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Etching mechanisms of POSS Copolymers
David EON, Gilles CARTRY, Vanessa RABALLAND, Christophe CARDINAUDand CRISPIES PARTNERS
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Aim of the present work
POSS etching in oxygen plasmas
v Measurement of etch rates and selectivityv Determination of the etch mechanismsv Surface roughness – Line Edge Roughnessv Dimension control during pattern transfer
Experiment
v ICP plasmav O2, 800W, 10 mTorr, 40 sccm – Bias voltage: 0 V or –100Vv In-situ ellipsometryv Quasi in-situ XPS
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O2 plasma - Bias voltage: 0V
Ä The etch resistance decreases with the silicon content
Ellipsometric measurements (I)
0 1 2 3 4 5 6 7 8
80
100
120
140
160
180
200
220
PDMS
40%
40% MA20
ICP O2 plasma, 800 W, 10 mTorr, 0 V
20%
30% MA20
60%
100%
N
orm
aliz
ed th
ickn
ess
(nm
)
Etching time (min)
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Ellipsometric measurements (II):Etch resistance
0
20
40
60
80
100
120
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.21 / (weight percentage of silicon)
Th
ickn
ess
loss
20%
30%(S69)40%
40% (S53)
60%
100%
10 min
7 min
5 min
3 min
1 min
12 s PDMSPMSQ
ICP O2 plasma, 800W, 10 mTorr, 0 V
Ä Si content (or Si-O) controls the etch resistance to oxygen plasma
O2 plasma - Bias voltage: 0V
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Ellipsometric measurements (III):Fluorinated materials
O2 plasma - Bias voltage: 0V – Fluorinated materials
0 1 2 3 4 5 6 7 8
80
100
120
140
160
180
200
220
20%
30% MA20
ICP O2 plasma, 800 W, 10 mTorr, 0 V
30% F-tBMA (S106)
30% FA30
(S83)
100%
N
orm
aliz
ed th
ickn
ess
(nm
)
Etching time (min)
Ä Fluorine significantly reduces material consumption
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Ellipsometric measurements (IV):Addition of a photo-acid generator
O2 plasma - Bias voltage: 0V – S47(40%POSS) / 5%PFOS
Ä PFOS slightly modify etch rates (19% at maximum)
0 1 2 3 4 5 6 7 8
130
140
150
160
170
180
190
200
210
S47 - S47PFOS
ICP O2 plasma, 800 W, 10 mTorr, 0 V
40% PFOS
40%
N
orm
aliz
ed th
ickn
ess
(nm
)
Etching time (min)
0
2
4
6
8
10
12
14
16
18
20
Difference (%
)
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Ellipsometric measurements (V):Bias effect
0 2 4 6 8 10 1220
40
60
80
100
120
140
160
180
200
220
100 % POSS
30%MA20%
40%MA20%
23 nm/min
7 nm/min
3 nm/min
1.6 nm/min
20 % POSS
40 % POSS
60 % POSS
PDMS
Thic
knes
s (n
m)
Etching time (min)
PDMS presents a different behavior
O2 plasma - Bias voltage: -100V
Ä POSS mat. present higher resistance to ion bombardment compared to PDMS
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0 2 4 6 8 10 1220
40
60
80
100
120
140
160
180
200
220
100 % POSS
30%MA20%
40%MA20%
23 nm/min
7 nm/min
3 nm/min
1.6 nm/min
20 % POSS
40 % POSS
60 % POSS
PDMS
Thic
knes
s (n
m)
Etching time (min)
O2 plasma - Bias voltage: -100V
Hydrocarbonresist (Novolac
AZ5214):630 nm/min
Selectivity:390
210
90
27
16
8
6
2
Ellipsometric measurements (VI):Selectivity
Ä 30% POSS is sufficient to ensure a satisfactory etch resistance
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v High initial etch rates suggest a surface oxidation of POSS copolymersv XPS is used to monitor surface atomic percentages versus etching time
0 2 4 6 8 10 12 14 16 18 20 22
10
20
30
40
50
60
70 S48 (40% POSS):
CARBON
SILICON
OXYGEN
Etching time (s)
Ato
mic
per
cent
ages
0 2 4 6 8 10 12 14 16 18 20 2210
20
30
40
50
60
S41 (100% POSS)
CARBON
SILICON
OXYGEN
Etching time (s)
Ato
mic
per
cent
ages
Oxidation of the surface !
