towards a better model for reactive magnetron sputter...
TRANSCRIPT
Towards a better model for reactive magnetron sputter deposition
Title
Towards a better model for reactive magnetron sputter deposition
D. Depla, R. De Gryse
Towards a better model for reactive magnetron sputter deposition
440
400
360
320
2802.01.51.00.50.0
0.65
0.60
0.55
0.50
0.45
0.402.01.51.00.50.0
flow (sccm) increasing flowdecreasing flow
dis
ch
arg
e v
olta
ge
(V
) to
tal p
ressu
re (
Pa
)
A first experiment
S (Pumping speed) : 16 L/sPAr (argon pressure) : 0.4 PaI (current) : 0.4 AStep : 2 minutesTarget : AluminiumReactive gas : oxygen
Towards a better model for reactive magnetron sputter deposition
Why does the voltage change ?
0discharge
0 i ISEE
WV
mE====
ε ε γε ε γε ε γε ε γ
W0 : effective ionisation energy
εεεεi : ion collection efficiency (for magnetron : almost 1)
εεεε0 : fraction of maximum possible number of ions (for magnetron : almost 1)
m : multiplication factor : accounts for ionisation in the sheath
E : effective ionisation probability : influenced by electron recapture
γγγγISEE : ion induced secondary electron emission coefficient
A voltage change is related to 1) a change of the plasma composition (W0) 2) and/or a change of the target composition (γγγγISEE / E)
G. Buyle, “Simplified model for the DC planar magnetron discharge (PhD, UGENT,2005)D. Depla et al. J. Appl. Phys. 101 (2007) 013301/1-013301/9
Depla D. et al. SCT 200 (2006) 4329 -4338
Towards a better model for reactive magnetron sputter deposition
Why does the pressure change ? The Berg model
Prof. Berg in Ghent
Effective gas pressure
targ
et
q0
qP
qt
qc
o p t cq q q q= + += + += + += + +
S. Berg et al., TSF 476 (2005) 215– 230
Towards a better model for reactive magnetron sputter deposition
How to calculate these flows ?
(((( ))))tt t tq F 1 A− θ− θ− θ− θαααα====
(((( ))))cc c cq F 1 A− θ− θ− θ− θαααα====
pq pS====+
Total reactive gas flow
The chemical reaction on the target and on the substrate is defined by chemisorption.
pF
2 kTM====
ππππThe effective reactive gas pressure defines the flux
The coverage of the target and substrate is given by θθθθ
tθθθθ
FF
oxide
oxide
metal
metal
For more about the chemisorption mechanism : C. Li et al. J. Phys. D: Appl. Phys. 37 (2004) 1065–1073
t1− θ− θ− θ− θ
c1− θ− θ− θ− θcθθθθ
Towards a better model for reactive magnetron sputter deposition
How to calculate the θ’s ?
(((( ))))t t d t c
2F1 I Y
zα − θ = θα − θ = θα − θ = θα − θ = θ
At the target side
F
A balance between chemisorption and compound sputtering
t
t
d c
2F
z2F
I Yz
ααααθ =θ =θ =θ =
+ α+ α+ α+ α
tθθθθ t1− θ− θ− θ− θ
Towards a better model for reactive magnetron sputter deposition
How to calculate the θ’s ?
(((( )))) (((( ))))(((( ))))d t m td t c t
c c c c
s s
I 1 Y AI Y A2F1 1 0
z A A
− θ− θ− θ− θθθθθα − θ + − θ − θ =α − θ + − θ − θ =α − θ + − θ − θ =α − θ + − θ − θ =
At the substrate side t
θt
1 θ−
cθ
c1 θ−
FFF1
F2
F4F3
A balance between chemisorption and deposition of compound and metal.
