The growing field of printed electronics combines liquid functional materials with state-of-the-art printing equipment to create semiconductor components and electronic circuits. The resulting devices are functionally similar to their traditional silicon-based counterparts. However, they're also less expensive and have a number of unique features that open the door to a wide range of new electronic applications, from tiny "smart labels" to full-body-sized medical imaging equipment.
Printed semiconductor technology delivers a sharp increase in productivity by building on a variety of established and familiar printing techniques. But while traditional graphic arts must only look good to the naked eye, electronics require precise electrical, mechanical, and optical properties. No matter which printing technology is selected, printed-electronics companies demand state-of-the-art equipment and procedures.
Virtually any printing technique can be adapted for semiconductor manufacturing, but certain processes better suit particular materials or applications. Each technology has its pros and cons. In practice, multiple printing processes may be combined in series to produce a single device.
One of the most popular technologies in printed electronics is ink-jet printing. A typical ink-jet printer has several print heads (one for each color or ink type), each with dozens of tiny nozzles that spray ink onto the substrate. Because it is a fully digital technology, it does not require any tooling. An electronic design can be directly converted into a printing file. It lends itself well to rapid prototyping and customized batch production, but it also can be used in a high-volume environment.
Ink-jet printing has many advantages, including fairly high resolution (80- to 100-Î¼m lines), flexibility, relatively low cost, and compatibility with almost any type of substrate. Printed electronics is driving further equipment development, as the newest ink-jet heads may be capable of 20-Î¼m feature sizes, which would greatly expand the use of ink-jet technology in electronics.
Another common technique in printed electronics is screen printing. A screen consists of a finely woven porous fabric or metal mesh stretched over a frame. A stencil on top of the screen blocks off the areas where ink should not pass. The screen is placed on top of the substrate, and ink is applied. A rubber blade pushes the ink through the open areas of the screen, and the screen is then lifted away.
Screen printing can be used with a variety of substrates. It's also possible to deposit thick films in a single pass. On the other hand, it cannot be used to deposit extremely thin layers. It was once considered a very low-resolution technique, but state-of-the-art screens can achieve features as small as 40 Î¼m, with sharper edges than ink-jet.
A relatively new technology that could be used in printed electronics is nanoimprint lithography. Based on traditional photolithography techniques used in the graphic arts, nanoimprint begins with a three-dimensional stamp.
A layer of liquid resist material is either spin-coated or dropped onto the substrate. The stamp is pressed onto the resist (Fig. 1a), and the material is hardened (Fig. 1b) with either heat or UV exposure. When the stamp is removed (Fig. 1c), the hardened resist maintains the shape of the stamp. The residual layer of resist may then be etched away (Fig. 1d).
The patterned resist can then be used as a mask to pattern subsequent layers and then be dissolved. Alternately, the resist material, properly formulated, can itself be a functional layer in the finished device. This promises to be an excellent high-resolution technique. Resolution is limited only by the stamp-making process and could be as small as 20 nm - several orders of magnitude below the resolution of ink-jet or screen print. The challenge may be formulating resist materials that also have the desired electrical and optical properties.
Because graphics printing is done on a wide variety of surfaces, today's commercial printing technology can print on nearly any material. The technology is ready to handle anything from thick glass to rough paper or plastic to thin plastic film. Even curved surfaces are possible.
This ability provides a number of advantages for electronics. Instead of being bound by thick, rigid silicon substrates, electronic components and circuits can be ultrathin, lightweight, bendable, and transparent. Substrate size is limited primarily by the printing technology, and commercial roll-to-roll printing equipment can print on surfaces 2 m wide and kilometers long.
The adoption of printing techniques requires liquid functional materials - conductors, semiconductors, insulators, and so forth. Although printed electronics is often discussed as if it were synonymous with organic electronics, in practice, both organic and inorganic materials may be used.
Printable inorganic materials include metallic nanoparticle materials such as silver and semiconducting materials like quantum dots in solution. The organic materials are based on the Nobel Prizewinning discovery in the 1970s that conjugated polymers have semiconducting properties. These materials can be tailored to have application-specific electrical and optical characteristics.
