双频容性耦合等离子体中电子无碰撞反弹共振加热
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摘要
双频容性耦合等离子体源(Dual Frequency Capacitively Coupled Plasma, DF-CCP)由于具有独立控制等离子体密度和轰击到极板表面上的离子能量以及能产生大面积均匀等离子体等优点,已被广泛地应用到微电子制造和平板显示器工业中的薄膜沉积和刻蚀工艺。在DF-CCP中,低频源的引入使得放电过程变得更加复杂,产生了许多新的物理问题,如高低频之间的耦合问题以及当高频频率较高时驻波效应引起的等离子体径向非均匀性问题等。低频源引起的射频干扰使得传统的补偿型朗缪尔探针诊断手段难以使用,这就对等离子体的诊断提出了更高的要求。在容性耦合等离子体中,由于电子加热是维持射频放电的根本,所以DF-CCP中电子的加热机制一直是当今研究的热点问题。
一般认为射频容性耦合放电中存在两种典型的电子加热机制：i)在体等离子体区,与电子和中性粒子碰撞相关的欧姆加热；ii)在鞘层-等离子体边界处,电子和振荡的鞘层边界相互作用产生的随机加热(或无碰撞加热)。前者在高气压放电情况下起主导作用,而后者主要是在低气压下维持等离子体放电。近年来,由于人们对低气压(-mTorr)下射频等离子体刻蚀工艺的逐渐重视,无碰撞加热机制成为当前的一个研究热点。唯像的“硬壁(hard wall)"模型经常被用来描述这种无碰撞加热,同时需要假设打破这种电子运动和电场协同作用的相位随机化机制的存在,来解释为什么大量电子能够在一个射频周期内从振荡的电场中获得净能量。一般来讲,早期大部分的研究都考虑电子仅仅和一个振荡的鞘层相互作用,不考虑之后还会发生什么现象。在1991年,Wood通过粒子模拟发现：在低气压容性放电中,存在着一种双鞘层协同加热机制,也就是所谓的反弹共振加热。该加热机制后来被人们通过理论,数值模拟和实验进行了广泛地研究。然而,这些研究主要是针对低能电子的反弹共振加热。这些共振电子的能量太小,远远低于激发或电离阈值,因此不会对等离子体的宏观性质产生影响。
本文研究的主要目的是：在DF-CCP (60/2MHz)放电装置上,对高能电子的反弹共振加热行为进行实验验证,并结合PIC/MCC (Particle-In-Cell and Monte-Carlo Collision)模拟,研究电极间隙、放电气压及双频电源参数(功率及频率)等对反弹共振加热过程及等离子体密度和发光强度的影响,从深层次上揭示电子反弹共振加热的物理机制,为半导体芯片制造工艺中等离子体刻蚀设备的优化提供科学依据。
在第一章绪论部分,介绍了双频容性耦合等离子体源的应用背景,以及几种常见等离子体源的特性,接着重点介绍了容性放电中电子加热机制的研究进展,以及现有的研究中存在的不足。
在第二章中,介绍了CCP中通常采用的探针、光谱和质谱三种诊断方法,简单综述了这几种实验诊断方法在CCP中的研究进展。
在第三章中,针对低气压DF-CCP中电正性氩气放电,开展了高能电子反弹共振加热行为的研究。在实验研究方面,分别利用全悬浮双探针、可移动光探针以及减速场能量分析仪,对不同放电条件下的等离子体密度、光谱强度以及下极板上的电子能量分布进行了测量。实验上首次观察到,在低气压放电下,当驱动频率和电极间隙满足一定的共振条件时,等离子体密度及发光强度在某一间隙下存在一个反常的共振峰,并且在这种情况下有大量的高能电子轰击到下极板。为了进一步深入理解这种共振峰产生的物理原因,我们利用PIC/MCC方法对这种放电过程进行了模拟,结果表明：由快速鞘层扩张产生的高能电子束经过半个射频周期穿过体等离子体区和对面扩张的鞘层相互作用,这时就发生了反弹共振加热现象。这样的电子束可以在两个交互扩张的鞘层边缘来回反弹,在每次和扩张的鞘层相互作用中获得能量。这些高能电子在反弹共振过程中与背景气体碰撞增强了激发和电离率,从而使光谱强度和等离子体密度产生共振峰。另外还揭示出,随着电极间隙的增加,参与共振的电子所需能量的最小值以及共振电子获得能量的最大值均增加,同时共振电子的数量锐减。在不同间隙下,共振电子的数量和能量共同决定着反弹共振加热的效率,这就使得当驱动频率为60MHz时,在电极间隙L=2cm时,反弹共振加热效应最强。最后,研究了反弹共振加热对等离子体的径向均匀性的影响,发现反弹共振加热能够增强极板中心区的放电,使得等离子体密度沿径向呈抛物分布,造成等离子体径向分布不均匀。
