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EE-194-PLA Introduction to Plasma Engineering

EE-194-PLA Introduction to Plasma Engineering. Part 1: Plasma Technology Part 2: Vacuum Basics Part 3: Plasma Overview Professor Jeff Hopwood ECE Dept., Tufts University. Part 1: Basic Plasma Technology.

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EE-194-PLA Introduction to Plasma Engineering

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  1. EE-194-PLAIntroduction to Plasma Engineering Part 1: Plasma Technology Part 2: Vacuum Basics Part 3: Plasma Overview Professor Jeff Hopwood ECE Dept., Tufts University

  2. Part 1:Basic Plasma Technology

  3. Plasma: an ionized gas consisting of atoms, electrons, ions, molecules, molecular fragments, and electronically excited species (informal definition) www.geo.mtu.edu/weather/aurora/

  4. plasma (electrons+ions) energy gas (steam) energy energy liquid (water) Plasma: the “fourth state of matter” solid (ice)

  5. ”sputtering” + + + + - - - - - + - - - - - - + - - Argon Electron Argon ion - + DC Plasma (AC Fluorescent Lamp…why AC?) Argon + Mercury @ ~0.01 atm. - + + lamp endcap Also, this is the heart of high powered gas lasers.

  6. Fluorescent Lamp SpectrumThe strong peaks of light emission are due to excited Hg:Hg + e- (hot) Hg* + e- (cold) Hg + light + e- photon http://www.chemcool.com http://en.wikipedia.org

  7. Integrated Circuit Fabricationand Plasma Technology

  8. (h) Microfabricationdeposit-pattern-etch-repeat (a) (e) (b) (f) (c) (g) Copper metallization on the PowerPC chip (d)

  9. Basic Plasma TechnologySputtering Magnetron DC Pulsed RF S S N Target S N N Substrate to pump

  10. Basic Plasma TechnologyCapacitively Coupled Plasma 0.4 – 60 MHz Hopwood and Mantei, JVST A21, S139 (2003)

  11. Cl2 Cl2 SiCl2 Cl+ SiCl2 Cl Plasma Etching S Simplified anisotropic etching Cl2 + e- Cl + Cl+ + 2e- Si(s) + 2Cl(g)+ ion energy  SiCl2(g)

  12. Cl+ Cl Si(s) + 2Cl(g)+ ion energy  SiCl2(g) The directional ion energy drives the chemical reaction only at the bottom of the microscopic feature. Anisotropyis due to directional ion bombardment Dry or Plasma Etching Wet Etching (in acid) wafer wafer In wet chemistry, the chemical reaction occurs on all surfaces at the same rate. Very small features can not be microfabricated since they eventually overlap each other.

  13. Trenches: etched and filled with copper Jason M. Blackburn, David P. Long, Albertina Cabañas, James J. Watkins Science 5 October 2001: Vol. 294. no. 5540, pp. 141 - 145

  14. SiH4 SiH4 SiHX+H2 SiH H2 SiH2 Plasma Deposition S Simplified plasma deposition SiH4 + e-  SiH3 + H + e- SiH3 + e-  SiH2 + H + e- SiH2 + e-  SiH + H + e- SiH + e-  Si + H + e- SiHx+ surface+ ion energy  Si (s) + Hx(g)

  15. Basic Plasma TechnologyElectron Cyclotron Resonance Plasma: Etch and Deposition Hopwood and Mantei, JVST A21, S139 (2003)

  16. Basic Plasma TechnologyInductively Coupled Plasma: Etch and Deposition 0.4 – 13.56 MHz Hopwood and Mantei, JVST A21, S139 (2003)

  17. Other applications:Xenon Ion Propulsion Deep Space 1 encounter with Comet Borrelly http://nmp.nasa.gov/ds1/images.html

  18. blue red green Other Applications : Plasma Display Panels (PDPs) Structure From S.S. Yang, et al, IEEE Trans. Plasma Sci. 31, 596 (2003).

  19. initiate breakdown (~ 300 volts) sustain plasma (~ 180 volts) surface + + + + + + Plasma Display Panels (PDPs) Basic Operation Sustain Electrode + + + + Bus Electrode h ~ 200 m l ~ 400 m d ~ 60 m From S.S. Yang, et al, IEEE Trans. Plasma Sci. 31, 596 (2003).

