气体放电和各种等离子体源

1、1气体放电 带电粒子的产生和消失过程 汤生放电和间隙击穿电压 （帕刑定律） 流注放电2 带电粒子的产生和消失过程3带电粒子的产生过程碰撞离化X+eX+e+eX*+eX+e+e热致离化X+X+KEX+X+e电子碰撞离化光致离化X+hvX+ehvfjn=1n=n=n=XX*n=1n=YX, Y: 气体原子或分子潘宁(Penning) 离化X*+YX+Y+e4带电粒子的消失过程电子附着过程X+eX- +hv 辐射附着XY+eX- +Y 分解附着X+Y+eX- +Y 三体碰撞附着X, Y, Z: 气体原子或分子复合过程X+eX*+hvX+e+eX*+eX+e+YX*+YXY+e(XY)*X*+YX+Y

2、- X+Y+hvX+Y- XY+hvX+Y- +ZXY+ZX+Y- X*+Y*辐射复合三体碰撞复合分解复合三体碰撞复合辐射复合电荷交换复合电子-离子离子-离子5电极表面现象Metalx0费米能级EFF真空能级E0电子分布函数, f电子能量, xT=0 K金属内真空肖特基效应(E108 Vm-1)量子隧穿效应 FowlerNordheim equation6二次电子发射粒子能量Fg效应一个入射粒子撞击后放出的二次电子数目称作g系数(g与材料和表面状态有关)应用例子光电子发射爱因斯坦光电效应光子能量Fhn金属金属高能粒子7汤生放电和帕刑定律8汤生放电电场dxn0dxnn+dn离子电子中性粒子初始电

3、子初始电子n0（来自宇宙射线或者电极表面辐射） 在电场作用下从阴极向阳极运动 能量E增加 E离化能，发生碰撞离化 电子移动单位距离产生的离化次数a， 称为一次汤生离化系数n=n0eaddn=nadx, d=0, n=n0a效应9汤生放电g效应 二次电子发射nA=n0ead/1-g(ead-1)=n0ead/(1-gead), ead1nA=nCeadDn=g(nA-nC)阳极电子数阴极二次发射电子数nC=n0+Dn阴极电子数IA=I0ead/(1-gead)1-gead=0, IA 趋近无穷大，发生间隙击穿(Breakdown)10帕刑定律 (间隙击穿电压)a=Ae-Bp/E, E=V/d1-

4、gead=0击穿条件汤生一次离化系数存在一个最小Vbd物理意义在帕刑最小右边: le随着p升高而降低，撞击次 数升高，电子难于获得大的能量，故击穿电压很高。在帕刑最小左边: le随着p降低而升高，电子 可获得很大能量，但撞击次数很低，难以发生击穿，也需要较大的击穿电压。电子自由径le1/p在此仅以d不变来做分析Vbd=Bpd/lnApd/ln(1/g)11流注放电（streamer discharge）汤生放电适用于 低气压，短电极距离，低过电压率此条件下，一个电子在电极间移动时，生成空间电荷数少，低于可影响空间电场的离子数临界值，Ncr (108) eadNcr例如在N2，p=760 Tor

5、r, d=1 cm直流击穿电压31 kV, E/p=41 V/(cm torr), 在10%的过电压率情况下， E/p=45 V/(cm torr) N=ead4106 （汤生放电）高气压，长电极距离，高过电压率 Ncr (108)流注放电理论适用12流注放电（streamer discharge）电场阴极阳极荷电粒子在碰撞离化雪崩增殖中，扩散的带电离化球半径满足 r=(4Dt)1/2D： 电子扩散常数由此离化球产生电场为 Es=eNs/(4pe0r2)Ns: 离化球中离子数高气压，长电极距离，高过电压率 Es 不可忽略，如有图所示。13流注放电（streamer discharge）阳极阳极

6、阴极正流注（阴极向）Meek mechanism1负流注（阳极向）Raether mechanism2阴极1 Meek, J. (1940). A Theory of Spark Discharge. Physical Review 57 (8): 722.2 Raether, H. (1939). Die Entwicklung der Elektronenlawine in den Funkenkanal. Zeitschrift fr Physik, 112: 464.二次电子雪崩光子14汤生放电-流注放电转变K：过电压率15小结放电中电子参与的 一些反应放电发生的具体过程帕邢定律流注放

7、电机制Types and Structures of DischargesTownsend dischargeNormal glowEnough ionization, discharge becomes self-sustaining.Gas begins to glow, the voltage drops accompanied by a sharp rise in current.Plasma spreads on the electrodes slowly with increasing power.Abnormal dischargeArcThe cathode gets hott

8、er. Now the thermionic emission of electrons exceeds that ofsecondary-electron emission and low-voltage arcs propagate.16Currentvoltage (i V ) characteristics of direct current (dc) electrical dischargeVb is the breakdown voltage, Vn is the normal operating voltage, and Vd is the operating voltage o

9、f arc discharge.17Glow discharge 18Sprite light in the atmosphere (left) and in a laboratory glow discharge tube (right). In both cases, the light near the positive (anode) end is red and arises from the collisional excitation of neutral nitrogen molecules by free electrons. Also in both cases, the

