Chapter 7 Plasma Basics - ntut.edu.tw

1 12006/4/121 Chapter 7 Plasma Basics2006/4/122 Objectives List at least three IC processes using Plasma Name three important collisions in Plasma Describe mean free path Explain how Plasma enhance etch and CVDprocesses Name two high density Plasma sources22006/4/123 Topics of Discussion What is Plasma ? Why use Plasma ? Ion bombardment Application of Plasma process2006/4/124 Applications of Plasma CVD Etch PVD Ion Implantation Photoresist strip Process chamber dry clean32006/4/125 What Is Plasma A Plasma is a ionized gas with equal numbersof positive and negative charges. A more precise definition:a Plasma is a quasi-neutral gas of charged and neutral particleswhich exhibits collective behavior. Examples:Sun, flame, neon light, of Plasma A Plasma consists of neutral atoms ormolecules, negative charges (electrons) andpositive charges (ions) Quasi-neutral:ni ne Ionization rate: ne/(ne+nn)42006/4/127 Neutral Gas Density Idea gas 1 mole = Litter = 104cm3 1 mole = 1023molecules At 1 atm, gas density is 1019cm 3 At 1 Torr, gas density is 1016cm 3 At 1 mTorr, gas density is 1013cm 3 RF Plasma has very low ionization rate2006/4/128 Ionization Rate Ionization rate is mainly determined byelectronenergyin Plasma , which is in turn controlled by theapplied power.

2 Also related to pressure, electrodespacing, gas species and chamber design. In most Plasma processing chambers, the ionizationrate is less than The ionization rate of high density Plasma (HDP)source such as inductively coupled Plasma (ICP) orelectron cyclotron resonance (ECR), is much higher,which is about 1~ 5%. Ionization rate in the core of sun is ~100%.52006/4/129 Parallel Plate Plasma SystemPlasmaRF powerDarkspaces orsheathlayersElectrodesTo Vacuum Pump2006/4/1210 Generation of a Plasma External power is needed Radio frequency (RF) power is the mostcommonly used power source with which avarying electric field is established Electrons and ions are continually generated andlost by collisions and recombination A Plasma is stabilized when generation rate ofelectrons is equal to loss rate of electrons Vacuum system is required to generate a stable RFplasma62006/4/1211 Ionization Processe + AA++ 2 e Ionization collisions generate electrons and ions It sustains a stable Plasma Electron collides with neutral atom or molecule Knock out one of orbital electron2006/4/1212 Illustration of IonizationFreeElectronFreeElectronsOrbit alElectronNucleusNucleus72006/4/1213 Excitation and Relaxatione + AA* + e where A* is excited stateA*A +h (Photons)light emission Different atoms or molecules have differencefrequencies, that is why different gases havedifferent glow colors.

3 The change of the glow colors is used for etchand chamber clean CollisionImpactelectronGroundedelectronE xcitedelectronNucleusNucleusImpactelectr on82006/4/1215 RelaxationGround Stateh h h: Planck Constant : Frequency of LightExcited State2006/4/1216 Dissociation Electron collides with a molecule, it canbreak the chemical bond and generate freeradicals:e + ABA + B + e Free radicals have at least one unpairedelectron and are very chemically reactive. Increasing chemical reaction rate Very important for both etch and Radicals2006/4/1218 Plasma Etch CF4is used in Plasma to generate fluorinefree radical (F) for oxide etche + CF4 CF3+ F + e 4F + SiO2 SiF4+ 2O Enhanced etch chemistry102006/4/1219 PlasmaEnhancedCVD PECVD with SiH4and NO2(laughing gas)e + SiH4 SiH2+ 2H + e e + N2O N2+ O + e SiH2+ 3O SiO2+ H2O Plasma enhanced chemical reaction PECVD can achieve high deposition rate atrelatively lower temperature2006/4/1220Q & A Why are dissociation not important in thealuminum and copper PVD processes?

