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Physics of Thin Films

PES 449 / PHYS 549

Sputter Deposition

Ohring: Chapter 3, sections 5 - 6

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Overview

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1. Atoms into gas state

at target:

• target atoms ejected
• target ions ejected (1 - 2 %)
• electrons emitted
  • helps keep plasma going
• Ar⁺ ions reflected as Ar neutrals
• Ar buried in target
• photons emitted

  We are most interested in the first of these: target atoms going into the gas phase

  Sputtering process

  • 
• momentum transfer process
  • involves top 10 Å
  • model as hard sphere collisions
    • good for energies < 50 keV
• 95 % of incident energy goes into target
  • => COOL the target
• 5 % of incident energy is carried off by target atoms
  • typical energies of 5-100 eV
• target atoms come off with a non-uniform distribution
  • more atoms normal to the surface
  • cosine distribution (like surface source)

  characterize process by sputter yield (S)

  S = number ejected / number incident

  S depends on

• target material
  • binding energy
  • mass of atoms
• sputtering gas
  • mass of atoms (S increases for heavier gasses)
  • incident energy (S increases for higher energies)
• geometry
  • most efficient 20-30 degrees from glancing

  

• for normal incidence sputtering:
  • 
    • maximum around 10 kV
    • sputtering threshold
    • S is about 1-10 typically

  For calculating S we need:

• number of atoms ejected
  • depends on momentum and energy transferred
    • these depend on relative masses and collision angle
    • maximum energy transferred to target atom in hard sphere collision
      • 
  • depends on binding energy of target atom
• number of layers involved in process
  • mean free path of ion in target
    • typically about 2 layers
• surface density of target atoms
• collision cross section of ion with target atom

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  Sputtering alloy targets

  composition of alloy in film is approximately the same as alloy in target (unlike evaporation)

  Why ?

• rapid mixing in liquids (evaporation)
• slow diffusion mixing in solids (sputtering)
  • target reaches steady state
  • surface composition balances sputter yield

  Process:

• Initial alloy of A and B .......................... ABABABABABABAB
• If S_A > S_B , remove more A
  • enriches surface in B .................... . BAB. B. BA. B . AB
• More B on surface => more B sputtered ....... ABABBABABBAB
• surface composition reaches steady state
  • surface enriched in B
  • bulk composition sputtered
  • f_AS_A / C_A = f_BS_B / C_B
    • where f = surface fraction and C = bulk composition

  alloy targets need to be conditioned by sputtering a few hundred Å before depositing

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2. Transport to substrate

• Target atoms pass through Ar gas and plasma environment
  • one Ar⁺ ion for every 10,000 Ar neutrals
  • electrons in plasma collide with Ar neutrals to form ions and more electrons
• Target atoms collide with Ar atoms, Ar⁺ ions and electrons
  • treat as random walk "diffusion" through gas
  • target atoms lose energy (down to 1-10 eV)
  • chemical reactions may occur in gas
  • not a line of sight process (unless pressure reduced)
    • can coat around corners

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3. Deposit on substrate

• target atoms and ions impinge
• electrons impinge
• Ar atoms impinge
  • Ar pressure about 0.1 torr
  • Ar may be incorporated into film
• energetic particles may modify growth
• substrates heat up
  • 100 - 200 C is common
  • for a thermally isolated sample (no heat conduction)
    • 
    • 
      • r = substrate density (g/cm³)
      • c = substrate heat capacity (J/gūC)
      • d = substrate thickness (cm)
      • W = film atomic volume (cm³/atom)
      • D-DOT = deposition rate (Å/min)
      • t = time (secs)
      • energies are in eV/atom
      • note: the constant does have units

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