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Physics of Thin Films

PES 449 / PHYS 549


Film Formation II

Ohring: Chapter 5, sections 4-6


5. Island Growth

observe 3 growth modes experimentally

1. Island growth (Volmer - Weber)

  • form three dimensional islands
  • source:
    • film atoms more strongly bound to each other than to substrate
    • and/or slow diffusion

    island growth model

    2. Layer by layer growth (Frank - van der Merwe)

  • generally highest crystalline quality
  • source:
    • film atoms more strongly bound to substrate than to each other
    • and/or fast diffusion

    layer by layer growth model

    3. Mixed growth (Stranski - Krastanov)

  • initially layer by layer
  • then forms three dimensional islands
  • => change in energetics

    mixed growth model

    When would we expect to see each of these ?

    surface tension balance

    equations for growth modes

    The layer growth condition with cosine greater than 1 looks odd. This is the case where the angle theta is undefined because for layer growth there really is no point where the substrate, vapor and film come together and therefore, no way to define the angle.


6. Island Coalescence

three common mechanisms:

1. Ostwald ripening

  • atoms leave small islands more readily than large islands
  • more convex curvature => higher activity => more atoms escape

Ostwald ripening

2. Sintering

  • reduction of surface energy

sintering

3. Cluster migration

  • small clusters (<100 Å across) move randomly
  • some absorbed by larger clusters (increasing radius and height)

cluster migration


7. Thick films - zone models

Further growth depends on:
  • bulk diffusion
  • surface diffusion
  • desorption
  • geometry:
    • shadowing (line of sight impingement)

      shadowing of film by roughness

    Relative importance of these processes depends on

  • substrate temperature (T)
  • deposition rate ()

    Plot these variables to find regions with similar film structure (similar properties)

    zone diagram of dep rate vs. Temp

    See handout in class (from J. A. Thornton, J. Vac. Sci. Technol. 12 (July/Aug 1975))

    Zone
    Temperature
    Diffusion
    Other Processes
    Structure
    1
    T < 0.2 - 0.3 TM
    limited
    .

    small grains, many voids

    T
    T < 0.2 - 0.5 TM
    surface

    renucleation during growth

    mixed small and large grains, fewer voids

    2
    T < 0.3 - 0.7 TM
    surface

    grain boundary migration

    columnar grains

    3
    T > 0.5 TM
    bulk (dominates) + surface

    grain boundary migration; recrystallization within grains

    large grains (sometimes columnar)

    more info: see article by Grovenor, Acta Metall. 32, 773 (1984) and book by Machlin

    Columnar structures

    • very common
    • from limited atomic mobility
    • often oriented slightly toward source

      tilted  columnar growth

    Films are typically lower density than bulk

    • more porosity at macro, micro and nano scales.

    Grain size dependence on deposition rate and substrate temperature

    • grain size typically increases with increasing film thickness, increasing substrate temperature, increasing annealing temperature, and decreaseing deposition rate.

      grain size vs other variables


Other factors affecting film growth

1. Substrate
  • not really a featureless plane
  • atomic structure => epitaxy
    • relationship of film crystal structure to substrate crystal structure
  • defects
    • nucleation sites

2. Contamination

from:
  • poor background pressure
  • impure deposition source
  • dirty substrate

changes the energies (surface energies, desorption energy, surface diffusion energy)

contamination

3. Impinging particle energy

0.5 eV -------------------> 10 - 20 eV --------> 100-1000 eV

thermal evaporation ----- sputtering --------- accelerated (bias)

interactions of incident particles with film/substrate produce:

  • sputter removal of surface atoms
  • insertion of particles into film or substrate
  • increased local temperature
  • defects
  • shock (pressure) waves


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