(((( ))))
d t c tc
sc
d t m td t c tc
s s
I Y A2F
z A
I 1 Y AI Y A2F
z A A
θθθθα +α +α +α +
θ =θ =θ =θ =− θ− θ− θ− θθθθθ
α + +α + +α + +α + +
Towards a better model for reactive magnetron sputter deposition
Final solution scheme
The “trick” to solve : choose a pressure and calculate a flow.
p F θθθθt
qtqp
θθθθc
qc q0
1.2
0.8
0.4
0.0
flow
(sccm
)
2.0x10-3
1.51.00.50.0
pressure (mbar)1.20.80.40.0
flow (sccm)
2.0x10-3
1.5
1.0
0.5
0.0
pre
ssu
re (
mba
r)
SWAP X and Y axis
Towards a better model for reactive magnetron sputter deposition
Is the S-shape real ?
AlOx: O2 Partial Pressure vs. Flow
IRESS Partial Pressure Control
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
O2 Partial Pressure, mTorr
O2 F
low
, sccm
Increasing
Decreasing
Target Power = 600 W
Pulse: 75 kHz, 2.5 µsec reverse
20 sccm Ar: 2.40 mTorr
Measurement by W.D. SproulSee also W.D. Sproul et al. SCT 89(1997) 10-15
Towards a better model for reactive magnetron sputter deposition
Interpretation : let’s play(1)
2.0
1.5
1.0
0.5
0.0
flow
(sccm
)
2.52.01.51.00.5
total flow (sccm)
to the pump to the target to the substrate
1.0
0.8
0.6
0.4
0.2
0.0
theta
2.52.01.51.00.5
total flow (sccm)
substrate condition target condition
Towards a better model for reactive magnetron sputter deposition
Interpretation : Take care
The abrupt change mechanism is related to substrate condition, and is only noticed when plotting against the flow. As a function of the pressure (or mole fraction), there is no abrupt change !
1.0
0.8
0.6
0.4
0.2
0.0
theta
targ
et
0.040.030.020.010.00
pressure (Pa)
Towards a better model for reactive magnetron sputter deposition
A second experiment : playing with the pumping speed
0.25
0.20
0.15
0.10
0.05
0.00543210
-160
-120
-80
-40
0 S =16 L /s
S =32 L /s
S =65 L /s
S =130 L /s
flow (sccm)
pre
ssure
diffe
rence (
∆P
a)
volta
ge d
iffe
rence (
∆V
) PAr (argon pressure) : 0.4 PaI (current) : 0.4 AStep : 2 minutesTarget : AluminiumReactive gas : oxygen
Towards a better model for reactive magnetron sputter deposition
Interpretation : let’s play(2)
0.25
0.20
0.15
0.10
0.05
0.00543210
-160
-120
-80
-40
0 S =16 L /s
S =32 L /s
S =65 L /s
S =130 L /s
flow (sccm)p
ressure
diffe
rence
(∆
Pa)
vo
lta
ge
diffe
ren
ce (
∆V
)
Model Experiment
Clear shift of the critical points
The critical points are fixed !
General trend is good :A more narrow hysteresis athigher pumping speed
0.04
0.03
0.02
0.01
0.00
pre
ssu
re (
Pa
)
3.53.02.52.01.51.00.5
flow (sccm)
16 L/s 32 L/s 65 L/s 130 L/s
Towards a better model for reactive magnetron sputter deposition
A more detailed look
0.25
0.20
0.15
0.10
0.05
0.00543210
-160
-120
-80
-40
0 S =16 L /s
S =32 L /s
S =65 L /s
S =130 L /s
flow (sccm)
pre
ssure
diffe
rence
(∆
Pa)
vo
lta
ge
diffe
ren
ce (
∆V
) Vincrease
Vdecrease
Remarkable constant
Plasma composition changes strongly
Conclusion : influence of oxygen presence is negligible on V increase
Towards a better model for reactive magnetron sputter deposition
What causes the voltage increase ?