4 Aluminum and copper sputtering processesonly use argon. Argon is a noble gas, whichexist in the form of atoms instead ofmolecules. Thus there is no dissociationprocess in argon plasma112006/4/1221Q & A Is there any dissociation collision in PVDprocesses? Yes. In TiN deposition process, both Ar andN2are used. In Plasma , N2is dissociated togenerate free radical N, which reacts withTi target to from TiN on the surface. Ar+ions sputter TiN molecules from the surfaceand deposit them on wafer Silane DissociationCollisionsByproductsEnergy of Formatione-+ SiH4 SiH2+ H2+ eVSiH3+ H + eVSi + 2 H2+ eVSiH + H2+ H + eVSiH2*+ 2H + eVSi*+ 2H2+ eVSiH2++ H2+ eVSiH3++ H + 2 eVSi++ 2H2+ 2 eVSiH++ H2+ H + 2 eV122006/4/1223Q & A Which one of collisions in Table is mostlikely to happen? Why? The one that requires the least energy is theone most likely to Free Path (MFP) The average distance a particle can travelbefore colliding with another particle.

5 N21 nis the density of the particle is the collision cross-section of the particle Larger molecules have shorter MFP becauseit is proportional to molecule size and cross-section132006/4/1225 MFP IllustrationLargeparticleSmallparticleLa rgeparticleSmallparticle(a)(b)2006/4/122 6 Mean Free Path (MFP) Effect of pressure: Higher pressure, shorter MFP 1p142006/4/1227Q & A Why does one need a vacuum chamber togenerate a stable Plasma ? At atmospheric pressure (760 Torr), MFP of anelectron is very short. Electrons are hard to getenough energy to ionize gases molecules. Extremely strong electric field can createplasma in the form of arcing (lightening)instead of steady state glow ofCharged Particle Electron is much lighter than ionme<< mime:mHydrogen=1:1836 Electric forces on electrons and ions are the sameF = qE Electron has much higher accelerationa = F/m152006/4/1229 Movement ofCharged Particle RF electric field varies quickly, electrons areaccelerated very quickly while ions react slowly Ions have more collisions due to their largercross-section that further slowing them down Electrons move much faster than ions in plasma2006/4/1230 Thermal Velocity Electron thermal velocity, 1eV = 11594 Kv= (kTe/me)1/2 RF Plasma ,Teis about 2 eVve 107cm/sec = 107mph(equivalent to airplane s speed)162006/4/1231 Magnetic Force and Gyro-motion Magnetic force on a charged particle.

6 F= qv B Magnetic force is always perpendicular to theparticle velocity Charged particle will spiral around themagnetic field line. of charged particleMagnetic Field Line172006/4/1233 Gyrofrequency Charged particle in gyro motion in magnetic fieldmqB Gyro radius Gyroradius of charged particle in a magneticfield, ,can be expressed as: = v / 2006/4/1234 Energy, Ef(E)2-3 eVElectrons withenough energyfor ionizationBoltzmann Distribution182006/4/1235 Ion Bombardment Electrons reach electrodes and chamber wall first Electrodes charged negatively, repel electronsand attract ions. The sheath potential accelerates ions towards theelectrode and causes ion bombardment. Ion bombardment is very important for etch,sputtering and PECVD Potential+-+-+-+-+-+-++-+-+-+-+-+-++-+-+ -+-+-+-++-+-+-+-+-+-++-+-+-+-+-+-++-+-+- +-+-+-++-+-+-+-+-+-++-+-+-+-+-+-++-+-+-+ -+-++++-+-+-+-++++-+-+-++++++-+-++++++-+ ++++++++++Sheath RegionVpVfxDark spaceBulk plasmaSheath PotentialElectrode192006/4/1237 Ion Bombardment Anything close to Plasma gets ion bombardment Mainly determined by RF power Pressure also can affect bombardment2006/4/1238 Applications of Ion bombardment Help to achieve anisotropic etch profile Damaging mechanism Blocking mechanism Argon sputtering Dielectric etch for gap fill Metal deposition Help control film stress in PECVD processes Heavier bombardment.

7 More compressive film202006/4/1239 Plasma Potential & DC BiasTimeVoltDC BiasRF potentialPlasma Potential2006/4/1240DC biases and RF powers0timePlasma potential0timePlasma potentialDC biasRF potentialsDC bias Lower RF power Smaller DC bias Higher RF power Larger DC bias212006/4/1241 Ion Bombardment Ion energy Ion flux (density) Both controlled by RF power2006/4/1242 Ion Bombardment Control Increasing RF power, DC bias increases, iondensity also increases. Both ion density and ion bombardment energyare controlled by RF power. RF power is the most important knob controllingion bombardment RF power also used to control film stress forPECVD processes222006/4/1243DC Bias of CVD Chamber PlasmaVp= 10 20 VRF hotGroundedDark spaces or sheath regions2006/4/1244DC Bias of Etch Chamber PlasmaDC bias0timeWafer PotentialPlasma potentialSelf bias232006/4/1245DC Bias of Etch Chamber PlasmaV2A2A1V1/V2=(A2/A1)V1= 200 to 1000 V4DC bias V12006/4/1246 Question and Answer If the electrode area ratio is 1:3, what is thedifference between the DC bias and the self-bias compare with the DC bias?

