Front-side metallization beyond silver paste:
Silicide formation / alternative technologies
Mónica Alemán, N. Bay, D. Rudolph, T. Rublack,
S. W. Glunz
Fraunhofer-Institute for Solar Energy Systems
ISE
Metallization workshop Crystal Clear
Utrecht, October 1st 2008
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Outlook
• Motivation: Low surface doping conc. (Ns) for high efficiencies
• Contact technology : Theory and praxis
• Interconnection technology
–
Silicide formation
–
Nickel silicides
–
Diffusion of Nickel in Silicon
• Alternatives
–
–
Electroless plating
•
Evaluation for shallow/ deep emitter
•
Application to industrial processes: Structuring of dielectric layers
Laser-induced metal deposition from a solution
•Conclusions
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Motivation: High efficiency cells
ƒ Low surface doping (Ns) Lower joe
for good passivated surfaces
Voc , LIMIT =
⎞
kT ⎛ jsc
ln⎜⎜
+ 1⎟⎟
q ⎝ joe ⎠
Check out also the work from:
King, Glunz, Sterk, Cuevas on emitters for high efficiency wafers
Source: Oliver Schultz PhD Uni- Konstanz 2005
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Contact emitters with higher efficiency potential (low Ns)
High Ns allows tunneling, but for low Ns
Lower contact resistances required!!
(as presented by A. Mette today)
How to contact lowly doped emitters?
-
Altering the Ag pastes to increase the crystallites # (or some other effects?)
(check out G. Schubert, D. Pysch, A. Mette, current research by M. Hörteis, A. Ebong)
-
Formation of the seed layer with alternative materials?
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Contact technology: Theory metal- silicon
FOCUS: Establishing the best contact to silicon
Barrier height also depends on
deposition method, and processing
due to differences in the interface
Theory ≠ Praxis!
“An Ohmic contact requires as small a barrier height as possible”
Measured Barrier heights [φB] as function of the metal work function [φB] for metal/n-Si and metal/p-Si contacts
Source: D. Schröder Solar cell contact resistance- A review-, IEEE trans. Electron devices ED31 (1984)
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Contact technology: What’s possible?
• Physical Vapor Deposition
–
–
Evaporation (low melting temperature materials)
Sputtering (many different metals)
• Chemical Vapor Deposition
–
Atomic Layer deposition (Cu)
• Chemical deposition
– Electroless plating : Co, Cu, Ni, Pd
–
Electrochemical plating: Ag, Cu
• Printing technologies (Ag pastes)
–
Screen-/ Tampon- / Aerosol- / InkJet- printing
• Laser enhanced deposition
–
–
–
Solid (powders) / or thin metal layers (Al, W, Co, Cr, Ni)
Solutions (Ni, Cu, Ag)
Gases
Front-side metallization beyond silver paste: silicide formation/alternative technologies
A look into the IC technology: Silicide formation
Chemical bond: metal & semiconductor
What happens to and with the interface??
Metal
Temperature
Silicon
Diffusion
Interface reactions
Metal
Silicide
Silicon
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Electrical properties of different silicides
Typical resistivity of common silicides used in IC technology (PVD)
Source: Nicolet, M.A., and Lau, S.S., “Formation and characterization of transition metal silicides” in
VLSI Electronics: Microstructure Science, Vol 6 , 1983
Compound
Type of sample
and orientation
ρ (293K)
[µΩ cm]
Ni2Si
Thin film
24
NiSi
Thin film
10.5
NiSi2
Thin film
34
Source: Maex, K., and Van Rossum, M., “Properties of Metal
silicides”, published by INSPEC , UK, 1995
Different ρ values for NiSi from different sources
Usually given as 13 µΩ.cm
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Nickel Silicide Formation
Bulk Reactions (thermodynamics)
vs.
Thin Film reaction (kinetics)
Non equilibrium
Source: Ottaviani, G., J. Vac Sci Technol. 116, 1112 (1979)
Surface preparation is a key parameter for the formation of silicides!!
Interface plays a major role!
