Army targets flexible electronics, displays

Dr. Eric W. Forsythe, staff physicist for the Army Research Laboratory, Adelphi, Md., agreed to discuss the development of flexible electronics and flexible displays with MICROmanufacturing. Forsythe is the team leader for display technologies and is an associate program manager for the Army’s Flexible Display Center. One of the Army’s goals is to develop electronics for use in advanced communications devices with flexible displays. One of the main challenges is to increase the performance levels of flexible electronics to more closely match those of traditional printed circuit boards. The ultimate goal is to procure reasonably priced devices that soldiers can use without the risk of them being easily damaged.

Dr. Eric W. Forsythe, Army Research Laboratory staff physicist

MICROmanufacturing: Why is the U.S. Army interested in micro flexible electronics?

Forsythe: For all the same reasons that we’re interested in flexible OLED (organic light emitting diode) displays. They help reduce device weight and make technology more rugged. It’s not just the weight of the substrates, but the weight of the packaging. We need electronics that are inherently more rugged and won’t break as often. We need to reduce the weight of devices and reduce the cost of supplying our war fighters with that technology.

Illustration of how flexible electronics may be employed in flexible displays for use by soldiers. Photo courtesy of Army Research Laboratory.

MICROmanufacturing: What is the Flexible Display Program?

Forsythe: The Flexible Display Program is composed of several elements, the largest of which is the Flexible Display Center at Arizona State University. It is both a pilot-product manufacturing line and a developer of manufacturing technology for flexible (non-glass) electronic displays. It is a public and private partnership with funding from the Army, the state of Arizona and 30 industry partners. We are leveraging a lot of existing intellectual property, with the focus on technology that can ultimately be manufactured at a reasonable cost to the Department of Defense. Once the manufacturing processes are in place, the goal is to transition them to industry so the DoD can purchase the technology back at costs comparable to today’s Apple iPads.

MICROmanufacturing: Are prototype flexible displays being manufactured? What technologies do they employ?

Forsythe: The Flexible Display Center can produce limited quantities of flexible displays that the Army and DoD can have a hard look at. It allows us to begin thinking about how to integrate the displays into concept demonstrators. For example, E Ink (electrophoretic ink), a type of electronic paper manufactured by E Ink Corp., is commonly used in mobile devices, such as e-readers. That kind of low-power technology is useful for military logistics, maps and situational-awareness devices. On the other hand, OLED technology, which requires more power than electrophoretic displays but displays vibrant colors and lots of motion, is used in about half of all smartphones today. We’re working with the Flexible Display Center to see how we can use versions of these technologies in Army applications. We need to get them to a point where they are commercially viable, and where the technical risks have been removed. It’s our hope that those devices will be available with flexible displays that can be integrated into novel DoD applications.

MICROmanufacturing: What improvements need to be made in the deposition processes for flexible electronics?

Forsythe: The performance of electronics directly fabricated on plastic substrates is limited by two factors: process temperature and design rules. The process temperature (200° to 300° C, as a function of substrate materials) implies that the transistor active channel material must be amorphous or, perhaps, microcrystalline. This limits the carrier mobilities to 10 to 20 centimeters squared per volt second (cm^2/V-s), or perhaps slightly higher in some systems. The design rule limits (geometric and connectivity restrictions that ensure sufficient margins) are 1 to 5µm. The combination of process temperature and design rules sets the electronic “speed” of flexible electronics at around 10 to 100MHz, significantly lower than standard silicon CMOS (complementary metal-oxide semiconductor) devices. However, these deposition technologies allow electronics to be fabricated over very large areas. An example would be video displays and digital X-ray imaging equipment with individual-pixel electronics. Thus, directly fabricated electronics on flexible substrates employ technology that includes distributed electronic controls, switches and amplifiers in large-area arrays.

More exotic approaches have been demonstrated where high-temperature processes are used to fabricate channel material. Once fabricated, the channel material is transferred to a plastic substrate for transistor-processing. This potentially could increase the carrier mobility but would not impact the design rule. This makes the overall transistor-performance limit well below that of today’s Si CMOS technology.

Such transfer approaches are essentially the same as “transferring” today’s Si CMOS high-performance electronics onto plastic substrates. For example, tab-bonds, flexible circuit boards and RFID tags are commercially available. By comparison, Si CMOS uses a very high temperature process technology, which produces crystalline channels. Electrons move fast in crystalline material, and since CMOS currently uses 22nm design rules, that means electrons need not move far—only a few atoms away. This combination leads to significantly higher performance and extremely high transistor/unit area densities. For example, electron speed is 1,000 times higher in Si CMOS than it is in displays. Also, design rules will soon drop to 14nm, which will pack even more transistors into a smaller footprint.

However, if many Si CMOS circuits are required over a large area (say, m^2) to control individual pixels in an array, then the cost to pick and place becomes very high. Likewise, utilizing Si CMOS processing for a large area is not possible with today’s conventional electronic manufacturing processes. Thus, the direct fabrication of electronics for pixel-array-integrated Si CMOS for high-end processing can produce unique hybrid systems, with displays being the first implementation.

MICROmanufacturing: What recent improvements in flexible electronic technology will be appearing soon in commercial products, and what improvements can micromanufacturers expect in the future?

Forsythe: The near-term improvements might be higher-mobility, directly fabricated transistors. More important will be the development of thin-film transistors with higher operating stability, which will increase the application space. Other improvements could be better techniques for integrating standard Si CMOS on flexible substrates and system-level packaging to leverage those technological bases. µ

For more information about the Flexible Display Center, contact Dr. Forsythe at (301) 394 0606, or at [email]eric.w.forsythe.civ@mail.mil[/email].