在第四章中,进一步研究了氩气放电中不同放电参数对反弹共振加热的影响。在较高的气压下,强烈的碰撞阻碍了持续的反弹共振加热,共振加热受到抑制。在电正性氩气放电中,高频电源的功率对共振加热影响比较小,然而高频频率对共振间隙有着显著的影响。PIC/MCC模拟发现,当电子反弹共振加热最强时,驱动频率和电极间隙的乘积为一个常数。虽然共振加热是电子和高频振荡鞘层相互作用的结果,但低频电源的功率和频率也对这种加热过程产生明显的影响。实验发现,随着低频功率的下降,共振主峰减弱,在更小间隙处出现另一个共振峰。模拟给出的解释是：鞘层的电压降主要由低频电压决定,所以较高的低频功律能够有效地阻止高能共振电子逃逸到极板上,这样就增加了共振电子的寿命,于是增强了共振加热效率。低频频率对共振加热的影响比较微妙,因为它改变了鞘层的扩张速度和鞘层的振荡波形等鞘层特性。实验发现,随着低频频率的降低,共振加热向小间隙移动,共振峰变强。
在第五章中,针对电负性氧气放电,实验上采用悬浮双探针、发卡探针和可移动光探针研究了DF-CCP中的电子反弹共振加热行为。实验发现,与电正性氩气放电相比,电负性氧气放电中的反弹共振加热行为发生在更大的电极间隙。PIC/MCC模拟表明,在氧气容性放电中,体等离子体区存在着较强的反向电场,并且它与鞘层电场的相位不一致。这样的电场能够减速穿越体等离子体区的高能共振电子,抑制电子反弹共振加热。实验结果还表明,对于氧气等离子体,当高频功率降低时,这种反弹共振加热被抑制。这主要是由于随着高频功率的降低,放电模式从电正性向电负性转换,这个过程中体等离子体区的反向电场逐渐增强。随着气压的增加,反弹共振加热效应也被抑制,这是因为在体等离子体区电子与中性粒子的碰撞变得频繁以及增强的体区反向电场对共振电子的阻碍作用增强。通过数值模拟我们发现,体等离子体区电场和鞘层区电场的相位关系对驱动频率有强烈的依赖性。当驱动频率为60MHz时,体区电场和鞘层电场的相位相反。当驱动频率下降到13.56MHz时,体区电场和鞘层电场相位趋于一致,这时体等离子体区电场会增强电子反弹共振加热。同电正性氩气放电类似,在电负性氧气放电中,电子反弹共振加热使得等离子体密度沿径向呈抛物分布,造成径向分布不均匀。
Dual-frequency capacitively coupled plasmas have been widely used in the semiconductor and flat panel display industries as thin film etching and deposition devices due to their ability of separately controlling plasma density and ion energy onto the electrode, and generating large area and homogeneous plasma. In DF-CCPs, the introduction of low-frequency source complicates discharge process, and gives rise to many new physical problems, e.g., the coupling between the high-frequency (HF) and the low-frequency (LF) sources and plasma radial non-uniformity caused by standing-wave effect, etc. And the strong rf disturbance induced by LF source disqualifies the usual rf compensation techniques for Langmuir probe, and hence, more sophisticated plasma diagnostics are demanded. In CCPs, electron heating by the time-varying fields is fundamental for the operation of various rf discharges, therefore the electron heating mechanism in DF-CCPs has always been under intensive discussion recently.
