Laser Induced Photo-detachment Diagnosis Of Capacitively Coupled Ar/O₂ Plasma Excited By Very High Frequency

Capacitively coupled electronegative plasma has been widely applied in the field of surface treatment.For example,a specific surface morphology can be obtained by plasma etching in the microelectronics industry,and some functional groups can be generated on the surface of the polymer.The formation and existence of negative ions have a significant effect on the kinetics and dynamic evolution of low temperature reactive plasma,especially on the balance of charged particle and the electron energy distribution function(EEDF).Therefore,it is necessary to study the electronegative characteristics of the capacitively coupled plasma.By using pulsed laser induced detachment technique assisted with a Langmuir probe,study on electronegative characteristics of the capacitively coupled Ar plasma doped with 5% O2 has been carried out.In this paper,we have firstly focused on the electrical signal of the probe after laser pulse induced negative ion detachment,and then analyzed characteristics of the probe signal with the probe bias below or above plasma space potential.According to the dependence of probe collection signal on bias,the electronegativity,which was used to describe the Ar+O2 plasma electronegative property,was defined as saturation ratio of current after pulsed laser radiation to that of collection probe at a potential above plasma spatial potential.Plasma electronegativity was diagnosed with discharge pressure,RF input power and axial position.The experiments showed that a decrease in electronegativity of plasma appeared with increasing input RF power and an increasing trend of electronegativity with plasma discharge pressure.Due to asymmetrical distribution of electrodes,the plasma electronegativity also took on asymmetric one with the axial position.Additionally,because of competition processes between negative ion production for collisional dissociation of oxygen molecules with high energy electron and negative ion loss for collisional detachment of negative ion with oxygen molecule,a higher electronegativity appeared in vicinity of the sheath boundary and a lower electronegativity at center of the plasma discharge.For the further understand of the electronegative plasma,the I-V characteristic curve of the Langmuir probe was analyzed in the experiment,and the electron density was calculated from the electron energy probability function(EEPF).Finally,the negative ion density was calculated by the combined electronegativity.The same as the electronegativity distribution,affected by asymmetric discharge,the axial distributions of the electron and negative ion density were non centrosymmetric.Electron density showed a high trend near the electrode and a low trend in the center.However,negative ion density was high only when close to the power electrode.Both of them were significantly affected by the pressure near the power electrode.