Highly flexible touch screen panel fabricated with silver-inserted transparent ITO triple-layer structures

A flexible and transparent amorphous-indium tin oxide/silver/crystalline-indium tin oxide (a-ITO/Ag/c-ITO) triple-layer structure was prepared as an electrode for capacitive-type touch screen panels (TSPs). A very thin metal film of silver (Ag) was inserted between two ITO layers, and the triple-layer structures were deposited on a colorless polyimide (CPI) substrate by a sputtering method. It was found that the tunable electrical and optical properties of the a-ITO/Ag/c-ITO triple-layer structures were critically affected by the thickness of the inserted Ag layer. The optimized flexible a-ITO/Ag/c-ITO triple-layer structure has low sheet resistance, high optical transmittance, and low surface roughness. In addition, during the 30 000 bending cycles, the resistance change (ΔR) of the flexible a-ITO/Ag/c-ITO triple-layer structure was 4.12%. For environmental reliability, the ΔR values of the flexible a-ITO/Ag/c-ITO triple-layer structure were 2.86% and 0.96% at the environmental temperature of 80 °C-50% and −40 °C, respectively. The above results indicate that the a-ITO/Ag/c-ITO triple-layer structure can be utilized to construct a promising TCO electrode. Finally, flexible and foldable capacitive-type TSPs were fabricated with multiple touch points using the a-ITO/Ag/c-ITO triple-layer electrode.


Introduction
Transparent conducting oxide (TCO) is essential for photosensitive electronic devices such as thin lm solar cells, 1 at panel displays, 2 touch screens, 3 and organic light-emitting diodes (OLEDs). 4 There are at least two important factors, including the resistivity and transmittance, that are used to evaluate the performance of a TCO thin lm. Low sheet resistance and high transmittance are required especially when TCO is used as an electrode material. Due to the great advance of experimental exploration, a large variety of TCO thin lms, including traditional metal oxides, 5,6 graphene, 7 carbon nanotubes, 8 and metal nanostructures, [9][10][11][12][13][14][15] have been successfully fabricated and widely applied in optoelectronic devices. Indium tin oxide (ITO) is the best choice for transparent conducting (TCO) lms because of its low resistivity (<10 À4 U cm) and high transmittance (>90%).
Touch screen panels (TSPs) have been considered as one of the key components in mobile communication devices. The capacitive-type TSPs are now being used instead of conventional resistive-type TSPs due to their capacity for multi-touch function and multitasking. Recently, exible capacitive-type TSPs have been extensively investigated for application in exible OLED displays. In these, the TCO electrode plays an important role in determining the performance of the exible TSPs. The exible TCO electrode should have mechanical robustness against substrate bending without resultant changes in its optical and electrical properties. In previous research, amorphous ITO thin lm were widely employed as a exible TCO material in exible optoelectronic devices due to their high conductivity and transparency in the visible spectral range. 16 However, cracks that easily form in the brittle amorphous ITO lms have been considered as a critical drawback in exible devices. 17 In addition, crystallized ITO lms showed a higher transmittance and lower resistivity than amorphous lms. 18,19 In order to eliminate the critical drawbacks of the ITO lms, ITO sandwiching of thin metal lms has been extensively investigated. 20,21 Compared with single-layer ITO lms, the ITO/ metal/ITO triple-layer structures can effectively suppress the reection from the metal layer in the visible range and yield better electrical conductivity, and therefore, this area of research has generated interest. The metal layer of TCO/metal/ TCO multilayer structures is oen comprised of gold (Au), silver (Ag), copper (Cu), or molybdenum (Mo). Triple-layer structures such as ITO/metal/ITO, 22,23 ZnO/Cu/ZnO, 24 IZO/Au/IZO, 25 ZnO/ Ag/ZnO, 26 AZO/Ag/AZO, 27 NTO/Ag/NTO, 28 and ZTO/Ag/ZTO 29 have been reported.
