Investigation Of Electromagnetic Effects And Plasma Uniformity In Very High Frequency Capacitive Discharges

Capacitively coupled plasmas(CCP)have been widely employed in the dielectric etching and thin film deposition processes in semiconductor industries due to various unique benefits.Particularly,discharges driven by dual or multiple frequency sources have been proved to be an effective approach to achieve separate control of the ion flux and ion bombarding energy,namely,the high-frequency(HF)source is used to control ion flux and the low-frequency(LF)source is used to control ion bombarding energy.Recently,large-area CCP reactors driven at very high frequencies become increasingly desirable in the semiconductor industry.Larger plasma reactors are required as wafer size increases;higher excitation frequencies produce reduced ion bombarding energy,required to minimize the substrate damage.However,when the reactor dimensions become comparable to the in-plasma wavelength of the excitation source,standing wave effect(SWE)will come into play and compromise the plasma uniformity,severely limiting the use of larger reactors and higher excitation frequencies.To overcome this challenge,it is imperative that the physics of SWE is fully understood.In this thesis,we investigate the SWE in single and dual frequency capacitive discharges by a combination of different approaches:(1)a floating double probe to measure the radial profile of the positive ion density;(2)a microwave resonance hairpin probe to measure the radial profile of the electron density;(3)phase resolved optical emission spectroscopy(PROES)measurements to provide access to the spatio-temporal dynamics of highly energetic electrons within the rf period;(4)a high-frequency magnetic probe to determine the spatial structure of the harmonic magnetic field;and(5)a nonlinear electromagnetics model to uncover the underlying physics of the coupling between the plasma series resonance and spatial wave resonance.This work seeks to explore a deeper physical understanding of SWE and provide a reference for the improvement of plasma uniformity in large-scale,very-high-frequency(VHF)CCP sources.This paper is organized as follows.In Chapter 1,we start by introducing the research background of the low-temperature plasma,followed by several most widely used low-temperature plasma sources in the semiconductor industry.Subsequently,research progresses with regard to electromagnetic effects and strategies to suppress the SWE are overviewed.At last,several existing problems in present researches are summarized.In Chapter 2,we first give a description of the experimental apparatus.And then,several commonly used plasma diagnostic tools,such as the Langmuir probe,microwave resonator probe,magnetic probe,and PROES technique,are described in detail.In Chapter 3,we propose two kinds of magnetic probes with relatively high signal-to-noise ratio:a tunable,compensable rf magnetic probe and a high-frequency differential magnetic probe.The tunable,compensable rf magnetic probe is based on a novel circuit,comprising two variable capacitors in a tunable series resonance circuit,with a center-tapped,step-up transformer.The output characteristics of the magnetic probe are predicted using two distinct equivalent circuit models,one for the differential mode and the other for the common mode.A Helmholtz coil and a Faraday cup are used for experimental validation of the predicted probe output.By tuning the two variable capacitors in the circuit,the magnetic probe can achieve improved signal-to-noise ratio by amplifying the inductive signal,while suppressing capacitive coupling interference.The high-frequency differential magnetic probe is fabricated using a semirigid coaxial cable.Based on the symmetry of the probe circuit,we exploited an on-line deferential technique to effectively eliminate the capacitive pickup,achieving an excellent probe signal-to-noise ratio.To this end,a visualized user interface software is developed for data acquisition and processing from the magnetic probe.Both magnetic probes are tested in a CCP reactor to further validate its performance,and the experimental results demonstrate that their magnetic measurements are reasonably reliable.In Chapter 4,a LF(2?8 MHz)power source is introduced into a VHF(100 MHz)capacitively coupled argon plasma reactor,in order to suppress the SWE and improve the plasma uniformity.The effect of the LF source parameters on the radial profile of plasma density has been investigated by utilizing a microwave resonance hairpin probe.The experimental results indicate that the plasma density profile exhibits different dependences on LF voltage ?L and LF frequency fL at different gas pressures or electrode driven types.(1)At lower pressures(e.g.,8 Pa),the pronounced SWE revealed in a VHF discharge can be suppressed at relatively high ?L or a low fL in case ?(the two rf sources are applied on one electrode),because the HF sheath heating is largely weakened due to strong modulation by the LF source.By contrast,?L and fL play insignificant roles in suppressing the SWE in case II(the two rf sources are applied on separate electrodes).(2)At higher pressures(e.g.,20 Pa),the opposite is true.The plasma density radial profile is more sensitive to ?L and fL in case ? than in case ?.In case ?,the SWE is surprisingly enhanced with increasing ?L;however,the center-high density profile caused by the SWE can be compensated by increasing fL due to the enhanced electrostatic edge effect dominated by the LF source.In contrast,the density radial profile shows a much weaker dependence on ?L and fL in case I.To understand the different roles of ?L and fL,the high-energy electron excitation dynamics in each case are analyzed by PROES measurements.In Chapter 5,the radial uniformity of plasma density in single and dual VHF capacitively coupled argon discharges is experimentally investigated by utilizing a floating double probe.The experimental results indicate that for single frequency discharges sustained at relatively low pressure,the plasma density radial profile exhibits a parabolic distribution at 90 MHz.whereas at 180 MHz,the profile evolves into a bimodal distribution,and both cases indicate poor uniformities.By contrast,when discharges are excited by two frequencies(i.e.,180 MHz+90 MHz),the plasma radial profile is simultaneously influenced by both sources.To gain a better plasma uniformity,it is necessary to consider the balance between the SWE,which leads to a maximum plasma density at the electrode center,and the edge field effect,which is responsible for a maximum density near the radial electrode edge.This balance can be controlled either by adjusting the voltage amplitude ratio ? of these two sources or by selecting a proper gas pressure.In Chapter 6,we investigate the nonlinear standing waves excited by plasma-series resonance-enhanced harmonics in VHF capacitively coupled argon plasma reactors,using a synergistic combination of experiments and simulation.We report the first-ever experimental observation of SWE excited by higher harmonics in low-pressure capacitive discharges by exploiting a magnetic probe.Special emphasis is placed on the role of higher harmonic excitations on plasma uniformity.To this end,we employed a high-frequency magnetic probe to determine the spatial structure of the harmonic magnetic field,in combination with a floating double probe to measure the radial distribution of the plasma density.The experimental results are used to validate a nonlinear electromagnetics model,which in turn is used to uncover underlying physics.At relatively low pressure,the nonlinear sheath motion generates high-order harmonics that can be strongly enhanced near the plasma series resonance frequencies.Satisfying certain conditions,such nonlinear harmonics induce radial standing waves,with voltage and current maxima on axis,resulting in center-high plasma density.Counter to intuition,excitation of the higher harmonics was suppressed at higher pressures,resulting in improved plasma uniformity.