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Diffusion in Silicon
Substitutional diffusion
The metal takes the place of the silicon in the crystal
Start at low temperatures
Slow process
Affected by vacancies density & defects
Interstitial diffusion
The metal goes through the lattice
higher temperatures
Critical for material optimization: Gettering
and material quality improvements
Very fast process
Not affected by the doping concentration
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Diffusion coefficient for nickel in silicon
Strong variation of the diffusion coefficient for low Vs. high temperatures
ion
s
fu
dif
he ge?
t
es han
o
d c
T ss
h
ic oce
h
w pr
At
1E-3
1E-6
L = D⋅t
1E-9
ti o
na
l
1E-12
itu
1E-15
Su
bs
t
Diffusion length
2
⎛ −Ea ⎞
⎜
⎟
⋅
k
T
⎝
⎠
D =D0 ⋅ e
Diffusion coefficient cm /s
Arrhenius equation
Interstitial
1E-18
Substitutional
Interstitial
1E-21
1E-24
0
200
400
600
800
1000
1200
1400
Temperature [C]
Diffusion of Nickel according to P. Bonzel, Phys Status Solidi, 20 (1967) 493. Cited by
G.L.P Berning & Levenson, Thin solid films 55 (1978) 473-482
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Example: Diffusion length
From the theory for evaporated nickel layers on intrinsic silicon
7
10
With an intrinsic diffusion
after 10 minutes @ 600C
nickel would go to the other side of
the wafer and back!
5
Diffusion length [nm]
10
Ref. line: Emitter 300nm
3
10
With the substitutional diffusion
After 10 min @ 600 C
the metal would reach 300 nm within
the silicon
1
10
Susbt. / Interstitial
-1
10
/
1 sec
/
1 min
/
10 min
-3
10
0
200
400
600
800
1000
1200
1400
Temperature [C]
Coefficients from Graff “Metal impurities in silicon devices” & P. Bonzel, Phys Status Solidi, 20 (1967) 493.
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Shunting issues: shallow junctions
• Cz with 3 different emitter profiles
• ARC SiNx
• IJ masking + etching
• Al BSF rear
• Electroless Ni deposition (seed)
• LIP
Results with fully plated Ag finger
Seed
E-less Ni
VOC
jSC
FF
η
[mV]
[mA/cm²]
[%]
[%]
614.1
35.6
77.7
16.7
M. Alemán, et al, EUPVSEC, Valencia 2008
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Shunting issues: shallow junctions
Process
• Only ~1-2 µm Ag on top of the nickel
• Sintering with increasing temp.
• Measured with Suns_Voc and IV
Results
• FF improves due to a reduction of Rs
(until 300 C)
• pFF starts to decrease ~300 C
• Deeper emitter is more resistant
• No difference in resistance for the P
doping @ Nd>1020cm-3
Metal diffuses very fast in the junction
M. Alemán, et al, EUPVSEC, Valencia 2008
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Deeper junction / Reference process: 120Ω/sq Emitter
Source for doping profile from Oliver Schulz PhD
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Evaporated references
Evaporated
VOC
jSC
FF
η
Metals
[mV]
[mA/cm²]
[%]
[%]
Ti-Pd-Ag
663.1
38.8
81.3
20.9
Ni-Ag
663.0
38.8
81.6
21.0
Al-Ag
662.5
38.7
81.2
20.8
• Front evaporated metals
Ti-Ag
662.9
38.9
81.5
21.0
• Rear evaporated Al + LFC
Cr-Ag
662.5
39.0
81.2
21.0
• Low Ns concentration
• FZ 120Ω/sq Emitter
• SiO2 rear passivation
• SiO2 as front ARC
• Area 4 cm2
Source: Ansgar Mette’s PhD Uni- Freiburg 2007
• Thickened by LIP
Very good results with different evaporated
metals converted into silicides!
Front-side metallization beyond silver paste: silicide formation/alternative technologies
120Ω/sq Emitter + E-less Ni Plating + Ag LIP + LFC rear
• Low Ns concentration
• FZ 120Ω/sq Emitter
• SiO2rear passivation
Seed
• Area 4 cm2
• Thickened by LIP
EQE, IQE, R
• Structuring: Photolithography
• LFC rear-side
jSC
FF
η
[mV]
[mA/cm²]
[%]
[%]
38.1
80.4
20.5
Eless Ni 669.1
• SiO2 as front ARC
• Evaporated Al
VOC
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
eqe
iqe
reflexion
300 400 500 600 700 800 900 1000 1100 1200
λ [nm]
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Structuring the dielectric layer:
Inkjet printing resist + wet etching
ƒ Narrow lines possible
~20µm
ƒ High throughput
ƒ Low cost
M. Alemán, et al, EUPVSEC Valencia, 2008
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Structuring the dielectric layer:
Laser Chemical Processing (LCP)
Laser guided in a solution
(with P)
Process:
ƒ Opening SiNx & local high
doping with LCP
SiNx
ƒ Seed layer + LIP to finish
The doping occurs while
opening the dielectric
ƒ A deep emitter is formed!