380
360
340
320
300
targ
et
vo
lta
ge (
V)
0.50.40.30.20.1
sputter time (s)
exposure time 5s 10s 20s 40s 80s 160s 320s 640s 1280s 2560s 4260s1.7x10-4 mbar
1
sta
tus
01
0
sta
tus
1
0
sta
tus
V
metallic mode
argon flow
oxygen flow
magnetron
target potential
∆t
Towards a better model for reactive magnetron sputter deposition
Chemisorption does !
oxid
efo
rmat
ion
chem
isor
ptio
n
mon
olay
erO
-20
-10
0
10
Vo
ltag
e ch
ange
(V)
102
103
104
105
oxygen exposure (L)
Constant pressure
experiments
Constant exposure
time
A chemisorption of oxygen results in a voltage increase.Oxide formation results in a voltage decrease, but …only at very high exposures
Towards a better model for reactive magnetron sputter deposition
Another problem
400
380
360
340
320
300
280
260
240
vo
lta
ge
(V
)
0.001 0.01 0.1 1 10time (s)
time
sta
tus
sta
tus
voltage
sta
tus
argon
oxygen
magnetron
VAr
VO2VoxAr
∆t
0.4 A
Thickness calculated is order of 2 nm
D. Depla et al. Solid Films 515 (2006) 468- 471
Towards a better model for reactive magnetron sputter deposition
Stability experiments Al/O2
time
sta
tus
sta
tus
voltage
sta
tus
argon
oxygen
magnetron
VAr
VO2VoxAr
∆t
-40
-38
-36
-34
-32
-30
V3
0m
s-V
Ar (V
)
6
102 4 6
1002 4 6
10002
waiting period (s)
The voltage changes without sputtering : Real ?
D. Depla et al. Plasma Sources Sci. & Techn. 10 (2001) 547 -555
Towards a better model for reactive magnetron sputter deposition
Stability experiments N2/Si
500
480
460
440
420
targ
et vo
ltag
e (
V)
8006004002000
time (s)
waiting period 0 s 1 s 12 s 120 s 1200 s 2430 s
Stability experiment for Si/N2
60 V
ch
an
ge !
D. Depla et al. Vacuum 66 (2002) 9-17
Towards a better model for reactive magnetron sputter deposition
Abrupt changes as a function of the mole fraction ?
40
30
20
10
0
-10
-20
-30
volt
age d
iffe
rence -∆
V(V
)
0.80.60.40.20.0mole fraction nitrogen
1.0
0.8
0.6
0.4
0.2
0.0
the
ta t
arg
et
0.040.030.020.010.00
pressure (Pa)
D. Depla et al. Vacuum 66 (2002) 9-17
Towards a better model for reactive magnetron sputter deposition
Some first conclusions
+ The Berg model shows the general trend.+ But … some details are hard to understand from this model.
Voltage increase by chemisorption. How does the target becomes oxidized ?
Sputter cleaning of the target : a much thicker layer than 1 monolayer is formed
Stability experiments : reaction and/or diffusion
Possible explanation : ion implantation and knock on implantation
Abrupt changes of the target as a function of the mole fraction
Towards a better model for reactive magnetron sputter deposition
True ? More proof.
1.8 MeV D+
14N(d,α0)12C
14N(d, α1)12C*
0.4 0.5 0.6 0.7 0.8
0
2
4
6
8
10
12
0
1
2
3
4
5
6
7
8
9
0.4 0.5 0.6 0.7 0.8
increasing N2 flow
decreasing N2 flow
Nitro
gen
pa
rtia
l pre
ssu
re
(10
-5 m
bar)
Nitrogen flow (sccm)
increasing N2 flow
decreasing N2 flow
Magnetron off
N
itro
gen
are
al de
nsity
(10
15 a
tom
s c
m-2)NRA measurements during
magnetron sputtering
See next talk for more details !
Guttler D., Abendroth B., Grotzschel R., Moller W., Depla DAPL 85 (2004) 6134 -6136
Towards a better model for reactive magnetron sputter deposition
Need for a new model : STEP 1
Implementation of ion implantation during reactive magnetron sputtering
First step : Let’s look to sputtering in combination with ion implantation
No chemical reaction !
Red : implantation profileGreen : the target
(((( )))) (((( ))))(((( ))))t
r S s0
ˆn x, t 2fI p x X x t t′ ′′ ′′ ′′ ′= + − ∂= + − ∂= + − ∂= + − ∂∫∫∫∫
Surface concentration ~ 1/Y
Towards a better model for reactive magnetron sputter deposition
Ag/N2 experiments
410 405 400 395 390binding energy (eV)
inte
nsi
ty (
a.u
.)
