氣體放電和各種等離子體源

1、1氣體放電 帶電粒子的產(chǎn)生和消失過程 湯生放電和間隙擊穿電壓 （帕刑定律） 流注放電2 帶電粒子的產(chǎn)生和消失過程3帶電粒子的產(chǎn)生過程碰撞離化X+eX+e+eX*+eX+e+e熱致離化X+X+KEX+X+e電子碰撞離化光致離化X+hvX+ehvfjn=1n=n=n=XX*n=1n=YX, Y: 氣體原子或分子潘寧(Penning) 離化X*+YX+Y+e4帶電粒子的消失過程電子附著過程X+eX- +hv 輻射附著XY+eX- +Y 分解附著X+Y+eX- +Y 三體碰撞附著X, Y, Z: 氣體原子或分子復(fù)合過程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*輻射復(fù)合三體碰撞復(fù)合分解復(fù)合三體碰撞復(fù)合輻射復(fù)合電荷交換復(fù)合電子-離子離子-離子5電極表面現(xiàn)象Metalx0費米能級EFF真空能級E0電子分布函數(shù), f電子能量, xT=0 K金屬內(nèi)真空肖特基效應(yīng)(E108 Vm-1)量子隧穿效應(yīng) FowlerNordheim equation6二次電子發(fā)射粒子能量Fg效應(yīng)一個入射粒子撞擊后放出的二次電子數(shù)目稱作g系數(shù)(g與材料和表面狀態(tài)有關(guān))應(yīng)用例子光電子發(fā)射愛因斯坦光電效應(yīng)光子能量Fhn金屬金屬高能粒子7湯生放電和帕刑定律8湯生放電電場dxn0dxnn+dn離子電子中性粒子初始電

3、子初始電子n0（來自宇宙射線或者電極表面輻射） 在電場作用下從陰極向陽極運動 能量E增加 E離化能，發(fā)生碰撞離化 電子移動單位距離產(chǎn)生的離化次數(shù)a， 稱為一次湯生離化系數(shù)n=n0eaddn=nadx, d=0, n=n0a效應(yīng)9湯生放電g效應(yīng) 二次電子發(fā)射nA=n0ead/1-g(ead-1)=n0ead/(1-gead), ead1nA=nCeadDn=g(nA-nC)陽極電子數(shù)陰極二次發(fā)射電子數(shù)nC=n0+Dn陰極電子數(shù)IA=I0ead/(1-gead)1-gead=0, IA 趨近無窮大，發(fā)生間隙擊穿(Breakdown)10帕刑定律 (間隙擊穿電壓)a=Ae-Bp/E, E=V/d1-

4、gead=0擊穿條件湯生一次離化系數(shù)存在一個最小Vbd物理意義在帕刑最小右邊: le隨著p升高而降低，撞擊次 數(shù)升高，電子難于獲得大的能量，故擊穿電壓很高。在帕刑最小左邊: le隨著p降低而升高，電子 可獲得很大能量，但撞擊次數(shù)很低，難以發(fā)生擊穿，也需要較大的擊穿電壓。電子自由徑le1/p在此僅以d不變來做分析Vbd=Bpd/lnApd/ln(1/g)11流注放電（streamer discharge）湯生放電適用于 低氣壓，短電極距離，低過電壓率此條件下，一個電子在電極間移動時，生成空間電荷數(shù)少，低于可影響空間電場的離子數(shù)臨界值，Ncr (108) eadNcr例如在N2，p=760 Tor

5、r, d=1 cm直流擊穿電壓31 kV, E/p=41 V/(cm torr), 在10%的過電壓率情況下， E/p=45 V/(cm torr) N=ead4106 （湯生放電）高氣壓，長電極距離，高過電壓率 Ncr (108)流注放電理論適用12流注放電（streamer discharge）電場陰極陽極荷電粒子在碰撞離化雪崩增殖中，擴散的帶電離化球半徑滿足 r=(4Dt)1/2D： 電子擴散常數(shù)由此離化球產(chǎn)生電場為 Es=eNs/(4pe0r2)Ns: 離化球中離子數(shù)高氣壓，長電極距離，高過電壓率 Es 不可忽略，如有圖所示。13流注放電（streamer discharge）陽極陽極

6、陰極正流注（陰極向）Meek mechanism1負(fù)流注（陽極向）Raether mechanism2陰極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: 464.二次電子雪崩光子14湯生放電-流注放電轉(zhuǎn)變K：過電壓率15小結(jié)放電中電子參與的 一些反應(yīng)放電發(fā)生的具體過程帕邢定律流注放

7、電機制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高氣壓: 紅色光線橫穿兩個電極，氣壓降低：紅色光線變粗

11、充滿兩電極間. 一個電極出現(xiàn) blue glow, 另一個出現(xiàn)space.繼續(xù)降氣壓， blue glow變成了薄薄的紅色sheath （A ）B : Negative glow, AB之間是Crookes dark spaceC: positive column （or plasma）D: 繼續(xù)抽真空 A/B/C都消失，出現(xiàn) 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