Fabrication and performance of an ultrafine silver grid film applied to flexible touch sensor
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The fabrication and performance of an ultrafine silver (Ag) grid film applied to flexible touch sensor as the transparent conductive electrode is reported. The ultrafine Ag grid film was fabricated based on the laser direct writing, electroforming, nano-imprint lithography. In the manufacturing process, firstly, the Nickel (Ni) mold used as the master mold was obtained by laser direct writing and electroforming technologies. Secondly, the micro-grooves were transferred from the Ni mold onto the surface of UV glue coated on the polyethylene terephthalate film through nano-imprinting technology. Lastly, the ultrafine Ag grid was generated through nano-imprint lithography with Ag paste filled into the micro-grooves on the UV glue. The result indicated that the ultrafine Ag grid film with size (L) 640 mm × (W) 520 mm had a uniform line width of 1.01 μm and showed excellent optoelectronic and mechanical properties, such as optical transmittance 90.00%, Haze 1.49%, sheet resistance 5.4 Ω/□, the variation ratio of the sheet resistance within 3% after 8000 bending cycles, and the almost negligible morphology change after the adhesion cross-cut test. Furthermore, the functional test was performed on a flexible touch sensor applying the ultrafine Ag grid film.
KeywordsUltrafine silver grid film Flexible touch sensor Transparent conductive electrode Nickel mold Laser direct writing
As a pivotal component of many photoelectric products described by Chung et al.  including solar cells studied by Yang et al. , liquid crystal displays (LCDs) reported by Blake et al. , organic light-emitting diodes(OLEDs) shown by Wu et al.  and touch screen panels (TSPs) discussed by Jeon et al.  and Kim et al. , conductive electrodes have attracted more and more attention. Indium tin oxide (ITO) is the most widely used transparent conductive electrode material owing to its excellent optoelectronic properties. However, as Hecht et al.  and Kwon et al.  proposed, many new applications have been limited due to its disadvantages, such as its brittleness and scarcity. Because of the flexible, abundant and cost-effective, transparent conductive electrode material will undoubtedly become an irresistible trend, which researched by Tang et al. , Li et al.  and Lai et al. .
Thus, the next generation of material with superior flexibility and abundant resources, without damaging the optoelectronic properties of transparent conductive electrode materials, is urgently needed. Current researches on alternative materials mainly focusing on carbon nanotubes reported by Yu et al. , graphene described by Bae et al.  and Tung et al. , conducting polymers investigated by Kirchmeyer and Reuter , polymer composite to improve the long-term stability of the conductive film was researched by Jiang et al. [16, 17], metal nanostructures shown by Gaynor et al. , Hong et al.  and Madaria et al. . While the conductivity and transmittance of carbon nano-materials are poorer than those of metal nanostructures, the same with the conductivity and stability of conducting polymers based on the researches of Hecht et al.  and Kumar and Zhou . And metal nanostructures, such as Ag nanowires or grids, have been successfully applied in touch screen panel based on the reports of Madaria et al. , Hong et al.  and Hyunjin et al. . However, there are still shortcomings in a few crucial properties of the Ag nanostructures in their reports, such as the line width of Ag nanowires, optical transmittance and haze, sheet resistance, mechanical stability and so on. Therefore, the further improvement of these properties will be critical to the effective application of Ag nanostructures in the practical industrial manufacturing.
In the present work, the fabrication and performance of an ultrafine silver (Ag) grid film applied to flexible touch sensor as the transparent conductive electrode was introduced. In order to improve the deficiencies of the above mentioned properties, and realize the application of Ag nanostructures in the large-area, high-throughput and low-cost production of flexible touch sensor. The ultrafine Ag grid film was fabricated based on the laser direct writing, electroforming, nano-imprint lithography. In the manufacturing process, the Nickel (Ni) mold used as the master mold was obtained by laser direct writing and electroforming technologies, and the micro-grooves were transferred from the Ni mold onto the surface of UV glue coated on the polyethylene terephthalate (PET) film through the nano-imprint lithography. Moreover, the ultrafine Ag grid was generated through nano-imprint lithography with Ag paste filled into the micro-grooves on the UV glue and baked at 130 °C for 1 h. In addition, the properties of the fabricated ultrafine Ag grid film were well studied and performed in order to evaluate the possibility of the fabricated ultrafine Ag grid film substituting for the conventional ITO electrode, and being adopted as a promising transparent conductive electrode applied to the flexible touch sensor.
2.1 Fabrication of Ni mold
2.2 Fabrication of ultrafine Ag grid film
The following fabricated process of the ultrafine Ag grid film in this work can be divided into the following three steps.
Firstly, a UV curable synthetic resin (UV glue) pre-polymer was coated on the surface of obtained Ni mold, and a PET film with size (L) 640 mm × (W) 520 mm × (T) 23 μm was imprinted with the UV glue by roller. In this process, the coating thickness of the UV glue was directly determined by the viscosity of UV glue, the pressure and rotation speed of the roller. In order to accurately obtain the expected UV glue thickness, a series of inquiry experiments on the relationship between these three impact factors and the coating thickness of the UV glue had been conducted, meanwhile the corresponding relationship model between them was established.
Lastly, the ultrafine Ag grid film was obtained with the Ag pastes filled into the micro-grooves on the UV glue (Fig. 2c) and baked at 130 °C for 1 h. In this process, the function and appearance of the product are determined by the pressure and speed of the squeegee used for printing, the leveling of the printing table, the time interval and the wiping force after the printing, the time and temperature of the post-bake, thus the experimental parameters corresponding to each step are the optimal values obtained from a series of optimization experiments before the actual mass production.
3 Results and discussion
3.1 Characterization of the micro-grooves
3.2 Characterization of the ultrafine Ag grid film
3.2.1 Morphological characterization
3.2.2 Optical performance
The optical properties of PET film and PET + Ag grid film
PET + Ag grid film
3.2.3 Mechanical and electrical properties
3.2.4 Functional test
In summary, we have demonstrated the fabrication and performance of an ultrafine Ag grid film applied to flexible touch sensor as the transparent conductive electrode. The ultrafine Ag grid film was fabricated based on the laser direct writing, electroforming and nano-imprint lithography, and the maximum size of the ultrafine Ag grid transparent conductor is (L) 640 mm × (W) 520 mm. The fabricated ultrafine Ag grid transparent conductive film shows excellent optoelectronic and mechanical properties, such as line width 1.01 μm, transmittance 90.00%, haze 1.49%, sheet resistance 5.4 Ω/□, the variation ratio of the sheet resistance within 3% after 8000 bending cycles and the almost negligible morphology change after the adhesion cross-cut test. Moreover, the flexible touch sensor applying this film was fabricated and worked successfully, as well as could identify the writing force. Thus, the fabricated ultrafine Ag grid film is a promising transparent conductive electrode that can substitute for the conventional ITO electrode applied in the large-area, high-throughput and low-cost production of flexible touch sensor.
The financial supports from the research and development projects of O-Film Technology Co., Ltd. are gratefully acknowledged.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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