Technical Field
This disclosure generally relates to an electronic system comprising a touch sensor and a method for manufacturing such system. This disclosure also generally relates to an electronic system comprising a transparent conductive electrode. This disclosure also generally relates to an optoelectronic system including a touch screen.
Description of Related Art
Since touch screens provide an easy interface for human-machine interactions, they recently have found wide range of applications in consumer electronics, such as mobile phones, tablets, global positioning systems (GPS), medical devices, laptops, point-of-sale terminals, point-of-information kiosks, industrial control units, and visual display systems.
Among many types of the touch screens, capacitive touch screens are getting more popular as compared to resistive touch screens due to their higher sensitivity to finger touch and good visibility for displays. The capacitive touch screens also allow users to perform functions not possible with resistive touch screens such as changing the orientation of images with thumb and forefinger since they can support multi-touch capability. For a summary of touch screen technologies and their features, for example, see publications by: Alfred Poor “How It Works: The Technology of Touch Screens” Computerworld, Oct. 17, 2012; Geoff Walker “Fundamentals of Touch Technologies” 2013 SID Touch Gesture Motion Conference, October 2013; and Trevor Davis “Reducing Capacitive Touchscreen Cost in Mobile Phones” Embedded, Feb. 25, 2013. The entire contents of these publications are incorporated herein by reference.
A capacitive touch screen system typically comprises a cover glass (or lens) with a screen printed decorative frame, and a touch sensor made from indium tin oxide (ITO) film deposited on another glass substrate. These two components are separately manufactured and assembled to form a single component by using an optically clear adhesive (OCA). Manufacturing of the currently available capacitive touch sensor involves in several process steps, including deposition of an ITO film on a glass surface by sputtering, then baking the ITO film above its melting point to create a conductive ITO layer, and finally etching the conductive ITO layer by photo or laser lithography to form a sensing circuit. Every manufacturing step adds to the cost of the final device, due to materials used and elongated manufacturing time. Since every step may have risks for causing defects, losses or decreasing production yield further contribute to the overall cost. In addition, as the size of the capacitive touch screen increases, so does its weight since the typical touch screen comprises two layers of glass. To achieve required touch sensitivity of large size touch sensor, sheet resistance of the transparent conducting electrode may need to be, for example, lower than 50 ohm/square. For ITO coatings on a glass substrate, such a low sheet resistance may be achieved by increasing thickness of the conductive layer while compromising transparency of the device.
Although use of ITO as an electrically conductive material dominates the manufacturing of the touch screens, the search for new materials that can replace ITO has been significantly intensified in the past few years, motivated by scarce supply of raw materials used in preparation of ITO films and ever increasing demand of consumer electronics product. Particularly, ITO based transparent conducting film may not meet the requirement of new products where light weight and great readability is essential.
Among several different approaches for manufacturing of alternative transparent conducting electrodes, nanomaterial based transparent conducting electrodes including carbon nanotubes, graphene, and especially metal nanowires are investigated as leading candidates. However, a number of challenges still exist before such an approach can meet full manufacturing specifications including optical/electrical properties and mechanical and environment stability. Especially lack of an efficient manufacturing process with high throughput capacity is one important hurdle.
Nanomaterials, especially metal nanowires may form a conductive network film by random organization of individual nanowires. Sheet resistance, or electric conductivity of the film, is largely limited by the contact among individual wires. In addition, adhesion of nanowire films to the transparent substrate may be weak due to weak molecular interaction between the metal wires and the transparent substrate, such as glass and polymer substrate. Efforts to improve adhesion without sacrificing the conductivity are reported in literature without much success. For example, in one approach, a binder material, vinyl chloride is added to a formulation conductive nanomaterial, carbon nanotube, as disclosed in Glatkowski et al. “Articles with Dispersed Conductive Coatings” U.S. Patent Application Publication No. 200610257638A1. The entire content of this disclosure is incorporated herein by reference.
Since most of the binder materials are insulators, they increase the contact resistance between nanowires or nanotubes. In another approach, as disclosed in Alden et al. “Transparent Conductors Comprising Metal Nanowires” U.S. Pat. No. 8,049,333; and “Nanowire Based Transparent Nano-Conductors” U.S. Patent Application Publication No. 200810286447A1, silver nanowires were deposited on a substrate to form a nanowire network, and then coated with a polymer matrix comprising acrylate and carboxy alkyl cellulose ether polymers. The entire contents of these disclosures are incorporated herein by reference. Although such approaches might partially solve the adhesion problem, the surface conductivity of nanowire film would be lost.
