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Title: Analysis of Al diffusion processes in TiN barrier layers for the application in silicon solar cell metallization

Abstract

An evaporated Al layer is known as an excellent rear metallization for highly efficient solar cells, but suffers from incompatibility with a common solder process. To enable solar cell-interconnection and module integration, in this work the Al layer is complemented with a solder stack of TiN/Ti/Ag or TiN/NiV/Ag, in which the TiN layer acts as an Al diffusion barrier. X-ray photoelectron spectroscopy measurements prove that diffusion of Al through the stack and the formation of an Al{sub 2}O{sub 3} layer on the stack's surface are responsible for a loss of solderability after a strong post-metallization anneal, which is often mandatory to improve contact resistance and passivation quality. An optimization of the reactive TiN sputter process results in a densification of the TiN layer, which improves its barrier quality against Al diffusion. However, measurements with X-ray diffraction and scanning electron microscopy show that small grains with vertical grain boundaries persist, which still offer fast diffusion paths. Therefore, the concept of stuffing is introduced. By incorporating oxygen into the grain boundaries of the sputtered TiN layer, Al diffusion is strongly reduced as confirmed by secondary ion mass spectroscopy profiles. A quantitative analysis reveals a one order of magnitude lower Al diffusion coefficientmore » for stuffed TiN layers. This metallization system maintains its solderability even after strong post-metallization annealing at 425 °C for 15 min. This paper thus presents an industrially feasible, conventionally solderable, and long-term stable metallization scheme for highly efficient silicon solar cells.« less

Authors:
; ; ; ;  [1]
  1. Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, 79110 Freiburg (Germany)
Publication Date:
OSTI Identifier:
22597814
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 120; Journal Issue: 2; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ALUMINIUM; ALUMINIUM OXIDES; ANNEALING; DEPLETION LAYER; DIFFUSION BARRIERS; GRAIN BOUNDARIES; ION MICROPROBE ANALYSIS; IONS; MASS SPECTROSCOPY; OXYGEN; PASSIVATION; SCANNING ELECTRON MICROSCOPY; SILICON SOLAR CELLS; SPUTTERING; SURFACES; TEMPERATURE RANGE 0400-1000 K; TITANIUM NITRIDES; X-RAY DIFFRACTION; X-RAY PHOTOELECTRON SPECTROSCOPY

