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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25495−25499
Effect of Inductively Coupled Plasma on Multilayer Electrodes for Flexible Single-Layer Touch Screen Panels Chan-Hwa Hong, Joon-Min Lee, Young-Hoi Kim, and Woo-Seok Cheong* ICT Materials & Components Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 34129, Korea
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ABSTRACT: We investigated the effect of inductively coupled plasma (ICP) on multilayer electrodes for flexible capacitive touch sensors. We found that using ICP during Ag deposition generally increased the conductivity and transmittance of multilayer electrodes. As a result, in the case of the multilayer electrode with an ICP power of 150 W during Ag deposition, 5.7 Ω/sq of sheet resistance and 89.6% of transmittance (550 nm) have been achieved. We demonstrate that the crystallization of the ICP supplied Ag layer in multilayer electrodes leads to the smooth surface roughness of the multilayer film; the smooth surface roughness provided low light scattering. As a result, the crystallized Ag thin film by ICP improved the sheet resistance and transmittance of multilayer electrodes. Finally, we fabricated a 221 × 130 mm (active layer)-sized single-layer touch screen panel (TSP) using multilayer electrodes with ICP on a corning glass and polyethylene terephthalate flexible substrate. The single-layer TSPs show high linearity and sensitivity with multitouches. KEYWORDS: ICP, single-layer TSP, multilayer electrode, flexible substrate, IPVD system
1. INTRODUCTION Capacitive-type touch sensors have attracted much interest in recent times because of their intuitive user interface for various electronic devices such as mobile displays.1 Ever since iPhone was unveiled, the touch industry has released the glass and film-based touch sensor at a high supply price. For fast tact time and low cost, a simple process and structure are necessary. Thus, a single-layer touch sensor which allows for Tx (transmitter) and Rx (receiver) electrodes on one side without the additional layer or electrode film is essential. Traditional single-layer touch sensors applied self-capacitive touch type. However, the self-capacitive touch type has lower signal-tonoise ratio (SNR) and immunity to liquid-crystal display noise than multicapacitive touch type.2 For these reasons, the development of mutual-capacitive single-layer touch screen panels (TSPs) was essential. To obtain a mutual-capacitive single-layer TSP with high quality, it needs a transparent conductive material (TCM) electrode with low sheet resistance and high transmittance. Among many TCMs, indium tin oxide (ITO) has been paid much attention because it is widely used for transparent conductive electrodes in various devices.3−6 However, ITO thin films are too brittle to be used in flexible applications.7,8 Furthermore, the conductivity of ITO is not enough to be applied for large-size single-layer touch sensors. Although lots of metal-based electrodes (mesh, nanowires, and carbon-based materials) have low sheet resistance, they have optical drawbacks such as moiré effect and starburst.9 Recently, several advantages of oxide/metal/oxide multilayer structures have been reported. The multilayer electrodes exhibit high © 2019 American Chemical Society
transparency due to the antireflection effect from the metal layer in the visible region, as well as metallic resistivity.10−14 However, their transmittance is not enough for utilizing the display devices as an electrode of the TSP. On the other hand, ionized physical vapor depositions (IPVDs) with inductively coupled plasma (ICP) reactors have improved the structural, electrical, and optical properties of transparent electrical materials by showing a high ionization rate and plasma density at a low temperature.15,16 In this study, we investigated the effects of ICP on the electrical and optical properties of zinc tin oxide (ZTO)/Ag/ ZTO electrodes at room temperature. In addition, we successfully fabricated a 221 × 130 mm sized glass and filmbased single-layer TSP.
2. EXPERIMENTAL SECTION ZTO and Ag thin films were deposited by the IPVD system (Figure S1). In order to fabricate multilayer structures, a 40 nm thick ZTO, 10 nm thick silver, and 40 nm thick ZTO layer was deposited on a 0.7 mm thick glass and an 180 μm thick polyethylene terephthalate (PET) substrate at room temperature, respectively. ICP has been applied only when Ag is deposited with various powers (0−200 W). An Ar flow ratio of 100 sccm with 0.5 sccm of oxygen gas and a working pressure of 5 mTorr were grown on the substrate at room temperature. The direct current powers for ZTO and Ag deposition were fixed to 900 and 200 W, respectively. The multilayer film was deposited without vacuum break. The sheet resistance, mobility, and Received: February 21, 2019 Accepted: June 25, 2019 Published: June 25, 2019 25495
DOI: 10.1021/acsami.8b20781 ACS Appl. Mater. Interfaces 2019, 11, 25495−25499
Research Article
ACS Applied Materials & Interfaces
Figure 1. Transmittance (a) and sheet resistance (b) of multilayer electrodes on a glass substrate as a function of the ICP power.