It is now generally recognized that there are two main mechanisms for electron heating in rf CCPs:i) the Ohmic heating due to electron-neutral (e-n) collisions in the bulk plasma; ii) the collisionless (or stochastic) heating at the oscillating plasma-sheath interface. The former dominates at relatively high pressures, while the latter is expected to sustain the plasmas at rather low pressures. In recent years, due to the increasing emphasis on industrial applications of rf plasma etching technology, the importance of understanding collisionless heating in rf discharges is growing. The "hard wall" model has been frequently employed as a paradigm to describe the collisionless heating, meanwhile a phase randomization mechanism, which breaks the phase coherence between electron motion and rf field, is generally assumed to explain the reason why the electrons gain net energy over an rf period. Previous studies generally considered electron collisions with only one oscillating sheath, regardless of what happened afterwards. In1991, Wood discovered that one "double-sheath coherent heating", now termed "bounce resonance heating (BRH)", could occur in low-pressure capacitive discharges under certain conditions. Since then, this type of heating mode has been studied both theoretically and experimentally. However, in some of these observations, the energy of resonant electrons is too small to have effect on the discharges.
The main motivation of this thesis is to study the BRH behavior due to high-energy electron in the DF-CCP (60/2MHz) reactor experimentally. Combining with Particle-In-Cell Monte-Carlo Collision (PIC/MCC) simulation, we investigate the effects of the electrode gap, pressure and DF sources (power and frequency) on the BRH, plasma density and emission intensity, and further to uncover the underlying physics of the BRH. This study will provide a scientific basis for optimizing plasma etching device in semiconductor chip manufacture.
In Chapter1, we first introduce the DF-CCP application background and several widely-used plasma sources. And then, we review the research progress of electron heating mechanism in CCPs and also summarize the existing problems.
In Chapter2, several usually-used diagnostic methods in CCPs are summarized, the probe, spectroscopy and mass spectrometer diagnostics are comparatively studied and the relative experimental studies are reviewed.
In Chapter3, we study the electron BRH in low-pressure Ar DF-CCP. In the experiments, we measure the plasma density, emission intensity and electron energy distribution on the bottom electrode at various discharge parameters by using floating double probe, optical probe and retarding field energy analyzer. For the first time we find experimentally that in low-pressure CCP, when the excitation frequency and electrode gap satisfy a certain resonance condition, both the plasma density and emission intensity appear an abnormal resonance peak at a certain gap, meanwhile substantial high-energy resonant electrons bombard the electrode. To further understand the underlying physics for the resonance peaks, we employ PIC/MCC method to simulate the physics phenomenon, and the results demonstrate that the high-energy beamlike electrons are generated by fast sheath expansion on one electrode, and they cross the bulk plasma in about half the rf circle and encounter the fast expansion of the other sheath, the BRH occurs. Thus, these electrons are bounced back and forth between two sheath edges and gain energy in collisions with either of the expanding sheath. In the course these high-energy electrons collide with background gas, producing intensive excitation and ionization, leading to the resonance peak in the plasma density and emission intensity. Moreover, PIC/MCC simulation also reveals that with increasing electrode gap, the minimum energy for an electron to join the BRH and the maximum energy for an election to obtain from the BRH increase, whereas the number of the resonant electrons drops significantly. At various electrode gaps, the number and the energy of the resonant electrons jointly determine the efficiency of the BRH. So the balance of the two factors determines the heating efficiency of the plasma to be the most significant at L=2cm when the driving frequency is60MHz. Finally, the effect of the BRH on the radial uniformity of the plasma is studied. It is found that the BRH can enhance the discharge in the central region between two electrodes, resulting in a parabolic profile of plasma density along radial direction.
