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Plasmas are essential for making computer chips

SHEATH. Plasmas are essential for making computer chips. For instance, for etching 60 nm features. UCLA. Etch rate is greatly enhanced by ions. UCLA. Why use radiofrequency power?. DC discharges: low density, internal electrodes

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Plasmas are essential for making computer chips

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  1. SHEATH Plasmas are essential for making computer chips For instance, for etching 60 nm features

  2. UCLA Etch rate is greatly enhanced by ions

  3. UCLA Why use radiofrequency power? • DC discharges: low density, internal electrodes • Microwave (ECR), 2.45 GHz: Large B-field, expensive plumbing, limited flexibility, energetic electrons. • RF (0.4 - 30 MHz): No internal electrodes needed, inexpensive power, years of development and experience. • VLF (30 - 300MHz): relatively undeveloped, short skin depth, finite-wavelength effects, circuit losses

  4. UCLA Types of RF plasma sources • History: old RIE parallel plate etchers • Inductively coupled plasmas (ICPs) • Dual frequency capacitively coupled plasmas (CCPs) • Helicon wave sources (HWS) and hybrids

  5. UCLA Schematic of a capacitive discharge

  6. UCLA 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)

  7. UCLA 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, not enough adjustments for controlling the ion and electron distributions and the plasma uniformity

  8. UCLA Dual-frequency CCPs are better W. Tsai et al., JVSTB 14, 3276 (1996)

  9. UCLA One advantage of a capacitive discharge Fast and uniform gas feed for depositing amorphous silicon on very large glass substrates for displays (Applied Komatsu)

  10. UCLA 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)

  11. UCLA Inductive coupling: The original TCP patent US Patent 4,948,458, Ogle, Lam Research, 1990

  12. UCLA The Lam TCP (Transformer Coupled Plasma) Simulation by Mark Kushner

  13. UCLA Top and side antenna types US Patent 4,948,458, Fairbairn, AMAT, 1993

  14. UCLA Applied Materials' DPS (Decoupled Plasma Source) US Patent 4,948,458, Fairbairn, AMAT, 1993

  15. UCLA Other antennas in AMAT patent US Patent 4,948,458, Fairbairn, AMAT, 1993

  16. UCLA B-field pattern comparison (1) Horizontal strips Vertical strips

  17. UCLA B-field pattern comparison (2) 3 close coils 2 separate coils

  18. UCLA B-field pattern comparison (3) Lam type AMAT type

  19. UCLA Antenna elements near axis are not necessary!

  20. How do ICPs really work? In MEMs etcher by Plasma-Therm (now Unaxis), density is uniform well outside skin depth

  21. UCLA UCLA experimental chamber

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

  23. UCLA 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)

  24. UCLA Anomalous skin effect (thermal motions) E.g., Kolobov and Economou, Plasma Sources Sci. Technol. 6, R1 (1997). Most references neglect collisions and curvature.

  25. UCLA Electron trajectories are greatly affected by the nonlinear Lorentz force

  26. UCLA 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.

  27. UCLA Electrons spend more time near center

  28. UCLA Density profile in four sectors of equal area Points are data from Slide 5

  29. UCLA A more compact ICP is possible Points are data from Slide 5

  30. UCLA 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

  31. UCLA Magnetic field above coil is wasted

  32.   H = J B = m H UCLA Coupling can be improved with magnetic cover

  33. Four configurations tested Meziani, Colpo, and Rossi, Plasma Sources Science and Technology 10, 276 (2001)

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

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

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

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

  38. Magnets Serpentine antennas (suggested by Lieberman)

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

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

  41. UCLA 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)

  42. A LAM Exelan oxide etcher

  43. High frequency controls plasma density Low frequency controls ion motions and sheath drop UCLA A dual-frequency CCP Thin gap. Unequal areas to increase sheath drop on wafer

  44. UCLA 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 ( - mode) • Why there is less oxide damage is not yet known

  45. The density increases with frequency squared (a) (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 .

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

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

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

  49. Plasma Application Modeling Group POSTECH Effect of increasing low-frequency drive Electron distribution changes from Druyvesteyn to bi-Maxwellian. (Secondary electron emission neglected,) KTe falls, density rises. The decrease of KTe is an unexplained collisionless effect. (H.C. Kim and J.K. Lee, PRL)

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