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Physics of Thin Films

PES 449 / PHYS 549


Sputter Deposition

Ohring: Chapter 3, sections 5 - 6


Overview

sputter deposition


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

    • sputter 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

    sputter yield vs. incident angle

  • for normal incidence sputtering:
    • sputter yield vs. ion energy
      • 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
        • equation for energy transfer
    • 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


    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 SA > SB , 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
    • fASA / CA = fBSB / CB
      • where f = surface fraction and C = bulk composition

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


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


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)
      • sources of sample heating and cooling
      • equation for temperature change
        • 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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