The basic functional difference between traditional silicon-based components and organic semiconductors is that silicon-based components have a 2D electrical charge interface, while components made with printable inks have a 3D interface. In a silicon device, donor and acceptor materials are separate layers. Only charges generated near the interface between the layers can contribute to current flow.
In organic semiconductors, on the other hand, donor and acceptor materials are mixed in a single layer, creating an interface over the entire 3D bulk of the layer. As a result, almost all generated charges contribute to current flow. Because the organic material is a blend of chemicals, simply varying the components in the mixture can alter the electrical and optical properties of the resulting device.
While there are some basic similarities to traditional silicon semiconductor fabrication, printed semiconductor production is much faster, simpler, and greener. What can take thousands of people weeks or months in a traditional multibillion-dollar silicon fab, a few dozen people at a printed semiconductor plant can generate in hours or days for a small fraction of the cost.
The production process begins with formulation of the functional materials into printable "inks" (Fig. 2). Each material is mixed for application-specific mechanical, electrical, and optical properties. The purpose is similar to that of the doping process in traditional fabs, but is much simpler and offers more flexibility. Additives may be needed to adjust drying time or viscosity.
For instance, ink-jet nozzles are extremely small, and the ink must have the right viscosity and surface tension to pass through. Screen printing, on the other hand, uses lower-viscosity ink. At the same time, the ink must function electrically. Viscosity and surface tension must be adjusted to avoid problems, such as drops that do not bond enough to conduct sufficiently, or an excessively rough surface that prevents the next functional layer from adhering.
Next, the substrate is cleaned and treated to promote adhesion. The functional layers are deposited using a specialized industrial printer (Fig. 3). The layers must align precisely. As a result, the substrate is mounted on a self-aligning X-Y table, a precision instrument with high-speed linear motors enabling precise control of movement with sub-micrometer accuracy.
Each layer must be dry before the next layer is printed. To speed drying time and improve electrical properties, each layer is cured. Conductive silver ink, for instance, contains silver nanoparticles encapsulated in a polymer for better printing. However, the encapsulation can prevent the particles from attaching to each other. Heat-curing evaporates the polymer, increasing conductivity.
Once the last layer has been deposited and cured, the devices may be encapsulated for robustness or longer shelf life, if the application requires. Devices are tested and then separated with a computer-controlled laser or glass dicing system. Finally, they go through final test and burn-in.
The new technical features available with printed electronics - thinness, light weight, flexibility, disposability, transparency, large size, and customizable optical characteristics - combined with low cost and faster turnaround, have enabled a wide range of new applications. These include displays and lighting: rollable, foldable displays for mobile devices, e-books, and large changeable billboards, as well as low-cost, high-efficiency lighting in any color on any surface (i.e., windows that become light sources at night).
Printed electronics also will play a role in sensors. Imagine large imaging sensors for medical and industrial applications (Fig. 4). The technology also will find use in thin, lightweight biometric recognition systems integrated into mobile devices. And, it could be used in tiny sensors on lab-on-a-chip systems for realtime point-of-care medical and environmental testing.
Next, just about everything can get smarter, as combinations of printed sensors, logic, memory, and communication appear in products that haven't included electronics. Applications include RFID tags for inventory control, interactive product packaging that talks or plays games, smart food packaging that changes the use-by date, drug packaging that monitors and communicates patient compliance, and clothing that monitors the wearer's vital signs and helps regulate body temperature.
Printed electronics could provide power as well. Flexible, high-efficiency photovoltaics could power mobile devices and commercial/residential power, while lightweight photovoltaics and thin-film batteries could power printed electronic devices.
Of course, this is only a sampling of the possibilities. In the 1950s, no one envisioned the pervasiveness of silicon technology. Yet today it's everywhere, in computers, cars, mobile electronics, appliances, and toys. Similarly, we don't yet know all of the ways that printed electronics will be used. Perhaps the greatest "killer app" is just around the corner. But we do know that it is a viable technology, enabling life-enhancing applications and creating new opportunities.
KLAUS G. SCHROETER is founder and CEO of Nanoident Technologies AG, which develops and manufactures printed semiconductor-based optoelectronic sensors. He has more than 20 years of CEO/CTO experience in VC-backed high-tech companies. He can be reached at [email protected]