Oxidation of the top surface (I): XPS measurements
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100 % POSS material before and after 5 min etching
800 1200 1600 2000 2400 2800 3200
0.00
2.50x105
5.00x105
7.50x105
1.00x106
1.25x106
1.50x106
1.75x106
Pure POSS monomer (S41)
After 5 min etching (0V)
Before etching
Wavenumber (cm-1)
abso
rban
ce (m
-1)
Oxidation of the top surface (II): FTIR measurements
Oxidation of the surface !
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Copolymer layerCarbon - Oxygen - Silicon
SiO2 layer No carbon
Top layer (I)
v We use a two layers model for XPS analysis
v The decrease of the carbon peak through the oxide layer is used to determine the oxide thickness versus etching time
v Once the oxide layer is determined it is possible to distinguish the oxygen and silicon peak contribution coming only from the oxide layer
Ä The chemical composition of the oxide layer can be determined
XPS analysis
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v There is oxidation of thetop surface during first seconds
v The chemical composition is closed to silicon oxide in all cases
v Ion bombardment seems to favor oxidation
Top layer (II)
Material 100% 60% 40%MA20 40% 30% IA10 30% IA20 30% MA20 30% FA30 30% IA10FA20 20%
Oxide thickness (nm) 2.4 2.0 1.7 1.7 1.5 1.6 1.6 1.4 1.5 1.4
Oxide thickness (nm) 3.2 2.7 2.4 2.2 2.4 2.5 2.2 2.4 2.5 1.8
O/Si ratio 1.9 1.7 1.9 1.8 1.6 1.7 1.8 1.7 1.8 1.9
O/Si ratio 2.2 2.1 2.0 2.0 1.9 2.0 2.0 1.9 2.0 2.0
Oxide thickness versus etching time
0 2 4 6 8 10 12 14 16 18 20 22
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5Estimation of SiO2 thickness using C1s peak
S41 - 100% S48 - 60% S71 - 30% IA20 S83 - 30% FA30
Etching time (s)
SiO
2 thi
ckne
ss (n
m)
0 V
100 V
0 V
100 V
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Silicon oxidation
Balance of Si atoms
F. Watanabe, Y. Onishi, JVSTB 4(1), Jan/Feb 1986, p422
dt
de
dtde polSi
poloxinSi
ox ρ−=ρ
Chemicals reaction with C and H forms volatiles compounds
Oxygen plasma
eox
Sputtering
Diffusion
SiXX
SiX pρ=ρ
Xρ Material densitySiXp Weight percent of
silicon
Oxidation model (I)
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Thickness loss with sputtering (bias: -100V)
( )tete ox)( +λ=∆
Sputtering of silicon dioxideLayer densification
Without sputtering (No bias: 0V)
( ) tktete sox )1()( λ++λ=∆
Oxidation model (II)
v To calculate λ, we need copolymers densities and silicon contentv Silicon content is always knownv Copolymer densities have been taken equal to 1.0 g/cm3
Sipol
Sipol
Siox
ρ
ρ−ρ=λ
SipolρSi
oxρ >
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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50
5
10
15
20
25
30
35
40
20%30%
40%
60%
100%
Oxide thickness (nm)
Lost
thic
knes
s (n
m)
( )tete ox)( +λ=∆ : theoretical values
Ä Thickness loss is measured by ellipsometry
Ä Oxide thickness have been measured by XPS
for 5s, 10s and 20s etching time
Ä Experimental curves ofthickness loss versus oxide thickness can be plotted
Oxidation model (III):Without ion bombardment
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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50
5
10
15
20
25
30
35
40
10s 20s5s
20%30%
40%
60%
100%
Oxide thickness (nm, XPS)
Lost
thic
knes
s (n
m)
( )tete ox)( +λ=∆ : theoretical and experimental values
Ä The model fits very well with the 100% POSS
material
Ä Thickness loss of the 100% POSS material is
due to layer densification following oxidation of the
top surface
Oxidation model (IV):Without ion bombardment
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( )tete ox)( +λ=∆ : theoretical and experimental values
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50
