at which the mode transition occurs,decreases with the increase of the working pressure.In CF4 discharge,due to the lower electronegativity at higher driving frequencies(>10 MHz),the change in electron density and light intensity with rf voltage is very similar to that in Ne discharge.By contrast,the electron density and emission intensity do not exhibit a simple dependence on the rf voltage at lower frequencies.In particular,the electron density exhibits a minimum at a certain rf voltage when the discharge mode is switching from DA mode to ? mode.And a minimum of the emission intensity can also be found at a higher rf voltage when a discharge is switching from a mode to ? mode.In chapter 5,the different discharge modes and the influence of several external parameters on mode transitions in dual-frequency(2 MHz&14 MHz)capacitively coupled Ar plasmas are investigated.The experiments show that as the low-frequency(LF)voltage increases,the electron density first decreases at low LF voltage and then increases slightly with LF voltage,suggesting that the plasma is dominated by a mode.When LF voltage exceeds some critical value,the electron density increases dramatically with LF voltage,indicating a a-y mode transition.And the discharge turns into ? mode at a lower LF voltage when high-frequency(HF)voltage is higher.When the discharge is sustained in a mode,the sheath thickness increases with the increase of LF voltage,and consequently,the axial maximum of electron density moves towards the ground electrode.When the discharge is sustained in y mode,the density peak moves axially towards the powered electrode due to reduced sheath thickness at higher pressures,while its distance to the powered electrode is almost independent of other external conditions.In addition,the ? mode can be significantly enhanced at a higher pressure and a larger electrode gap,A more significant increase in the electron density in ? mode can be seen at a higher SEE coefficient,while,in a mode,different SEE coefficients have basically no influence on the discharge.