By Dr. Phil Garrou, Contributing Editor
Apple has acquired 24 tech companies in the last 18 months. Recently, Apple acquired LuxVue, a start-up focused on low power micro-LED displays. Although Apple has not disclosed any details of the acquisition, not even the purchase price, one can easily envision where micro LED displays could play a big part in Apples thrust into wearable electronics such as the i-watch. Reportedly the LuxVue display is 9 times brighter than both LED and LCD screens. Such µ LED displays would be compatible with curved surfaces and would save power and thus increase battery life. Brighter, lower power displays could have applications in other Apple products and products such as Google glasses.
Little is known about the LuxVue technology other than the patents that have been issued. They have no web page and have made no public presentations that I can find. The do have several dozen patents many of which deal with transferring micro devices, which specifically for displays would be LED devices as small as 10 x 10 miron and placing them on 10 micron pitch.
Their preferred transfer device consists of a substrate with an arrangement of protruding mesas. Each mesa contains electrodes with a thin coating of dielectric. By providing a charge between the electrodes, an electrical field is created which electrostatically attracts the miniature chips or LEDs [for example, see USP 8333860 B1].
It is proposed that using this approach one can selectively choose to remove specific micro devices from a source substrate by applying voltage only to the projecting mesas corresponding to the positions of the device to be transferred.
In essence, this is massively parallel pick and place. We are all familiar with conventional pick-and-place assembly using vacuum collets and pin ejectors. For devices that are on the scale of 10 micron however, manipulation and accurate placement are significantly more difficult with today’s tools. This LuxVue “electrostatic chuck” mechanism is one way to deal with manipulation of such small devices.
If massively parallel pick-and-place sounds familiar, recall similar technology Semprius and more recently X-Celeprint have been developing. These startups are based on the work of John Rogers at U. Ill. Which uses PDMS stamps rather than the more complex electrostatic chuck. [link 1] [link 2]
I have compared the two technologies in the figure below:
I contacted Professor Rogers and X-Celeprint CTO Chris Bower, and they agreed that microassembly of such LEDs is indeed the sweet spot for X-Celeprint technology. They sent the images below which show some early examples of transparent and flexible devices using Micro-Transfer-Printed inorganic LEDs. At this point, they were just willing to say that this was an area “of active interest.”
Double printing the RGB LEDs allows circuit redundancy, so a bad LED or connection does not produce a bad pixel. This is the technique already in use by display manufacturers [link 1] and more recently 3DIC practitioners to insure that a bad TSV does not result in a failed chip [link 2].
As devices continue to get smaller, we can expect to see micro transfer technologies such as these take a more prominent role in their assembly.
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