In this research, we studied the structural, electrical, and optical properties of the ITO/metal/ITO triple-layer structures, which were sputtered on a colorless polyimide (CPI) substrate. The transmittance, coefficient of thermal expansion, and hardness of the CPI were outstanding for plastic material as compared to that of polyimide (PI), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyester (PES). The effects of the thin metal lm thickness of the ITO/metal/ITO triple-layer structures were investigated. Under optimized conditions, we fabricated a exible capacitive-type TSP using the ITO/metal/ITO triple-layer structure. The capacitive-type TSPs have been substituted for conventional resistive-type TSPs due to their capacity for multi-touch function and multitasking. The multi-touch function of TSPs is critically dependent on the resistance and optical transparency of the touch sensor electrodes. Thus, this study can provide a practical solution to the existing problems characterizing exible TCO materials and their TSP application.

Fabrication of the ITO/Ag/ITO triple-layer structures
In previous reports, the higher transmittance of the TCO-Ag-TCO structure compared with that of the TCO-Au-TCO structure was explained by Ag having a lower absorption than Au in the visible region of 400-700 nm (5% versus 8%). [30][31][32] In this investigation, a thin metal lm of Ag was inserted between two exible ITO thin lms to form a triple-layer structure. The ITO/Ag/ITO triple-layer structures were prepared using a radio frequency (RF) magnetron sputtering method on a colorless polyimide (CPI) substrate under optimized top and bottom ITO thin lm deposition conditions as a function of Ag thickness. The thickness of the CPI substrate was 10 mm. The distance between the CPI substrate and the target was approximately 5 cm, and the substrate holder rotated so that a uniform lm morphology was achieved during deposition. The sputtering deposition parameters of the ITO lm and Ag thin metal lm were base pressure of 3 Â 10 À6 torr, pure Ar ow rate of 50 sccm, and working pressure of 5 Â 10 À3 torr. The RF power of the ITO thin lm was 80 W, and for the Ag thin metal lm, was 50 W. The thicknesses of the Ag thin metal lm were 1.5 nm, 2.9 nm, 5.7 nm, 8.5 nm, 11.4 nm, 14.2 nm, and 17.2 nm with deposition times of 10 s, 20 s, 40 s, 60 s, 80 s, 100 s and 120 s, respectively. The detailed deposition parameters of the ITO thin lm and the Ag thin metal lm are shown in Table 1. Finally, the ITO/Ag/ITO triple-layer structures were annealed in the air for 1 h at 300 C.

Analysis of the ITO/Ag/ITO triple-layer structures
Using a UV-Vis spectrometer, the optical transmittances of the ITO/Ag/ITO triple-layer structures were measured at wavelengths between 300 and 1100 nm. The sheet resistances of the ITO/Ag/ ITO triple-layer structures were measured using four probe methods. The microstructure of the optimized ITO/Ag/ITO triplelayer structures was examined using high resolution transmission electron microscopy (HR-TEM). The TEM images were obtained from a cross-sectional HR-TEM specimen prepared via focus ion beam (FIB) milling. The resistivity (r), carrier concentration (n), and mobility (m) of the ITO/Ag/ITO triple-layer structures were obtained from Hall-effect measurements. The mechanical and temperature reliability of the ITO/Ag/ITO triplelayer structures were investigated using a computer-controlled bending test machine and environmental reliability test system, respectively.
To investigate the feasibility of the ITO/Ag/ITO triple-layer structure as an electrode for capacitive-type TSPs, we fabricated a single-sided ITO (SITO) structure of the TSPs using an ITO/Ag/ITO electrode. The ITO/Ag/ITO electrode was directly deposited, and then patterned by a conventional photolithography and wet-etching process. The exible printed circuit board (FPCB) bonding with the metal pattern progressed using anisotropic conductive lm (ACF), and the FPCB was connected to the touch circuit controller. The touch circuit was designed by HannsTouch Solution Incorporated, in which the touch integrated circuit (IC) was purchased from Synaptics. Aer the touch sensor was connected to the touch circuit controller, rmware tuning was required. Finally, the plastic cover lens was attached to the touch sensor, and then the fabrication of the TSP was completed.

