Printable electronic materials show considerable promise

Rapid progress in microelectronics has led to exponential increases in computational power over the last 40 years, accompanied by comparable decreases in both the size and cost of electronic devices. This has enabled the spread of electronics and computer technology – once the sole domain of scientists, engineers, and dedicated hobbyists – into every corner of our lives.

Much of this change has been fuelled by intense R&D in the electronics industry, resulting in the evolution of chip designs with ever-increasing technical sophistication. However, despite these advances, the basic process used in chip manufacture remains the same: deposition of materials onto rigid, highly pure substrates, followed by masking and etching steps, with the whole cycle repeated as needed. Although highly effective for the production of sophisticated electronic architectures, this process has a very high capital requirement, and its batch-based workflow limits the amount of customization that can be introduced at the fabrication level.

Progress in electronics has also played a large role in driving changes in the way documents are printed. The same revolution in office and home printing that turned inkjet and laser printers into common household items has taken root in the world of commercial printing, where the plate-based lithographic printing processes first developed centuries ago are increasingly being replaced by digital technologies that deposit ink on paper (or other substrates) without a plate, under the control of systems that ensure consistently high resolution and colour accuracy.

Printing know-how turned toward electronics industry

The marking materials used in these printers are themselves the products of many years of materials chemistry R&D, which has led to inks and toners with highly controlled particle sizes, shapes, and surface features. This know-how is now being turned back toward the electronics industry, providing opportunities for electronics fabrication that circumvent some of the limitations of current techniques.

Relative to the traditional process, printing electronics provides a manufacturing method with lower capital costs, fewer steps, and the ability to work under ambient conditions. Conductive and semi-conductive materials can be formulated into inks, which are then printed via inkjet directly onto substrates (which need not be rigid) without masking or etching. While best suited for low-end, disposable electronic devices, printing electronics enables a degree of flexibility and customization not yet available in traditional fabrication.

“Any printable electronic material

must meet certain criteria to be

useful in fabricating electronic devices”

To be useful in fabricating electronic devices, any printable electronic material must meet certain criteria. Printed lines must be narrow, smooth, and (for conductors) conductive enough to enable the preparation of functional device arrays of reasonable density (such as organic thin-film transistors). The height of the printed material on the substrate must be sufficient for conductivity, but not too thick for multilayer device printing. Finally, printed features must be stable and robust, whether printed directly on the substrate itself or on top of pre-deposited materials.

Designing inks that enable rapid printing of electronic devices

As part of our R&D program in printable electronic materials, we focused on designing inks that enable rapid printing of electronic devices on a variety of substrates, ranging from glass to plastic or even paper, while avoiding these potential pitfalls. Key to this has been the development of a silver nanoparticle ink consisting of stabilized silver particles (5-10 nm in diameter) dispersed in a hydrocarbon vehicle. Xerox nanosilver ink, which is now routinely produced in 40-litre batches, can be printed in narrow lines on both hydrophilic and hydrophobic substrates. Viscosity and metal loading are optimized to provide maximum resolution and eliminate the formation of satellite drops and broken or uneven lines, and the extremely small size of the silver nanoparticles allows the metal to be sintered at temperatures far below the melting point of bulk silver. This provides smooth, conductive silver lines and films at plastics-compatible temperatures (120ºC).

*See Figure 1.

Proper ink formulation can enable beneficial printing properties. For example, silver nanoparticle inks containing surface-modifying agents give rise to self-correcting behaviour – as the ink drop recedes during drying, the underlying surface becomes resistant to further deposition of silver nanoparticle ink, ensuring even spacing between adjacent features. In addition, carefully choosing the solvent vehicle for the ink enables printing of conductive features on top of existing, solvent-sensitive layers. These techniques have been used to print a silver-based source-drain array with 400 µm pixel size on top of a layer of organic semiconductor which itself remains undisturbed during the printing process. In this example, the self-correcting effect provides an organic thin-film transistor array with even channel widths.

*See Figure 2.

While not yet equal to the performance of traditionally fabricated devices, printable electronic materials show considerable promise. As this field matures, the prospect of simple printing of electronic devices – on the bench-top or in a printing facility, using commercially available printers, inks, and flexible substrates – will enable even more life-changing applications.

www.xerox.ca/xrcc