In Chapter4, the effects of various plasma control parameters on the BRH are further investigated. At high pressure, frequent collisions prevent electrons from bouncing continuously between two sheath edges, and thus the BRH is suppressed. In an electropositive argon discharge, the HF power has a small effect on the BRH, whereas the HF frequency affects the resonance gap significantly. PIC/MCC simulation shows that when the BRH occurs, the product of driving frequency and resonance gap is a constant. Although the energetic beamlike electrons are resonantly heated by the HF oscillating sheaths, the LF power and frequency have very significant effects on the BRH. The experiments show that with the decrease of the LF power, the main resonance peak is weakened while another excitation peak emerges at a much smaller gap. The explanation by PIC/MCC simulation shows that the sheath potential drop is determined mainly by the LF power, and so high-energy resonant electrons are restricted effectively by the sheath barrier at high LF power, which increases the life-time of resonant electrons and thus the BRH efficiency is enhanced. The LF frequency has a subtle effect on the BRH, since it has considerably changed the sheath properties, e.g., the sheath expansion speed as well as probably the waveform of sheath oscillation. The experiments also indicate that with the decrease in LF frequency, the BRH tends to occur at a smaller gap and resonance peak strengthens.
In Chapter5, we study the BRH behavior in low-pressure electronegative oxygen DF-CCP by a combined method of floating double probe, hairpin probe and optical probe. The experiments show that in comparison with electropositive argon discharges, the BRH tends to happen at larger electrode gaps. PIC/MCC simulations reveal that in oxygen discharges the bulk electric field reversal inside the bulk becomes quite strong and out of phase with sheath field. Such an electric field would retard high-energy resonant electrons when they cross the bulk, resulting in a suppressed BRH. The experiments show that in oxygen plasmas the BRH is suppressed with the decrease in HF power. It is attributed to an enhanced reversal bulk electric field when the discharge mode transits from electropositive to electronegative. As the pressure increases, the BRH is suppressed. This is caused by a combined effect of the increased e-n collisions and enhanced reversed electric field in the bulk. By simulation, we find that the phase relationship between bulk electric field and sheath field has a strong dependence on the driving frequency. At driving frequency of60MHz, the bulk electric field is out of phase with sheath field. With the driving frequency reduced to13.56MHz, the bulk electric field is completely in phase with the sheath field, in which regime the BRH will be enhanced by the bulk electric field. Similar to the electropositive Ar discharge, in the electronegative oxygen discharge the BRH also causes a parabolic profile of plasma density along radial direction.