After Ar sputter cleaning
After nitrogen gas exposure
After nitrogen implantation
No chemical reaction between nitrogen and silver
Depla D. et al. VACUUM 69 (2003) 529 -536
Ion implantation of N2+
in a silver target, mounted in a magnetron
Towards a better model for reactive magnetron sputter deposition
Ag/N2 experiments
Experimental set-up
Magnetron
Circular target : 2 inch
Target material : Ag
Ion source
shutter
Ion implantation of N2+
in a silver target, mounted in a magnetron
Towards a better model for reactive magnetron sputter deposition
Ag/N2 experiments
333.2
332.8
332.4
332.0abso
lute
tar
get
volt
age
(V)
80604020
sputter time (s)
Ion implantation of N2+
in a silver target, mounted in a magnetron
After implantation : magnetron running in pure argon
After implantation : waiting period + magnetron running in pure argon
350
345
340
335
abso
lute
tar
get
volt
age
(V)
806040200
sputter time (s)
increasing
implantation doseLonger waiting period
Towards a better model for reactive magnetron sputter deposition
Need for a new model : STEP 2
Chemical reaction between implanted reactive gas and target materialNo chemisorption
x 0
s b
D D-s
Ta
rge
t su
rfa
ce
q
x 0
implantation depth D
D D-s
θθ θθsm
θθ θθsr
θθ θθb
mθθ θθ
br
ions
What occurs in the target ?Implantation defined by the profileChemical reaction : reaction rate = knmnR with k the reaction rate constantMovement towards the target surface.
Towards a better model for reactive magnetron sputter deposition
Solution of step 2 in steady state
0
s b
D D-s
Ta
rge
t su
rfa
ce
q
x 0
implantation depth D
D D-s
θθ θθsm
θθ θθsr
θθ θθb
mθθ θθ
br
0
s b
D D-s
Ta
rge
t su
rfa
ce
q
x 0
implantation depth D
D D-s
θθ θθsm
θθ θθsr
θθ θθb
mθθ θθ
br
Start situation Start situation + ∆∆∆∆t
In steady state : when the slab reaches the surface, the chemical reaction finishes and the oxidation degree at the target surface must be equal to θθθθsr
The slab moves at a speed given by the erosion rate.The sputter yield of the target : Ys=θθθθrsYr+ θθθθmsYm defines the erosion rate.
D. Depla et al. J. Phys. : Appl. Phys. D, 40 (2007) 1957–1965
Towards a better model for reactive magnetron sputter deposition
Two ways of testing
ϕϕϕϕ
Oblique reactive ion implantation
Constant mole fraction f=1Variable metal sputter yield
Reactive magnetron sputtering
Variable mole fraction fConstant metal sputter yieldChemisorption/Knock-on possible
In both experiments the target condition is defined by the implantation of reactive species and the sputtering process
Towards a better model for reactive magnetron sputter deposition
Two ways of testing the model
40
30
20
10
0
-10
-20
-30
volt
age d
iffe
rence -
∆V
(V)
0.80.60.40.20.0mole fraction nitrogen
Oblique reactive ion implantation Reactive magnetron sputtering
1.0
0.8
0.6
0.4
0.2
0.0
de
gre
e o
f o
xid
atio
n
706050403020100angle of incidence
Experiment : 5 keV O2+ ion beam on Si
XPS measurement of degree of oxidationIon current density : 1015 - 1016 ions/(cm2.s)
Experiment : 50 W Sputtering Si/N2
Discharge voltage changecharacteristic to target conditionIon current density : 1017 ions/(cm2.s)
D. Depla et al. Vacuum 66 (2002)
Alay et al. Phys. Rev 50 (1994)
Towards a better model for reactive magnetron sputter deposition
First way
Implantation profiles : SRIM
Sputter yield dependency : H. De Witte UAntwerp, PhD (2001)k = 1x10-22 cm3/(at.s)
1.0
0.8
0.6
0.4
0.2
0.0
de
gre
e o
f o
xid
atio
n
806040200
angle of incidence(°)
experimental data (taken from Alay et al. Phys. Rev. 1994)
θb
θs
Towards a better model for reactive magnetron sputter deposition
Second way
Using the same reaction rate constant buta constant sputter yield for metal and compound
No abrupt change ?
