Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations


Presentation on theme: "EE-194-PLA Introduction to Plasma Engineering Part 1: Plasma Technology Part 2: Vacuum Basics Part 3: Plasma Overview Professor Jeff Hopwood ECE Dept.,"— Presentation transcript:

1 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

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)

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

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

6 Fluorescent Lamp Spectrum The strong peaks of light emission are due to excited Hg: Hg + e - (hot) Hg * + e - (cold) Hg + light + e - photon

7 Integrated Circuit Fabrication and Plasma Technology

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

9 Basic Plasma Technology Sputtering Magnetron N S N S N S Target Substrate DC Pulsed RF to pump

10

11 Basic Plasma Technology Capacitively Coupled Plasma 0.4 – 60 MHz Hopwood and Mantei, JVST A21, S139 (2003)

12 Plasma Etching Cl 2 Cl + Cl SiCl 2 Cl 2 SiCl 2 Simplified anisotropic etching Cl 2 + e - Cl + Cl + + 2e - Si(s) + 2Cl(g)+ ion energy SiCl 2 (g) S

13 Anisotropy is due to directional ion bombardment Cl + Cl Si(s) + 2Cl(g)+ ion energy SiCl 2 (g) The directional ion energy drives the chemical reaction only at the bottom of the microscopic feature. Dry or Plasma EtchingWet Etching (in acid) 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. wafer

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

15 Plasma Deposition SiH 4 SiH SiH 2 H2H2 SiH 4 SiH X +H 2 Simplified plasma deposition SiH 4 + e- SiH 3 + H + e- SiH 3 + e- SiH 2 + H + e- SiH 2 + e- SiH + H + e- SiH + e- Si + H + e- SiH x + surface+ ion energy Si (s) + H x (g) S

16 Basic Plasma Technology Electron Cyclotron Resonance Plasma: Etch and Deposition Hopwood and Mantei, JVST A21, S139 (2003)

17 Basic Plasma Technology Inductively Coupled Plasma: Etch and Deposition 0.4 – MHz Hopwood and Mantei, JVST A21, S139 (2003)

18 Other applications: Xenon Ion Propulsion Deep Space 1 encounter with Comet Borrelly

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

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

21 Part 2: Basic Vacuum Concepts

22 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,

23 Ultrahigh Vacuum High Vacuum Rough Vacuum Typical High Pressure Plasma Typical Low Pressure Plasma Processing Vacuum (units) 1 atm. 1.3x x x Torr 1 Torr 1 mTorr1x10 -6 Torr 1 Torr = 1 mm-Hg 101,333 Pa133 Pa Pa 0.133x10 -3 Pa 1 Pascal = 1 N/m 2

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

25 High Vacuum Pumping Cryopumps condense gases on cold surfaces to produce vacuum Typically there are three cold surfaces: (1)Inlet array condenses water and hydrocarbons ( Kelvin) (2)Condensing array pumps argon, nitrogen and most other gases (10-20 K) (3)Adsorption is needed to trap helium, hydrogen and neon in activated carbon at K. These gases are pumped very slowly! 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 (Campbell)

26 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

27 High Vacuum Pumping Process chamber Heater/Pumping Fluid Foreline -to mech pump 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. Water- cooled walls adapted from Lesker.com

28 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 requires 50 cm -3 min -1 (760Torr/5x10 -3 Torr)(300/273)(1min/60sec)(1/10 3 ) = 140 liters/sec of pumping speed at the chamber pump port

29 Conductance Limitation 50 sccm 5 mTorr 140 l/s = Q/(P 1 – P 2 ) Fixed Throughput, Q: Q = Torr x 140 l/s = 0.7 Torr-l/s > 140 l/s …since P 2

> a, L (see Mahan, 2000)

30 Pressure Measurement Vacuum Gauge Selection adapted from Lesker.com 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) Ion Gauge: Sensitive to gas composition, but a good choice for base pressures Ionization of Gas RGA: A simple mass spectrometer

31 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? H 2 O is a polar molecule that is difficult to pump from the walls --> bake-out the chamber Source: Pfeiffer vacuum products Leak?