The second major challenge is the stability of the nanowire films, particularly that of films comprising silver nanowires. When exposed to the ambient atmosphere, pollutants in air, such as H2S, may react with silver nanowires to form electrically non-conductive silver sulfide. Oxidation of silver by oxygen to form silver oxide would contribute another factor for instability of the touch sensors. An antioxidant is also incorporated into the overcoat formulation to prevent direct contact of the silver nanowire film with atmospheric pollutants. Such overcoats might slow down the penetration of the pollutants to the silver nanowire film. The effectiveness of protection depends on the porosity of the overcoat and its thickness. However, as the thickness of the overcoat increase, the surface conductivity of the nanowire would also be lost.
The third major challenge is related to the application of nanowire film. Continuous conductive electrode may need to be processed into a patterned sensor by widely used method such as photolithography or laser ablation. Effective photo lithography methods to etch both overcoat polymer and silver nanowire to form a touch sensor is disclosed in Allemand et al. “Nanowire-Based Transparent Conductors and Applications Thereof” U.S. Patent Application Publication No. 2014/0338735. The entire content of this disclosure is incorporated herein by reference. Feasibility of the laser ablation of the silver nanowire has been demonstrated, for example, see Hong Sukjoon et. al, Journal of Nanoscience and Nanotechnology, volume 15, no. 3, pages 2317-2323. The entire content of this publication is incorporated herein by reference. However, because the overcoat layer is transparent, it may not be ablated by the laser to allow evaporation and thereby removal of silver from the coating. Therefore most of the silver vapor formed by laser ablation may be trapped underneath the overcoat, leading to crosstalk of the patterned lines and device failure.
To reduce the cost and the weight of the touch screen, several different touch screen structures are being developed, such as sensor on-cell type touch screens, sensor in-cell type touch screens, glass lens/film sensor type touch screens, and sensor on glass lens or one glass solution (OGS) type touch screens. In these structures, main target is to reduce number of layers of glass incorporated into the system, thereby reducing the touch screen weight and costs.
However, there are still significant technical barriers for in-cell and on-cell type touch screens. For the on-cell type touch screens, the primary issue is the noise injected from the display module, such as liquid crystal display (LCD). As the touch sensor is structured to be closer and closer to the thin film transistor (TFT) switching elements of LCD, this noise substantially grows. In the case of in-cell type touch screen, the touch sensor is implemented within the TFT structure, which is complicated to manufacture, and therefore this type of touch screen is only used for a few high end applications today.
The glass lens/film type touch sensors are also manufactured by using two separate processes to prepare cover lenses and film sensors, and assembling these two components by using an optically clear adhesive. Achieving required sheet resistance with polymer film substrates is more difficult since the polymer films usually have lower thermal stability than the glass substrates. And the ITO coating layer need to be annealed at a high temperature to achieve lower sheet resistance. Most widely available ITO coatings on PET films have the sheet resistance of 150 ohm/square at acceptable transparency of higher than 85% at 550 nm. Such a high sheet resistance and a low transparency may only find applications in small size touch sensors. ITO coatings on PET films with sheet resistances lower than 50 ohm/square are rarely available and expensive.
The sensor on glass lens or one glass solution (OGS) approach may reduce the weight in overall device. This approach consolidates multilayer touch sensor system into a simpler structure and keeps supply chains intact for consumer electronics manufacturers. However, it still faces a number of technical challenges.
To be used as a glass lens, regular glass must be strengthened to prevent the breakage during the device use. The glass lens usually includes a silk screen printed decorative frame on its inner surface. This frame is used to hide the circuitry of the device. These two features of glass lens pose processing difficulties during the process scale up for commercialization. If the process scale up involves sputtering of an ITO layer on a large strengthened glass followed by patterning of the ITO layer, there may be substantial losses during cutting of the large strengthened glasses into small devices, decreasing the process yield. If the process involves small pieces of the strengthened glass, the productivity may dramatically drop.
Furthermore, the silk screen printed decorative frame usually has about 5 micrometers to 10 micrometers thickness. This frame prevents the ITO layer to form a uniform and continuous film during the ITO sputtering process across the glass and over the silk screen printed area. Any disruption in the conductive layer, at the frame to the glass transition regions, would cause device failures. This process may therefore be unsatisfactory.