  20. Part 2:Basic Vacuum Concepts

  21. Goals • To review basic vacuum technology • Pressure, pumps, gauges • To review gas flow and conductance • To understand the flux of vapor phase material to a substrate • To understand mean free path, l

  22. Typical High Pressure Plasma 1.3x10-9 1.3x10-3 1.3x10-6 1 atm. 1 Torr = 1 mm-Hg 1 Torr 1x10-6 Torr 1 mTorr 760 Torr 1 Pascal = 1 N/m2 0.133x10-3 Pa 0.133 Pa 133 Pa 101,333 Pa Typical Low Pressure Plasma Processing Ultrahigh Vacuum Rough Vacuum High Vacuum Vacuum (units)

  23. Rough Vacuum • “Mechanical Pumps” typically create a base pressure of 1-10 mTorr or 0.13-1.3 Pa Warning: Certain process gases are incompatible with pump fluids and pose severe safety risks! Rotary Vane Pump (Campbell)

  24. High Vacuum Pumping • Cryopumps condense gases on cold surfaces to produce vacuum • Typically there are three cold surfaces: • Inlet array condenses water and hydrocarbons (60-100 Kelvin) • Condensing array pumps argon, nitrogen and most other gases (10-20 K) • Adsorption is needed to trap helium, hydrogen and neon in activated carbon at 10-12 K. These gases are pumped very slowly! (Campbell) Warning: all pumped gases are trapped inside the pump, so explosive, toxic and corrosive gases are not recommended. No mech. pump is needed until regen. adapted from www.helixtechnology.com

  25. High Vacuum Pumping Process chamber Turbomolecular Pump High rotation speed turbine imparts momentum to gas atoms Inlet pressures: <10 mTorr Foreline pressure: < 1 Torr Requires a rough pump Good choice for toxic and explosive gases – -gases are not trapped in pump All gases are pumped at approx. the same rate Pumping Speeds: 20 – 2000 liters per sec foreline adapted from Lesker.com

  26. High Vacuum Pumping Diffusion Pump The process gas is entrained by the downward flow of vaporized pumping fluid. Benefits: Low cost, reliable, and rugged. High pumping speed: ~ 2000 l/s Caution: The process chamber will be contaminated by pumping fluid. A cold trap must be used between the diffusion pump and the process chamber. Not recommended for “clean” processes. Process chamber Water- cooled walls Foreline -to mech pump Heater/Pumping Fluid adapted from Lesker.com

  27. Flow Rate Typically gas flows are cited in units of standard cubic centimeters per minute (sccm) or standard liters per minute (slm) “Standard” refers to T=273K, P = 1 atm. Example: Process gas flow of 50 sccm at 5 mTorr (@300K) requires 50 cm-3min-1(760Torr/5x10-3Torr)(300/273)(1min/60sec)(1/103) = 140 liters/sec of pumping speed at the chamber pump port

  28. Conductance Limitation 50 sccm Conductance depends on geometry and pressure (use tabulated data) 5 mTorr 140 l/s = Q/(P1 – P2) Fixed Throughput, Q: Q = 0.005 Torr x 140 l/s = 0.7 Torr-l/s > 140 l/s …since P2<P1 Corifice = ¼ (pa2)<v>l/s Ctube = pa2 (2a<v>/3L) …if mean free path >> a, L (see Mahan, 2000)

  29. Convectron Gauge: Initial pumpdown from 1 atm, and as a foreline monitor Thermal Conductivity of Gas Baratron: Insensitive to gas composition, Good choice for process pressures True Pressure (diaphragm displacement) Ionization of Gas Ion Gauge: Sensitive to gas composition, but a good choice for base pressures Pressure Measurement RGA: A simple mass spectrometer Vacuum Gauge Selection adapted from Lesker.com

  30. Residual Gas Analysis Low pressure systems are dominated by water vapor as seen in this RGA of a chamber backfilled with 4x10-5 torr of argon Why? H2O is a polar molecule that is difficult to pump from the walls --> bake-out the chamber Leak? Source: Pfeiffer vacuum products

  31. Gas Density (n) Ideal Gas Law PV = NkT Gas density at 1 Pascal at room temp. N/V = n = P/kT = (1 N/m2)/(1.3807x10-23J/K)(300 K) = [1 (kg-m/s2)/m2]/[4.1x10-21 kg-m2/s2] = 2.4x1020 atoms per m3 = 2.4x1014 cm-3 …at 1Pa Rule of Thumb n(T) = 3.2x1013 cm-3 x (300/T) …at a pressure of 1 mTorr

  32. Gas Kinetics Maxwellian Distribution Average speed of an atom: Flux of atoms to the x-y plane surface: Very important! (Campbell)

  33. Example A vacuum chamber has a base pressure of 10-6 Torr. Assuming that this is dominated by water vapor, what is the flux of H2O to a substrate placed in this chamber? n = 3.2x1013 cm-3/mTorr * 10-3 mTorr = 3.2x1010 cm-3 <v> = (8kT/pM)1/2 = 59200 cm/s Gz = (¼)n<v>= 4.74x1014 molecules per cm2 per sec! This is approximately one monolayer of H2O every second at 10-6 Torr base pressure.

