Nikkei Electronics Asia -- November 2007
Cover Story
Transparent Electronic Products Soon a Reality

Since the arrival of low-cost transparent transistors, R&D into transparent electronics has progressed rapidly. It will soon be possible, for instance, to embed transparent electronic circuits into large areas like windows, enabling the display of video imagery.

Displaying video imagery when required on windows, automobile windshields, or eyeglass lenses ... at last, the technology to make this possible is on the verge of becoming reality. Transparent electronics - which comprises transparent electrodes and semiconductors, along with various transistors, circuits and other elements - is expected to become commercially possible as early as a year or two from now.

It has been impossible to make transparent electronic circuits until now, because most circuits, electrodes and other elements are made of materials such as metal or silicon. If transparent electronic circuits can be made, it will become possible to embed them into large areas like windows. While a pane of glass might look like an ordinary window, it could actually incorporate a new type of electronics with a range of functions. Even places like walls, desktops and other locations could be equipped with "stick-on" electronics. The development seems likely to bring electronic circuits into every phase of our lives.

These transparent electronic circuits will probably not be complete products, to start with, but rather will appear as transparent electronic circuit components for items such as thin-film transistors (TFT). They will make possible a range of improvements in liquid crystal display (LCD) panels, organic light-emitting diode (OLED) panels and other displays, including larger screens, finer definition, high aperture ratio and simpler manufacturing.
After the industry has built up sufficient experience in using these components, the first transparent electronic products will finally appear.

The "Second Wave"
Technology to utilize transparent materials in electronic equipment has been around for a while. Transparent electrodes such as indium tin oxide (ITO) have become indispensable in touch panels, LCD panels, solar cells and more.

The technologies that are attracting so much attention now, however, are transparent versions of active, functional devices, such as transistors. As such, the development represents the "Second Wave" (Fig 1).
Some possibilities are next-generation electronic paper based on a flexible substrate, or TFTs to drive an OLED panel.

Korea Takes Lead
Some of the companies pushing this Second Wave are Canon Inc and Toppan Printing Co Ltd, both of Japan, Samsung Electronics Co Ltd of Korea, LG Electronics Inc of Korea, and Hewlett-Packard Co (HP) of the US. These manufacturers were among the first to recognize the potential of transparent TFTs, resulting in a product development competition.

LG Electronics is significantly ahead of the pack, though, in the degree of maturity of its developments. The firm revealed active-matrix OLED panels at the Society for Information Display (SID) 2007 symposium and exhibition held in the US in May 2007, and again at the European Materials Research Society (E-MRS) 2007 international symposium in France. It also held a demo showing actual video. One Japanese engineer who attended E-MRS 2007 explained his astonishment: "It's really at a commercial level already. I couldn't help but wonder how far they'd run with it by the next society presentation."

Close on the heels of the second wave, however, is the "Third Wave", which will use transparent p-type semiconductors. The development thrust here is aimed at complementary metal-oxide semiconductor (CMOS) integrated circuit (IC) manufacturing, and devices to compete with GaN blue, white or other light-emitting diodes (LED). It is likely that by this time there will not only be transparent components, but also transparent electronic products. Some possibilities are transparent smart cards, windowpanes with transparent solar cells, heads-up displays (HUD) showing driver support information directly on a vehicle's windshield, and so-called "head-mount displays" where the lenses of goggles or eyeglasses serve as displays.

Japanese universities have driven the development of the basic technology underlying the Second and Third Waves, but application technology is being developed around the world today. Korea, for example, launched the "Smart Window with Transparent Electronic Devices" national project in 2006 to develop windowpanes with active functions, and has already involved manufacturers in research and development (R&D).

Oxides Play Key Role
One major reason why there has been such interest and activity in transparent electronics recently is that there has been a sharp jump in the carrier mobility of transparent semiconductors, which determines transparent TFT characteristics. This now exceeds the carrier mobility of materials such as low-temperature poly-Si (LTPS) and amorphous Si used in LCD panels.

Even better, it means lower cost. Transparent semiconductors such as GaN and diamond are already known, but they come at high cost (materials, manufacturing, etc) which makes them impossible to use in transparent electronic devices demanding relatively large screens, such as displays.

