p-type PERC cells > 20% achieved in 2012. [1] R. Russel et al, 27th EUPVSEC-
2DP1.3. [2] A. Lachowicz et al, 27th EUPVSEC-2CV6.16. [3] L. Tous et al, PIP ...
Correlation between SiNx laser ablation and nickel silicide formation by Excimer Laser Annealing for two steps Ni-Cu metallization
S. Gall1, J-F. Lerat2, J. Vibert1, S. Manuel1, M. Pirot1, T. Emeraud2 1 CEA,
LITEN, INES, 50 avenue du Lac Léman, BP 332, 73377 Le Bourget du Lac, France Group NV, Kempischesteenweg 305 bus 2, 3500 Hasselt, Belgium
2 EXCICO
4th Metallization Workshop - 2013, 7th of May
CEA CONFIDENTIAL
Content 1. Motivation for NiCu metallization • •
Cost reduction Performance potential
2. Challenges 3. Process flow 4. Laser ablation • Surface morphology • Emitter modification • Laser damages
5. Nickel silicidation by ELA 6. Summary 4th Metallization Workshop - 2013, 7th of May
2
Motivation for NiCu metallization Market dominating Ag pastes [SP] closer to its limits: Cost reduction has a limit Limitation to highest efficiencies 2003 - 2013
2011-Cabrera- 3rd Metallization Workshop
Cu expected to replace Ag from 2015: ideal conductor: 92% of σ(Ag) cheap: 6€/kg vs 700€/kg enabler for higher efficiency
p-type PERC cells > 20% achieved in 2012 [1] R. Russel et al, 27th EUPVSEC-2DP1.3 [2] A. Lachowicz et al, 27th EUPVSEC-2CV6.16 [3] L. Tous et al, PIP 2013 DOI: 10.1002/pip.2362
2012-Imec-27th EUPVSEC 4th Metallization Workshop - 2013, 7th of May
3
Challenges for NiCu metallization 1.
ARC opening
2.
Thickness uniformity Deposition selectivity
NixSiy formation
4.
Parameter optimization
Ni deposition
3.
Induced defects Surface roughness
Diel. cSi
Cu Ni Ni NixSiy
2013-Rublack-3rd SiliconPV
Crucial for adhesion Ni diffusion: emitter shunts Ni1Si1 preferred (lowest Rcontact)
Ni/Cu/Sn plating
Reliability
Focus on NiSi formation in the laser ablated areas 4th Metallization Workshop - 2013, 7th of May
2013-Rauer-3rd SiliconPV
4
Process Flow
1E22
3
P concentration [at/cm ]
Dielectric deposition Laser opening Chemical treatment Ni plating
P concentration [at/cm²]
Emitter formation
cSi
1E21
1E20
1E21
1E20
1E19 0.00
0.04
0.08
depth [µm]
0.12
1E19
1E18
% (POCl3 (60 ohm/sq)) 1E17 0.0
0.1
0.2
0.3
0.4
depth [µm]
NiSi formation
•
industrial shallow emitter: 350nm deep
•
No drive-in steps with high-thermal oxidation
•
no selective emitters
4th Metallization Workshop - 2013, 7th of May
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Process Flow SiNx
Emitter formation Dielectric deposition Laser opening Chemical treatment Ni plating
cSi
Standard ARC a:SiNx(H) layer • Thickness 70nm
• PECVD RF 40 kHz • Deposition temperature: 450°C • Refractive index: 2.1
NiSi formation
4th Metallization Workshop - 2013, 7th of May
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Process Flow SiNx cSi
Emitter formation Dielectric deposition ARC opening
Reference
Talisker UV
Talisker Green
No nitride, no ablation
355nm, 10ps, 200kHz
532nm, 10ps, 40kHz
1/e2 = 1.3 ±0.3mm
1/e2 = 1.4 ±0.3mm
Chemical treatment Ni plating NiSi formation
SiNx
Post-ablation characterization: •
Surface morphology: polished and textured cSi
•
Emitter sheet resistance
•
Lifetime
4th Metallization Workshop - 2013, 7th of May
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Laser ablated areas: morphology
Source : UV, ps: SiNx on polished cSi 0,08 W
0,10 W
SEM + EDX
0,12 W
85%w Si 15%w N
0,14 W
100%w Si 0%w N
100%w Si 0%w N
1/e2 beam diameter F1= cSi damages F2= SiNx ablation
0,18 W
0,22 W
100%w Si 0%w N
For UV, ps laser source: •
Fth ablation ≈ 0.10W
•
Suspected cSi damages Fth > 0.14W
F3= Heat-Affected Zone (HAZ) 4th Metallization Workshop - 2013, 7th of May
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Laser ablated areas: morphology Source : UV, ps: SiNx on polished cSi 0,08 W
0,10 W
0,22 W
0,18 W
0,14 W
0,12 W
F3 F2
5
F1
4 25
20
Spot width (µm)
HAZ SiNx ablation cSi damages
6
3
2
1
7
8 Ablation depth
15
10
0 0.06
For UV, ps laser source:
Damage -free process window
5
0.08
0.10
0.12
0.14
0.16
0.18
Laser Power (W)
0.20
0.22
0.24
•
Fth ablation ≈ 0.10W
•
Suspected cSi damages Fth > 0.14W
Damage-free process window?
