Sobolewski Mark A, Wang Yicheng, Goyette Amanda
National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Appl Phys Lett. 2016;109(2). doi: 10.1063/1.4958836.
Simple kinematic considerations indicate that, under certain conditions in radio-frequency (rf) plasmas, the amplitude of the low-energy peak in ion energy distributions (IEDs) measured at an electrode depends sensitively on ion velocities upstream, at the presheath/sheath boundary. By measuring this amplitude, the velocities at which ions exit the presheath can be determined and long-standing controversies regarding presheath transport can be resolved. Here, IEDs measured in rf-biased, inductively coupled plasmas in CF gas determined the presheath exit velocities of all significant positive ions: CF , CF , CF, and F. At higher bias voltages, we detected essentially the same velocity for all four ions. For all ions, measured velocities were significantly lower than the Bohm velocity and the electropositive ion sound speed. Neither is an accurate boundary condition for rf sheaths in electronegative gases: under certain low-frequency, high-voltage criteria defined here, either yields large errors in predicted IEDs. These results indicate that many widely used sheath models will need to be revised.
简单的运动学分析表明，在射频（rf）等离子体的某些条件下，在电极处测量的离子能量分布（IED）中低能峰的幅度敏感地取决于离子在预鞘层/鞘层边界上游的速度。通过测量这个幅度，可以确定离子离开预鞘层的速度，并且可以解决关于预鞘层输运的长期争议。在这里，在CF气体中通过射频偏置的电感耦合等离子体测量的IED确定了所有重要正离子（CF⁺、CF₂⁺、CF₃⁺和F⁺）的预鞘层出口速度。在较高的偏置电压下，我们检测到所有四种离子的速度基本相同。对于所有离子，测量到的速度明显低于玻姆速度和正电离子声速。这两者都不是电负性气体中射频鞘层的准确边界条件：在此处定义的某些低频、高压标准下，任何一个都会在预测的IED中产生很大误差。这些结果表明，许多广泛使用的鞘层模型将需要修订。
