Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11675117, 11175127) and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, China
Received Date:12 December 2020
Accepted Date:24 January 2021
Available Online:27 May 2021
Published Online:05 June 2021
Abstract:The capacitively coupled Ar plasma containing oxygen, driven by a radio frequency of 27.12 MHz, is investigated by laser-induced photo-detachment technique assisted with a Langmuir probe. The plasmas with different amounts of oxygen are obtained by changing the flow of Ar and oxygen, each of which is controlled by a mass flow controller. The axial distribution of plasma characteristic can be measured by changing the relative axial position of the Langmuir probe between the parallel electrodes. The electron density and electron temperature are calculated from the current-voltage curve measured by the scanning Langmuir probe, and the electronegativity is obtained from the current curves of the probe with the laser-induced photo-detachment technique. The negative ion density can be calculated from the electron density and the electronegativity. It is shown that with oxygen flow rate increasing, the dissociative attachment of oxygen molecules with electrons will consume the electrons with the middle energy in the electron energy probability function (EEPF) measured by Langmuir probe. The EEPF evolves from Druyvesteyn to Maxwellian distribution due to the thermalization by the e-e interaction with applied power increasing. It is worth mentioning that a depression in the EEPF curve will appear when discharging high-pressure Ar gas containing oxygen. This depression can also be caused by the dissociative attachment of oxygen molecules with electrons where the threshold energy is around 4.5 eV. The axial profile of the electron density is calculated from the EEPF changing from a linear rise in pure Ar plasma to a flater phase of the distribution due to the negative ions such as oxygen introduced into the plasma. The electron temperature changes a little at different axial positions. The rise of negative ion density nearby the sheath of powered electrode is due to the dissociative attachment caused by the collision of oxygen molecules with energetic electrons. In addition, the axial distribution of electronegativity takes on a shape of spoon, which results from the consequence of generation and loss of negative ions in the process of the ambipolar-electric-field-driven diffusion to the plasma center. Keywords:laser induced photo-detachment/ Langmuir probe/ electronegative plasma/ electronegativity
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2.实验装置与诊断方法安装有Nd:YAG激光发生器和射频补偿朗缪尔探针的容性耦合等离子体发生装置如图1所示, 等离子体被限制在高为300 mm、直径为350 mm的圆柱形腔室内. 频率为27.12 MHz的功率电源通过匹配箱连接于上极板, 下极板则接地, 上下极板面积相等, 其直径均为200 mm. 为了确保等离子体被约束于极板之间, 上下电极通过聚四氟乙烯材料连接到接地的气体导流板上. 实验用气体为纯度为99.999%的氩气与氧气, 可以通过质量流量计来控制通入气体的流量. 机械泵与涡轮分子泵组合成为装置的真空抽气系统, 在实验前将腔室抽至低于5 × 10–4 Pa的本底真空. 图 1 安装有Nd:YAG激光器和朗缪尔探针的电容耦合等离子体实验装置 Figure1. The capacitively coupled plasma experimental device equipped with a Nd:YAG laser and a Langmuir probe.
实验使用朗缪尔探针对掺入不同含量氧气的容性耦合Ar等离子体进行诊断, 含氧量为氧气流量与气体总流量的比值. 探针针尖位于激光光斑的轴向位置并置于放电中心, 通过同时调节上下极板来改变探针的相对轴向位置, 以得到等离子体参数的轴向分布情况. 极板间距设置为34 mm. 将两极板中心位置设为0, 以功率电极方向为正向, 探针可以在–8—+8 mm之间进行调节. 朗缪尔探针装置如图2所示, 针尖是由直径为150 μm、长为10 mm的铂丝制作而成. 针尖与一级和二级LC滤波电路相连接, 减少了27.12 MHz及其谐波信号带来的射频干扰. 探针顶部连接有直径25 mm的环状铜丝, 铜丝直径为400 μm, 并通过10 nF电容连接至探针扫描电路, 这可以使噪声信号进一步降低. 图 2 朗缪尔探针结构示意图 Figure2. The schematic diagram of the structure of Langmuir probe.