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Physics of Thin Films

PES 449 / PHYS 549

Evaporation

Ohring: Chapter 3, sections 1 - 4

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Physical Vapor Deposition

• Evaporation
• Sputter Deposition

  Process:

1. source material -> gaseous state
2. transport source atoms to substrate
3. deposit atoms on substrate

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Evaporation

Overview:

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1. Atoms to gas state

• heat source until P_vapor > 10^-4 torr
• some sources sublime from solid, others evaporate from liquid
• compounds may break apart
  • produce films with different stoichiometry
  • SiO₂ --> SiO_2-x
• metal alloy sources do not give same alloy in film
  • components evaporate independently based on each separate vapor pressure
  • could try to adjust source composition
  • BUT composition of alloy source changes with time
• describe evaporation rate (flux) from kinetic theory
  • 
  • where
    • P_vap = vapor pressure (Torr)
    • M = molecular weight
    • cm² => area of source
• can convert this to mass flux
  • 
  • at P_vap = 10^-2 torr, mass flux = 10^-4 grams/cm² sec

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2. Transport to surface

• line of sight deposition
• want to avoid collisions in gas
  • long mean free path
  • good vacuum
  • let h = source to substrate distance
    • for h of 10 - 100 cm, want P < 10^-5 torr
    • bigger h => lower P
• Particles have energies comparable to evaporation temperature
  • 1000 C is about 0.2 eV
• distribution of evaporant
  • depends on geometry of source
  • consider 2 geometries
    • Point Source
      • 
        • q = tilt of dA_S from radial direction
        • projection of dA_S onto sphere of radius r = dA_Scosq
        • dM_S = mass hitting dA_S
        • M_e = total evaporated mass
      • 
        • distribution depends on r and q
    • Surface Source
      • 
      • For many materials, this is equivalent to Knudsen cell
      • 
      • if directions are random, only dA_S cosq / 4ır² are headed in right direction
        • integrate over time and source
        • 
        • now distribution depends on horizontal position as well
    • Experimentally observe
      • 
      • greater n => more directed evaporation

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3. Deposition onto substrate

Consider film thickness and purity

THICKNESS

since dM/dA_s depends on r, q, f, so does film thickness (d)

consider flat substrate, perpendicular to source

for this geometry: q = f , cosq = h/r , r = (h² + l²)^1/2

in general:

point source:

surface source:

surface source has slightly poorer thickness uniformity

better uniformity:

• decrease sample size (l)
• inrease distance to substrate (h)
  • need bigger chamber
  • need better vacuum
  • wastes evaporant
• use multiple sources
• move substrate during deposition
• use rotating amsk to reduce evaporant near center
  • 
• put source and substrate on same sphere surface
  • 
  • 
  • No dependence on q, f, or r !!!
  • planetary fixtures

FILM PURITY

• PROBLEM: contamination from source materials
  • SOLUTION: use pure materials (99.99999%)
• PROBLEM: contamination from source or substrate heaters
  • SOLUTION: use materials with low diffusion
    • see tables of crucibles for each material
      • on line tables: http://www.lesker.com/mfiles/m_tech_deposition_techniques.html
• PROBLEM: residual gas in chamber
  • gives two "sources" impinging
    • evaporant:
    • residual gas:
  • impurity concentration in film:
    • 
    • P in Torr; M_gand M_e are molecular weights
  • SOLUTION:
    • better vacuum
    • higher deposition rate
    • note: P and T_g are not independent

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