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.

2 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. Also related to pressure, electrodespacing, gas species and chamber design.

3 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

4 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. 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.

5 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? Aluminum and copper sputtering processesonly use argon. Argon is a noble gas, whichexist in the form of atoms instead ofmolecules.

6 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?

7 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. 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.

8 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.

9 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.

10 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