相位角对容性耦合电非对称放电特性的影响

电非对称效应作为一种新兴技术，被广泛用于对离子能量和离子通量的独立调控.此外，在改善等离子体的径向均匀性方面，电非对称效应也发挥了重要作用.本文采用二维流体力学模型，并耦合麦克斯韦方程组，系统地研究了容性耦合氢等离子体中当放电由多谐波叠加驱动时，不同谐波阶数k下的电非对称效应，重点观察了相位角θ_n对自偏压以及等离子体径向均匀性的影响.模拟结果表明：在同一谐波阶数下，自偏压随相位角θ_n的变化趋势不尽相同，且当k增大（k>3）时，自偏压随最高频相位角θ_k的变化范围逐渐减小.此外，通过调节相位角θ_n，可以改变轴向功率密度和径向功率密度的相对关系，进而实现对等离子体径向均匀性的调节.研究结果对于利用电非对称效应优化等离子体工艺过程具有一定的指导意义.

Effect of phase angle on plasma characteristics in electrically asymmetric capacitive discharge

In addition to the separate control of the ion energy and ion flux, the so-called electrical asymmetry effect (EAE) also plays an important role in improving the plasma radial uniformity. In this work, a two-dimensional fluid model combined with a full set of Maxwell equations is used to investigate the plasma characteristics in an electrically asymmetric capacitive discharge sustained by multiple consecutive harmonics. The effects of the phase angle θ_n on the dc self-bias (V_dc) and on the plasma radial uniformity for different numbers of consecutive harmonics k are discussed. The simulation results indicate that the phase angles of different harmonics θ_n have different influences on the dc self-bias V_dc. For instance, V_dc varies almost linearly with θ₁ with a period π in dual frequency discharge, and the period is 2π for other discharge conditions. Besides, the modulation of V_dc becomes less obvious by changing the phase angle of the highest harmonic θ_k, especially for k>3. In addition, both the axial component of the power density P_z and the radial component of the power density P_r vary with θ_n, thus the plasma radial uniformity can be adjusted. When the total power density at the radial edge becomes comparable to that in the discharge center, the plasma distribution becomes uniform. For instance, when k=2, the plasma radial uniformity is the best at the phase angle θ₁=π/2 and θ₂=π. However, for k=3, the best radial uniformity is observed at θ₁=3π/2, and the nonuniformity degree α is only 0.41% under this condition. It is worth noting that at k=8, the maximum of α is seven times higher than the minimum by changing the phase angles θ₁ and θ₂, which means that the plasma radial uniformity can be adjusted effectively. However, the modulation induced by θ_k(k>3) becomes less obvious, especially for k=8. Indeed, the electron density shows an edge-high profile, and the radial uniformity is always bad for all θ₈ investigated. The results obtained in this work can help us to gain an insight into the optimization the plasma process by utilizing the EAE.