The Optoelectronics Industry Development association (OIDA) roadmap is aiming for SSL sources to reach 150 lm/W by 2012. For white LEDs to challenge traditional technologies such as incandescent (15 lm/W)and linear fluorescent lamps (85 lm/W) then improvements are needed in several areas, including internal quantum efficiency, light extraction efficiency and phosphor conversion efficiency.The efficacy of commercially available whiteLEDs is currently of the order 65 lm/W. The main efficacy gains made during the past several years can be attributed mostly to improvements in light extraction efficiency from the chip. Although new phosphor blends have been used to improve colour properties,they have not contributed much to the luminous efficacy gains. This may be mainly due to the fact that the commonly used YAG:Ce phosphor is very efficient. Here are discussed some of the recent techniques that have been used to improve light extraction efficiency:

• Silicone encapsulants

• Flip chip

• Thin-film (TF) technology

• Remote phosphors

Silicone encapsulants
A key aspect of good HB LED package design is the physical and optical characteristics of the material used to bond and hold adjacent components together when used as encapsulants, phosphor coatings and lenses.
Silicone-based materials offer many such advantages and are ideally suited for the HB LED market demands of high brightness, long lifetimes, lead-free solder components and manufacturing process for high-throughput packaging.

What are the optical characteristics of silicones that match silicones to the market demands?

Transmission: Silicones have less than 1% absorption in the UV-visible wavelength region with very little scattering loss; light that is produced by the LED is transmitted efficiently through the silicone material. Note that certain silicones grades are more prone than others to degradation after prolonged UV exposure which is observed as a characteristic yellowing of the material

Refractive index: The silicone polymers can be synthesised as a linear polymer with varying organic groups attached to the silicon atom; this group tailors the refractive index. Hence, the difference in refractive index between adjacent layers in the package can be minimised to reduce the losses from fresnel reflections or the refractive index difference can be optimised to control the amount of refraction at an interface.

Exceptional physical properties ideal for HB LEDs: In addition to their optical properties, silicones possess:

• A wide range of cured moduli from gels to hard resins – soft compliant gels are used because of their thermal-stress relieving characteristics.
• Good adhesion between substrate and various components.
• A variety of cure chemistries for easy processing – can be offered in either one-part or two-part compositions; can be accelerated with heat; shows little to no cure shrinkage; and there are no cure by-products.
• Can tolerate high lead-free solder reflow temperatures (260oC max).

Flip Chip
There are two methods for mounting a chip in an LED package. The conventional method is chip-up mounting, in which the active lightemitting ‘p-layer’ of the chip faces up, and the substrate is attached to the heat sink. Alternatively, there is chip-down (or flip-chip) mounting, in which the active p-layer of the chip faces down. Light is reflected up from the epilayer-substrate interface. The chipdown approach enables superior heat sinking which may enable the phosphor and encapsulant to remain cooler, since they are placed on the substrate side. Chip-down also removes the wire bond from the top face. However, chip-up has fewer processing steps and is less expensive to manufacture.

The Cree XBright is such a commercially available flip-chip package that combines highly efficient InGaN materials with Cree’s proprietary G·SiC® substrate. A highly reflective layer is added to the bottom of the chip during the wafer fabrication process to increase the luminous intensity. A metal dieattach layer is added which serves to protect the reflector and terminates with gold/tin.

Figure 1 Schematic of Cree Xbright and XThin chip structure

More recently, SemiLEDs Corp has successfully developed and commercialised a novel device structure for high-power blue LEDs. The metal vertical photon LED (MvpLED)TM has a p-down epitaxial structure mounted on a reflector layer which is attached to a metal alloy substrate. Details of the fabrication process are unavailable due to patents pending. The MvpLEDTM chip has a total thickness of 80 μm, with the substrate accounting for 75 μm. The metal alloy substrate provides very high thermal conductivity of 400 W/m·K, allowing highcurrent operation. The chip has a patterned surface with ‘photon-injecting nozzle’ microstructures to enhance light extraction in the forward direction. This thin structure could lead to further miniaturisation of personal electronic products. SemiLEDs can supply MvpLEDTM (SL-V-B40AA) blue chips in production today for 60 lumens per dollar.

Figure 2 SemiLEDs’ new MvpLEDTM device structure

SemiLEDs has collaborated with NeoPac Lighting to package SemiLEDs’ chips in a NeoPac fixture: eg four SemiLEDs MvpLEDTM (SL-V-B40AA) chips, packaged in NeoPac’s single-packaged, point-light-source NeoBulbTM light engine and operating at 4 W, can produce more than 240 lm with a luminous efficacy >60 lm/W. Or, using eight chips and NeoPacTM package platform, the NeoBulbTM light engine operating at 8 W can produce more than 460 lm with a luminous efficacy >58 lm/W.

