MSN 551 THIN FILM DEPOSITION
EVAPORATION

What is evaporation?
• Material to be evaporated is heated to increase vapor pressure • In a reasonably high vacuum, material atoms fly to a target and stick onto the surface • Source material is coated onto the target surface

Vapor pressures at their melting points (approx., in torr): Gallium (essentially zero; too low to measure) Tin (less than 1e-11 torr) Indium (less than 1e-11 torr) Lithium 1e-10 Bismuth 2e-10 Lanthanum 3e-10 Aluminum 2e-9 Lead 3e-9 Chromium 5 Uranium 1e-8 Magnesium 2 Sodium 1e-7 Manganese 1 Mercury 2e-6 Zinc 1e-1 Iron 2e-2 Titanium 3e-3 Nickel 2e-3 Copper 3e-4

Physical vapor deposition (PVD): thermal evaporation
N = No expHeat S ources Resistance e-beam RF Laser

Φe The number of molecules kT leaving a unit area of evaporant per second
Advantages No radiation Low contamination No radiation No radiation, low contamination Dis advantages Contamination Radiation Contamination Expensive

6

E-beam evaporation

Physical vapor deposition (PVD): thermal evaporation
N (molecules/unit area/unit time) = 3. 513. 10 22 Pv (T)/ (MT) 1/2
Si
Arbitrary surface element

This is the relation between vapor pressure of the evaporant and the evaporation rate. If a high vacuum is established, most molecules/atoms will reach the substrate without intervening collisions. Atoms and molecules flow through the orifice in a single straight track,or we have free molecular flow :
K n = λ/D > 1

Resist

β

d

θ
Evaporant container D with orifice diameter D

The fraction of particles scattered by collisions with atoms of residual gas is proportional to:
1-exp (+d/ λ)
The source-to-wafer distance must be smaler than the mean free path (e.g, 25 to 70 cm)

The cosine law
A ~ cosβ cos θ/d2

Physical vapor deposition (PVD): thermal evaporation
From kinetic theory the mean free path relates to the total pressure as: λ = (πRT/2M) 1/2 η/PT cos β 1 t1 = ≈ 3 t2 cos β 2

Substrate

t2 t1

Surface feature β1 = 0 0

β 2 = 70 0

Since the thickness of the deposited film, t, is proportional To the cos β, the ratio of the film thickness shown in the Figure on the right with θ = 0° is given as:

Source

t1/t2 =cosβ1 /cosβ2
Shadow

Source