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Physics of Thin Films

PES 449 / PHYS 549


Evaporation

Ohring: Chapter 3, sections 1 - 4


Physical Vapor Deposition

  • Evaporation
  • Sputter Deposition

    Process:

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


Evaporation

Overview:

evaporation


1. Atoms to gas state

  • heat source until Pvapor > 10-4 torr
  • some sources sublime from solid, others evaporate from liquid
  • compounds may break apart
    • produce films with different stoichiometry
    • SiO2 --> SiO2-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
    • equation for flux of evaporant
    • where
      • Pvap = vapor pressure (Torr)
      • M = molecular weight
      • cm2 => area of source
  • can convert this to mass flux
    • mass flux of evaporant
    • at Pvap = 10-2 torr, mass flux = 10-4 grams/cm2 sec


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
        • point source
          • q = tilt of dAS from radial direction
          • projection of dAS onto sphere of radius r = dAScosq
          • dMS = mass hitting dAS
          • Me = total evaporated mass
        • equations for mass distribution
          • distribution depends on r and q
      • Surface Source
        • surface source
        • For many materials, this is equivalent to Knudsen cell
        • Knudsen cell
        • if directions are random, only dAS cosq / 4ır2 are headed in right direction
          • integrate over time and source
          • equations for distribution from surface source
          • now distribution depends on horizontal position as well
      • Experimentally observe
        • experimentally observed distribution
        • greater n => more directed evaporation


3. Deposition onto substrate

Consider film thickness and purity

THICKNESS

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

consider flat substrate, perpendicular to source

thickness of evaporated film

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

in general: equation for thickness

point source:

equations for point source

surface source:

equations for 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
    • evaporation mask
  • put source and substrate on same sphere surface
    • evaporation on sphere
    • equations for evaporation on sphere
    • 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
  • PROBLEM: residual gas in chamber
    • gives two "sources" impinging
      • evaporant: equation for evaporant
      • residual gas: equation for residual gas
    • impurity concentration in film:
      • impurity concentration
      • P in Torr; Mgand Me are molecular weights
    • SOLUTION:
      • better vacuum
      • higher deposition rate
      • note: P and Tg are not independent


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