ITO/Au/ITO multilayer thin films on transparent polycarbonate with enhanced EMI shielding properties

https://doi.org/10.1016/j.cap.2020.01.012Get rights and content

Highlights

  • ITO/Au/ITO thin films were deposited without substrate heating by magnetron sputtering.

  • Transparent polycarbonate was used as substrate to converge aerospace applications.

  • EMI SE of 27 dB was achieved without sacrificing much of the transparency.

  • Experimental results were confirmed by a simulation study.

Abstract

ITO/Au/ITO multilayer thin films were deposited onto polycarbonate substrate via magnetron sputtering technique without intentional heating. The deposition times of both ITO and Au layers were studied to optimize the overall transparency and conductivity. As-prepared thin films were characterized using X-ray diffraction analysis, secondary ion mass spectroscopy, scanning and transmission electron microscopy, atomic force microscopy and physical property measurement system. The optical measurement results revealed that the transmittance of the films were enhanced by increasing the gold deposition time up to 15 s. Beyond this point, further increasing the duration caused a decrease in optical transmittance. Upon optimization of the Au deposition time, the deposition duration of ITO layers was also studied to increase electromagnetic interference (EMI) shielding effectiveness (SE). Maximum EMI SE in this work was measured as 26.8 dB, yielding 99.8% power attenuation, which was verified by simulation results.

Introduction

Transparent conducting oxide (TCO) films play a key role in a number of applications including displays, smart windows, solar cells, photodetectors, electroluminescence devices and organic light emitting diodes [1,2], IR-reflective coatings [3], heatable layers in defrosting windows [4] and dissipating static charge [5,6]. To date, various alternative TCOs such as indium tin oxide (ITO) [7], indium zinc oxide (IZO) [8], gallium zinc oxide (GZO) [9] and aluminum zinc oxide (AZO) [10,11] have been proposed to be used in these applications. Among all alternatives, ITO dominates the field due to its performance in high visible region transmission and low resistivity [7].

Thermoplastic polymers are used in many industrial applications due to their impact resistance, toughness, durability and price advantage. However, they do not withstand the temperatures required for some widely used deposition steps in semiconductor processing. Magnetron sputtering with a heat treatment is the most common deposition method for semiconductor processing, as it allows large scale deposition [12]. However, heating is not applicable when the substrate is a thermoplastic material. Therefore, in the case of thermoplastic substrates, the electrical and optical properties should be improved by other means, rather than improving crystallinity via substrate heating [13]. One of the most effective ways to realize improved optical transmission and electrical conductivity is to use sandwich type structures such as dielectric/metal/dielectric (D/M/D) multilayer films [14]. In this structure, a very thin metal film is embedded as an interlayer to improve the electrical conductivity without deteriorating the transparency of the dielectric material. Metals such as Au, Ag, Cu, Al and their alloys have been extensively studied for interlayer of D/M/D films [15]. While the surface of other metals is easily altered in air/moisture environments due to the formation of oxides, gold retains its shiny aspect over time because gold oxides are not stable [16]. It is already reported that the sheet resistance of sandwich structures including Au interlayer remains constant for an exposure time of 600 h at 60 °C, in air having a relative humidity of 90% [13].

Regarding to the quality of a thin metal interlayer between dielectric components, the fabrication process generally gives rise to aggregated gold islands before a homogeneous layer is deposited [17]. Sytchkova et al. [18] and Guske et al. [19] both reported that the thickness of metal film should exceed 10 nm to prevent discontinuity of the interlayer coat. Based upon this limitation, numerical studies have been conducted to obtain the best thickness values of individual layers with further correlation with experimental studies [20]. Although plenty of work has been done on flexible substrates [13,21], research towards robust thin film deposition onto structural polymer substrates is very limited. Especially, a sandwich structure on polymer substrates consisting of a metal interlayer between TCO layers, providing optimum optical transparency and electrical conductivity with a focus on effective EMI shielding remains elusive.

In this work, ITO/Au/ITO multilayer films were systematically deposited on polycarbonate (PC) and Soda Lime Glass (SLG) substrates using magnetron sputtering technique. The deposition times of Au and ITO layers were optimized to provide highest electrical conductivity and visible light transparency. The structural, electrical, and optical properties of the sandwich films were studied and the EMI shielding behavior of the D/M/D films were evaluated via both numerical and experimental methods. Through a detailed study on the factors influencing EMI shielding performance like individual film thicknesses and electrical conductivity, as well as carrier mobility and carrier concentration, the maximum shielding effectiveness of about 26.8 dB at 8–12 GHz, providing 99.8% power attenuation could be achieved by ITO/Au/ITO multilayer thin films.

Section snippets

Deposition of multilayer films

Deposition of ITO and Au thin films were performed in a magnetron sputtering system that was equipped with two cathodes [22]. DC power was applied to both ITO and Au targets. The ITO target used in this work was an In2O3 (90%) and SnO2 (10%) containing disc with 2 in. diameter and 0.25 in. thickness. A 99.99% pure disk with 2 in. diameter and 0.0625 in. thickness was used as gold target for interlayer deposition. ITO bi-layer and ITO/Au/ITO trilayer films were deposited on Soda Lime Glass (SLG,

Results and discussion

The crystal structure plays an important role on the optical and electrical characteristics of the prepared thin films. Fig. 1 shows the GIXRD patterns of the films deposited with different stacking sequences onto PC. Peaks located at 30.1 ± 0.05° and 35.00 ± 0.05° can be indexed to the (222) and (400) planes of ITO with bixbyite crystal structure (JCPDS File No. 89–4596). No peak belonging to SnO or SnO2 was detected. It is previously reported that the polycrystalline ITO films are only

Conclusions

D/M/D sandwich coatings, which greatly relieves the trade-off between visible light transparency and EMI shielding, were successfully prepared by magnetron sputtering. Unlike most of the previous studies, the sputtering was conducted in the absence of substrate heating as PC substrates were used to converge industrial application. It was found that the presence of the metal interlayer improved the electrical conductivity and the optical transparency significantly by tailoring crystallinity and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was partially supported by Scientific and Technological Research Council of Turkey (TUBİTAK) under contract number of 5189901 and Turkish Aerospace. The authors thank to the collaboration between Turkish Aerospace and Bilkent University Institute of Materials Science and Nanotechnology (UNAM). The authors also thank to Enver Kahveci and Mustafa Guler for the assistance in the GIXRD and FIB litography, respectively. The authors are indebted to Dr. H. Volkan Demir and Dr. Metehan

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