Results and discussion
In this investigation, a thin metal lm of silver (Ag) was inserted between exible indium tin oxide (ITO) thin lms to form ITO/ Ag/ITO triple-layer structures. The transmittance spectra of the exible ITO/Ag/ITO triple-layer structures as a function of Ag deposition time in wavelengths ranging from 300 to 1100 nm are shown in Fig. 1. In Fig. 1, the transmittance of the CPI  substrate is 98.1% in the visible region and near-infrared region (NIR) without deposition of the exible ITO/Ag/ITO triple-layer structures. At Ag deposition times of 10, 20, and 40 s, the transmittance of the exible ITO/Ag/ITO triple-layer structures shows a sharp increase in the near-ultraviolet (NUV), and then slightly decreases in the visible range, with a subsequent transmittance increase in the NIR. The exible ITO/Ag/ITO triple-layer structures show a high transmittance in the visible region, with a subsequent slight decrease in the transmittance in the NIR as the Ag deposition time increases from 60 to 80 s. Further increasing the Ag deposition time from 100 to 120 s, the transmittance decreases again in the visible region and rapidly decreases in the NIR. It is obvious that the overall transmittance of the ITO/Ag/ITO triple-layer structures is highly inuenced by the Ag deposition time. Fig. 2  When an optimum Ag thin metal lm is embedded between two oxide layers, the resulting oxide/metal/oxide (OMO) triplelayer structure can suppress reections from the Ag thin metal lm and results in a high transmittance in the visible wavelength region. [33][34][35][36] Therefore, the maximum transmittance at 500 nm (88.7%) and 400-700 nm (86.2%) of the exible ITO/ Ag/ITO triple-layer structure was obtained with the Ag deposition time set to 80 s. As the deposition times of the Ag are further increased, the transmittance and average transmittance of the exible ITO/Ag/ITO triple-layer structures decreases due to the reection on the thicker Ag metal lm. In particular, the optical transmittance in the infrared wavelength region significantly decreased with increasing Ag thin metal lm thickness, and this phenomenon was caused by the plasma resonance frequency effect. 37 Consequently, by insertion of Ag thin metal lms with different deposition times between the ITO, the effective plasma resonance frequency of the ITO/Ag/ITO triplelayer structure can be tuned accordingly. Fig. 3 presents the resistivity (r), carrier concentration (n), and mobility (m) of the exible ITO/Ag/ITO triple-layer structures as a function of Ag deposition time. The resistivity of the exible ITO/Ag/ITO triple-layer structures decreased from 9.12 Â 10 À4 to 2.2 Â 10 À5 U cm as the Ag deposition time increased from 10 to 120 s. The decrease in resistivity is mainly due to the increases in both carrier concentration and mobility with the increase in Ag deposition time because the resistivity of the exible ITO/Ag/ITO triple-layer structures is proportional to the reciprocal value of the product of the carrier concentration and the mobility. 38 The carrier concentration increased from 2.65 Â 10 20 cm À3 to 2.21 Â 10 22 cm À3 as the Ag deposition time increased from 10 to 120 s. As discussed by Alford Klöppel et al., the metal interlayer can act as an electron source for the oxide layer in the TCO/ metal/TCO structure. 39 Therefore, the electron in the Ag layer can be easily injected into the ITO layer due to downward band bending at the contact by the difference in work functions between Ag (4 M ¼ 4.4 eV) layer and ITO (4 O ¼ 4.5-5.1 eV) in the exible ITO/Ag/ITO triple-layer structures. 40,41 A schematic of the energy band diagram of the ITO and Ag prior to and aer their contact is shown in Fig. 4(a) and (b), respectively. The mobility of the exible ITO/Ag/ITO triple-layer structures  increased from 4.2 cm 2 V À1 to 74.3 cm 2 V À1 as the Ag deposition time increased. The mobility increase is largely ascribed to the reduction of scattering at the interface regions between the metal and oxide layers, because the interface scattering is considered the main scattering mechanism in the exible ITO/ Ag/ITO triple-layer structure. 42 Fig . 5 shows the gure of merit (FOM) and sheet resistances of the exible ITO/Ag/ITO triple-layer structures as a function of Ag deposition time. To obtain the best combination of high transmission and low resistivity, the FOM for the lms was calculated using the Haacke equation, 43 where T av is the optical transmittance of the exible ITO/Ag/ITO triple-layer structures at the 550 nm wavelength, and R s is the sheet resistance. T av can be estimated using eqn (2): where T(l) is the transmittance, and v(l) is the photopic luminous efficiency function dening the standard observer for photometry. 44 In Fig. 5, the maximum value of the FOM at 12.2 Â 10 À3 U À1 can be observed at 80 s Ag deposition time. The sheet resistance and transmittance at 550 nm were 13 U , À1 and 88.7%, respectively. From the above results, high optical transmittance and low resistance of the exible ITO/Ag/ITO triple-layer structure were obtained as the Ag deposition time was set to 80 s [abbreviation as ITO/Ag(80)/ITO].
