Kaganovich I D, Ramamurthi B N, Economou D J
Plasma Processing Laboratory, Department of Chemical Engineering, University of Houston, 4800 Calhoun Road, Houston, Texas 77204-4004, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2001 Sep;64(3 Pt 2):036402. doi: 10.1103/PhysRevE.64.036402. Epub 2001 Aug 27.
The spatiotemporal evolution of charged species densities and wall fluxes during the afterglow of an electronegative discharge has been investigated. The decay of a plasma with negative ions consists of two stages. During the first stage of the afterglow, electrons dominate plasma diffusion and negative ions are trapped inside the vessel by the static electric field; the flux of negative ions to the walls is nearly zero. During this stage, the electron escape frequency increases considerably in the presence of negative ions, and can eventually approach free electron diffusion. During the second stage of the afterglow, electrons have disappeared, and positive and negative ions diffuse to the walls with the ion-ion ambipolar diffusion coefficient. Theories for plasma decay have been developed for equal and strongly different ion (T(i)) and electron (T(e)) temperatures. In the case T(i)=T(e), the species spatial profiles are similar and an analytic solution exists. When detachment is important in the afterglow (weakly electronegative gases, e.g., oxygen) the plasma decay crucially depends on the product of negative ion detachment frequency (gamma(d)) and diffusion time (tau(d)). If gamma(d)tau(d)>2, negative ions convert to electrons during their diffusion towards the walls. The presence of detached electrons results in "self-trapping" of the negative ions, due to emerging electric fields, and the negative ion flux to the walls is extremely small. In the case T(i)<<T(e), the spatiotemporal dynamics is more complicated due to the presence of negative ion density fronts. During the afterglow, although negative ions diffuse freely in the plasma core, the negative ion fronts propagate towards the chamber walls with a nearly constant velocity. The evolution of ion fronts in the afterglow of electronegative plasmas is important, since it determines the time needed for negative ions to reach the wall, and thus influence surface reactions in plasma processing.
研究了电负性放电余辉期间带电粒子密度和壁通量的时空演化。含有负离子的等离子体的衰减包括两个阶段。在余辉的第一阶段，电子主导等离子体扩散，负离子被静电场捕获在容器内部；负离子向壁的通量几乎为零。在这个阶段，在存在负离子的情况下，电子逃逸频率显著增加，最终可以接近自由电子扩散。在余辉的第二阶段，电子消失，正离子和负离子以离子-离子双极扩散系数扩散到壁上。已经针对离子（T(i)）和电子（T(e)）温度相等和差异很大的情况开发了等离子体衰减理论。在T(i)=T(e)的情况下，粒子的空间分布相似，并且存在解析解。当余辉中离解很重要时（弱电负性气体，例如氧气），等离子体衰减关键取决于负离子离解频率（gamma(d)）和扩散时间（tau(d)）的乘积。如果gamma(d)tau(d)>2，负离子在向壁扩散的过程中会转化为电子。由于出现电场，离解电子的存在导致负离子的“自陷”，并且负离子向壁的通量极小。在T(i)<<T(e)的情况下，由于存在负离子密度前沿，时空动力学更加复杂。在余辉期间，尽管负离子在等离子体核心中自由扩散，但负离子前沿以几乎恒定的速度向腔室壁传播。电负性等离子体余辉中离子前沿的演化很重要，因为它决定了负离子到达壁所需的时间，从而影响等离子体处理中的表面反应。