Thin-film (TF) technology
Osram too has steadily improved optical extraction through shaped InGaN-on-SiC devices (the ATON chip) and its flip-chip variant, NOTA. More recently, Osram Opto has developed a thin GaN chip based on a new approach – thin-film (TF) technology – which has been incorporated into high-power products such as the Golden Dragon and Ostar. The device is reported to offer an extraction efficiency of around 75%, compared with 52% and 60% for the ATON and NOTA variants, respectively.

The key advantages of the TF process are:

• The substrate, used for crystal growth, absorbs a large proportion of the generated light. The TF process allows the substrate to be removed.

• The luminance (a measure of the brightness) of a chip can be scaled by increasing the chip area. In shaped chips, such as Lumileds’ TIP-LED and Cree’s XBright device, as the chip area is increased, sidewall emissions become less effective, so that the extraction efficiency and hence the luminance is reduced. An Osram TF chip does not rely on sidewall emission, and allows the luminance to be scaled with chip area. In the fabrication of TF devices, after growth, microprisms are etched into the epitaxial layer structure, and this structure bonded to a metallised carrier substrate, after which the original epitaxial growth substrate is removed by laser lift-off.

In the final device structure, a p-contact isformed at the base of each microprism. Lightemission is isolated to the active layer within each microprism but does not occur in the area directly underneath the n-contact bond pad. Furthermore, the geometry of the microprisms causes light to be efficiently reflected towards the top surface.

Figure 3 A schematic outlining TF devices

First-generation amber (615 nm) TF devices measuring 1 x 1 mm, that have been commercially available for about a year, have generated an output of 44 lm at 600 mA. Third-generation prototype 618 nm TF devices have demonstrated efficacy of 96 lm/W (at 20 mA), with a wall-plug efficiency of 33%. In comparison, Lumileds has reported an efficacy of 100 lm/W for its 605 nm TIP-LED chip, which has a much larger size of 1 x 1 mm.

Remote phosphors
The familiar process for LEDs to generate white light uses a blue-emitting chip (or die) coated with a phosphor material; the yellow phosphor absorbs light from the LED and reemits it at a different (longer) wavelength. The phosphor is in contact with the blue LED either in a thin conformal coating or in a thick cup. However, more than half of the photons produced by the phosphor are diverted back towards the LED chip where much of the light is both lost to absorption and adds to the chip’s thermal load. This reduces the white LED’s overall light extraction efficiency and is detrimental to its lifetime.

The LRC at RPI reported in April 2005 a remote phosphor technique known as scattered photon extraction (SPE) to significantly improve extraction efficiency and alleviate the associated thermal loading. The new SPE method works by moving the phosphor away from the die and by shaping the primary optic surrounding the die to extract a significant proportion of the back-scattered light before it is absorbed by the package. The remote phosphor concept is not new; it was first introduced by Chen in 1999 and more recently developed by Noguchi in 2004. But SPE is the first method to demonstrate efficient extraction of backtransferred light that results in over 60% improvement in light output and efficacy compared to similar commercial white LEDs, where the phosphor closely surrounded the chip. The key aspects of SPE optical design are illustrated in Figure 5.

Figure 4 White LED using SPE concept 

Further refinement of the remote phosphor technique was reported in September 2005 by LPI, who presented a new dual-opticbased remote-phosphor configuration. One disadvantage of remote phosphors recently showcased was that their phosphor area was much larger than the chip, which greatly reduced source luminance. LPI designed and developed a new dual-optic-based design that increased the source luminance by:

• Minimal increase in phosphor etendue over
that of the source.
• Greatly improved spatial uniformity.
• The yellow phosphor back-emission is recycled with a blue-pass mirror that re-illuminates the phosphor to increase its luminance.

The result is a new white-light source with superior luminance, efficacy and uniformity.

One commercially available device incorporating the remote-phosphor system is manufactured in a joint venture (JV) between GE Lighting and Emcore. The 1 W GELcore TetraTM Power White LED Lighting System contains one (1 x 1 mm) chip in a domeshaped package. The remote-phosphor technology allows various lamp forms and it has been scaled to 4 W (containing three chips). One key difference is that GELcore uses near-UV LED chips emitting at around 405 nm, in combination with phosphor blends that emit at selected wavelengths across the visible spectrum. This has several benefits for reasons discussed in the section on near-UV LED and phosphors.

Source: DTI Global watch mission report
Author: Mike Bean, Optical Design Engineer at Carclo Technical Plastics Plc