2. Types of RF plasma sources
Old RIE parallel plate etcher (GEC reference cell)
Inductively coupled plasmas (ICPs)
New dual frequency capacitively coupled plasmas (CCPs)
Helicon wave sources (HWS)
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3. Schematic of a capacitive discharge
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4. The GEC Reference Cell
In the early days of plasma processing, the Gaseous Electronics Conference standardized a capacitive discharge for 4-inch wafers, so that measurements by different groups could be compared.
Brake et al., Phys. Plasmas 6, 2307 (1999)
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5. Problems with the original RIE discharge
The electrodes have to be inside the vacuum
Changing the power changes both the density and
the sheath drop
Particulates tend to form and be trapped
Densities are low relative to the power used
In general, too few knobs to turn to control the ion
and electron distributions and the plasma uniformity
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6. Dual-frequency CCPs are better
W. Tsai et al., JVSTB 14, 3276 (1996)
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7. One advantage of a capacitive discharge
Fast and uniform gas feed for depositing amorphous silicon on very large glass substrates for displays (Applied Komatsu)
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8. Types of RF plasma sources
Old RIE parallel plate etcher (GEC reference cell)
New dual frequency capacitively coupled plasmas (CCPs)
Helicon wave sources (HWS)
Inductively coupled plasmas (ICPs)
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9. Inductive coupling: The original TCP patent
US Patent 4,948,458, Ogle, Lam Research, 1990
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10. The Lam TCP (Transformer Coupled Plasma)
Simulation by Mark Kushner
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11. Top and side antenna types
US Patent 4,948,458, Fairbairn, AMAT, 1993
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12. Applied Materials' DPS (Decoupled Plasma Source)
US Patent 4,948,458, Fairbairn, AMAT, 1993
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13. Outside
What the DPS looks like
Inside
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14. Other antennas in AMAT patent
US Patent 4,948,458, Fairbairn, AMAT, 1993
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15. B-field pattern comparison (1)
Horizontal strips
Vertical strips
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16. B-field pattern comparison (2)
3 close coils
2 separate coils
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17. B-field pattern comparison (3)
Lam type
AMAT type
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18. How do ICPs really work?
In MEMs etcher by Plasma-Therm (now Unaxis), density is uniform well outside skin depth

19. UCLA
In the plane of the antenna, the density peaks well outside the classical skin layer
Data by John Evans

20. Anomalous skin effect (thermal motions)
E.g., Kolobov and Economou, Plasma Sources Sci. Technol. 6, R1 (1997).
Most references neglect collisions and curvature.
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21. Nonlinear effects have been observed
Collisionless power absorption
(Godyak et al., Phys. Rev. Lett. 80, 3264 (1998)
Second harmonic currents
Smolyakov et al., Phys. Plasmas 10, 2108 (2003)
Ponderomotive force
Godyak et al., Plasma Sources Sci. Technol. 10, 459 (2001)
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22. Electron trajectories are greatly affected by the nonlinear Lorentz force
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23. Without FL, electrons are fast only in skin
Reason: The radial FL causes electrons to bounce off the sheath at more than a glancing angle.
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24. Electrons spend more time near center
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25. Density profile in four sectors of equal area
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Density profile in four sectors of equal area
Points are data from Slide 5

26. Disadvantages of stove-top antennas
Skin depth limits RF field penetration. Density falls
rapidly away from antenna
If wafer is close to antenna, its coil structure is seen
Large coils have transmission line effects
Capacitive coupling at high-voltage ends of antenna
Less than optimal use of RF energy
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27. B-field pattern comparison (2)
3 close coils
2 separate coils
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28. Coupling can be improved with magnetic cover
  H = J
B = m H
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29. Four configurations tested
Meziani, Colpo, and Rossi, Plasma Sources Science and Technology 10, 276 (2001)

30. The dielectric is inside the vacuum
Meziani, Colpo, and Rossi, Plasma Sources Science and Technology 10, 276 (2001)

31. Iron improves both RF field and uniformity
(Meziani et al.)

32. Magnets are used in Korea (G.Y. Yeom)
SungKyunKwan Univ. Korea

33. Both RF field and density are increased
SungKyunKwan Univ. Korea

34. (suggested by Lieberman)
Serpentine antennas
(suggested by Lieberman)
Magnets

