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Physics of Thin Films

PES 449 / PHYS 549

Film Formation II

Ohring: Chapter 5, sections 4-6

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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

  

  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

  

  3. Mixed growth (Stranski - Krastanov)

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

  

  When would we expect to see each of these ?

  

  

  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.

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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

2. Sintering

• reduction of surface energy

3. Cluster migration

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

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7. Thick films - zone models

Further growth depends on:

• bulk diffusion
• surface diffusion
• desorption
• geometry:
  • shadowing (line of sight impingement)

    

  Relative importance of these processes depends on

• substrate temperature (T)
• deposition rate ()

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

  

  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

    

  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.

    

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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)

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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