一般认为射频容性耦合放电中存在两种典型的电子加热机制：i)在体等离子体区,与电子和中性粒子碰撞相关的欧姆加热；ii)在鞘层-等离子体边界处,电子和振荡的鞘层边界相互作用产生的随机加热(或无碰撞加热)。前者在高气压放电情况下起主导作用,而后者主要是在低气压下维持等离子体放电。近年来,由于人们对低气压(-mTorr)下射频等离子体刻蚀工艺的逐渐重视,无碰撞加热机制成为当前的一个研究热点。唯像的“硬壁(hard wall)"模型经常被用来描述这种无碰撞加热,同时需要假设打破这种电子运动和电场协同作用的相位随机化机制的存在,来解释为什么大量电子能够在一个射频周期内从振荡的电场中获得净能量。一般来讲,早期大部分的研究都考虑电子仅仅和一个振荡的鞘层相互作用,不考虑之后还会发生什么现象。在1991年,Wood通过粒子模拟发现：在低气压容性放电中,存在着一种双鞘层协同加热机制,也就是所谓的反弹共振加热。该加热机制后来被人们通过理论,数值模拟和实验进行了广泛地研究。然而,这些研究主要是针对低能电子的反弹共振加热。这些共振电子的能量太小,远远低于激发或电离阈值,因此不会对等离子体的宏观性质产生影响。
本文研究的主要目的是：在DF-CCP (60/2MHz)放电装置上,对高能电子的反弹共振加热行为进行实验验证,并结合PIC/MCC (Particle-In-Cell and Monte-Carlo Collision)模拟,研究电极间隙、放电气压及双频电源参数(功率及频率)等对反弹共振加热过程及等离子体密度和发光强度的影响,从深层次上揭示电子反弹共振加热的物理机制,为半导体芯片制造工艺中等离子体刻蚀设备的优化提供科学依据。
在第一章绪论部分,介绍了双频容性耦合等离子体源的应用背景,以及几种常见等离子体源的特性,接着重点介绍了容性放电中电子加热机制的研究进展,以及现有的研究中存在的不足。
在第二章中,介绍了CCP中通常采用的探针、光谱和质谱三种诊断方法,简单综述了这几种实验诊断方法在CCP中的研究进展。
在第三章中,针对低气压DF-CCP中电正性氩气放电,开展了高能电子反弹共振加热行为的研究。在实验研究方面,分别利用全悬浮双探针、可移动光探针以及减速场能量分析仪,对不同放电条件下的等离子体密度、光谱强度以及下极板上的电子能量分布进行了测量。实验上首次观察到,在低气压放电下,当驱动频率和电极间隙满足一定的共振条件时,等离子体密度及发光强度在某一间隙下存在一个反常的共振峰,并且在这种情况下有大量的高能电子轰击到下极板。为了进一步深入理解这种共振峰产生的物理原因,我们利用PIC/MCC方法对这种放电过程进行了模拟,结果表明：由快速鞘层扩张产生的高能电子束经过半个射频周期穿过体等离子体区和对面扩张的鞘层相互作用,这时就发生了反弹共振加热现象。这样的电子束可以在两个交互扩张的鞘层边缘来回反弹,在每次和扩张的鞘层相互作用中获得能量。这些高能电子在反弹共振过程中与背景气体碰撞增强了激发和电离率,从而使光谱强度和等离子体密度产生共振峰。另外还揭示出,随着电极间隙的增加,参与共振的电子所需能量的最小值以及共振电子获得能量的最大值均增加,同时共振电子的数量锐减。在不同间隙下,共振电子的数量和能量共同决定着反弹共振加热的效率,这就使得当驱动频率为60MHz时,在电极间隙L=2cm时,反弹共振加热效应最强。最后,研究了反弹共振加热对等离子体的径向均匀性的影响,发现反弹共振加热能够增强极板中心区的放电,使得等离子体密度沿径向呈抛物分布,造成等离子体径向分布不均匀。
在第四章中,进一步研究了氩气放电中不同放电参数对反弹共振加热的影响。在较高的气压下,强烈的碰撞阻碍了持续的反弹共振加热,共振加热受到抑制。在电正性氩气放电中,高频电源的功率对共振加热影响比较小,然而高频频率对共振间隙有着显著的影响。PIC/MCC模拟发现,当电子反弹共振加热最强时,驱动频率和电极间隙的乘积为一个常数。虽然共振加热是电子和高频振荡鞘层相互作用的结果,但低频电源的功率和频率也对这种加热过程产生明显的影响。实验发现,随着低频功率的下降,共振主峰减弱,在更小间隙处出现另一个共振峰。模拟给出的解释是：鞘层的电压降主要由低频电压决定,所以较高的低频功律能够有效地阻止高能共振电子逃逸到极板上,这样就增加了共振电子的寿命,于是增强了共振加热效率。低频频率对共振加热的影响比较微妙,因为它改变了鞘层的扩张速度和鞘层的振荡波形等鞘层特性。实验发现,随着低频频率的降低,共振加热向小间隙移动,共振峰变强。
在第五章中,针对电负性氧气放电,实验上采用悬浮双探针、发卡探针和可移动光探针研究了DF-CCP中的电子反弹共振加热行为。实验发现,与电正性氩气放电相比,电负性氧气放电中的反弹共振加热行为发生在更大的电极间隙。PIC/MCC模拟表明,在氧气容性放电中,体等离子体区存在着较强的反向电场,并且它与鞘层电场的相位不一致。这样的电场能够减速穿越体等离子体区的高能共振电子,抑制电子反弹共振加热。实验结果还表明,对于氧气等离子体,当高频功率降低时,这种反弹共振加热被抑制。这主要是由于随着高频功率的降低,放电模式从电正性向电负性转换,这个过程中体等离子体区的反向电场逐渐增强。随着气压的增加,反弹共振加热效应也被抑制,这是因为在体等离子体区电子与中性粒子的碰撞变得频繁以及增强的体区反向电场对共振电子的阻碍作用增强。通过数值模拟我们发现,体等离子体区电场和鞘层区电场的相位关系对驱动频率有强烈的依赖性。当驱动频率为60MHz时,体区电场和鞘层电场的相位相反。当驱动频率下降到13.56MHz时,体区电场和鞘层电场相位趋于一致,这时体等离子体区电场会增强电子反弹共振加热。同电正性氩气放电类似,在电负性氧气放电中,电子反弹共振加热使得等离子体密度沿径向呈抛物分布,造成径向分布不均匀。
Dual-frequency capacitively coupled plasmas have been widely used in the semiconductor and flat panel display industries as thin film etching and deposition devices due to their ability of separately controlling plasma density and ion energy onto the electrode, and generating large area and homogeneous plasma. In DF-CCPs, the introduction of low-frequency source complicates discharge process, and gives rise to many new physical problems, e.g., the