1. Thin Film Deposition Processes
Usually source materials are transported to a substrate in the gas phase
Exceptions: LPE
Source Material
Tsource
Vapor Phase
Mass transport to substrate
Substrate
Tsubstrate < Tsource
Condensation, Reaction

2. Thin Film Deposition Technology
PVD
CVD
Source material enters the vapor phase by physical means (e.g., evaporation, sputtering)
No chemical reactions
Source material is supplied in the form of a precursor gas
Chemical reactions

3. (Thermal Evaporation)
PVD
PVD
Resistance-Heated
(Thermal Evaporation)
Sputtering
E-beam Evaporation

4. Thermal Evaporation Tungsten Filament Sheet/Boat Sources Crucible
Molten beads of metal are retained by surface tension forces on W wires
Sheet/Boat
Sources
W, Ta, or Mo form boats that contain the evaporant
Crucible
Sources
Oxide, pyrolytic boron nitride (PBN), or refractory metal containers
Heated by W wire wrapped around the crucible
from Mahan,
Fig. I.1, p. 2

5. Thermal Evaporators
from Ohring, Fig. 3-11, p.97

6. Thermal Evaporators Typical bell jar evaporator
From

7. Thermal Evaporators
Requirements :
Heater, crucible, etc. must not contaminate, react, or alloy with the source material
Heater, crucible, etc. must not vaporize
Typical materials: graphite, Ta, Mo, W, SiO2, Al2O3, BeO, ZrO2, pyrolyric boron nitride (BN)

8. Thermal Evaporators
Typically need Tsource ~ Tmelt for most metals to achieve reasonable VP for deposition (10-4 to 10-2 Torr)
from Mahan, Fig. V.14, p. 143

9. Thermal Evaporators
from Ohring, Table 3-3, p. 102

10. Thermal Evaporators
from Ohring, Table 3-3, p. 103

11. Effusion cell = Knudsen cell (1909) A special type of crucible source
Effusion Cells
Effusion cell = Knudsen cell (1909)
A special type of crucible source
charge
from Panish & Temkin, Fig. 3.2, p. 58

12. Resistively heated using Ta or Mo wire
Effusion Cells
Resistively heated using Ta or Mo wire
Uses inert containers; e.g., pyrolytic BN
from Mahan, Fig. I.2, p. 2

13. Effusion Cells
Used as solid sources in molecular beam epitaxy (In, Ga, Al, Sb, Si, Be)
from Ohring, Fig. 7-21(a), p. 338

14. Effusion Cells
Effusion: the process by which molecular flow occurs from a region of high vapor pressure toward a region of low vapor pressure
Effusion
Cell
Molecular flow P1
P2 < P1

15. Effusion Cells P1 P2 < P1 No collisions Dss Effusion Cell
Fluid Flow
Molecular
(MFP) >> Dss
(no collisions
in the gas)
Viscous
<< Dss
No collisions
Dss
Effusion
Cell
Molecular flow
(molecular beam) P1
P2 < P1

16. Molecular Flow vav radius, r density, n
· Collision rate, R = # collisions per unit time
R ~ (volume per unit time swept out by particle) x (gas density)
= (particle cross-section) x (average particle speed) x (gas density)
= pr 2 vav n
vav
radius, r
density, n

17. Mean Free Path
Collision rate, R ~ vav Pc
vav = average particle speed
= distance traveled per unit time
Pc = probability of a collision per unit distance traveled
Therefore, Pc = R/vav = pr2 n

18. · Mean free path of gas particles before a collision, l: l = 1 / Pc
l = 1 / Pc
Pc = probability of a collision per unit distance traveled
l = 1 / Pc = (pr2 n)-1

19. Mean Free Path
Ideal gas law: n = P / kT gives :
l = (kT / pr2)
~ 5 x 10-3 / P for ambient air (N2)
with [l] = cm, [P] = Torr
e.g., at 760 Torr, l = 650 Å P

20. molecular flow: Dss < 5 x 10-3 / P
Mean Free Path
molecular flow: Dss < 5 x 10-3 / P
from Ohring, Fig. 2-3, p. 56

21. Mean Free Path
· For a source-to-substrate distance >10 cm, molecular flow requires chamber pressure < 10-3 Torr
from Ohring, Fig. 2-3, p. 56