5
10
15
20
25
30
35
40
10s 20s5s
20%30%
40%
60%
100%
Oxide thickness (nm)
Lost
thic
knes
s (n
m)
Ä The model fits well with the experiments: Thickness losses seem
to come from layer densification
Ä The model points out the effect of surface
segregation on plasma etching
Oxidation model (V):Without ion bombardment
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( ) tktete sox )1()( λ++λ=∆
Oxidation model (VI)With ion bombardment
v A second term is added to the material densification term
v This second term is positive and takes into account the sputtering
v For a sake of clarity, we present results only after 10 seconds etching
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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50
5
10
15
20
25
30
35
40
45
50
55
60
100V0V
100V
0V
10 seconds
20%
30%
40%
60%
100%
Oxide thickness (nm)
Lost
thic
knes
s (n
m)
Ä No sputtering is observed !
(except for S43)
Ä Ion bombardment seems to favor oxidation
Oxidation model (VII)With ion bombardment
( ) tktete sox )1()( λ++λ=∆
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Main ideas of the model
v The flux of oxidant species at the interface between silicon oxide and polymer is equals to the oxidation flux:
v This gives the layer growth function f(eoxide)The oxide layer is then determined by the following differential equation:
v Its solution give the oxidised layer growth function
Oxidised layer growth: Deal & Grove model
sputteringoxideoxide kef
te
−=∂
∂)(
1
cF D
z∂
= −∂ ( )2
Si Cox ox i ox iF k k c k c= + ==
( )1ln 1 expox s
s s
B Be k At
A k k
= − − +
Three free parameters: A, B, ks
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0
20
40
60
80
100
120
0 1 2 3 4 5 6 7 8Temps (min)
Epa
isse
ur t
otal
e pe
rdue
(nm
) 20%
0
2
4
6
8
10
12
14
0 1 2 3 4 5 6 7 8Temps (min)
Ep
aiss
eur
tota
le p
erd
ue
(nm
) 100%
Results of the oxidation layer growth model
Ä A, B and kS parameters are the fitting parameters
Ä kS, the sputtering rate is assumed independant of the material
Ä kS is found lower than for silicon oxide !
Ä B is determined for all copolymers.Ä B could give the reaction coefficient between Oand Si at interface if Oplasma is measuredÄNeed for mass spectrometry and optical actinometry
ÄA is determined for all copolymersÄA could give the carbon etching coefficient if DO-SiO2 is known
If these coefficients are known, the Demokritos profile simulator may be applied to POSS copolymer pattern
transfers
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Conclusions
Ä Etch rates and selectivity have been measured for a all the copolymers (>30% for good selectivity)
ÄWe have shown that the copolymer top surface is converted into an oxide like layer during first plasma seconds
ÄWe have developped an oxidation model:ØWithout bias, thickness loss comes from layer densificationØ The ion bombardment favors the formation of the oxide layer but do not induce a strong sputtering
ÄWe have determined for all copolymers the oxidised layer growth function and we may be able to estimate rate coefficient of atomic oxygen at interface
ÄWe want to apply all these results to:Ø The developement of an etching strategy without LERØ The control of dimension during pattern transferØ The study of new materials
ÄWe still need:Ø New etching conditionsØ Plasma measurements (ion flux, oxygen concentration)Ø Pattern transfer experiments
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0 2 4 6 8 10 12 14 16 18 20 221.0
1.5
2.0
2.5
3.0
3.5
Surface segregation effect ?
Estimation of SiO2 thickness using C1s peak
S41 - 100% S48 - 60% S47 - 40% S43 - 20%
Etching time (s)
SiO
2 thi
ckne
ss (n
m)
Silicon oxide thickness versus time