Improvement of IBAD-MgO texturing for high throughput of buffered substrate

doi:10.1016/j.physc.2011.05.100

Physica C: Superconductivity and its Applications

Volume 471, Issues 21–22, November 2011, Pages 963-965

https://doi.org/10.1016/j.physc.2011.05.100 Get rights and content

Abstract

The requirements from the market on two important factors of performance and cost need to be satisfied for commercialization of the coated conductors. Highly biaxially grain texturing with high production rate should be realized from the perspective of buffer layers processing. IBAD-MgO process is one of the major techniques which are possible to satisfy those requirements. The structure of our buffered substrate is IBS-GZO/IBAD-MgO/RFsputter-LaMnO₃/PLD-CeO₂. The PLD-CeO₂ process is the rate limiting and cost dominant one in this architecture. It is proposed that the self-texturing CeO₂ layer thickness could be reduced by optimization of the MgO processing due to higher MgO texturing and/or effective growth of self-texturing CeO₂. Influence of the IBAD beam conditions and deposition time has been studied to optimize the IBAD conditions. Optimized IBAD conditions were decided from the viewpoints of in-plane grain texturing and the stability to obtain high texturing on fabrication. The Δ $ϕ$ value of CeO₂ layer was improved from 4–5° to 3–3.5° by the optimization. This buffered substrate gave high and uniform I_c values of 524–565 A/cm-width for 50 m long GdBCO (1.5 μm) tape, indicating uniform distribution of Δ $ϕ$ (CeO₂). This improvement of Δ $ϕ$ (CeO₂) enables to reduce the CeO₂ thickness down to 300 nm without making Δ $ϕ$ (CeO₂) > 5°, which improves CeO₂ throughput from 10 m/h to 30 m/h. A 50 m long patch sample showed more uniform Δ $ϕ$ distribution around 4° even by high speed of 30 m/h as CeO₂ through-put. Highly and uniformly textured CeO₂ buffered substrate was obtained in 100 m long cost-effectively by optimization of IBAD-MgO processing.

Introduction

Coated conductors are composed of metal substrates, buffer layers, superconducting and stabilizing layers. Improvement of superconducting performance and cost is a major issue for practical applications of the coated conductors. The architecture of the buffer layers we have been developed is IBS-GZO/IBAD-MgO/RFsputter-LaMnO₃/PLD-CeO₂. In these four layers, IBAD-MgO gives biaxially texturing [1], [2] and CeO₂ cap layer improves the texturing [3], Δ $ϕ$ (CeO₂) value, to less than 5° by the self-texturing effect. The Δ $ϕ$ (CeO₂) value becomes smaller with thickening the CeO₂ layer. Smaller Δ $ϕ$ (CeO₂) helps improving I_c property, however, grain boundary angle <5° does not cause weak-link [4]. The thicker film usually requires longer deposition time resulting in decrease of production rate and increase of the deposition cost. The CeO₂ deposition process is the cost-dominant process among the buffer layers, therefore, reduction of CeO₂ thickness with maintaining high texturing is effective for cost reduction. In this study, IBAD-MgO processing has been studied to improve the grain alignment of MgO so that the CeO₂ production rate to obtain Δ $ϕ$ (CeO₂) < 5° became faster. The investigation of suitable fabricating condition of IBAD-MgO as template layer could promote effective appearance of CeO₂ self-texturing effect and helps stably making thin and high textured CeO₂ layer in long length fabrication.

Section snippets

Experimental

Two ion guns of 6 cm × 22 cm were used for the IBAD process. The IBAD assist beam current and deposition time have been varied to deposit MgO on Gd–Zr–O (110 nm) materials on metal substrate (100 μm): Sputter beam voltage and current were 1200 V and 300 mA, Assist beam voltage and current were 950 V and 90–125 mA. Deposition time was varied from 30 to 150 s. The MgO surface morphologies were observed by AFM with 1 μm square area. LMO (10 nm) and CeO₂ (500 nm) layers were deposited on the IBAD-MgO one.

Results and discussion

Fig. 1 shows the relationship between the assist beam current and the Δ $ϕ$ value of CeO₂ layer. MgO surface images (1 μm × 1 μm) and roughness (Ra) for each data point are shown as well. Ra value decreases monotonously with increase of the assist current value from 1.13 nm to 0.27 nm. The assist ion beam has two effects of texturing and sputtering on deposition of MgO layer. Due to the sputtering effect, higher assist beam current results in a thinner film. The Ra value of the Gd–Zr–O layer was 0.24 nm

Conclusion

In this study, IBAD-MgO processing has been studied to improve the grain alignment of MgO layer so that the CeO₂ production rate to obtain Δ $ϕ$ < 5° became higher. We attempted short samples prepared by several IBAD beam conditions and deposition time. IBAD conditions were optimized from the viewpoints of the Δ $ϕ$ (CeO₂) and the stability to obtain high grain texturing. As a result, by optimization of IBAD assist beam current and deposition time, Δ $ϕ$ (MgO) value was improved from 17.4 to 12.4°, measured

Acknowledgment

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).

