Repetition: Evaporation of Alloys
Transcript of Repetition: Evaporation of Alloys
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Repetition: Evaporation of Alloys
Evaporation of an alloy corresponds to a fractional destillation. The reason for this is the unhindered material transport within the source.
100
10
1
0.1
1
k=10
k=2
k=1 n/n
log(R /R )A
A B
A
00 0
0
B
B
A B
Alloy composition: A:B=1:1A is the more volatile material (p > p )
particle number for t = 0total number of evaporated particles
n = n + nn = n + n
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Repetition: Sputtering
Solid source, i. e. arbitrarysource geometry
Low deposition temperature
Wide parameter field
High deposition rates can bereached
Good coating adhesion
Coating composition =source composition
Interesting film propertiesSource (water cooled)
+
+
+
+ +
Substrate
-600V
Ground
+Deposition materialWorking gas, neutral or reactive
Elementary Processes: Characteristics:
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Repetition: Gas Discharge
Experimental set-up:
I/V characteristic:I: Ohmic behaviorII: Saturation regionIII: collisional ionization/
Townsend-dischargeIV: normal glowV: anormal glow
secondary electronemission
-
-
+
+
d = 0.1 - 1 m
p = 0.1 - 10 Pa
U = 1 - 5 kV
U
Iself sustainedgas discharge
non-self sus-tained gas discharge
I
IVV
I II II
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Repetition: RF-Sputtering
An excess electron current is generated by the higher electron mobility. It leads to a negative net voltage at the target, idependent wether the target is conductive or not.
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Repetition: Magnetron-Sputtering
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Repetition: I/V Characteristics
)ln( UkIR
Empirical correlation:
R = Erosion rateI = Discharge currentU = Discharge voltage
Magnetron discharges work at significantly lower gas pressures!
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Sputter Yield I
Yn
n
<n> = mean number of particles emitted per impingementn+ = number of impinging ions
Y is dependent on several parameters of the ions and of the target material.
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Sputter Yield II
Dependence on :
Target materialIon energy
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Sputter Yield III
Dependence on:
Ion impingement angleIon mass
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Sputtering Regimes: Single Knock On
+
Ion energy small,and/or ion mass small
110: YM
E eV YEU
100
:
U0 = Surface binding energy
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Sputtering Regimes: Linear Collision Cascade I
Ion energy: 0.1 - 10 keVCollision potentials: E+ 0.1 - 1 keV: Born-MayerE+ 1 - 10 keV: Thomas-Fermi
02
4UE
MMMMY
t
t
Mt = Mass of target atoms
+
Ejection volumeapprox. 1 nm3
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Sputtering Regimes: Linear Collision Cascade IIPerpendicular impingement:
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Sputtering Regimes: Linear Collision Cascade IIIOblique impingement:
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Sputtering Regimes: Thermal Spike
+
Ion energy > 10 keV
YUk TB
exp 0
i. e. an evaporation-characteristic of the ejection volume
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Linear Collision Cascade: Global Characteristics
Y = 0,5 - 4
+
Ejection volumeapprox. 1 nm3
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Sputtering Regimes: Simulation
www.srim.org
Stopping Range of Ions in Matter
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Energy Distribution of Ejected Particles
The energy distribution of sputtered particles is significantly different from that of thermally evaporated ones.
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Linear Collision Cascade: Energy Distribution
E-2
E
n(E)dE
U /20 EmaxE
dE
UEEdEEn 3
0
)(
Emax = maximum energy, E Emax
E = mean emission energy
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Linear Collision Cascade: Angular Distribution
n<1n=1
n>1
n n( ) cos n 1 E < 1 keVn 1 E > 1 keV
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Sputtering of Alloys: Different Y
In the case of the homogenous distribution of the constituents the vapor composition is (after a transient regime) identical to the target composition.
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Sputtering of Alloys: Cone Formation I
If a low yield material is present in the form of macroscopic preciptates, cones can be formed on the target surface.
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Sputtering of Alloys: Cone Formation II
The terminating surfaces of the cones are often low index crystal planes or have an inclination corresponding to surfaces with maximum sputter yield.
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Sputtering of Single Crystals: Channelling
Ions may penetrate a single crystal more or less deep in dependence on their impingement direction.
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Sputtering of Single Crystals: Wehner-Spots
Focusing of the impulse along densly packed crystallographic directions:
Y = maximum along these directions! If a hemispherical collector is placed above the target, one can detect the so-called "Wehner Spots".
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Reactive Processes IIn the case of reactive sputtering processes compounds of the sputtered material and the reactive gas are formed at the target and the substrate.
Gas flows of thereactive gas, qi :
pct0 qqqq
Berg-model
q0 ... Total flowqt ... Flow to targetqc ... Flow to wallqp ... Flow to pump
q
p
Reaktivegas (N )
(Ti)-target
(RF)-voltage
Pump
Recipient wall/substrate
t
N
2
c
0
p
q
q
q
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Reactive Processes IIBalance of areal coverages and particle flows:
1 ... Reacted surface target2 ... Reacted surface WallF1,3 ... Flows of reaction productF2,4 ... Flows of metal particles
J ... Flow of workong gasF ... Flow of reactive gas
Result: system of numerically solvable balance equations
J J F
F
F F F F
ATarget
Substrate/Wall
Q
Q
1- Q
1- Q
A
1
1
2 2
1
2 3 4
t
c
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Reactive Processes: Example TiN IErosion rate at the target in dependence on the N2 -flow:
Hysteresis at the transition from the metallic to the nitridic mode.
0
D
C B
A
0
2
4
6
8
10metallic
unstable
nitridic
12
14
0.2 0.4 0.6N -flow [sccm]
Ero
sion
rate
[a. u
.]
2
0.8 1 1.2 1.4
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Reactive Processes: Example TiN IIPressure in the chamber in dependence on the N2 - flow:
At first all N2 is consumed; the unstable operating point A would be the optimum working condition
0
C
B
00.2
0.2
0.1
0.4 0.6N -flow [sccm]
Tota
l pre
ssur
e [P
a]
2
0.8 1 1.2 1.4D A
no plasma
metallic
nitridic
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TiN: Experimental DataThe hysteresis in the relation betweem N2 -flow and total pressure is well visible.
0,4 0,6 0,8 1,0 1,2 1,4 1,6
0,22
0,22
0,23
0,23
0,24
0,24
0,25
Tota
l pre
ssur
e [P
a]
B
A
C
D
increase N -flowdecrease N -flowtheory
2
2
N -flow [sccm]2
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Reactive Processes: Large PlantsSputtering plant for the reactive deposition of solar cell materials.
1.5 m
Reactive sputtering processes have recently been accepted as suitable methods for the deposition of oxidic, nitridic and carbidic materials.