Preservation of Si Surface Structure by Ag/Al Contact Spots – an...
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Preservation of Si Surface Structure by Ag/Al Contact Spots – an Explanatory Model S. Fritz 1 , S. Riegel 1 , A. Herguth 1 , M. König 2 , M. Hörteis 2 , G. Hahn 1 1 Universität Konstanz 2 Heraeus Precious Metals GmbH & Co Heraeus Precious Metals GmbH & Co. 5 th Metallization Workshop Konstanz, 20 10 2014 5 th Metallization Workshop, Konstanz 2014 S. Fritz 1 20.10.2014
Transcript of Preservation of Si Surface Structure by Ag/Al Contact Spots – an...
Preservation of Si Surface Structure by Ag/Al Contact Spots – an Explanatory Model
S. Fritz1, S. Riegel1, A. Herguth1, M. König2, M. Hörteis2, G. Hahn1
1Universität Konstanz2Heraeus Precious Metals GmbH & CoHeraeus Precious Metals GmbH & Co.
5th Metallization Workshop Konstanz, 20 10 2014
5th Metallization Workshop, Konstanz 2014 S. Fritz1
20.10.2014
Motivation
G i i t t i t ti + Si ith i ti t
• Contact formation of pure Ag pastes to n+ emitters well
• Growing interest in contacting p+ Si with screen-printing pastes, but only basic understanding of contact formation
• Contact formation of pure Ag pastes to n emitters well investigated
• Ag screen-printing pastes on p+ Si: specific contact resistanceAg screen printing pastes on p Si: specific contact resistance ϱc > 20 mΩcm2
• Addition of Al to paste reduces ϱc[1]
• Problem of Al addition: deep metal spikes can penetrate emitter
Better understanding of contact formation process of Al-containing Ag screen-printing pastes necessary
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[1] Kopecek et al., Proc. 15th Int. Photovoltaic Science and Engineering Conf., Shanghai, 2005.
.
Outline
S l ti (3 diff t t t )• Sample preparation (3 different textures)
• SEM and EDX analysis- Top view analysis
- Cross-sectional analysis
• Model for contact formation
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Sample Preparation
3 Ωcm n-type Cz-Si
BBr3 diffusion (50 Ω/, Nsurface ≈ 3·1019 cm-3)
Iso-textureAlkaline texture Plasma texture
SiNx:H (75 nm, PECVD)
Screen-printing Ag/Al paste
Contact firing (Tpeak,wafer ≈ 800°C)
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Characterization
Mech Polished
Top View Samples Cross Sections
HF Aqua regiaHF Aqua regia Mech. Polished
SEM and EDX Analysis
HF Aqua regiaHF Aqua regia
Hydrofluoric acid Aqua regiaHydrofluoric acidEtches glass layer, bulk metal on top of glass is also removed
Aqua regiaAqua regia etches only bulk metal
Si surface + residual metal Glass layer remains Si surface
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Top View Analysis
Sample etched in hydrofluoric acid
Alkaline textured surface
• Ag/Al contact spots in shape of pyramids grown into Si surface
Si
into Si surface
Ag/Al contactcontact spots
2 µm
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Top View Analysis
Sample etched in hydrofluoric acid
Alkaline textured surface
• Ag/Al contact spots in shape of pyramids grown into Si surface
Si
into Si surface
• Sharp edged Si pyramids around contact spots – notaround contact spots – not etched by glass frit
2 µm
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Top View Analysis
Sample etched in hydrofluoric acid
Alkaline textured surface
• Ag/Al contact spots in shape of pyramids grown into Si surface
Si
into Si surface
• Sharp edged Si pyramids around contact spots – notaround contact spots – not etched by glass frit
• Rounded Si pyramidsRounded Si pyramids away from contact spots2 µm
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Top View Analysis
Sample etched in hydrofluoric acid
Plasma textured surface
• Ag/Al contact spots: surface shows small holes like plasma texture
Si
like plasma texture
2 µm
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Top View Analysis
Sample etched in hydrofluoric acid
Plasma textured surface
• Ag/Al contact spots: surface shows small holes like plasma texture
Si
like plasma texture
• Si surface not corroded close to contact spots –close to contact spots –Si not etched by glass frit
2 µm