To produce exible ITO/Ag(80)/ITO triple-layer structures with higher transmittance, the different deposition parameters of the ITO thin lm were investigated. Fig. 6 shows the transmittance spectra of the ITO/Ag(80)/ITO triple-layer structures at the deposition temperature of 160 C for the bottom ITO thin lm [abbreviated as ITO/Ag(80)/ITO(160)]. The transmittance at 550 nm and the average transmittance at 400-700 nm of the exible ITO/Ag(80)/ITO(160) triple-layer structures were 91.4% and 87.5%, respectively. The transmittance and the average transmittance of the exible ITO/Ag(80)/ITO(160) triple-layer structure are higher than that of the exible ITO/Ag(80)/ITO triple-layer structure. This was caused by the crystallized ITO thin lm, which showed a higher transmittance and lower resistivity than the amorphous lm. 45,46 The sheet resistance of the exible ITO/Ag(80)/ITO(160) triple-layer structure is 6.4 U , À1 , and it is lower than that of the exible ITO/Ag(80)/ ITO structure. Table 2 gives previously published values for the optical transmittance at 550 nm, sheet resistance, and FOM values of transparent conductive lms deposited on exible substrates. It can be seen that the FOM value obtained in the present work is slightly higher than previously reported values.
The surface roughness value of the exible ITO/Ag(80)/ ITO(160) triple-layer structure is very important for future   applications. Fig. 7 shows the root mean square (RMS) roughness (1 mm Â 1 mm) of the exible ITO/Ag(80)/ITO(160) triple-layer structure by atomic force microscopy (AFM). The RMS roughness of the exible ITO/Ag(80)/ITO(160) triple-layer structure is 1.39 nm. The RMS roughness of the top ITO layer is small and demonstrates the amorphous structure and decrease in the surface roughness. Therefore, the exible ITO/ Ag(80)/ITO(160) triple-layer structure is benecial for further TSP device fabrication. Fig. 8 shows cross-sectional transmission electron microscopy (TEM) images and the high-resolution TEM image of the ITO/Ag(80)/ITO(160) triple-layer structure. The gray and black images clearly show the triple-layer structure consisting of bottom ITO, Ag, and top ITO thin lms without interfacial layers, as shown in Fig. 8(a). The thickness of the bottom ITO, Ag, and top ITO thin lms are 21.1 nm, 11.4 nm, and 24.7 nm, respectively. In the case of the ITO/Ag/ITO triple-layer structure in Fig. 8(b), the image shows that the bottom ITO thin lm has a complete crystallization structure due to its deposition at the substrate temperature of 160 C. It shows the bottom ITO and Ag layers without an interfacial layer. There are no interface reactions of interfacial oxide layers between the bottom ITO and  Ag layers due to the use of a continuous sputtering process without breaking the vacuum. In addition, before deposition of the Ag thin lm, the 160 C substrate temperature would be cooling down to room temperature. Finally, the amorphous structure of the top ITO thin lm was obtained at deposition at room temperature. From the above results, it was evident that the crystallization of the ITO thin lm enhances the transmittance of the ITO/Ag/ITO triple-layer structure, and the amorphous ITO thin lm reduces the surface roughness.
