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By Daniel Gamota, iNEMI
The term large-area flexible electronics encompasses a wide array of devices and applications manufactured on flexible substrates. These products can be fabricated using a full range of materials (organic, inorganic, hybrid) and processes (printing, vacuum, lithography). One of the most compelling aspects of large-area flexible electronics is that they can be produced using high-volume, low-cost, and roll-to-roll (R2R) processes. This article touches on functional inks, processing platforms, and in-line characterization tools.
Several factors are driving the growth of this market segment:
- • availability of higher-performance materials functional inks that are solution-processable (organic and inorganic) and provide intrinsic bulk electrical, thermal, chemical, or optical properties;• commercialization of large-area processing equipment compatible with functional inks;• development of large-area, lower-cost manufacturing processes leveraging R2R equipment infrastructure;• and growing demand for renewable energy systems, low-cost sensors, low-power and highly efficient lighting arrays, low-frequency wireless devices, innovative signage, and flexible displays.
Although promising, there are several technology needs that must be addressed to facilitate widespread deployment of large-area flexible-electronics-based products. This article touches on three of those areas: functional inks, processing platforms, and in-line characterization tools.
Functional Inks Large-area flexible electronics technology is maturing rapidly with the advent of higher-performing electrically functional inks. These inks have enabled the use of low-cost printing platforms, such as inkjet, flexographic, gravure, offset and screen printing, which are traditionally used for printing books, packaging and newspapers. Such graphic arts printing platforms promise to greatly reduce the cost of printed electronics such as photovoltaic modules, displays, sensors and simple wireless products (e.g., RFID tags).
Functional inks include, for example, solution-processable pentacene, inorganic silicon pre-cursors, nanoscale silicon fibers, and other novel inorganic and organic dispersions. One type of conductive ink receiving much attention is silver nanoparticle inks. These inks require reduced curing temperatures, enabling use with plastic substrates. They also provide increased conductivity, can be used to fabricate transparent conductive structures, and demonstrate more robust ease of use. The recent success and commercial promise of silver conductive inks have led to additional investments in the development of large-area-format manufacturing equipment and hardware/software processing tools. These investments may help reduce manufacturing costs and, ultimately, lower prices. Table 1 identifies some of the technology needs and potential solutions for functional inks.
Platforms & ProcessesImprovements in process techniques, combined with printing platforms, are enabling more accurate delivery of semiconductor, conductor, and dielectric functional inks, thus allowing smaller device feature sizes and higher-performance products.
Metal- and polymer-based inks, along with advances in nanoparticle technology, will continue to produce materials that not only provide enhanced electrical attributes, but also lend themselves to printing and R2R processes. These materials must offer suitable physical characteristics to support the delivery and curing constraints of high-speed large-area-format manufacturing equipment.
The manufacturing process will continue progressing. More accurate placement, reduced device feature sizes, and tighter registration will allow higher-density electrical circuit designs. Improved techniques in material spraying, stamping, photochemical assembly and self-assembly (along with conventional printing methods), may also play a role. Several technology needs and potential solutions for printing platforms are outlined in Table 2.
Curing methods currently in development may offer improvements in the performance of functional inks. These simultaneously could reduce system costs by enabling use of non-thermally-stable substrates, which are less expensive. Thermal curing via convection or IR ovens is the dominant approach for most functional inks. Its major drawback is that some inks require high temperatures and/or long curing times. This requires expensive, thermally stable substrates. Various low-temperature cure technologies offer potential solutions.
UV radiation curing requires the formulation of functional inks to incorporate photo-initiators. UV-curable metallic-based inks currently in development will enable printed electronics to further benefit from the economies of scale offered by high-throughput R2R manufacturing. The potential savings in cost and cycle time has led to the adoption of UV curing by product manufacturers (e.g., composite substrate manufacturing and automobile components manufacturing). However, this method of curing raises potential materials compatibility issues, as photo-initiators can degrade electrical performance.
Laser and e-beam curing technologies are also being explored for printable electronics. Laser-based systems can cure a large variety of inks, but the systems are expensive and have low throughput. E-beam systems cure a limited number of inks, but have high throughput, and are widely available.
Photonic curing is a new cure method for metallic inks that require sintering.1 The approach uses a brief (~1 ms), intense pulse of light from a xenon flash lamp to selectively heat and sinter the metal nanoparticles without heating the substrate. Photonic cure is similar to that of laser-based systems, but less expensive and with higher throughput. Since the nanoparticles preferentially absorb the radiation, precise registration is not required. Photonic curing has been successfully demonstrated with various silver and copper nanoparticle conductive inks on substrates such as polyester, paper, polycarbonate, polyimide, polyethylene, and polystyrene. Ongoing research is investigating the applicability of photonic curing for semiconductive, dielectric, and resistive materials.
In-line Characterization ToolsOne technical hurdle is getting real-time quality control monitoring in place during production, via in-line characterization tools. Attributes of printed electronics such as as-printed device dimensions and electrical continuity require constant monitoring to ensure high product yield during high-throughput printing. In-line systems for printed electronics should draw tooling and best practices from both print manufacturing and IC fab manufacturing. Table 3 lists some of the technology needs and potential solutions for in-line characterization tools.
ConclusionThe large-area flexible electronics industry is still young and several technology and infrastructure needs must be addressed to fully realize the advantages it promises. Among these needs are continued development of materials and processes to adapt lower-cost large-area-format processes to the more stringent requirements for producing electronic devices, circuits, and components. Industry experts maintain that there is a strong need for the design of hybrid manufacturing platforms and associated workflow processes that incorporate best practices from the electronics and printing industries. Since the publication of the 2009 iNEMI Roadmap, we have seen increased activity to address this need.
REFERENCES:1. Schroder, K., McCool, S., and Furlan, W., "Broadcast Photonic Curing of Metallic Nanoparticle Films," NSTI Nanotech 2006, www.nsti.org.
Daniel Gamota is chair of the Large-Area Flexible Electronics chapter of the 2009 iNEMI Roadmap. He is president of Printovate Inc., a company positioned to bring novel large-area printed electronics products to the market, including lighting, energy, displays, communications, and life sciences applications. Contact him at email@example.com. For more information about the iNEMI Roadmap, visit www.inemi.org/cms/roadmapping/2009_Roadmap.html.
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