technology LEDs
Figure 2. LEDs of (a) inverted pyramidal and (b) inverted conical
geometries
shapes. Considering the fact that LED chips in the market are invariably diced into squares or rectangles, device designers should give serious thought to re- designing the chip.
Our findings from these simulations are backed up by measurements on packaged but un-encapsulated LED chips laser micro-machined into various shapes (see Figure 1). These results show that the square LED is on average 16 percent less efficient than other polygons. Of course, one could argue that the chip packing density, and thus chip count across a wafer, might be compromised. Therefore, we propose the use of triangles and hexagons; such shapes can be closed-packed into any array without sacrificing chip space and thus make economic sense.
An additional feature introduced into our optical setup enables machining of three-dimensional freeform structures. A mirror inserted into the optical path between the focusing optics and the machining plane allows the focused beam to strikes the wafer at an
oblique angle. LEDs of truncated pyramidal (TP) structure, as depicted in Figure 2(a), are formed simply by applying four successive oblique cuts along the four edges of devices. On average light enhancement is increased by 89 percent over similar chips with vertical facets, consistent with our theoretical prediction of a 85 percent gain obtained by ray-tracing.
The inclined sidewalls serve as reflectors to redirect laterally-propagating photons into the escape cone. Devices of TP-geometry also emit with a wider divergence angle. Combining oblique angle machining and rotary machining, LEDs of truncated conical structures can also be formed (see Figure 2(b)). Subsequently, a reflective metallic layer is deposited onto the bottom and inclined sidewall surfaces of the chip to form an integrated reflector cup.
This structure is particularly beneficial for building white LEDs with homogeneous emission. In a typical white LED, an epoxy mixture containing phosphor particles is coated prior to encapsulation. The phosphor mixture is allowed to reflow in order to cover both the chip surface and sidewall facets, resulting in a dome-shape-like non- uniformity, the effects of which are particularly prominent at the edges. Such thickness non-uniformities give rise to non-homogeneity of colour emission at different viewing angles.
With the integrated reflector cup design, light emission via sidewalls is suppressed effectively. The majority of the light rays are emitted from the top planar surface, so that the divergence angle of the device can be reduced by 16°. Consequently, only the top planar surface needs to be phosphor coated; planar coating produces significantly improved uniformity that paves the way for superior colour homogeneity.
Figure 3. (a) A fibre coupled hemispherical LED assembly and (b) RGB LED stack
26
www.compoundsemiconductor.net January / February 2011
Coupling LED emission to fibres It is possible to incorporate a hemispherical lens by inverting and flip-chip bonding the truncated conical chip to a package. The lens can be attached via liquid capillary bonding to produce a nanometre scale air-gap
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