Figure 2. Transmittance (a) and sheet resistance (b) of multilayer electrodes on a flexible substrate as a function of the ICP power. carrier density of multilayer films were measured using a four-point probe and Hall measurement system (HL5500PC). The surface morphology of the multilayer was measured with a field-effect scanning electron microscope. The touch sensitivity such as linearity, time constant, SNR, and scan rate was measured at the Zinitix Co., Ltd.
3. RESULTS AND DISCUSSION Figure 1 shows the transmittance (Figure 1a) at a wavelength of 400−700 nm and sheet resistance (Figure 1b) of multilayer electrodes on a glass substrate with an ICP power of 0−200 W. As a result, the transmittance increased from 88.85 to 90.06% with an ICP power of 150 W and the sheet resistance decreased from 8.1 to 6.0 Ω/sq (ρ ≈ 5.4 × 10−5 Ω·cm) with an ICP power of 175 W. In the case of the multilayer electrode on the PET film, the transmittance increased from 88.74 to 89.85% with an ICP power of 125 W and the sheet resistance decreased from 8.4 to 5.7 Ω/sq (ρ ≈ 5.13 × 10−5 Ω·cm) with an ICP power of 150 W as shown in Figure 2a,b. From these results, we indicated that the ICP power on the Ag layer of the multilayer electrode generally leads to both the diminution of optical loss and improved electrical properties. Furthermore, we demonstrated that electrons are easily injected from the Ag layer because of energy band bending at the contact between Ag and ZTO. In other words, there is no barrier to electron flow between the Ag- and ZnO-based materials because the work function of Ag is smaller than that of ZTO.17 In order to find out the best condition for the multilayer electrodes as a transparent electrode, the figure of merit (ΦTC) was calculated from the sheet resistance and the transmittance at a wavelength of 550 nm, as shown in Figure 3. The ΦTC was defined by Haacke as in eq 1.18 ΦTC = T10/R s
Figure 3. ΦTC of multilayer electrodes as a function of substrates and ICP power.
were 58.0 × 10−3 Ω−1 (ICP power of 175 W) and 58.5 × 10−3 Ω−1 (ICP power of 150 W), respectively. Figure 4 shows the
Figure 4. Carrier concentration and mobility of multilayer electrodes with various ICP powers.
carrier concentration and mobility of multilayer electrodes as a function of the ICP power at room temperature. The carrier concentration of multilayer electrodes generally increased from 2.3 × 1021 to 6.9 × 1021 cm−3 with increasing ICP power. However, the mobility of multilayer electrodes increased from 27.5 to 39.5 cm2/V s with an ICP power of 50 W. Then, the mobility of multilayer electrodes generally decreased from
(1)
where T is the transmittance and Rs is the sheet resistance of the multilayer electrodes. As shown in Figure 3, the highest ΦTCs of multilayer electrodes on the glass and PET substrate 25496
DOI: 10.1021/acsami.8b20781 ACS Appl. Mater. Interfaces 2019, 11, 25495−25499
Research Article
ACS Applied Materials & Interfaces
microscopy (TEM) image of multilayer films without ICP (Figure 6a) and with an ICP power of 150 W (Figure 6b). The TEM image indicated that the agglomeration of Ag film has not occurred even if the thickness of the Ag layer was 10 nm. In addition, we found that using ICP during Ag deposition of the multilayer film may increase the grain size of Ag layers, as shown in Figures 5b and 6b. From these results, we indicated that the decreased roughness by the crystallization of the Ag thin film leads to low light scattering and enhanced the transmittance.21 To demonstrate the carrier scattering mechanism, we measured X-ray diffraction of the multilayer film deposited without/with ICP as shown in Figure 7. As shown in Figure 7,
39.5 to 11.5 cm2/V s. These results can be explained that the carrier mobility of the multilayer thin film increased with low ICP power because the proportion of well-ordered crystalline structures to amorphous structures gradually increased. Lee et al. demonstrate that two kinds of grain boundary scattering could be considered: grain boundary scattering between the crystal and the amorphous grains in state I and ionized impurity scattering between the crystal grains in state II. In the case of state I, the carrier concentration and mobility increased. However, in the case of state II, the carrier concentration increased and mobility decreased caused by a different electron scattering mechanism.19 From this mechanism, we can expect that the dominant electron scattering mechanism of the multilayer film with low ICP power (