The candidate materials attracting the most interest can be broadly divided into two oxide categories (see "Why Transparent Oxides Conduct Electricity" on p26). The first group is zinc oxide (ZnO), and the second is amorphous oxides with heavy metal content, such as amorphous InGaZnO4 (a-IGZO). Both pass visible light and are almost completely transparent. The carrier mobility of a TFT made with ZnO is 250cm2/Vs, significantly higher than the 100cm2/Vs achieved by LTPS. A TFT made with a-IGZO ranges from 1cm2/Vs to 100cm2/Vs, again significantly higher than the 1cm2/Vs max that amorphous Si provides. The pace of R&D has been accelerating in the last few years (Fig 2), with growth in ZnO carrier mobility especially rapid and manufacturers actively developing applications based on a-IGZO. Announcements like that of LG Electronics at E-MRS 2007 are based on a-IGZO.

A comparison of ZnO and a-IGZO shows that ZnO has the lead when it comes to carrier mobility. At present, though, a-IGZO is the material of choice for large-area displays, electronic paper utilizing low-temperature processing, etc. There are even some organic transparent semiconductor materials, but even the best only achieve a carrier mobility of around 5cm2/Vs. Organic semiconductors are therefore limited to applications with larger area where the lower cost can be leveraged.

Additional Strengths
Transparency is an asset not previously found in electronics. As Dr Masashi Kawasaki, Professor, Institute for Materials Research, Tohoku University of Japan explained, "Transparency makes it fully possible for these materials to compete with existing silicon materials." If high performance can be joined by transparency in equipment development, new products will be created with which conventional technology can't compete.

At the same time, though, transparent electronics "... do not have very many immediate applications that really utilize transparency," said Prof Hideo Hosono, Materials & Structures Lab, Dept of Materials Science & Engineering, Tokyo Institute of Technology of Japan. Development activity remains strong even without a range of target applications, however, because ZnO and a-IGZO have other strengths in addition to transparency (Fig 3). R&D projects are also very interested in things like (1) the wide band gap, which means that it may be possible to create ultraviolet (UV) LEDs, white LEDs with high color purity and other devices, (2) the high carrier mobility, which could lead to replacements for amorphous Si, LTPS, etc, and (3) the fact that low-temperature process means it is possible to manufacture light, flexible products. Development efforts focusing on these characteristics may converge into transparent electronic devices in the future, in the form of constituent technology.

LG Electronics, a participant in the Korean "Smart Window" national project, is first aiming at practical targets to refine constituent technologies, and plans to apply the technologies gained to actual transparent products later. As Honggyu Kim, chief research engineer, Conformable Devices Group, Display Research Lab there, explained, "Lately we're more interested in developing TFT drive arrays for OLED panels than in transparency itself. Things like transparent displays and functional windowpanes, however, are vital mid-range and long-range development goals."

Next-Gen Electronic Paper
Panel manufacturers are eagerly eyeing transparent oxide semiconductors as materials for TFTs to drive products like OLED panels, electronic paper and LCD panels. The carrier mobility required for a TFT is about 1cm2/Vs to 20cm2/Vs. The a-IGZO transparent semiconductor being so intensively researched has almost cleared this threshold, and engineers in the field don't see any more major obstacles looming (Table 1).

Evaluations of a-IGZO are high, with R&D into a-IGZO-based transparent TFTs under way at companies including Toppan Printing, LG Electronics and Canon. ZnO-based TFTs are only being researched by a few manufacturers, including Sharp Corp and Casio Computer Co Ltd, both of Japan. The general consensus is that a commercial a-IGZO TFT will be achieved first. According to LG Electronics' Kim, "We haven't found any major problems with a-IGZO TFTs yet. They offer the high uniformity of amorphous Si TFTs, simple device architecture, and the high carrier mobility of low-temperature poly-Si TFTs."

a-IGZO TFTs for E-Paper
The display application using transparent TFTs most likely to appear first in the form of a commercial product is next-gen electronic paper. The use of transparent TFTs means that the device is light and flexible, and will not break even if dropped. It can also display color. Toppan Printing has been interested in a-IGZO for quite some time, and is pushing ahead with electronic paper applications. It has developed prototypes of flexible electronic papers since 2005 with an active matrix display driven by a-IGZO TFTs. In March 2006 the firm announced that it plans to develop a commercial-grade prototype in fiscal 2008.