Citation Formats

Kumm, J., Samadi, H., Chacko, R. V., Hartmann, P., and Wolf, A., E-mail: andreas.wolf@ise.fraunhofer.de. Analysis of Al diffusion processes in TiN barrier layers for the application in silicon solar cell metallization. United States: N. p., 2016. Web. doi:10.1063/1.4954684.
Kumm, J., Samadi, H., Chacko, R. V., Hartmann, P., & Wolf, A., E-mail: andreas.wolf@ise.fraunhofer.de. Analysis of Al diffusion processes in TiN barrier layers for the application in silicon solar cell metallization. United States. doi:10.1063/1.4954684.
Kumm, J., Samadi, H., Chacko, R. V., Hartmann, P., and Wolf, A., E-mail: andreas.wolf@ise.fraunhofer.de. 2016. "Analysis of Al diffusion processes in TiN barrier layers for the application in silicon solar cell metallization". United States. doi:10.1063/1.4954684.
@article{osti_22597814,
title = {Analysis of Al diffusion processes in TiN barrier layers for the application in silicon solar cell metallization},
author = {Kumm, J. and Samadi, H. and Chacko, R. V. and Hartmann, P. and Wolf, A., E-mail: andreas.wolf@ise.fraunhofer.de},
abstractNote = {An evaporated Al layer is known as an excellent rear metallization for highly efficient solar cells, but suffers from incompatibility with a common solder process. To enable solar cell-interconnection and module integration, in this work the Al layer is complemented with a solder stack of TiN/Ti/Ag or TiN/NiV/Ag, in which the TiN layer acts as an Al diffusion barrier. X-ray photoelectron spectroscopy measurements prove that diffusion of Al through the stack and the formation of an Al{sub 2}O{sub 3} layer on the stack's surface are responsible for a loss of solderability after a strong post-metallization anneal, which is often mandatory to improve contact resistance and passivation quality. An optimization of the reactive TiN sputter process results in a densification of the TiN layer, which improves its barrier quality against Al diffusion. However, measurements with X-ray diffraction and scanning electron microscopy show that small grains with vertical grain boundaries persist, which still offer fast diffusion paths. Therefore, the concept of stuffing is introduced. By incorporating oxygen into the grain boundaries of the sputtered TiN layer, Al diffusion is strongly reduced as confirmed by secondary ion mass spectroscopy profiles. A quantitative analysis reveals a one order of magnitude lower Al diffusion coefficient for stuffed TiN layers. This metallization system maintains its solderability even after strong post-metallization annealing at 425 °C for 15 min. This paper thus presents an industrially feasible, conventionally solderable, and long-term stable metallization scheme for highly efficient silicon solar cells.},
doi = {10.1063/1.4954684},
journal = {Journal of Applied Physics},
number = 2,
volume = 120,
place = {United States},
year = 2016,
month = 7
}
  • Conducting diffusion barrier layers play a critical role in high-density memory integration. We recently demonstrated that Ti--Al can be used as a diffusion barrier layer for the integration of ferroelectric capacitors with complementary metal--oxide semiconductor devices for the fabrication of nonvolatile ferroelectric random access memories (NVFRAMs). Here, we discuss results from systematic studies designed to understand Ti--Al film growth and oxidation processes using in situ ion beam sputter deposition in conjunction with complementary in situ atomic layer-resolution mass spectroscopy of recoil ion (MSRI) and surface sensitive x-ray photoelectron spectroscopy (XPS). The concurrent MSRI/XPS analysis revealed that amorphous Ti--Al layers producedmore » by tailored sputter-deposition methods are resistant to oxidation to at least 600{sup o}C, and that oxidation occurs only when the a-Ti--Al layers are exposed to oxygen at >600{sup o}C, via the segregation of Ti species to the surface and TiO{sub 2} formation. The a-Ti--Al layers discussed in this letter could be used in the double functionality of a bottom electrode/diffusion barrier for the integration of ferroelectric capacitors with Si substrates for the fabrication of NVFRAMs and other devices. {copyright} 2001 American Institute of Physics.« less
  • The properties of gold--silicon and gold oxide--silicon Schottky-barrier solar cells are compared. With an optimum oxide thickness =19 AA the conversion efficiency of the MOS device is 35% greater than that of the corresponding M-S device.
  • We have studied the matrix effects in Si{sub 1-x}Ge{sub x} structures under O{sub 2}{sup +} and Cs{sup +} bombardments. Matrix effects are practically suppressed with Cs{sub 2}Ge{sup +} secondary ions, for Ge concentrations between 0 and 100 at. %. A procedure for the accurate quantification of the Ge concentration in Si{sub 1-x}Ge{sub x} alloys using Cs{sub 2}Ge{sup +} and CsGe{sup +} clusters has been proposed. For structures in which the Ge content is constant over several hundreds of nanometers, both methods provide very similar results, with an excellent agreement between the Ge concentrations measured by secondary ions mass spectrometry andmore » x-ray diffraction. However, for continuously varying Ge concentration profiles, the nonlinear response of the CsGe{sup +} normalized intensity and the persistence of strong matrix effects for CsSi{sup +} ions lead to differences between the Ge concentration profiles measured with the CsGe{sup +} method compared to the Cs{sub 2}Ge{sup +} one. The latter is therefore the only reliable method for the study of Ge indiffusion into Si from a pure Ge layer grown by chemical vapor deposition. An application of this method to the analysis of Ge indiffusion in Si at 900 deg. C is also reported.« less
  • A novel tungsten nitride (WN{sub x}) film for diffusion barrier applications has been prepared by nitridation of a fine grain chemical vapor deposited tungsten (CVD-W) film. The fine grain CVD-W is deposited at 300{degree}C in a low pressure chemical vapor deposition reactor with a SiH{sub 4}/WF{sub 6} flow rate of 12.5/5 sccm under a total gas pressure of 100 mTorr. The subsequent nitridation process is executed in nitrogen plasma at 300{degree}C without breaking vacuum. The thickness of WN{sub x} layer as examined by secondary ion mass spectroscopy is 50 nm after 5 min exposure to nitrogen plasma. X-ray photoelectron spectroscopymore » spectra shows that the atomic ratio of tungsten to nitrogen in WN{sub x} layer is 2:1. According to the analysis by Auger electron spectroscopy and the measurement of n{sup +}p junction leakage current, the Al/WN{sub x}/W/Si multilayer maintains excellent interfacial stability after furnace annealing at 575{degree}C for 30 min. The effectiveness of W{sub 2}N barrier is attributed to stuffing grain boundaries with nitrogen atoms which eliminates the rapid diffusion paths in fine grain CVD-W films. {copyright} {ital 1997 American Institute of Physics.}« less