References (4)

Y. Yamada et al.
Physica C
(2009)
M. Yoshizumi et al.
Physica C
(2009)

There are more references available in the full text version of this article.

Cited by (3)

Effects of sputtering and assisting ions on the orientation of titanium nitride films fabricated by ion beam assisted sputtering deposition from metal target
2016, Materials Letters
Citation Excerpt :
IBAD-MgO require only 10 nm to develop texture, whereas another usual buffer layer, yttria-stabilized zirconia, needs at least 500 nm. Therefore IBAD-MgO buffer layer have been investigated extensively [5–9]. However, the coated conductors based on IBAD-MgO templates cannot achieve self-protection from quench, for MgO buffer layer is insulating.
Ion beam assisted titanium nitride (TiN) film has attracted much attention because its fast texture and high conductivity can be effectively applied in all-conductive superconducting coated conductor. In this work, TiN films were prepared by ion beam sputtering deposition from a metal titanium target. Effects of sputtering ion energy, assisting ion energy, assisting ion current and deposition temperature on the orientation and surface morphology were analyzed. The results indicate that assisting ion is an important factor in orientation selection, and high assisting ions and low assisting ion current could enhance the crystallinity. However, too high assisting ion energy and current can destroy the crystallinity in IBAD–TiN. This orientation selection can be attributed to the energy exchange between assisting ions and adatoms.
Orientation selection in MgO thin films prepared by ion-beam-deposition without oxygen gas present
2015, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Citation Excerpt :
IBAD–MgO buffer has great efficiency for its fast formation (only 10 nm needed) of biaxial texture [15,16]. Much work has been done to investigate the texturing process of IBAD–MgO [17–21], and improvement in the texture and surface of MgO films has been achieved. Current understanding of IBAD texturing in MgO has been reviewed recently by Matias and Hammond [22].
Ion assisted MgO film has attracted much attention because of its extensive application. Most of the related work was focused on ion assisted evaporation. In this work, MgO thin films were prepared by ion beam sputtering deposition without oxygen gas present. Effects of sputtering energy, deposition temperature, deposition angle and assisting ions on the orientation were analyzed. The orientation of MgO films is found strongly dependent on these parameters. As the parameters vary, (1 1 0), (1 1 1), (1 0 0) orientation is preferred in turn. The results surface morphology indicate that assisting ions can flatten the film surface and enhance the crystallinity. This orientation selection can be attributed to the energy exchange between assisting ions, deposition atom and adatoms. The results give the possibility to modulate the orientation to meet different demands.
Influence of assisting ion beam current on the microstructure and surface morphology of IBAD-MgO thin films for YBCO coated conductors
2014, Surface and Coatings Technology
Citation Excerpt :
While further increase of assisting ion beam current, the reduction rate became slow. The assisting ion beam had two effects on the deposition of MgO layer, one was texturing and the other was sputtering [21]. Higher assisting ion beam current resulted in a thinner film owing to the sputtering effect.
MgO thin films were grown on Y₂O₃/Al₂O₃ buffered C276 tapes by ion beam assisted deposition (IBAD) method and the effects of assisting ion beam current on their microstructure and surface morphology were investigated. In order to evaluate the quality of IBAD-MgO thin films, CeO₂ cap layer was deposited directly on the IBAD-MgO template and then YBCO superconducting layer was grown by pulsed laser deposition (PLD) method. Film characterization was performed by in-situ reflection high energy electron diffraction, X-ray diffraction, and atomic force microscopy. The critical current I_c of the YBCO film was measured by the conventional four-probe method. It was found that the orientation, texture degree and surface morphology of the IBAD-MgO thin films were very sensitive to assisting ion beam current. By optimizing the assisting ion beam current, IBAD-MgO thin films had pure (002) orientation, excellent biaxial texture and very smooth surface with a root mean square surface roughness of less than 1 nm. The PLD-CeO₂ layer had the best out-of-plane texture of Δω = 1.6° and in-plane texture of Δφ = 4.8° at 140 mA of assisting ion beam current. Under optimized experimental condition, high quality YBCO coated conductor with the critical current density J_c of 4.5 × 10⁶ A/cm² was fabricated at 77 K and self field.

View full text