In May 2007 Toppan Printing showed a 4-inch color electronic paper prototype for the first time at E-MRS 2007. Dr Manabu Ito, research leader of the company's Product Innovation Laboratory, Technical Research Institute there commented that the display still "has a few defects"; however, it did achieve high definition, flexibility and color imagery (Fig 4). The image is quarter video graphics array (QVGA; 320 x 240 pixels), with a resolution of 100ppi, which is at least twice that of past monochrome prototypes, and delivers at least four times the definition of past color prototypes. There are four colors - red (R), green (G), blue (B) and white (W) - providing a small subpixel diagonal of only 125um. Toppan Printing's Ito said, "It was only possible because the TFTs are transparent." The electronic paper was developed jointly with
E Ink Corp of the US.

Alignment Unnecessary
The only way to attain this level of definition in a color display until now has been to align color filters and TFT array with precision, of several micron meters horizontally, and sandwich them together. The difficulty of this process has been a major obstacle.

Similar alignment is required in LCD panels, but there the color filters and TFT array have only a 4um to 6um spacer between them, making high-precision fairly simple. The situation is a bit different when it comes to electronic paper, though, because the display film from E Ink uses micro-capsules 40um to 50um in diameter. When the encapsulation film is included, said Toppan Printing's Ito, "Difficulty in focusing under the microscope made high-precision alignment very difficult." Another problem, he added, was that "The films were glued to each other with an adhesive, and it was therefore impossible to repair a misalignment."

Toppan Printing resolved the problem by utilizing the transparency of a-IGZO TFTs. Until 2006, the company had formed color filters on top of the E Ink display film, and then bonded the display film to the TFT array film. The new "front drive structure" electronic paper instead positions the TFT array between the color film and the display film. The TFT array is transparent, allowing light to pass through the display film from the color filter size without interfering with electronic paper functionality.

The manufacturing process for the new structure uses sputtering to form the TFT array on top of the color filter, making an integrated subassembly that is then sandwiched to the display film. "This approach eliminates the need for alignment during sandwiching, and really simplifies manufacturing," said Ito. Standard sputtering technology is sufficient when forming the TFT array on the color filter.

a-IGZO TFTs for OLED
a-IGZO TFTs seem to be the best choice for OLED panels as well as for electronic paper. LG Electronics revealed a 3.5-inch active matrix OLED panel with a-IGZO TFT drive at E-MRS 2007 (Fig 5).

LG Electronics has already developed and begun sample-shipping active matrix OLED panels, using low-temperature poly-Si TFTs. One of the reasons the company remains so interested in a-IGZO TFTs in spite of this is the worries about yield for low-temperature poly-Si TFTs. "Low-temperature poly-Si TFTs show considerable variation in characteristics, so you need a circuit to compensate for it. That makes the device structure, manufacturing process and other points pretty complex," said the firm's Kim. This complexity affects yield more than manufacturing cost, he explained, then added, "a-IGZO has about the same characteristic uniformity as amorphous Si TFTs. There's no need for a corrective circuit, so device structure can be as simple as amorphous Si TFTs, too."

LG Electronics mentions the low leakage current between source and drain as another advantage of a-IGZO TFTs. This feature makes them a good choice for driving OLED panels. The only real problem to be resolved before commercial application, according to LG Electronics' Kim, is that "Volume production requires dedicated sputtering systems for a-IGZO TFTs. Whether or not to make the investment is a management decision."

by Tetsuo Nozawa

Websites:
Canon: www.canon.com
Casio Computer: world.casio.com
E Ink: www.eink.com
E-MRS: www.emrs-strasbourg.com
HP: www.hp.com
LG Electronics: www.lge.com
Samsung Electronics: www.samsung.com
Sharp: sharp-world.com
Toppan Printing: www.toppan.co.jp/english

Free Subscription to Nikkei Electronics Asia Newsletter

Nikkei Electronics Asia magazine is available each month free of charge to engineers, managers and other qualified readers.

• Magazine Subscription Information
• The NE ASIA News Update e-newsletter

MOST POPULAR ARTICLES

24 Hours | 7 Days