射频容性耦合Ar/O_(2)等离子体的轴向诊断

Axial diagnosis of radio-frequency capacitively coupled Ar/O_(2) plasma

实验通过朗缪尔探针辅助激光诱导解离技术对27.12 MHz驱动的不同含氧量条件下容性耦合Ar等离子体进行了诊断研究.通过质量流量计改变通入Ar与O_(2)的流量以得到不同含氧量的等离子体.结果表明,由于氧气的分解吸附反应需要消耗电子,致使朗缪尔探针测得的电子能量概率函数(EEPF)的中能部分随着含氧量的上升而下降.射频输入功率增加时电子密度的上升引起了电子-电子碰撞热化,从而使EEPF由Druyvesteyn分布向麦克斯韦分布转变.在功率电极附近,由于鞘层边界附近的电子氧气分子碰撞时的分解吸附反应使得鞘层区附近的负离子密度较高.另外,负离子密度沿着轴向呈现勺型分布的特征.这主要是由于负离子在双极电场作用下向等离子体放电中心扩散的过程中所存在的负离子产生与损失的反应过程导致.

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-electricfield-driven diffusion to the plasma center.