电非对称双频容性耦合CF_4/Ar放电电极间距对放电模式和刻蚀剖面的影响

Gap length effect on discharge mode and etching profiles in asymmetric dual frequency capacitive CF4/Ar discharges

容性耦合等离子体技术广泛应用在半导体工业、生物医药、环境等领域,是不可替代的核心处理技术.基于电非对称双频源驱动的容性耦合等离子体因其可以实现离子能量和离子通量的相对独立调控而一直被寄予厚望.本文采用一维流体耦合蒙特卡罗模型和刻蚀槽模型,对基于电非对称效应的双频容性耦合CF_4/Ar混合气体放电进行了模拟研究.研究表明,随着电极间距的增大,自偏压的绝对值和电负性均减小.此外,由于放电区域变大但边界损失保持不变,会导致外部馈入功率显著增加和等离子体密度增大.本文采用CF_4/Ar混合气体,其中Ar含量很高,气体电负性不是很高,因此放电模式在不同的电极间距下都是a模式和双极扩散模式的共同作用.在电极间距逐渐变大的过程中,因为自偏压的变化导致离子能量分布中最大离子能量减小并且离子能量展宽变窄,功率极板处离子通量的变化不明显,中性基团通量的增大十分显著,这些变化最终导致相同时间内的刻蚀速率和形貌发生改变.所以,在未来研究中,不仅离子能量和通量的独立控制,优化离子通量和中性基团通量的协同作用达到调节刻蚀速率、改善刻蚀形貌同样是重要研究内容.

The capacitive CF4/Ar discharges driven by a dual frequency source based on the electrical asymmetry effect(EAE) are studied by using a one-dimensional fluid coupled with Monte-Carlo(MC) model and a twodimensional trench model. The effects, induced by varying the relative gap distance, on self-bias voltage,electronegativity, ion flux, neutral flux and other plasma characteristics are systematically discussed. In this asymmetric discharge, as the gap distance increases, the absolute value of the self-bias voltage and electronegativity decrease. Meanwhile, the plasma density and absorption power increase accordingly because the effective discharge area expands but the boundary loss is still limited. In addition, both α mode and driftambipolar(DA) mode can play their important roles in the discharges with different gap distances, though DA mode is weakened in large gap discharge. Owing to the fact that the self-bias is larger and electronegativity is stronger for the case of smaller gap distance, the sheath expansion electric field at the powered electrode and the bulk electric field heat the electrons, leading the ionization rate to greatly increase near the collapse of the sheath at the grounded electrode. Besides, at the larger gap distance, the maximum value of the ionization rate decreases due to the reduction of electrons with relatively high-energy, and the ionization rate near the grounded electrode is reduced evidently. Moreover, with the increase of the gap distance, the maximum ion energy decreases and the ion energy distribution width becomes smaller due to the reduction of the self-bias voltage. Meanwhile, the etching rate increases a lot since the neutral flux increases significantly near the powered electrode. However, as the gap distance increases to 5 cm, the etching rate stops increasing and the trench width at the bottom becomes narrow because the neutral flux increases greatly compared with ion flux,forming a thick layer of polymer. So, besides separately controlling the ion energy and flux, optimizing the synergistic effect of ion flux and neutral group flux to adjust the etching rate and improve the etching morphology is also an interesting topic in the asymmetric CF4/Ar discharges.