Research On Characteristics Of The Magnetized Electronegative Plasma Sheath

In the technologies widely used in industry,such as plasma material surface modification,etching,thin film deposition and etc.,a negative bias voltage is applied to the workpiece so that a plasma sheath is formed on the surface of the workpiece.Negative ions usually produced in the plasma sheath,which play important roles in many systems and technologies,including plasma chemistry,surface treatment,gas lasers,gas discharge,ionospheric phenomena and so on.Therefore,research on physical characteristics of plasma sheath is of direct significance to the application of the interaction between plasma and the material surface.The influence of the magnetic field on the sheath cannot be ignored in many cases.In addition,the external magnetic field may be used to realize effective control of the techniques.At present,the common plasma sheath has been studied thoroughly,but the electronegative plasma sheath with the external magnetic field has no further details.In this paper,the magnetized electronegative plasma sheath and the magnetized electronegative plasma sheath considering collisions between positive ions and neutral particles are theoretically and numerically investigated respectively.The fluid model and the presheath-sheath boundary velocity induced by positive ions E ?B? ? drift are adopted.The main content is as follows:Firstly,the Bohm criterion and the structure of the electronegative plasma sheath consisting of hot electrons,hot negative ions and cold positive ions are studied by theoretical derivation and numerical simulation,and an applied magnetic field is taken into account at the same time.An exact expression of sheath criterion is obtained by studying the Sagdeer potential.The results of numerical simulation reveal that the higher negative ion concentration of the sheath edge,the lower negative ion temperature in the sheath and the bigger magnetic field angle would result in the smaller lower limit of ion Mach number and the positive ions would enter into the sheath more easily.The changes of magnetic field intensity pose little effect on the sheath structure while the magnetic field is weak(B<0.04T)or strong(0.04T<B<0.4T).But obvious variations of the sheath structure take place under the condition of extremely strong magnetic field(0.4T<B<4T).As the increasing of negative ions concentration or the decreasing of negative ions temperature,the electrons density and negative ions density goes down to zero faster,the peak of net space charge density moves to the sheath edge,the positive ions kinetic energy when arriving the surface of workpiece increases,the spatial potential decreases and the thickness of the plasma sheath diminish.While discussing the effects of the other variables which can result in the variety of the ion Mach number on the sheath formation,a different conclusion may be made when ion Mach number is adopted either as variable or invariable value.Besides,that the ion Mach number is fixed as different value satisfying the Bohm criterion has an influence on the sheath structure.Secondly,the Bohm criterion and the structure of the magnetized electronegative plasma sheath consisting of hot electrons,hot negative ions and cold positive ions are discussed by theoretical derivation and numerical simulation,and the collisions between positive ions and neutral particles are considered.An exact expression of the Bohm criterion is obtained through theoretical deduction and it is found that the ion Mach number has not only the lower limit but also the upper limit.The numerical results show that: a)the upper limit and the lower limit of ion Mach number get down as the increasing of collision parameter,magnetic field angle or negatives ion concentration of the sheath edge;b)as the decreasing of negative ions temperature,the lower limit of ion Mach number get down but the upper limit remain unchanging;c)as the increasing of collision parameter or dimensionless parameter varying from-1 to 0,negative ions density does not change obviously,but positive ions density decreases more slowly and electrons density drops to zero faster,the peak of net space charge density increases and broadens and moves to the sheath edge,the spacial potential reduces,positive ions kinetic energy when arriving the surface of workpiece decreases and the thickness of the sheath diminishes;d)with the strengthening of sheath edge electric field,positive ions density,negative ions density and electrons density drop more quickly to zero,the peak of net space charge density decreases and shifts to the sheath edge,the spacial potential decreases,the positive ions kinetic energy when arriving the surface of workpiece increases and the sheath thickness diminishes.Finally,the Bohm criterion and the structure of the electronegative plasma sheath consisting of hot electrons,hot negative ions and hot positive ions are researched by theoretical derivation and numerical simulation,and positive ions temperature and electron impact ionization are considered.An exact expression of the Bohm criterion is obtained through theoretical deduction.The numerical simulation results show that: a)with the increasing of positive ions temperature,the upper limit of ion Mach number does not change obviously,but the lower limit increases,positive ion density decreases faster near the sheath edge and more slowly far from the sheath edge,electrons density and negative ions density decreases more slowly,net space charge density increases near the sheath edge,the peak of net space charge density decreases and leaves the sheath edge,the spacial potential increases,positive ions kinetic energy when arriving the surface of workpiece decreases and the sheath thickness increases;b)with the electron impact ionization rate increasing,the upper limit of ion Mach number does not change significantly,but the lower limit decreases,positive ions density raises at the sheath edge and reduces apart from the sheath edge,electrons density and negative ions density decreases,net space charge density increases,the peak of net space charge density increases and moves to the sheath edge,the spacial potential decreases,positive ions kinetic energy when arriving the surface of workpiece increases and the sheath thickness diminishes.