10、light near the negative (cathode) end is blue and arises from the collisional excitation of N2+ ions by free electrons. Sprite and Glow Discharge Tube19Two metal plates /10k V/and vacuum the tube. Regions in the DC Glow Discharge Tube20Regions in the DC Glow Discharge Tube高气压: 红色光线横穿两个电极，气压降低：红色光线变粗

11、充满两电极间. 一个电极出现 blue glow, 另一个出现space.继续降气压， blue glow变成了薄薄的红色sheath （A ）B : Negative glow, AB之间是Crookes dark spaceC: positive column （or plasma）D: 继续抽真空 A/B/C都消失，出现 Green Fluorescent Light.21Regions in the DC Glow Discharge TubeCathodeAston dark Very thin: containing low energy electrons and high en

12、ergy positive ionsCathode glow De-excitation of positive ions through neutralization is the probable mechanism of light emission here.Cathode dark (Crookes)little ionization, this region is dark. Most of the discharge voltage is dropped across the cathode dark space. Referred to as the cathode sheat

13、h. Negative glowVisible emission due to interactions between secondary electrons and neutrals with attendant excitation and de-excitation.Faraday darkPositive columnAnode dark Commonly referred to as the anode sheath.Anode22Yu. P. Raizer. Gas Discharge Physics. Springer, Berlin, 1991.Light intensity

14、Potential VField ECurrent densityCharge densityCharge density (total)Potentials along the TubeJ=J+JeJ+= n+eu+EJe=-neeueE23Fundamental of Plasma PhysicsPlasma species: ne, ni, noElectron has highest velocityElectrically neutral: ne = niDegree of gas ionization: fi = ne/(ne+no)fi = 10-4 for glow disch

15、argeParticle energies and temperature:For glow discharge: Electron energy = 1 to 10 eV (typically 2 eV)Effective characteristic temperature Te = E/kB = 23000 KNeutral gas energy = 0.025 eV, (To = 293 K)Low pressure glow discharge is usually a nonequilibrium cold plasma.(if in equilibrium: Ti = To =

16、Te = T)24Motion of Plasma SpeciesElectrical current densityParticle fluxChargeSurface are charged negatively due to greater electron bombardment. vevi25Charging of Surface in a PlasmaThe implication of this calculation is that an isolated surface within the plasma charges negatively initially becaus

17、e of the greater electron bombardment.Subsequently, additional electrons are repelled while positive ions are attracted.Therefore, the surface continues to charge negatively at a decreasing rate until the electron flux equals the ion flux and there is no net current.26Mobility: velocity per unit ele

18、ctric fieldMobility in an Electric FieldCollision frequencyElectric forceFrictional dragIn the steady state,Typical mobilities for gaseous ions at 1 torr and 273 K range from 4 102 cm2/V-s (for Xe+) to 1.1 104 cm2/V-s (for H+).27DiffusionCharge Neutrality, Electric field developed by separation of c

19、hargeAmbipolar diffusion coefficientApplied an electric field to a plasma, then28Electron Motion in Combined Electric FieldClearly, magnetic fields prolong the electron residence time in the discharge and enhance the probability of ion collisions.29Electron Motion in Combined Electric FieldElectrons

20、 emitted normally from the cathode ideally do not even reach the anode but are trapped near the electrode where they execute a periodic hopping motion over its surface.30Electron TraceElectrons repeatedly return to the cathode at time intervals of /c. q (Coul)E (V/m)wcme (Kg)B (Gauss)1.60E-19100002.

21、80E+079.11E-341031Electron Motion in Glow Discharge PlasmaElectrons repeatedly return to the cathode at time intervals of /c.Electron motion is strictly confined to the cathode dark space where both fields are present.However, if electrons stray into the negative glow region where E is small, they d

22、escribe a circular orbit before collisions may drive them either back into the dark space or forward toward the anode.Confinement in crossed fields prolongs the electron lifetime over and above that in crossed fields, enhancing the ionizing efficiency near the cathode. A denser plasma and larger dis

23、charge current result.32Radial Electric Potential around an Isolated Positive Ion33AC Effects in PlasmasMaximum electron displacement amplitudeAssuming no collisions of electrons with neutrals,Ionization energyField Strength required to ionize gas.Eo = 11.5 V/cm is an easily attainable field in typi

24、cal plasma reactors.No power is absorbed in the collisionless harmonic motion of electrons, however.For electrons undergo inelastic collisions,Electron motion is randomized and power is effectively absorbed from the RF source.Even smaller values of Eo can produce ionization if, after electron-gas co

25、llisions, the reversal in electron velocity coincides with the changing electric-field direction.Through such effects RF discharges are more efficient than their DC counterparts in promoting ionization.34Electrodeless ReactorsInductive couplingCapacitive couplingHowever, for the deposition of films

26、by RF sputtering, internal cathode targets are required.35Electrode SheathsBoth anode and cathode surfaces will be at a negative floating potential (Vf) relative to the plasma potential (Vp)Lower electron density in the sheath means less ionization and excitation of neutrals. Less luminosity there.Large electric fields are restricted to the sheath regions.It is at the sheath-plasma interface that ions begin to accelerate on their way to the target during sputtering.The