This document discusses various topics related to interdiffusion, reactions, and transformations in thin films: 1. It explains the concepts of diffusion, nucleation and grain growth that can occur in thin films. Diffusion is impacted by factors like temperature, grain boundaries, and dislocations. 2. Interdiffusion in thin films occurs much faster than in bulk materials due to diffusion along grain boundaries and dislocations. This can impact thin film integrity over time. 3. Reaction and compound formation at interfaces between thin films is discussed, like the growth of SiO2 at the Si/O2 interface or intermetallic compound formation. 4. Phase transformations and the use of diffusion barriers to prevent undesirable

1. Interdiffusion, Reactions, and Transformations
in Thin Film
Prepared by
Md Ataul Mamun
EE&CS Department, SDSU
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2. •Impurity atom at position A must posses an energy EA to push
the host atoms away and move into the neighboring void at B
•EA is the activation energy
Diffusion of an impurity atom in a solid
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3. Diffusion and diffusion length
d2 =4Dt
d= “distance” diffused after time t,
D = diffusion coefficient
D=
DO = constant
d
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4. (a) Nucleation
(b) Growth
(c) Solidified polycrystalline solid
For simplicity cubes represent atoms.
Nuclei, Grain, and Grain Boundary
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5. What is
Interdiffusion
?
Solid to solid diffusion
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6. Interdiffusion facts for thin film
• Mass-transport differs from bulk to small scale
• Diffusions and corresponding reactions can alter the thin film
integrity
• May lead to device malfunction
• Capable of doing open and short circuits
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7. How much Interdiffusion in thin film differs
from the bulk?
At 300C, Cu-Ni diffusion coefficient 3.8x10-24 cm2/s. For 0.1µm Ni film
what will be the Interdiffusion time?
d2 =4Dt
t=1/4(d2/D)
More than 200,000 years!!!
Experiment shows inter mixing happens in less than 1 hour!!!!
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8. Considerable areas for diffusion
• Grain boundary (GB) Activation energy, EGB
• Dislocation (d)Activation energy, Ed
• Plain Surface (s)Activation energy, Es
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9. Grain, Grain boundaries, and dislocations in
polycrystalline
• Grain-boundary slabs (δ) act as short circuit
diffusion path
• At dislocation cores (ꓕ), diffusion rate is
Just lower than grain-boundary
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10. Dominant diffusion
regimes
TM =Melting point
From figure, as temperature rises
dominance goes from GB to L
However, for typical metal films with
a grain size of 1um or less, grain-
boundary diffusion dominates at all
practical temperature
Figure: Regimes of dominant diffusion mechanism in
FCC metal films as a function of temperature
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11. T vs D
Figure: Diffusion coefficients (D) of various elements in Si
and GaAs as a function of temperature
D=
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12. What should one do when diffusion parameters in
polycrystalline thin films must be estimated but
experimental data are lacking?
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13. How to estimate
DGB/DL
• Draw horizontal line at T/TM
• Draw vertical line on grain size, log
d= log(2√DLt)
• Draw parallel line to beta through
intersection
Figure: Master Curves for estimating DGB/DL
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14. Thin Film
Diffusion
couple
D=CADB+CBDA
CA and CB are respective concentrations
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15. Effect of Infinite diffusion couple
• Consider O2 and Si wafer (They have an infinite
diffusion couple)
• SiO2 grows on surface and thickens with time
• The resulting amorphous SiO2 films serves critical
function of the gate oxide dielectric in FET
• Passivates the Si
• Allows a very low current leakage
• Leaves minimum dangling bonds
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16. But not always great
• Difficult to solder on Al due to quick
oxidization
• Difficulty overcome by evaporated
Cr-Cu-Au thin film structure
• Solder joins with Cu easily, but Cu adheres
poorly with Al, so Cr is introduced
• To protect Cu, introduce Au
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17. Interdiffusion in Miscible Alloy Systems
Figure: Palladium concentration profiles in a Au-Pd thin-film
Profile is symmetric when intrinsic
Atomic diffusivities are equal
ie, DA=DB
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18. Grain Boundary
Diffusion in Thin-
Films
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19. Intermetallic
compound formation
Figure: RBS spectrum showing formation of AuAl2 and
Au2Al Phases at 230C
Figure: Thickness relationship with time and temp
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20. Compound Formation
in Films
End phases depend on whether
dAl>dAu or dAu>dAl
Figure: Compound formation sequence in Al-Au
thin film couples 20

21. Phase Transformation
in Thin Films
• Means changing state from one to another at
a fixed temperature
• Example: Amorphous CoSi2 to Crystalline
CoSi2
Figure: Transformed fraction of CoSi2 as a function of
time measured by change in resistivity
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22. Metal-Semiconductor Reactions
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23. Diffusion
Barriers
• Thin films used to separate materials from
coming into direct contact to prevent them
from reacting
• Similar to paint and electrodeposited layers to
protect underlying materials
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24. Barrier
Requirements
Constitute a kinetic barrier to the traffic of A and B across
it (The diffusivity of A and B in X should be small)
Kinetic
barrier
Be thermodynamically stable with respect to A and B at
the highest temperature useStable
Have low contact resistance with A and B (and possess
high electrical and thermal conductivity)Low R
Be easy to deposit, adhere to the involved films, possess
low stress, and compatible with other processing
Easy to
deposit
Example: For Si-Al interconnection TiC, TiN
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25. Silicides, Salicides,
Polycides
• Silicide: A silicide is a compound that has silicon
with (usually) more electropositive elements
(Example: Na2Si, TiSi2)
---Used in MOS/CMOS processes for ohmic
contacts of the source, drain, and poly-Si
gate
• Salicide: Self-Aligned silicide. Lithographic
patterning processes not required
• Polycide: Combination of a silicide and polysilicon
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26. Salicides
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27. Achieving Salicides
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