4th Metallization Workshop - 2013, 7th of May
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Laser ablated areas: Rsheet UV, ps
Green, ps
Speed (mm/s) S. Gall et al., 25thEUPVSEC, 2CV.2.39
260
300mm/s 500mm/s 1000mm/s 2000mm/s
240
Rsheet (Ohm/sq)
220 200
300mm/s 500mm/s 1000mm/s 2000mm/s
240 1E21 220 200
1E20
180 160
180 160
1E21
1E20
1E19 0.00
120
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
0.12
120
POCl3 emitter Post-abl. emitter
80
80
0.08
140
1E18 100
100
0.04
depth [µm]
1E19
140
60 0.06
260
P concentration [at/cm²]
Power (W) 3 P concentration (at/cm ) Rsheet (Ohm/sq)
1E22
1E17 60 0.0
0.03
0.10.04
Power (W)
0.05 0.2
0.06 0.3
0.4
Power (W) Depth (µm)
•
Higher Rsheet with increasing power and decreasing speed
•
Higher Rsheet after (UV, ps) vs (Green, ps) ablation
•
Lower doping surface concentration 4th Metallization Workshop - 2013, 7th of May
0.07
10
Process Flow SiNx
SiNx
cSi
Emitter formation Dielectric deposition Laser opening Chemical treatment
•
Various cleaning solutions & bath duration: •
Clean_1 & Clean_2: Surface defects cleaning
•
Clean_3: cSi light cSi etching
Ni plating NiSi formation
4th Metallization Workshop - 2013, 7th of May
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Laser ablated areas: Lifetime UV, ps
Surface defects
Green, ps
Power (W)
Speed (mm/s)
Clean_1 Clean_2 Clean_3 UV, ps Green, ps
X
X
ok
X
X
light
•
Improved lifetime for specific etching solution & laser ablation (highest power)
•
Complete ablation mandatory for damage etch
•
Deeper defects with green laser
4th Metallization Workshop - 2013, 7th of May
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Laser ablated areas: Lifetime Deep defects: Lifetime vs etching depth 500
Source : UV, ps laser Clean + new SiN deposition
400
ARC opening
Emitter depth Max initial lifetime
300
Lifetime (µs)
Clean_3 SiNx deposition
200
Min initial lifetime
0.22W 300mm/s 0.30W 300mm/s 0.12W 300mm/s
100
Lifetime
0 0.0
Etching depth
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Measured etching depth (µm)
•
Improved lifetime with etching depth
•
Defects deeper than emitter for P> 0.22W (300-600nm)
Lower laser power to be further studied (P~0.12W) 4th Metallization Workshop - 2013, 7th of May
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Process Flow SiNx
Emitter formation
Ni
cSi
Dielectric deposition Laser opening Chemical treatment
• Uniform deposition required
PVD, 50nm
Ni plating NiSi formation
4th Metallization Workshop - 2013, 7th of May
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Process Flow SiNx cSi
Emitter formation
Laser opening Chemical treatment Ni plating NiSi formation
Excimer Laser annealing: 308nm, 150ns • Large area beam >cm², with top-hat beam profile • Ni1Si1 formation w/o emitter shunting [1] • No passivation damages [2] Reflectivity at =308nm [%]
Dielectric deposition
Ni NixSiy
SiNx / SiOx / c-Si
60
SiNx / c-Si
50 40
50nm Ni
30 20 10 0 0
10
20
30
40
50
60
70
80
90
100
[1] L. Tous et al, Prog. Photovolt: Res. Appl. (2013) DOI: 10.1002/pip.2362 [2] J-F. Lerat et al, Proceedings 27th EUPVSEC
SiNx thickness [nm]
4th Metallization Workshop - 2013, 7th of May
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Nickel silicidation 1) NiSi formation on bare cSi (No SiNx ablation) 55 50
ELA 0.6J/cm² ELA 0.9J/cm² ELA 1.1J/cm²
45 40
Count (%)
35 30 25 20 15 10
ELA 0.6J/cm²
5
2013-Rauer-3rd SiliconPV
0 0
50
100
150
200
250
NiSi thickness (nm)
•
Smooth and continuous NixSiy layer from top to valleys of the pyramids
•
With increasing ELA Energy Density:
- Thicker NixSiy layer - Wider dispersion
4th Metallization Workshop - 2013, 7th of May
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Nickel silicidation 2) NiSi formation in ARC openings • 220
energy density
200
Ref cSi 0.055W 0.062W 0.070W Ni/cSi Ni/SiNx
180
Rsheet (Ohm/sq)
160 140 120 100 80 60 40
Ni2Si NiSi2
NiSi
20 0 0.2
Typical Rsheet decrease with increasing ELA
0.4
0.6
0.8
1.0
1.2
1.4
•
Formation of various NixSiy silicide
•
Nitride = barrier layer to NiSi formation
•
After green, ps laser ablation: •
Similar trend as reference
•
Lower Rsheet for lower laser power
Effect of the surface doping concentration? 1.6
1.8
2.0
ELA Energy density (J/cm²)
4th Metallization Workshop - 2013, 7th of May
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Nickel silicidation 2) NiSi formation in ARC openings
Green, ps + ELA 1.1J/cm²
25
No ablationps + ELA 1.1J/cm² Green,
Green, ps (0.22W) + ELA (1.1J/cm²)
Count (%)
20
15
10
5
Green, ps + ELA 1.1J/cm²
0 0
50
100
150
200
250
NiSi thickness (nm)
•
Effective NiSi formation all over the surface
•
Similar NiSi thickness dispersion
4th Metallization Workshop - 2013, 7th of May
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Summary Conditions of 1) ablation, 2) cleaning/etching and 3) silicidation interdependent Laser ablation: No complete ablation w/o induced defects into cSi
Cleaning/etching Etching step required for damage removal Deeper emitter required? Partial ablation combined with etching could lead to no defects, esp. UV laser
NiSi formation by ELA: Effective NiSi formation on textured silicon and laser ablated areas Thin NiSi layers (