Shake-and-Bake Technique Builds Displays

Daniel S. Burgess

Researchers at the University of Minnesota in Minneapolis and at Harvard University in Cambridge, Mass., have developed a self-assembly technique that may enable the manufacture of novel displays. According to the scientists, the shake-and-bake technique could lead to displays mounted on curved surfaces.

Researchers at Harvard University and the University of Minnesota are ordering their LEDs shaken, not stirred, in a process that may enable the manufacture of curved displays. Courtesy of Heiko O. Jacobs. Reprinted with permission from Science.

"The problem with current manufacturing techniques is that they work only on planar surfaces," said Heiko O. Jacobs, an assistant professor at the University of Minnesota who worked with Harvard's George M. Whitesides on the project. "If you want planar, it's OK to use traditional techniques, such as serial pick-and-place, serial wire bonding, serial packaging. But self-assembly is three-dimensional, so it can work on cylindrical or curved surfaces."

He also noted two other advantages to self-assembly. Current manufacturing techniques do not tolerate parallel assembly, making such approaches relatively slow and, thus, costly. Moreover, they cannot assemble arrays using elements that are smaller than 100 µm. The new technique, in contrast, should be faster and scalable for use with smaller emitters.

To build an LED display by self-assembly, the researchers begin with a flexible copper-polyimide substrate that features an array of solder-covered spots to which LEDs will selectively bond (A and B, below). They insert the substrate into a cylindrical vial with hot water and acetic acid to melt the solder and remove any metal oxide, respectively, and add the LEDs. Shaking the mix for up to two minutes ensures that the components assemble on the solder receptors (C). A subsequent agitation in another water bath removes any LEDs that have bonded incorrectly.

To attach the top electrode, which also is the flexible, transparent composite, they wet the copper wires in the substrate with solder as well as the exposed contacts on the bonded LEDs (D). Sandwiching the two pieces and heating them above the melting point of the solder fuses the LEDs to the top electrode (E).

To demonstrate the technique, the researchers fabricated a monochrome display with 113 280 x 280 x 200-µm GaAlAs LEDs (F). In tests using silicon blocks to simulate the LEDs, they found that they could similarly fabricate 5-cm², 1600-element assemblies.

Jacobs said that self-assembly also could be used to manufacture full-color displays. Three approaches could be employed; for example, utilizing three-color emitters, defining the receptors in different shapes to accept particular colors of LEDs or selectively activating groups of the receptors to bond with particular colors.

Because self-assembly can fabricate curved displays, he said, it could enable the manufacture of novel consumer-level displays, such as sunglass-mounted monitors or pen/cell phone combinations with a screen around the stylus. Although the team has no plans to commercialize the technique for such applications, it is in discussions with several companies to license it.