120 Ω/sq Emitter
1
2
3
open +
dope
seed
growth
D. Kray et al., S. Diego 33rd PV Conference 2008
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Structuring the dielectric layer:
Laser ablation
ƒ Structuring of ~8µm lines
possible
ƒ Ablation of oxides and nitrides
with UV Lasers
ƒ Further diffusion of P within the
surface after laser process!!
Check:
S.A.G.D. Correia & K. Neckermann,
EUPVSEC-22, Milan 2007
A. Knorz, submitted to PiP 2008
V. Rana, EUPVSEC-23, Valencia, 2008
M. Alemán PVSEC-17, Fukuoka, 2007
SEM view of a laser ablated line with Nickel on top
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Structuring the dielectric layer
Yet other ideas:
ƒ Depositing the nitride through a mask
ƒ Printing an etching paste
ƒ Any other??
ƒ Open the nitride and deposit metal with the laser during the
same step?
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Electrical contacts by Laser micro-sintering
Using alternative materials:
From metal powders
Achieved:
ƒ Very good aspect ratio
Width ~40µm - Thickness ~15-18 µm
ƒ Good adhesion on textured
samples
M. Alemán: EUPVSEC-21, Dresden 2006
Inhomogeneous powder layer
T. Rublack: EUPVSEC-23 Valencia 2008
SEM section view: interface of a tungsten seed contact on top of a
silicon solar cell before plating.
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Laser-induced metal deposition from an electrolyte
ƒ Metal is in an electrolyte with or without reducing agents
ƒ A contact is formed through the nitride
ƒ Well defined surface for the laser process
Laser
Glas
Contacts
Wafer
Metal
salt
D. Rudolph, EUPVSEC-23 Valencia 2008
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Laser-induced metal deposition from an electrolyte
First experiments already show metal deposition
50µm
Microscope & SEM views of a metal finger on top of a silicon wafer after
laser processing
D. Rudolph, EUPVSEC-23 Valencia 2008
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Laser-induced metal deposition from an electrolyte
First results!
ƒ 1Ω cm Cz p-type
Results
Voc
jsc
FF
η
[mV]
[mA/cm2]
[%]
[%]
NiCl2
589.6
31.1
47.7
8.8
NiSO4
594.9
33.4
67.7
13.4
ƒ 50Ω/sq emitter
ƒ Al-BSF Rear
ƒ SiNx as ARC
ƒ No texture
D. Rudolph, EUPVSEC-23 Valencia 2008
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Conclusions
ƒ Emitters with low surface doping (Ns) allow higher efficiency potential
ƒ Contacting low Ns emitters is possible by using materials like Ti, Ni, W
ƒ Shunting shallow emitters while sintering with e-less Nickel plating is a risk!
ƒ
Independent of the surface concentration (in the range analyzed)
ƒ
Higher resistance to the process with deeper junctions
ƒ With electroless nickel plating & lowly doped emitters, solar cells have been
manufactured reaching FF = 80.5% & η = 20.5%
ƒ Alternative structuring processes for the ARC are being developed right now!
We’re working on several ideas!!
ƒ A very innovative technology is presented by the laser-induced deposition of metal
from an electrolyte. Encouraging results have been achieved!
Front-side metallization beyond silver paste: silicide formation/alternative technologies
Acknowledgments:
Antonio Leimenstoll, Elisabeth Schäffer, Luca Gautero, Andreas Grohe, Jan
Specht, Daniel Kray, Annerose Knorz, Rainer Neubauer, Dominik Barucha,
Sonja Seitz, Marc Retzlaff, Norbert Kohn, David Stuewe, Anke Herbolzheimer,
Denis Erath, Jonas Bartsch, Sybille Hopman, Kuno Mayer, Ansgar Mette, and
all the PV people at the Fraunhofer ISE for all the support and good work!