32 Gas Density (n) Ideal Gas Law PV = NkT Gas density at 1 Pascal at room temp. N/V = n = P/kT = (1 N/m 2 )/(1.3807x J/K)(300 K) = [1 (kg-m/s 2 )/m 2 ]/[4.1x kg-m 2 /s 2 ] = 2.4x10 20 atoms per m 3 = 2.4x10 14 cm -3 … at 1 Pa Rule of Thumb n(T) = 3.2x10 13 cm -3 x ( 300 / T ) …at a pressure of 1 mTorr

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

34 Example A vacuum chamber has a base pressure of Torr. Assuming that this is dominated by water vapor, what is the flux of H 2 O to a substrate placed in this chamber? n = 3.2x10 13 cm -3 /mTorr * mTorr = 3.2x10 10 cm -3 = (8kT/ M) 1/2 = cm/s z = (¼)n = 4.74x10 14 molecules per cm 2 per sec! This is approximately one monolayer of H 2 O every second at Torr base pressure.

35 Collisions and Mean Free Path Gas Density n = P/ kT n Cross-section ~ d 2 d Rigorous Hard Sphere Collisions: = kT / 2 d 2 P Ar cm 8 / P (mTorr) Ar

36 Part 3: Plasma Basics

37 Paschen Curve F. Paschen, Ann. Phys. Chem., Ser. 3 37, 69 (1889). V DC d Too few ionizing collisions: >d Too many collisions Electron energy

38 What do we need to know about plasma? substrate radicals, molecular fragments ions Wall gas (n g ) Gas flow pumping electrons n e, T e Power excited atoms and molecules light reaction products secondary electrons PLASMA

39 Power Absorbed substrate radicals, molecular fragments ions Wall gas (n g ) Gas flow pumping electrons n e, T e Power excited atoms and molecules light reaction products secondary electrons PLASMA

40 Power Absorbed: DC DC power –General electrical mobility and conductivity –Mobility: e = q /m = q/ m m e Where is the average time between collisions and m is the collision frequency (collisions per second) –Electron Conductivity: DC = qn e e = q 2 n e / m m e –DC power absorbed:

41 Power Absorbed: RF RF/microwave power –Ohmic Heating –Generic electron-neutral collision frequency m ~ 5x10 -8 n gas T e 1/2 (s -1 ) … n gas (cm -3 ), T e (eV). –Example: Find the pressure at which rf ohmic heating becomes ineffective: m = 0.1 T e = 2eV = MHz * 2 = 85.2Mrad/s n gas = 0.1*85.2x10 6 /5x10 -8 (2) 1/2 = 1.2x10 14 cm -3 = 3.7 mTorr V RF An electron oscillates in a rf electric field without gaining energy unless electron collisions occur f=13.56 MHz Hopwood and Mantei, JVST A21, S139 (2003)

42 Stochastic Heating an electron enters and exits a region of high field for a fraction of an rf cycle t 0 << 2 Reflecting Boundary (plasma sheath) - E max E ~ 0 E RF x z v x (t 0 ) > v x (0) The usual mechanism for heating electrons using RF electric fields at low pressures

43 Wave/Resonant Heating x -E x --- t1t1 t2t2 t3t3 k B DC x y v F = q(vxB) E=0 Electron cyclotron frequency: ce = qB/m e = 1.76x10 7 B(gauss) If ce and E RF is perpendicular to B DC, then the electron gains energy from E x in the absence of collisions. Ex. f=2.45 GHz --> B=875 G E RF W/cm 3 Hopwood and Mantei, JVST A21, S139 (2003)

44 Electron Collisions substrate radicals, molecular fragments ions Wall gas (n g ) Gas flow pumping electrons n e, T e Power excited atoms and molecules light reaction products secondary electrons PLASMA

