Ultrathin Inorganic LEDs

There is now a new process under development to create ultrathin, ultrasmall inorganic light-emitting diodes (LEDs) and assembling them into large arrays, which offers new classes of lighting and display systems with interesting properties.

Applications for the arrays, which you can print onto flat or flexible substrates ranging from glass to plastic and rubber, include general illumination, high-resolution home theater displays, wearable health monitors, and biomedical imaging devices.

“Our goal is to marry some of the advantages of inorganic LED technology with the scalability, ease of processing, and resolution of organic LEDs,” said John Rogers, the Flory-Founder chair professor of Materials Science and Engineering at the University of Illinois.

Compared to organic LEDs, inorganic LEDs are brighter, more robust, and longer-lived. Organic LEDs, however, are attractive because you can form them on flexible substrates, in dense, interconnected arrays. The researchers’ new technology combines features of both.

“By printing large arrays of ultrathin, ultrasmall inorganic LEDs and interconnecting them using thin-film processing, we can create general lighting and high-resolution display systems that otherwise could not be built with the conventional ways that inorganic LEDs are made, manipulated, and assembled,” Rogers said.

To overcome requirements on device size and thickness associated with conventional wafer dicing, packaging, and wire bonding methods, researchers developed epitaxial growth techniques for creating LEDs with sizes up to 100 times smaller than usual. They also developed printing processes for assembling these devices into arrays on stiff, flexible, and stretchable substrates.

As part of the growth process, a sacrificial layer of material embeds beneath the LEDs. When fabrication is complete, a wet chemical etchent removes this layer, leaving the LEDs undercut from the wafer but still tethered at anchor points.

To create an array, a rubber stamp contacts the wafer surface at selected points, lifts off the LEDs at those points, and transfers them to the desired substrate.

“The stamping process provides a much faster alternative to the standard robotic ‘pick and place’ process that manipulates inorganic LEDs one at a time,” Rogers said. “The new approach can lift large numbers of small, thin LEDs from the wafer in one step, and then print them onto a substrate in another step.”

By shifting position and repeating the stamping process, LEDs can transfer to other locations on the same substrate. In this fashion, you can create large light panels and displays from small LEDs made in dense arrays on a single, comparatively small wafer. And, because the LEDs can be placed far apart and still provide sufficient light output, the panels and displays can be nearly transparent. The thin device geometries allow the use of thin-film processing methods, rather than wire bonding, for interconnects.

In addition to solid-state lighting, instrument panels, and display systems, the new method also allows for flexible and even stretchable sheets of printed LEDs, with potential use in the health-care industry.

“Wrapping a stretchable sheet of tiny LEDs around the human body offers interesting opportunities in biomedicine and biotechnology,” Rogers said, “including applications in health monitoring, diagnostics, and imaging.”

LED Backlighting

It is said that energy-saving flat-panel television sets are about to become common in shops, spawning a whole new range of technical words to understand in Berlin.

Most manufacturers believe the best way to reduce TV power consumption is to change the type of lamp at the back of the flat panel, as well as to devise clever ways to reduce wasted light output.

The newest liquid-crystal-display (LCD) television sets are to feature LED backlights instead of the cold cathode fluorescent lamps (CCFL) which have done the job in the past. LED stands for light- emitting diode.

That is where the confusion starts, because at the same time, the electronics industry has been trying, without much success, to develop TV-sized displays where the image itself is formed by a matrix of LEDs.

LED backlighting has got nothing to do with that technology: all it changes is the light source that shines through the LCD screen.

Word has been spreading for a decade that LED light bulbs are more efficient than fluorescent lamps, so it is no surprise that TV manufacturers are also turning to this new light source.

Philips, for example, claims an energy saving of 40 per cent on its televisions.

The different ways of configuring this new type of backlight are sure to set off more confusion.

The simplest way to deploy the LEDs is around the four edges of the screen and let the light diffuse across the back of the screen. This is cheaper, and salespeople will make a point of explaining that these ‘edge-lit’ displays are even thinner than their predecessors.

‘For the bigger screens, this requires about 500 LEDs,’ explains Peter Koch of LG Germany.

More expensive are the so-called direct LED backlights. Instead of being placed around the edges, these LED lamps are arrayed right across the back of the screen. Direct-LED backlighting is a smart idea because the intensity of the light can be dimmed behind dark parts of the image. This ‘local dimming’ creates deeper, more natural blacks.

‘If the image is of people under a night sky,’ all the LEDs behind the sky will be turned off so that it really seems dark,’ explains Sascha Lange of Toshiba Germany.

This matters, because LCD televisions are often thought to be a degree inferior when compared to plasma flat-panel televisions. The black on existing LCD screens is generally a dark grey, and colours generally seem washed out when viewed off-centre.

Over time, local dimming also helps to save electricity and keep the TV set cooler.

The new backlights generally use LEDs that give off white light, but there is a third variant, the so-called RGB backlight system, which uses a mixture of red green and blue LEDs.

This will only be offered in the most expensive sets, aimed at buyers who want the very best. In fact, television broadcasts do not demand such a subtle graduation of colours, but the difference will be visible while watching the highest-quality high-definition films from Blu-ray discs.