Correlation between SiNx laser ablation and nickel silicide formation ...

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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

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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

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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

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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

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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

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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

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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?

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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

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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

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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

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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]

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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

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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)

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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 (