45 Electron Collisions Elastic Collisions: –Ar + e Ar + e –Gas heating: energy is coupled from e to the gas Excitation Collisions –Ar + e hot Ar * + e cold, Ar * Ar + h –Responsible for the characteristic plasma glow –E electron >E exc (~11.55 eV for argon) Ionization Collisions: –Ar + e hot Ar + + 2e cold –Couples electrical energy into producing more e _ –E electron > E iz (15.76 eV for argon) Dissociation: –O 2 + e hot 2O + e cold or O 2 + e hot O + O + + 2e cold –Creates reactive chemical species within the plasma –E electron > E diss (5.12 eV for oxygen)

46 Collision Cross Sections Unlike the hard sphere model, real collision cross sections are a function of electron kinetic energy (E), or electron velocity (v). We must find the expected collision frequency by averaging over all E or v. becomes (cm 3 s -1 )

47 Graphically f(E) or (E) Electron energy, E Ar (E) f(E)f(E) TeTe E iz Note: the exponential tail of energetic electrons is responsible for ionization The RATE CONSTANT: K iz (T e ) K izo exp(-E izo /T e ) curve fitting

48 Graphically f(E) or (E) Electron energy, E Ar (E) f(E)f(E) TeTe E iz Note: the exponential tail of energetic electrons is responsible for ionization The RATE CONSTANT: K iz (T e ) K izo exp(-E izo /T e ) curve fitting Hot electrons – more ionization

49 Examples of Numerically Determined Rate Constants (Lieberman, 2005)

50 Generation Rate of Plasma Species by Electron Collisions y + e x + e dn x /dt = K x n e n y For example, Ar + e Ar + + e + e dn e /dt = K iz n e n gas is the number of electrons (and ions) generated per cm 3 per second

51 Electron-Ion Recombination Three-Body Problem: e + Ar + + M Ar + M the third body is needed to conserve energy and momentum in the recombination process - + M M - + M wall recombination dominates at low pressure because three body collisions are rare wall recombination volume recombination

52 Transport to Surfaces substrate radicals, molecular fragments ions Wall gas (n g ) Gas flow pumping electrons n e, T e Power excited atoms and molecules light reaction products secondary electrons PLASMA n = ¼ n

53 n e n i 0 Electron and Ion Loss to the Substrate and Walls - the plasma sheath - electrons are much more mobile than ions e = q /m e >> q /m i = i n e n i 0 n e n i chamber

54 Electron and Ion Loss to the Substrate and Walls - the plasma sheath - (x) x ++ n e = n i n e <

55 Ion Flux The ion flux to a solid object is determined by the Bohm velocity (or sound speed) of the ion: u B = (kT e /m i ) 1/2 = 9.8x10 5 (T e /M) 1/2 cm/s =9.8x10 5 (3 eV/40 amu) 1/2 ~ 2.5x10 5 cm/s … and the ion flux is given by i = u B n i (cm -2 s -1 ) (this is the ion speed at the edge of the sheath)

56 Electron Flux Only the most energetic electrons can overcome the sheath potential, V s. e = ¼ n e exp (qV s /kT e ) f(E) Electron energy, ETeTe qV s flux to surfaceBoltzmann factor

57 Sheath Potential, V s In the steady state, the electron and ion fluxes to the chamber/substrate must be equal, if there is no external current path e = i ¼ n e exp (qV s /kT e ) = u B n i = (kT e /m i ) 1/2 n e giving V s = -T e ln(m i /2 m e ) ~ -5T e This is often called the floating potential: Isolated surfaces have a negative potential relative to the plasma.