40
30
20
10
0
-10
-20
-30
volt
age d
iffe
rence -∆
V(V
)0.80.60.40.20.0
mole fraction nitrogen
1.0
0.8
0.6
0.4
0.2
0.0
de
gre
e o
f o
xid
atio
n
0.160.120.080.040.00
mole fraction oxygen
θrb
θrs
Towards a better model for reactive magnetron sputter deposition
Second way (2)
Fits with the experiment : N has a lower affinity for Si than O.
1.0
0.8
0.6
0.4
0.2
0.0
degre
e o
f oxid
ation
0.60.50.40.30.20.10.0
mole fraction reactive gas
θrb
θrs
k=1
k=3.2k=10
k=100
k values in unit of 1x10-23
cm3/(at.s)
Towards a better model for reactive magnetron sputter deposition
Need for a new model : knock on and chemisorption
Chemical reaction between implanted reactive gas and target materialWith chemisorption and knock-on implantation
x 0
s b
D D-s
Ta
rge
t su
rfa
ce
q
0
implantation depth D
D D-s
θθ θθsm
θθ θθsr
θθ θθb
mθθ θθ
br
ions
neutrals
θθ θθsc
knock-on
Simplification : knock-on profile is identical to implantation profile
sm2F
z
αθ
smI
z
βθ
Towards a better model for reactive magnetron sputter deposition
Influence chemisorption
Fixed reaction rate constant : 1x10-23 at/(cm2.s)Knock-on probability ββββ=0
1.0
0.8
0.6
0.4
0.2
0.0
degre
e o
f oxid
ation
1.00.80.60.40.20.0
mole fraction reactive gas
α=0α=0.001α=0.01
α=0.05α=0.1α=0.5α=1
Walking from Uppsala to Ghent
Towards a better model for reactive magnetron sputter deposition
Influence knock-on
Fixed reaction rate constant : 1x10-23 at/(cm2.s)Sticking probability αααα = 0.01
1.0
0.8
0.6
0.4
0.2
de
gre
e o
f o
xid
atio
n
1.00.80.60.40.20.0
mole fraction reactive gas
sticking coefficient α=0.01
β=0β=0.001β=0.01β=0.05β=0.1β=0.5β=1
Towards a better model for reactive magnetron sputter deposition
Knock-on vs. ion implantation
Sputter yield Subsurface oxidation
Knock on vs. direct reactive ion implantation
0.5
0.4
0.3
0.2
0.1
targ
et
sp
utt
er
yie
ld
0.40.30.20.10.0
reactive gas mole fraction
chemisorption/knock-onreactive ion implantationchemisorption
1.0
0.8
0.6
0.4
0.2
0.0d
eg
ree
of
oxid
atio
n0.40.30.20.10.0
reactive gas mole fraction
chemisorption/knock-onreactive ion implantation
Towards a better model for reactive magnetron sputter deposition
How to calculate the flows ?