coupling between the high-frequency (HF) and the low-frequency (LF) sources and plasma radial non-uniformity caused by standing-wave effect, etc. And the strong rf disturbance induced by LF source disqualifies the usual rf compensation techniques for Langmuir probe, and hence, more sophisticated plasma diagnostics are demanded. In CCPs, electron heating by the time-varying fields is fundamental for the operation of various rf discharges, therefore the electron heating mechanism in DF-CCPs has always been under intensive discussion recently.
It is now generally recognized that there are two main mechanisms for electron heating in rf CCPs:i) the Ohmic heating due to electron-neutral (e-n) collisions in the bulk plasma; ii) the collisionless (or stochastic) heating at the oscillating plasma-sheath interface. The former dominates at relatively high pressures, while the latter is expected to sustain the plasmas at rather low pressures. In recent years, due to the increasing emphasis on industrial applications of rf plasma etching technology, the importance of understanding collisionless heating in rf discharges is growing. The "hard wall" model has been frequently employed as a paradigm to describe the collisionless heating, meanwhile a phase randomization mechanism, which breaks the phase coherence between electron motion and rf field, is generally assumed to explain the reason why the electrons gain net energy over an rf period. Previous studies generally considered electron collisions with only one oscillating sheath, regardless of what happened afterwards. In1991, Wood discovered that one "double-sheath coherent heating", now termed "bounce resonance heating (BRH)", could occur in low-pressure capacitive discharges under certain conditions. Since then, this type of heating mode has been studied both theoretically and experimentally. However, in some of these observations, the energy of resonant electrons is too small to have effect on the discharges.
The main motivation of this thesis is to study the BRH behavior due to high-energy electron in the DF-CCP (60/2MHz) reactor experimentally. Combining with Particle-In-Cell Monte-Carlo Collision (PIC/MCC) simulation, we investigate the effects of the electrode gap, pressure and DF sources (power and frequency) on the BRH, plasma density and emission intensity, and further to uncover the underlying physics of the BRH. This study will provide a scientific basis for optimizing plasma etching device in semiconductor chip manufacture.
In Chapter1, we first introduce the DF-CCP application background and several widely-used plasma sources. And then, we review the research progress of electron heating mechanism in CCPs and also summarize the existing problems.
In Chapter2, several usually-used diagnostic methods in CCPs are summarized, the probe, spectroscopy and mass spectrometer diagnostics are comparatively studied and the relative experimental studies are reviewed.