58 Ion Energy (after Mahan, 2000) -1kV s x V 0 v Ex: Assuming argon with T e = 3 eV, the ion energy at the cathode is E i = q(1 kV + 4.7T e ) = 1014 eV ignoring ion-neutral collision within s, and the ion energy at the anode is E i = 4.7 T e = 14 eV Ion mean free path: i = 1/n gas i ~ 3/p (cm) for Ar + …where p is the pressure in mTorr Here i = 3/100 cm or torr NOTE: s>> i E i << 1014 eV!

59 Particle Conservation and Electron Temperature A simple model for electron temperature can be found for a steady state plasma: # of ions created/sec = # of ions lost/sec K iz n gas n e V = u B n i A eff K iz /u B = K izo e -E iz /kT e /(kT e /m i ) 1/2 = A eff /(V n gas ) =1/d eff n gas (V=plasma volume, A eff = effective chamber area, d eff = V/A eff ) n e =n i

60 The electron temperature (T e ) is a unique function of 1.gas density, n gas (pressure) 2.chamber size, d eff = V/A eff 3.gas type: K iz, E iz Ar+e Ar * +e Ar * + e Ar + + 2e Ar + e Ar + + 2e Example: Two large parallel plates separated by 2 cm are used to sustain an argon plasma at 25 mTorr. Find T e. d eff = V/A eff ~ R 2 d / ( R 2 + R 2 ) = d/2 n gas d eff ~ (25*3.2x10 19 m -3 )(0.01m) =0.8e+19 m -2 T e = 3 eV (Note: we have assume that the plasma density is uniform)

61 Power Conservation and Electron Density, n e Power Absorbed by the Plasma = Power Lost from the Plasma P abs = [qn i u B E ion +q(¼n e e V s /kT e )E elec ]A eff +(P heat +P light +P diss ) qn e u B A eff (E ion + E elec + E c ) where E C is the collisional energy lost in creating an electron-ion pair due to ionization, light, dissociative collisions, and heat: E C = [ iz E iz + ex E ex + diss E diss + m (3m e /m i )T e ]/ iz qV s 2T e P ion P electron

62 C Collisional Energy Loss

63 Electron Density Example Continuing with the previous example A plasma is sustained in argon at 25 mTorr between two parallel plates separated by 2 cm. The radius of the plates is 20 cm and the power absorbed by the plasma is 100 watts. Find n e. 100 W = qn e u B A eff (E ion + E elec + E c ) = (1.6x C)n e (2.5x10 5 cm/s)(2 x 20 2 cm 2 ) x (5T e + 2T e + 35 eV) n e = 1.3x10 10 cm -3 Find n e if the gas is N 2, assuming that T e ~ 3 eV 100 W = (1.6x C)n e (2.5x10 5 cm/s)(2 x 20 2 cm 2 )(5T e + 2T e eV) n e = 2.3 x 10 9 cm -3

64 Example (contd) Repeat the previous example using argon, BUT include an electrode voltage of 1000v that is applied to one plate to sustain the plasma. 100 W = qn e u B A eff (E ion + E elec + E c ) = (1.6x C)n e (2.5x10 5 cm/s)( x 20 2 cm 2 ) x {(5T e + 2T e + 35 eV)+[(1000 eV+5T e ) + 2T e + 35 eV]} n e = 1.7x10 9 cm -3 anodecathode

65 Secondary Electrons e = sec i, where sec ~ and E e ~ qV s substrate radicals, molecular fragments ions Wall gas (n g ) Gas flow pumping electrons n e, T e Power excited atoms and molecules light reaction products secondary electrons PLASMA secondary electrons

66 Summary substrate radicals, molecular fragments ions Wall gas (n g ) Gas flow pumping electrons n e, T e Power excited atoms and molecules light reaction products secondary electrons PLASMA

67 Conclusion Basics of Vacuum –n g,, n,, Plasma Generation and Simple Models –T e, n e, n i, i Basic Plasma Generation –DC (sputter deposition systems) –AC < 400 kHz (plasma displays, lighting) –Radio Frequency 0.4 900 MHz


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

Similar presentations


Ads by Google