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Top View Analysis
Sample etched in hydrofluoric acid
Plasma textured surface
• Ag/Al contact spots: surface shows small holes like plasma texture
Si
like plasma texture
• Si surface not corroded close to contact spots –close to contact spots –Si not etched by glass frit
• Corroded Si surfaceCorroded Si surface away from contact spots2 µm
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Top View Analysis
Sample etched in hydrofluoric acid
Iso-textured surface
• Ag/Al contact spots surface shows sharp edges of iso texture
Si2 µm
of iso-texture
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Top View Analysis
Sample etched in hydrofluoric acid
Iso-textured surface
• Ag/Al contact spots surface shows sharp edges of iso texture
Si2 µm
of iso-texture
• Si: sharp-edged structure of iso-texture around contactiso-texture around contact spots
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Top View Analysis
Sample etched in hydrofluoric acid
Iso-textured surface
• Ag/Al contact spots surface shows sharp edges of iso texture
Si2 µm
of iso-texture
• Si: sharp-edged structure of iso-texture around contactiso-texture around contact spots
• Corroded Si surfaceCorroded Si surface away from contact spots
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Top View Analysis
Samples etched in hydrofluoric acidSamples etched in hydrofluoric acid
Iso-texturedPlasma texturedAlkaline textured
• Ag/Al contact spots show surface structure of Si texture
• Si surface not etched by glass frit close to contact spots
• Corroded Si surface away from contact spots
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• Corroded Si surface away from contact spots
Top View Analysis
Samples etched in aqua regia
5 µm
Plasma textured surface
5 µm
Iso-textured surface
Si5 µm
Si
5 µm
Si
Glass Glass
• Opening in glass layer: inverted Si pyramids
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Top View Analysis
Samples etched in aqua regia
5 µm
Plasma textured surface
5 µm
Iso-textured surface
5 µm 5 µm
Glass Glass
• Opening in glass layer: inverted Si pyramids• Si spots surrounded by bright glass, Al & N detected
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Top View Analysis
Samples etched in aqua regia
5 µm
Plasma textured surface
5 µm
Iso-textured surface
5 µm 5 µm
Glass Glass
• Opening in glass layer: inverted Si pyramids• Si spots surrounded by bright glass, Al & N detected
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• Away from Si spots: dark glass, no Al & N
Top View Analysis
Sample etched in aqua regiaSample etched in hydrofluoric acid p q gSample etched in hydrofluoric acid
S/ inverted Si pyramidsAg/Al contact spots
intact Si surface texture bright glass, Al & N
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dark glass, no Al & Ncorroded Si surface texture
Cross-Sectional Analysis
Two regions can be distinguished:
• Homogeneous microstructure
Two regions can be distinguished:
g
• Intact or no Al particles
• Pure Ag phaseAg
Pure Ag phase
Al
Si 5 µm
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Cross-Sectional Analysis
Two regions can be distinguished:
Ag/AGlass • Homogeneous microstructure
Two regions can be distinguished:
Ag/AGlass with Al
g
• Intact or no Al particles
• Pure Ag phasePure Ag phase• Inhomogeneous microstructure
• Molten Al particles mix with
Ag/AMolten Al particles mix with silver Ag/Al phase
• Glass containing AlSi 5 µm
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Cross-Sectional Analysis
• Ag/Al contact spots below• Ag/Al contact spots below Al-containing parts of finger2 µm
Ag/AlSi
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More details: Fritz et al., IEEE J. Photovoltaics, accepted for publication.
Cross-Sectional Analysis
• Ag/Al contact spots below• Ag/Al contact spots below Al-containing parts of finger
• No Ag/Al contact spots below 2 µm
g pparts of finger without Al
No contact spotsSi
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More details: Fritz et al., IEEE J. Photovoltaics, accepted for publication.