To evaluate the mechanical reliability of the exible ITO/ Ag(80)/ITO(160) triple-layer structure for TSP application, we measured resistance changes (DR) in the exible ITO/Ag(80)/ ITO(160) triple-layer structure during dynamic outer bending. Table 3 shows the bending reliability analysis of the exible ITO/Ag(80)/ITO(160) triple-layer structure using a computercontrolled bending test machine with the bending radius set to 3 mm. The critical bending radius of 3 mm was dened by the standard commercial test created by HannsTouch Solution Incorporated. The sheet resistance of the exible ITO/Ag(80)/ ITO(160) triple-layer structure is 6.4 U , À1 . Changes in the sheet resistance of the ITO/Ag(80)/ITO(160) triple-layer structure can be expressed as DR (%) ¼ (R 1 À R 0 )/R 0 , where R 0 is the initial resistance and R 1 is the measured resistance aer bending. During the 15 000 and 30 000 bending cycles, the DR values of the exible ITO/Ag(80)/ITO(160) triple-layer structure were 2.06% and 4.12%, respectively. The dynamic outer bending of the exible ITO/Ag(80)/ITO(160) triple-layer structure showed less change in sheet resistance aer 30 000 bending cycles, demonstrating good exibility of the ITO/ Ag(80)/ITO(160) triple-layer structure. This exibility of the ITO/Ag(80)/ITO(160) triple-layer structure can be attributed to the ability of the metallic Ag thin interlayer between the ITO layers to withstand high strain, and even the local delamination of or crack formation in the top and bottom ITO thin lms.
Environmental reliability is a very important test for commercialization of a material. The resistance change (DR) of the exible ITO/Ag(80)/ITO(160) triple-layer structure was measured by using the environmental reliability test system. The high and low temperature are set to 80 C-85% and À40 C, respectively. The resistance of the exible ITO/Ag(80)/ITO(160) triple-layer structure is 6.4 U , À1 . During the environmental reliability test of 80 C-85% and À40 C, the DR of the exible ITO/Ag(80)/ITO(160) triple-layer structure are 2.86% and 0.96% with 240 h as shown in Table 4, respectively. The exible ITO/Ag(80)/ITO(160) triple-layer structure showed less change in resistance aer 80 C-85% and À40 C environmental reliability test, demonstrating the good reliability of the exible ITO/Ag(80)/ITO(160) triple-layer structure. Fig. 9 shows an optical microscopy (OM) image of the exible ITO/Ag(80)/ITO(160) triple-layer structure of the fabricated capacitive-type TSPs. To fabricate the exible capacitive-type TSPs, the ITO/Ag(80)/ITO(160) electrode was patterned by  Paper a photolithography and wet etching process. The wet etchant is a phosphoric acid and nitric acid mixture. Using the well patterned ITO/Ag(80)/ITO(160) electrode, exible capacitivetype TSPs were fabricated. Fig. 10 shows the multiple touch point operation of a exible capacitive-type TSP fabricated with the ITO/Ag/ITO electrode. The capacitive-type TSP was operated by exact sensing of X-Y coordinates and characteristics of linearity. Operation of the exible capacitive-type TSP with the ITO/Ag/ITO electrode indicates that the ITO/Ag/ITO triple-layer structure with low sheet resistance and high optical transmittance is a promising transparent electrode to substitute for the conventional ITO electrode. In this investigation, the maximum bending angle of the exible capacitive-type TSP was 180 , as shown in Fig. 10. To evaluate the practical usage of the exible capacitive-type TSP, we lightly touched the TSP, and the four touch points were displayed on a notebook monitor, as shown in Fig. 10. This test veried that this exible capacitive-type TSP can detect multiple touch points. We are currently in the process of fabricating small-sized capacitive-type TSPs for smartphones and applications combined with organic light emitting diode (OLED) displays.

Conclusion
An amorphous-indium tin oxide/silver/crystalline-indium tin oxide (a-ITO/Ag/c-ITO) triple-layer structure with low resistivity and high transmittance was studied as a substitute for conventional ITO electrodes for exible capacitive-type touch screen panels (TSPs). Under optimized conditions, the a-ITO/ Ag/c-ITO (21.1 nm/11.4 nm/24.7 nm) triple-layer structure grown on a colorless polyimide (CPI) substrate exhibited a sheet resistance of 6.4 U , À1 , an optical transmittance of 91.4%, a surface roughness of 1.39 nm, a resistance change (DR) with bending reliability of 4.12%, and DR of 2.86% and 0.96% with environmental reliability of 80 C-50%/À40 C, which is much better than conventional ITO thin lm. In this investigation, the exible and foldable capacitive-type TSP fabricated with a-ITO/ Ag/c-ITO electrode demonstrated multiple touch points function.

Conflicts of interest
There are no conicts to declare.