As Expectations for LED Lighting Grow, So Must Advances in Fabrication Technology

Clive Beech
LED Components Manager
As Expectations for LED Lighting Grow, So Must Advances in Fabrication Technology
LED technology has advanced a long way in recent years - from he small LED indicators used years ago, LED lighting is now a major technology providing everything from television displays to domestic and commercial lighting as well as having many automotive applications.

LED lighting is no longer a novelty – consumers, businesses and governments now appreciate the very real energy-saving and environments benefits this technology is capable of. Users have also been won over on the advantages of LED lights compared to other energy-saving alternatives, so much so that they are now demanding even better solutions for an ever-wider range of applications.

The battle for improved energy efficiency, especially in the home, began some 30 years ago with the compact fluorescent lamp (CFL), which offered an energy reduction of up to 75% and, with an 8 times life expectancy, promised significant cost savings over the bulb’s life despite an initial high cost. But convincing consumers to adopt CFLs wasn’t easy and, in the early years, often required governments to subsidise bulb costs via energy utilities to encourage uptake.

Even so, consumers weren’t happy with the poor light quality, slow start-up and warm-up times, lack of dimmability, and the unattractive appearance of CFL bulbs. This led to a resurgence of filament technology with the halogen bulb, certainly in niche applications, despite it only being 10-20% more efficient than a standard incandescent bulb.

It is not surprising then that solid-state lighting technology has had an immediate appeal. LED bulbs offer instant-on, full-bright performance in more natural colours, with energy-saving benefits every bit as good or better than CFLs, and a life expectancy 3 times that of CFLs and up to 25x that of incandescent bulbs. LED technology also provides a more robust solution with more innovative designs possible due to the small size of the LED chips, so not just a replacement technology for existing bulbs.

However, to misuse that common expression ‘success can be its own worst enemy’, what we are seeing now is a case of ‘just because it’s good, doesn’t mean it can’t be better’. Like CFLs, cost was an initial hurdle for LEDs despite a similar lifetime savings argument. And the dramatic reduction seen in LED bulb prices in just the last few years doesn’t stop customer demanding even lower unit prices. Similarly, there is a demand for even better light quality and output levels, more uniform and more controllable.

Fortunately, once again technology comes to the rescue as demonstrated by advances in the design and fabrication of chip-on-board (COB) LEDs, which provide the basis for many forms of LED lighting.

Today’s Established COB Manufacturing Process and Its Limitations

The high-power LEDs required for lighting are actually manufactured as blue LEDs using gallium nitride (GaN) semiconductor material, with a phosphor lens used to convert the optical output to white light. In volume, GaN devices are most commonly fabricated on a sapphire substrate as this is much less expensive than the alternatives of silicon carbide or even GaN itself. A number of the singulated LEDs produced in this way are then typically mounted together on a common substrate and connected in series with wirebonds. This chip-on-board process produces devices that provide a higher light output than the individual LED die and at a lower cost than comparable higher-power LEDs. Also, the series connection allows the COB LED to operate from a higher supply voltage than individual LEDs.

Light extraction from COB LEDs is usually enhanced with a silicone lens applied over the LED dies but forming the beam, using secondary optics to maximise utilisation of the emitted optical flux from a multi-die COB LED, can be more challenging than with a single LED die. This often results in a luminaire with a relatively large lens and reflector assembly, which in turn contributes to its size, weight, and overall cost. Furthermore, the spacing of multi-die COB LEDs can significantly reduce the light-emitting surface (LES) area, especially at the optical centre of the device, as can be seen in the four-die example shown above when compared to a single die that can also use smaller secondary optics. Although the design of the lens and reflector can compensate for this to some extent, that is at the expense of increased size and weight.

LEDs fabricated with conventional GaN on sapphire technology also suffer from thermal and optical performance issues. Sapphire’s poor thermal conductivity (27 W/m-K) poses a challenge in maintaining die temperature while trying to maximize optical output by driving LEDs at full rated current. Avoiding the reliability impact of excessive temperatures requires the use of heatsinks to control the LED junction temperature at high currents but these heatsinks further contribute to the cost and physical size of a luminaire.

The way that the LED active layers are fabricated in a conventional stack on a sapphire substrate results in a relatively poor optical efficiency. This, combined with the light absorption of sapphire can account for the loss of as much as 20% of the luminous flux from a GaN-on-sapphire LED.

The Answer is GaN-on-Silicon

Silicon is a long and well-established semiconductor technology, which accounts for the majority of semiconductor devices produced in the world today. As a substrate, silicon’s thermal conductivity (149 W/m-K) is over five times better than sapphire and, because of the shear volumes involved and the equipment and infrastructure associated with producing regular silicon chips, fabrication costs can be substantially lower. The improved conductivity allows a smaller heatsink to be used when operating die at the same current and temperature, with obvious size and cost benefits.

GaN-on-silicon fabrication uses a patented silicon-etch process that begins with indium gallium nitride (InGaN) layers grown on the silicon substrate to provide a buffer layer on which the GaN layer is grown, followed by an n-GaN layer, multiple quantum wells and a p-GaN final layer. Electrodes to connect to the top surface can be formed with additional metal and insulation layers. Then the wafer is flipped over so that the original substrate can be removed, allowing what is now the new top surface to be patterned to enhance light extraction, with top contacts finally being made for both the anode and cathode.

Plessey has developed the GaN-on-silicon process based on benefits from the maturity and stability of established 6-inch CMOS wafer manufacturing technology that is considerably less expensive than GaN‑on‑sapphire. Further economies are gained because the GaN-on-silicon process requires fewer masks and mask alignment is less critical. More importantly, when compared to the existing approach to producing multi-die COB LEDs, the process allows monolithic multi-emitter fabrication that both eliminates the need for inter-die bonding and enables much closer spacing of the emitters in a series connected array, resulting in a greater LES area at the centre of the LED.

Plessey’s multi-emitter GaN-on-silicon LEDs provide the basis for its PLW7070 series LEDs, which deliver superior optical performance and electrical efficiency and can be used with a smaller secondary lens and reflector than is usually required with a conventional COB LED. Further optical efficiency is achieved as a result of the surface etching of the silicon and wafer-level phosphor deposition, which uses a proprietary thin-film process. Testing of this LED structure has already demonstrated excellent reliability at elevated temperature and at various stress current levels, while the lumen maintenance achieved over 5000 hours at a stress current of 1250 mA.

Conclusion

Plessey’s PLW7070 series offers a multi-emitter LED using GaN-on-silicon technology that fits the industry standard 7070 footprint and can use any standard secondary optics, while delivering a 30% increase in light output compared to multi-die COB LEDs when. Furthermore, the economies offered by its patented GaN-on-silicon process and other proprietary techniques achieve a 50% reduction in cost compared to other power LED solutions.

The superior thermal performance of the silicon substrate allows lighting designers more freedom to create luminaires with smaller heatsinks and other optical components, enabling innovative new lighting developments for industrial, commercial and consumer applications. 

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