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Metal foils offer several advantages over plastic, or polymer substrates, says Hatalis. Because metal can withstand much higher temperatures, it is easier to fabricate electronics, especially high-performance electronics, on metal foils than it is on plastics. Metal foils are compatible with most wet-chemistry processes, and their resistance to moisture makes them ideal for OLEDs.
In addition, metal-foil substrates can accommodate more components, such as display drivers. This improves reliability and adds functionality, thus eliminating the need for extra electronics.
The superior thermal properties of metals, says Hatalis, make them amenable to the polysilicon TFT technology that his group uses. Most metal-foil researchers work with amorphous silicon TFT technology, says Hatalis, but poly-Si TFT offers higher current and faster switching speed.
“For flexible displays, poly-Si TFTs can be used for both pixel-switching and integrated display-driver circuitry,” Hatalis wrote in an article published late last year in Applied Physics Letters. “Such integration not only increases the system’s reliability but also reduces the physical size of the flexible display.”
A full-service shop
Inside the Display Research Lab, Hatalis’s team designs, fabricates, characterizes and tests electronic circuitry as well as display and sensor systems on metal-foil substrates.
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“We have a unique infrastructure for building prototypes,” says Hatalis. “We design, fabricate and test all of our systems. We have demonstrated fully functional systems for both displays and sensors. Only a handful of labs worldwide have this capability.”
“We work beyond R&D to make a prototype system,” says Abbas Jamshidi-Roudbari, a graduate student who joined Hatalis’s team in 2003. “That’s why government agencies and industry are asking us to develop new electronic systems on stainless-steel substrates. We can do flexible circuitry and systems on a stainless-steel substrate only 100 microns thick.”
Hatalis’s group orders high-precision thin foils and photolithographic masks from outside vendors. Everything else is done in house.
The group has overcome several challenges posed by metal foils. These include the inherent roughness of stainless steel as well as changes in the microstructure caused by grain growth during annealing, or heating.
“Commercially available metal foils are very rough,” says Hatalis. “Any surface bump greater than 3 nm in dimension can cause an electrical short. We have learned to smooth the surface by polishing it and coating it with an insulating layer.
“Also, because the foil is not rigid, it presents issues during annealing, cleaning, lithography and other stages of fabrication. We have learned how best to handle the metal-foil substrate during processing.”
“There have been extensive studies of amorphous silicon TFTs on flexible substrates under mechanical strain,” Hatalis’s group reported last year in the Applied Physics Letters article. “However, no relevant investigation was reported for poly-Si TFTs in the past.”
In the article, the group reported that high-performance circuitry could be bent to a 1-cm radius of curvature and function without problems. At that radius, “all TFTs remained functional under strain applied up to 1.13% with no obvious physical damage observed. This shows that the poly-Si TFT technology on stainless steel foil is suitable for flexible electronics applications.”
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| In addition to providing visual information quickly, says Hatalis, flexible displays can be bent, folded, spindled and even embedded in clothing. |
Dreaming big
What will those applications be?
“In the next five to 10 years,” says Hatalis, “we can expect to see cell phones and other electronic gadgets no thicker than credit cards. Some iPods are this way already.
“Some companies have demonstrated prototype displays that unroll out of a box. We think we can also integrate flexible displays onto clothing, such as soldiers’ uniforms, and onto backpacks.
“In the movie Red Planet, three astronauts on Mars are shown rolling out a display of the Martian horizon to help them determine where they are. This kind of technology is not yet in electronics stores but it’s coming.
“Wherever people need to access visual information quickly, on unbreakable electronic systems that are easy to carry, flexible displays are going to make a big difference.”
Flexible electronics will also find biomedical applications.
Hatalis is collaborating with Lehigh chemistry professor Kamil Klier and Robert Dorner of the Royal Institution of Great Britain on a new generation of TFTs made from chalcogenide semiconducting materials.
“Based on our technology, we believe it would be possible to develop a disposable ‘computer patch’ that could monitor basic physiological functions. In just one example, the patch could be used to monitor babies and avoid sudden infant death syndrome.”