In Chapter3, we study the electron BRH in low-pressure Ar DF-CCP. In the experiments, we measure the plasma density, emission intensity and electron energy distribution on the bottom electrode at various discharge parameters by using floating double probe, optical probe and retarding field energy analyzer. For the first time we find experimentally that in low-pressure CCP, when the excitation frequency and electrode gap satisfy a certain resonance condition, both the plasma density and emission intensity appear an abnormal resonance peak at a certain gap, meanwhile substantial high-energy resonant electrons bombard the electrode. To further understand the underlying physics for the resonance peaks, we employ PIC/MCC method to simulate the physics phenomenon, and the results demonstrate that the high-energy beamlike electrons are generated by fast sheath expansion on one electrode, and they cross the bulk plasma in about half the rf circle and encounter the fast expansion of the other sheath, the BRH occurs. Thus, these electrons are bounced back and forth between two sheath edges and gain energy in collisions with either of the expanding sheath. In the course these high-energy electrons collide with background gas, producing intensive excitation and ionization, leading to the resonance peak in the plasma density and emission intensity. Moreover, PIC/MCC simulation also reveals that with increasing electrode gap, the minimum energy for an electron to join the BRH and the maximum energy for an election to obtain from the BRH increase, whereas the number of the resonant electrons drops significantly. At various electrode gaps, the number and the energy of the resonant electrons jointly determine the efficiency of the BRH. So the balance of the two factors determines the heating efficiency of the plasma to be the most significant at L=2cm when the driving frequency is60MHz. Finally, the effect of the BRH on the radial uniformity of the plasma is studied. It is found that the BRH can enhance the discharge in the central region between two electrodes, resulting in a parabolic profile of plasma density along radial direction.
In Chapter4, the effects of various plasma control parameters on the BRH are further investigated. At high pressure, frequent collisions prevent electrons from bouncing continuously between two sheath edges, and thus the BRH is suppressed. In an electropositive argon discharge, the HF power has a small effect on the BRH, whereas the HF frequency affects the resonance gap significantly. PIC/MCC simulation shows that when the BRH occurs, the product of driving frequency and resonance gap is a constant. Although the energetic beamlike electrons are resonantly heated by the HF oscillating sheaths, the LF power and frequency have very significant effects on the BRH. The experiments show that with the decrease of the LF power, the main resonance peak is weakened while another excitation peak emerges at a much smaller gap. The explanation by PIC/MCC simulation shows that the sheath potential drop is determined mainly by the LF power, and so high-energy resonant electrons are restricted effectively by the sheath barrier at high LF power, which increases the life-time of resonant electrons and thus the BRH efficiency is enhanced. The LF frequency has a subtle effect on the BRH, since it has considerably changed the sheath properties, e.g., the sheath expansion speed as well as probably the waveform of sheath oscillation. The experiments also indicate that with the decrease in LF frequency, the BRH tends to occur at a smaller gap and resonance peak strengthens.
In Chapter5, we study the BRH behavior in low-pressure electronegative oxygen DF-CCP by a combined method of floating double probe, hairpin probe and optical probe. The experiments show that in comparison with electropositive argon discharges, the BRH tends to happen at larger electrode gaps. PIC/MCC simulations reveal that in oxygen discharges the bulk electric field reversal inside the bulk becomes quite strong and out of phase with sheath field. Such an electric field would retard high-energy resonant electrons when they cross the bulk, resulting in a suppressed BRH. The experiments show that in oxygen plasmas the BRH is suppressed with the decrease in HF power. It is attributed to an enhanced reversal bulk electric field when the discharge mode transits from electropositive to electronegative. As the pressure increases, the BRH is suppressed. This is caused by a combined effect of the increased e-n collisions and enhanced reversed electric field in the bulk. By simulation, we find that the phase relationship between bulk electric field and sheath field has a strong dependence on the driving frequency. At driving frequency of60MHz, the bulk electric field is out of phase with sheath field. With the driving frequency reduced to13.56MHz, the bulk electric field is completely in phase with the sheath field, in which regime the BRH will be enhanced by the bulk electric field. Similar to the electropositive Ar discharge, in the electronegative oxygen discharge the BRH also causes a parabolic profile of plasma density along radial direction.
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