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Super-tiny LEDs could pack a big punch in lighting applications

Super-tiny LEDs could pack a big punch in lighting applications

The usual approach of making LEDs by dicing up a wafer and connecting up individual diodes eliminates a lot of the most appealing configurations for LED lighting, argue researchers from the University of Illinois, Northwestern University, and the Miami University. Writing in the Proceedings of the National Academy of Sciences, they say taking a fresh approach to manufacturing LEDs could usher in applications involving large collections of ultrasmall, thin devices distributed uniformly, but sparsely, over emissive areas of large modules. End uses, among other things, could include direct replacements for troffers currently used in fluorescent building lights.

The researchers have come up with new materials, etching strategies, interconnection methods, thermal management techniques, and schemes for wavelength conversion and light distribution that could change the way LEDs are manufactured and, in so doing, make practical numerous uses that heretofore were impractical.

The process begins with removal of InGaN epitaxial material grown on silicon wafers using lithographically defined structures and anisotropic wet chemical etching, in ways that bypass conventional laser lift-off techniques and wafer dicing. When implemented with fully formed LEDs, researchers say, these ideas can be combined with deterministic assembly via transfer printing to allow high-throughput manipulation of devices with geometries that are orders of magnitude smaller than those compatible with robotic pick-and-place procedures that are the norm for assembling LED-based products today.

What the researchers have in mind are self-aligned techniques for thin-film metallization that exploit the large band-gap of GaN and which, they say, provide remarkably simple routes to interconnect large collections of devices. The outcome could effectively consist of a blanket of light: Finely distributed sources of illumination that naturally manage the thermal aspects of operation through dramatically accelerated rates for passive heat spreading, consistent with analytical models for heat flow.

Laminating such systems with patterned layers of phosphors and film-type optical diffusers yields thin, flexible lighting modules whose formats make them attractive for wide ranging applications in general illumination, both conventional and unconventional.

Researchers used multilayer InGaN epitaxial stacks grown on Si wafers rather than on conventional substrates such as sapphire or SiC because the silicon processing could potentially be quicker. Rather than cut LEDs from the wafer with a dicing saw. researchers used chemical etching. The procedure for releasing these devices from the underlying substrate exploits the large differences in etching rates (>100×) for one crystal plane compared to another. Chemical etching baths were of potassium hydroxide (KOH) or tetramethylammonium hydroxide. To take advantage of this effect, the arrays are configured such that two sides of each device lie perpendicular to an etching plane. The devices are tightly packed in this direction (i.e., spacing of 10 μm for this example, but with values that can be as small as 2 μm), and somewhat less so in the orthogonal direction. Immersion in a hot, aqueous solution of KOH rapidly removes silicon along one plane exposed by the mesa etch, thereby undercutting the devices without etching into the depth of the silicon wafer. Because the etching proceeds only along one particular plane, relief structures of silicon remain in the orthogonal direction between devices.

A pair of small supporting structures (i.e., anchors) of GaN, also defined during the mesa etch, connects each of the devices to the silicon in these regions (i.e., anchor bars), to yield freely suspended configurations after the KOH etching self-terminates. At this stage, the devices can be removed, in a nondestructive, high-speed and parallel operation, using soft stamps and the techniques of transfer printing.

In this way, assembly into arrayed layouts on glass, plastic, or other classes of substrate can be achieved at room temperature, with throughputs of millions of devices per hour and micron-scale positioning accuracy, in deterministic and adjustable ranges of pitch over areas that can be much larger than those defined by the devices on the source wafer.

The LEDs formed in this manner have emission areas and thicknesses that can be up to 1,600× and 100× smaller, respectively, than conventional devices (i.e., 1 × 1 mm2).

The full paper, "Unusual strategies for using indium gallium nitride grown on silicon (111) for solid-state lighting," can be found here: http://www.pnas.org/content/108/25/10072

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