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
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
Motivation: High efficiency cells Low surface doping (Ns) Lower joe for good passivated surfaces V oc, LIMIT kt j sc = 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
Contact emitters with higher efficiency potential (low Ns) High N s allows tunneling, but for low N s 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?
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 [φ B ] as function of the metal work function [φ B ] for metal/n-si and metal/p-si contacts Source: D. Schröder Solar cell contact resistance- A review-, IEEE trans. Electron devices ED31 (1984)
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
A look into the IC technology: Silicide formation Chemical bond: metal & semiconductor What happens to and with the interface?? Metal Silicon Temperature Diffusion Interface reactions Metal Silicide Silicon
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 Ni 2 Si NiSi NiSi 2 Type of sample and orientation Thin film Thin film Thin film ρ (293K) [µω cm] 24 10.5 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
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!
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
Diffusion coefficient for nickel in silicon Strong variation of the diffusion coefficient for low Vs. high temperatures Arrhenius equation D = D 0 e E a k T Diffusion length L = D t Diffusion coefficient cm 2 /s 1E-3 1E-6 1E-9 1E-12 1E-15 1E-18 1E-21 At which T does the diffusion process change? Substitutional 1E-24 0 200 400 600 800 1000 1200 1400 Temperature [C] Interstitial Substitutional Interstitial 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
Example: Diffusion length From the theory for evaporated nickel layers on intrinsic silicon Diffusion length [nm] 10 7 10 5 10 3 10 1 10-1 Ref. line: Emitter 300nm Susbt. / Interstitial / 1 sec / 1 min / 10 min With an intrinsic diffusion after 10 minutes @ 600C nickel would go to the other side of the wafer and back! With the substitutional diffusion After 10 min @ 600 C the metal would reach 300 nm within the silicon 10-3 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.
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 V OC j SC FF η Seed [mv] [ma/cm²] [%] [%] E-less Ni 614.1 35.6 77.7 16.7 M. Alemán, et al, EUPVSEC, Valencia 2008
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 @ N d >10 20 cm -3 Metal diffuses very fast in the junction M. Alemán, et al, EUPVSEC, Valencia 2008
Deeper junction / Reference process: 120Ω/sq Emitter Source for doping profile from Oliver Schulz PhD
Evaporated references Low Ns concentration FZ 120Ω/sq Emitter SiO 2 rear passivation SiO 2 as front ARC Front evaporated metals Rear evaporated Al + LFC Area 4 cm 2 Thickened by LIP Evaporated Metals Ti-Pd-Ag Ni-Ag Al-Ag V OC j SC FF η [mv] [ma/cm²] [%] [%] 663.1 38.8 81.3 20.9 663.0 38.8 81.6 21.0 662.5 38.7 81.2 20.8 Ti-Ag 662.9 38.9 81.5 21.0 Cr-Ag 662.5 39.0 81.2 21.0 Source: Ansgar Mette s PhD Uni- Freiburg 2007 Very good results with different evaporated metals converted into silicides!
120Ω/sq Emitter + E-less Ni Plating + Ag LIP + LFC rear Low Ns concentration FZ 120Ω/sq Emitter SiO 2 rear passivation SiO 2 as front ARC Structuring: Photolithography Evaporated Al LFC rear-side Area 4 cm 2 Thickened by LIP Eless Ni EQE, IQE, R Seed [mv] [ma/cm²] [%] [%] 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 V OC 669.1 eqe iqe reflexion j SC 38.1 FF 80.4 η 20.5 0,0 300 400 500 600 700 800 900 1000 1100 1200 λ [nm]
Structuring the dielectric layer: Inkjet printing resist + wet etching Narrow lines possible ~20µm High throughput Low cost M. Alemán, et al, EUPVSEC Valencia, 2008
Structuring the dielectric layer: Laser Chemical Processing (LCP) Laser guided in a solution (with P) Process: Opening SiN x & local high doping with LCP SiN x Seed layer + LIP to finish The doping occurs while opening the dielectric A deep emitter is formed! 120 Ω/sq Emitter 1 open + dope 2 seed 3 growth D. Kray et al., S. Diego 33rd PV Conference 2008
Structuring the dielectric layer: Laser ablation Structuring of ~8µm lines possible Ablation of oxides and nitrides with UV Lasers 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
Structuring the dielectric layer Yet other ideas: Depositing the nitride through a mask Printing an etching paste Any other?? Open the nitride and deposit metal with the laser during the same step?
Electrical contacts by Laser micro-sintering Using alternative materials: From metal powders Achieved: Very good aspect ratio Width ~40µm - Thickness ~15-18 µm 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.
Laser-induced metal deposition from an electrolyte Metal is in an electrolyte with or without reducing agents A contact is formed through the nitride Well defined surface for the laser process Laser Glas Contacts Wafer Metal salt D. Rudolph, EUPVSEC-23 Valencia 2008
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
Laser-induced metal deposition from an electrolyte First results! 1Ω cm Cz p-type 50Ω/sq emitter Al-BSF Rear SiN x as ARC No texture Results V oc j sc FF η [mv] [ma/cm 2 ] [%] [%] NiCl 2 589.6 31.1 47.7 8.8 NiSO 4 594.9 33.4 67.7 13.4 D. Rudolph, EUPVSEC-23 Valencia 2008
Conclusions Emitters with low surface doping (N s ) allow higher efficiency potential Contacting low Ns emitters is possible by using materials like Ti, Ni, W Shunting shallow emitters while sintering with e-less Nickel plating is a risk! Independent of the surface concentration (in the range analyzed) Higher resistance to the process with deeper junctions With electroless nickel plating & lowly doped emitters, solar cells have been manufactured reaching FF = 80.5% & η = 20.5% Alternative structuring processes for the ARC are being developed right now! We re working on several ideas!! A very innovative technology is presented by the laser-induced deposition of metal from an electrolyte. Encouraging results have been achieved!
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!