35. Density uniformity in two directions
G.Y. Yeom, SKK Univ., Korea

36. Effect of wire spacing on density
Park, Cho, Lee, Lee, and Yeom,
IEEE Trans. Plasma Sci. 31, 628 (2003)

37. Godyak: All RF lamps use iron cores
Philips QL Lamp: MHz, 85W (equiv. to 350W lamp)
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38. Types of RF plasma sources
Old RIE parallel plate etcher (GEC reference cell)
Inductively coupled plasmas (ICPs)
New dual frequency capacitively coupled plasmas (CCPs)
Helicon wave sources (HWS)
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39. A LAM Exelan oxide etcher

40. Thin gap. Unequal areas to increase sheath drop on wafer
A dual-frequency CCP
Thin gap. Unequal areas to increase sheath drop on wafer
High frequency controls plasma density
Low frequency controls ion motions and sheath drop
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41. Most of volume is sheath
Electrons are emitted by secondary emission
Ionization mean free path is shorter than sheath thickness
Ionization occurs in sheath, and electrons are
accelerated into the plasma
Why there is less oxide damage is not yet known
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42. The density increases with frequency squared
(b)
Density
Debye length
(c)
(d)
Reason: The rf power is  I2R, where I is the electron current escaping through the sheath. Since one bunch of electrons is let through in each rf cycle, <Irf> is proportional to .

43. Effect of frequency on plasma density profiles
13.56 MHz
27 MHz
40 MHz
60 MHz

44. Effect of frequency on IEDF at the smaller electrode
27 MHz
(a)
(b)
13.56 MHz
(c)
(d)
60 MHz
40 MHz

45. IEDF at Wall – Pressure Variation
10 mTorr
20 mTorr
30 mTorr
50 mTorr
Plasma Application
Modeling Group
POSTECH

46. Types of RF plasma sources
Old RIE parallel plate etcher (GEC reference cell)
Inductively coupled plasmas (ICPs)
New dual frequency capacitively coupled plasmas (CCPs)
Helicon wave sources (HWS)
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47. A helicon source requires a DC magnetic field..
U. Wisconsin

48. ...and is based on launching a circularly polarized wave in the plasma
Much higher density at given power than ICPs
Density peak occurs downstream from the antenna
Magnetic field provides adjustment for uniform density
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49. Axial density and temperature profiles
Density increases greatly as B-field is added.
The density peak is detached from the source.
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50. Two commercial helicon reactors
The PMT (Trikon) MØRI source
The Boswell source
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51. The Coil Current Ratio shapes the plasma
The MØRI source
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52. How do helicon source really work?
A cyclotron (TG) wave at the surface rapidly damps the RF energy
Typical radial deposition profile
Direct detection of the TG peak in the RF current
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53. There are actually 2 types of helicon discharges
The Big Blue Mode
The Low Field Peak
Low density, low B-field
Ideal for plasma processing
B > 800G, n > 1013 cm-3
Due to an neutral depletion instability
No important application yet
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54. Reflection from end causes the L.F. peak
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55. A 7-tube array of stubby helicon sources
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56. Gives good uniformity and high density
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57. 2-D density scans show no m = 6 asymmetry
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58. Helicon tools have been modeled
MØRI tool: Kinder and Kushner, JVSTA 19, 76 (2001)

59. Bose, Govindan, and Meyyappan, IEEE Trans. Plasma Sci. 31, 464 (2003)
TG mode is seen
Power deposition
Bose, Govindan, and Meyyappan, IEEE Trans. Plasma Sci. 31, 464 (2003)
Plasma density

60. What next for RF sources?
Control of KTe, species production, ion velocities
Electron filtering, pulsed plasmas, gas feed and pumping, additive gases to absorb electron groups, shaped bias voltage, electronegative optimization. etc.
Understanding and eliminating oxide damage
Large area sources for FPDs, not wafers
Eventual widespread adoption of helicon sources
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