8 The DC bias isV1,the self-bias isV1 V2,therefore, the difference is[V1 (V1 V2)]/V1=V2/V1= (A1/A2)4= (1/3)4= 1/81 = and A Can we insert a fine metal probe into theplasma to measure the Plasma potentialV2? Yes, we can. However, it is not very accuratebecause of sheath potential near probe surface Measurement results are determined by thetheoretical models of the sheath potential,which have not been fully developed, Bombardment and Electrode Size Smaller electrode has more energetic ionbombardment due to self-bias Etch chambers usually place wafer onsmaller electrode252006/4/1249 Advantages of Using Plasma Plasma processes in IC fabrication: PECVD CVD chamber dry clean Plasma Etch PVD Ion implantation2006/4/1250 Benefits of Using Plasma inCVD Process High deposition rate at relatively lowertemperature. Independent film stress control Chamber dry clean Gap fill capability262006/4/1251 Comparison of PECVD and LPCVDP rocessesLPCVD (150 mm)PECVD (150 mm)Chemical reactionSiH4+ O2 SiO2 +.

9 SiH4+ N2O SiO2 + ..Process parametersp =3 Torr, T=400 Cp=3 Torr, T=400 C andRF=180 WDeposition rate100 to 200 /min 8000 /minProcess systemsBatch systemSingle-wafer systemWafer to wafer uniformityDifficult to controlEasier to control2006/4/1252 Gap Fill by HDP-CVD Simultaneously deposition and sputtering Tapering the gap opening Fill gap between metal lines bottom up272006/4/1253 HDP CVD Void-free Gap m, A/R 4:12006/4/1254 Benefits of Using Plasma ForEtch Process High etch rate Anisotropic etch profile Optical endpoint Less chemical usage and disposal282006/4/1255 Benefits of Using Plasma ForPVD Process Argon sputtering Higher film quality Less impurity and higher conductivity Better uniformity Better process control Higher process integration capability. Easier to deposit metal alloy films2006/4/1256 PECVD and Plasma Etch Chambers CVD: Adding materials on wafer surface Free radicals Some bombardment for stress control Etch: Removing materials from wafer surface Free radicals Heavy bombardment Prefer low pressure, better directionality of ions292006/4/1257 PECVD Chambers Ion bombardment control film stress Wafer is placed grounded electrode Both RF hot and grounded electrodes haveabout the same area It has very littleself-bias The ion bombardment energy is about 10 to20 eV, mainly determined by the RF power2006/4/1258 Schematic of a PECVD ChamberPlasmaChuckRFWafer302006/4/1259 Plasma Etch Chambers Ion bombardment Physically dislodge break chemical bonds Wafer on smaller electrode Self-bias Ion bombardment energy on wafer (RF hot electrode): 200 to 1000 eV on lid (ground electrode).

10 10 to 20 Etch Chambers Heat generation by heavy ion bombardment Need control temperature to protect masking PR Water-cool wafer chuck (pedestal, cathode) Lower pressure not good to transfer heat fromwafer to chuck Helium backside cooling required Clamp ring or electrostatic chuck (E-chuck) tohold wafer312006/4/1261 Plasma Etch Chambers Etch prefer lower pressure longer MFP, more ion energy and less scattering Low pressure, long MFP, less ionizationcollision hard to generate and sustain Plasma Magnets are used to force electron spin andtravel longer distance to increase collisions2006/4/1262 Schematic of an Etch ChamberProcess gasesPlasmaProcesschamberBy-products tothe pumpChuckRF powerBacksidecooling heliumMagnet coilsWafer322006/4/1263 Remote Plasma Processes Need free radicals Enhance chemical reactions Don t want ion bombardment Avoid Plasma -induced damage Remote Plasma systems2006/4/1264 ProcessgasesPlasmaMW or RFProcesschamberBy-products tothe pumpRemoteplasmachamberFree radicalsHeated plateRemote Plasma System332006/4/1265 Photoresist Strip Remove photoresist right after etch O2and H2O chemistry Can be integrated with etch system In-situ etch and PR strip Improve both throughput and yield2006/4/1266 Photoresist Strip ProcessH2O.