  34. Rigorous Hard Sphere Collisions: l = kT / 2pd2P  lAr(cm) ~ 8 / P (mTorr) Collisions and Mean Free Path Gas Density n = P/ kT Cross-section s ~ pd2 l = 1/sn d Ar

  35. Part 3: Plasma Basics

  36. VDC Too many collisions Electron energy<ionization energy d Too few ionizing collisions: l>d Paschen Curve F. Paschen, Ann. Phys. Chem., Ser. 3 37, 69 (1889). http://www.duniway.com/images/pdf/pg/Paschen-Curve.pdf

  37. light Power Gas flow gas (ng) excited atoms and molecules electrons ne, Te ions radicals, molecular fragments reaction products secondary electrons pumping pumping What do we need to know about plasma? Wall Wall PLASMA substrate

  38. light Gas flow gas (ng) excited atoms and molecules ions radicals, molecular fragments reaction products secondary electrons pumping pumping Power Absorbed Power Wall Wall PLASMA electrons ne, Te substrate

  39. Power Absorbed: DC • DC power • General electrical mobility and conductivity • Mobility: me = q<t>/m = q/nmme Where <t> is the average time between collisions and nm is the collision frequency (collisions per second) • Electron Conductivity: sDC= qneme = q2ne/nmme • DC power absorbed:

  40. VRF Power Absorbed: RF • RF/microwave power • Ohmic Heating • Generic electron-neutral collision frequency nm ~ 5x10-8 ngasTe1/2 (s-1) … ngas (cm-3), Te(eV). • Example: Find the pressure at which rf ohmic heating becomes ineffective: nm = 0.1w, Te = 2eV w = 13.56 MHz * 2p = 85.2Mrad/s ngas = 0.1*85.2x106/5x10-8(2)1/2 = 1.2x1014 cm-3 = 3.7 mTorr f=13.56 MHz An electron oscillates in a rf electric field without gaining energy unless electron collisions occur Hopwood and Mantei, JVST A21, S139 (2003)

  41. Stochastic Heatingan electron enters and exits a region of high field for a fraction of an rf cyclet0 << 2p/w Reflecting Boundary (plasma sheath) Emax ERF z x - E ~ 0 vx(t0) > vx(0) The usual mechanism for heating electrons using RF electric fields at low pressures

  42. -Ex t1 t3 t2 - - - x BDC Electron cyclotron frequency: wce = qB/me = 1.76x107 B(gauss) If w = wce and ERF is perpendicular to BDC, then the electron gains energy from Ex in the absence of collisions. Ex. f=2.45 GHz --> B=875 G k ERF E=0 v y F = q(vxB) x W/cm3 Wave/Resonant Heating Hopwood and Mantei, JVST A21, S139 (2003)

  43. light Gas flow gas (ng) excited atoms and molecules ions radicals, molecular fragments reaction products secondary electrons pumping pumping Electron Collisions Power Wall Wall PLASMA electrons ne, Te substrate

  44. Electron Collisions • Elastic Collisions: • Ar + e  Ar + e • Gas heating: energy is coupled from e to the gas • Excitation Collisions • Ar + ehot Ar* + ecold, Ar* Ar + hn • Responsible for the characteristic plasma “glow” • Eelectron>Eexc (~11.55 eV for argon) • Ionization Collisions: • Ar + ehot Ar+ + 2ecold • Couples electrical energy into producing more e_ • Eelectron > Eiz (15.76 eV for argon) • Dissociation: • O2 + ehot 2O + ecold or O2 + ehot O + O+ + 2ecold • Creates reactive chemical species within the plasma • Eelectron > Ediss(5.12 eV for oxygen)

  45. Collision Cross Sections • Unlike the hard sphere model, real collision cross sections are a function of electron kinetic energy s(E), or electron velocity s(v). • We must find the expected collision frequency by averaging over all E or v. becomes (cm3s-1)

  46. Graphically f(E) f(E) or s(E) sAr+(E) Note: the exponential tail of energetic electrons is responsible for ionization Te Eiz Electron energy, E The RATE CONSTANT: Kiz(Te) Kizoexp(-Eizo/Te) curve fitting

  47. Graphically Hot electrons – more ionization f(E) f(E) or s(E) sAr+(E) Note: the exponential tail of energetic electrons is responsible for ionization Te Eiz Electron energy, E The RATE CONSTANT: Kiz(Te) Kizoexp(-Eizo/Te) curve fitting

  48. Examples of Numerically Determined Rate Constants (Lieberman, 2005)

  49. Generation Rate of Plasma Species by Electron Collisions y + e  x + e dnx/dt = Kxneny For example, Ar + e  Ar+ + e + e dne/dt = Kiznengas is the number of electrons (and ions) generated per cm3 per second

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