Effective gas
pressure
targ
et
q0
qP
qt
qc
o p c tq q q q= + += + += + += + + Essential the same as in the Berg modelbut one must account for knock on and ion implantation
sct sm s rb t
I zq F Y A
2 2
βθβθβθβθ = αθ − + θ= αθ − + θ= αθ − + θ= αθ − + θ
Towards a better model for reactive magnetron sputter deposition
Let’s model
Calculation of the critical points
Ac ααααt
k ββββ ααααc
380
(150)
2.0E-23
(4.9E-26)
0.22
(0.093)
0.35
(0.13)
0.17
(0.034)
Acααααt kββββααααc
3087 combinations6 results in a good fit
Towards a better model for reactive magnetron sputter deposition
Discharge voltage increase
-160
-120
-80
-40
0 S =16 L /s
S =32 L /s
S =65 L /s
S =130 L /s
vo
lta
ge
diffe
ren
ce (
∆V
) Vincrease
Vdecrease
0.14
0.12
0.10
0.08
θcs
,max
16 32 65 130
pumping speed (L/s)
25
20
15
10
5
dis
cha
rge
vo
ltag
e d
iffere
ren
ce(V
)
Towards a better model for reactive magnetron sputter deposition
Stability experiment
-40
-38
-36
-34
-32
-30
V3
0m
s-V
Ar (V
)
6
102 4 6
1002 4 6
10002
waiting period (s)
0.15
0.10
0.05
0.00
θrb
,20
s-θ
rb,s
tead
y s
tate
0.250.200.150.100.050.00
mole fraction oxygen
Target oxidation after 20 s without ion bombardment
Target oxidation during ion bombardment
Towards a better model for reactive magnetron sputter deposition
The future : things we should think about
… and the future :
+ The plasma contains more then neutral reactive gas molecules+ The deposition is not uniform+ The target erosion is not uniform+ Other types of magnetrons behave differently
Conclusions :
The implementation of reactive ion implantation is necessary to understand the details
Towards a better model for reactive magnetron sputter deposition
The future : deposition profile
0.08
0.06
0.04
0.02
0.00
oxyg
en
pre
ssu
re (
Pa)
1.81.61.41.21.00.80.6
flow (sccm)
0.9
0.7
0.5
0.3
0.1
0.9
0.7
0.5
0.3
0.1
0.9
0.7
0.5
0.3
0.1
0.9
0.7
0.5
0.3
0.1
1.00.80.60.40.2
θc
Profile Homogeneous
0
0.2
0.4
0.6
0.8
1
-90
-75
-60
-45
-30-15015
30
45
60
75
90
Implementing the deposition profile by segmenting the substrate. The future is combining this with a simulation program for transport
Mahieu S. et al.NIMB 243 (2006) 313 -319
Towards a better model for reactive magnetron sputter deposition
The future : erosion profile
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
-25 -15 -5 5 15 25position on the target (mm)
de
pth
(m
m)
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.001.71.51.31.10.9
flow (sccm)
oxyge
n p
ressu
re (
Pa)
1.00.80.60.40.20.0
θsr
Implementing the erosion profile by segmenting the target based on the measured race track. The future is to measure the effective current density on the target.
Towards a better model for reactive magnetron sputter deposition
The future : rotating magnetrons
250
270
290
310
330
350
370
0 0.5 1 1.5 2 2.5
oxygen flow (sccm)
dis
ch
arg
e v
olta
ge
(V
)
0 RPM
1.1 RPM
2.4 RPM
5.8 RPM
8.9 RPM
19.5 RPM
26.5 RPM
40.1 RPM
64.1 RPM
Towards a better model for reactive magnetron sputter deposition
Time plays an important role
0.1
1
10
100
flow
(sccm
)
0.01 0.1 1 10time (s)
Berg model presented model
Same hysteresis, and some steady state, but different kinetics !
J. Musschoot et al.J. Phys. D : Appl. Phys. 39 (2006) 3989-3993
Towards a better model for reactive magnetron sputter deposition
Some things to remember
RSD2007, Leoben, Austria, 6 and 7 December 2007
ICTF14, November 2008in Ghent, Belgium
tθ
t1 θ−
cθ
c1 θ−
FFF1
F2
F4F3
tθ
t1 θ−
cθ
c1 θ−
FFF1
F2
F4F3 Berg model : a good starting point
x 0
s b
D D-s
Targ
et
su
rfa
ce
q
x 0
implantation depth D
D D-s
θθ θθs
mθθ θθ
sr
θθ θθb
mθθ θθ
br
ions
More detailed and necessary to describe the kinetics
Some things to remember …
Reactive Sputter Deposition (ed. Depla, Mahieu) : Beginning 2008, Springer
Towards a better model for reactive magnetron sputter deposition
Acknowledgements
+ R. De Gryse : introduction to the fun of magnetrons+ J. Haemers : development of sputter sources+ S. Mahieu : simulation and measuring of the deposition profiles+ L. Xiuyang : rotating cylindrical magnetrons+ G. Buyle : discharge voltage interpretation
+ S. Berg : long and nice discussions about reactive magnetron sputtering+ W. Möller : NRA measurements