Cross-Sectional Analysis
• Ag/Al contact spots below• Ag/Al contact spots below Al-containing parts of finger
• No Ag/Al contact spots below 2 µm
g pparts of finger without Al
• N detected between Al containing parts of finger and Si surface / Ag/Al contact spots (thin layer) + N p ( y )+ N
Si
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More details: Fritz et al., IEEE J. Photovoltaics, accepted for publication.
Cross-Sectional Analysis
• Ag/Al contact spots below• Ag/Al contact spots below Al-containing parts of finger
• No Ag/Al contact spots below 2 µm
g pparts of finger without Al
• N detected between Al containing parts of finger and Si surface / Ag/Al contact spots (thin layer) + N p ( y )
• No N detected below parts of contact without Al
+ Nno NSi
Conclusion: SiNx:H layer not completely etched away
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More details: Fritz et al., IEEE J. Photovoltaics, accepted for publication.
Cross-Sectional Analysis
Where does the Si go?
2 µm
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Cross-Sectional Analysis
Where does the Si go?• Dark spots and lamellas
in Ag/Al phase of inhomo-geneous contact region
2 µmgeneous contact region contain high amount of Si
• No Si in Ag/Al phase Si
g paround Si spots
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Cross-Sectional Analysis
Where does the Si go?• Dark spots and lamellas
in Ag/Al phase of inhomo-geneous contact region
2 µmgeneous contact region contain high amount of Si
• No Si in Ag/Al phase
• Hole in SiNx:H layer
g paround Si spotsResidual
SiNx:H layerx y
direct connection between contact spot and metal finger
Conclusion: Exchange of material through opening in SiNx:H layer
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Si precipitates in Ag/Al phase during cooling down
Cross-Sectional Analysis
How does this result in the surface structure of the contact spots?
• Ag/Al contact spots grow through openings in SiNx:H layer
As SiN H is deposited on te t red Si s rface it presents a mo ld of
• Contact spots expand below layer, growth stops there
• As SiNx:H is deposited on textured Si surface it presents a mould of Si surface
Conclusion:
p p y , g p
Conclusion: SiNx:H acts as a mould for the growth of the Ag/Al contact spots giving them the structure of the Si surface
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Model for Contact Formation
1. Before firing 2. T ≥ 660°C 3. T ≥ 660°C
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4. T = 660-800°C 5. T = 660-800°C 6. After cool downMore details: Fritz et al., IEEE J. Photovoltaics, accepted for publication.
Model for Contact Formation
1. Before firing 2. T ≥ 660°C 3. T ≥ 660°C
Ag/Al adopts g pformer Si surface
shape
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4. T = 660-800°C 5. T = 660-800°C 6. After cool downMore details: Fritz et al., IEEE J. Photovoltaics, accepted for publication.
Model for Contact Formation
1. Before firing 2. T ≥ 660°C 3. T ≥ 660°C
intact Si surface
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4. T = 660-800°C 5. T = 660-800°C 6. After cool downMore details: Fritz et al., IEEE J. Photovoltaics, accepted for publication.
Summary
• SEM and EDX analysis of Ag/Al screen-printed S a d a a ys s o g/ sc ee p edcontacts on differently structured surfaces
• Model for contact formation of Al-containing Ag screen-printing pastes through SiNx:H layer
Inhomogeneous microstructure in Al containing part of contactAl-containing part of contact
Ag/Al contact spots grow into Si surface exclusively below Al-containing parts ofexclusively below Al containing parts of contact
SiNx:H layer not completely etched away contact spots grow through openings in
the layer below SiNx:H SiNx:H acts as mould of the Si surface
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xstructure for the Ag/Al contact spots
Thank you for your attention!Thank you for your attention!
Part of this work was supported by the German Federal Ministry for thePart of this work was supported by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety
(FKZ 0325426 and FKZ 0325581).
See you in Konstanz in March 2015!
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See you in Konstanz in March 2015!
