As LEDs continue to displace traditional lighting technologies, there are an increasing number of applications that require high lumen output, high-power LEDs. While the major manufacturers of LEDs offer single-chip power LED products, they are unfortunately still very expensive compared to mid-power class products. Taking advantage of advances in LED chip-scale packaging, manufacturers have typically combined four mid-brightness LED chips into one package to increase lumen output while keeping costs low. The downsides are that multi-die LED products have different optical characteristics to single-die LEDs, and more demanding thermal properties compared to multiple separately packaged LEDs. This rules them out from some applications that require high brightness combined with tight beams, including high bay lighting, floodlighting, street lighting and handheld portable torches.
The answer to this problem seems improbable: simply manufacture some single-die power LEDs that can produce the required lumen output at the same time as overcoming the thermal and optical issues of multi-die solutions, while also being significantly cheaper. However, with cutting-edge process technology and device design, Plessey has made this happen. Our PLW7070 single-chip LEDs are 50% cheaper than any other power LED solution, comparable in price with ordinary mid-power LED products.
These dramatic cost savings are enabled primarily by Plessey’s MaGIC LED process technology, which builds GaN LED devices on silicon. Elsewhere in the industry, LEDs are built on expensive sapphire or SiC substrates, since growing GaN layers on top of cheaper silicon wafers is particularly difficult. The lattice spacing of atoms in the silicon wafer is a complete mismatch with GaN, so buffer layers of materials with intermediary lattice spacings must be used to bridge the gap. This is a complex and difficult area of process technology; incorrect buffering means wafers will bend and crack due to the excessive strain placed on the GaN layers. Plessey gained extensive knowledge and expertise in this technique when it acquired CamGaN in 2012.
Once the buffer layers are complete, n-GaN, multiple quantum well (MQW) and p-GaN layers are grown along with metal layers for interconnection and an insulation layer. The whole assembly is then bonded to another wafer and flipped upside down, the original substrate is removed, and the newly exposed surface is patterned to allow for maximum light extraction. Negative and positive contacts are also formed at this point. The wafer is metallised on the back with a nickel–silver layer to allow for good die adhesion and thermal conduction to the package (Figure 1).
High Power LED manufacturing flow schematic
Plessey’s process allows LED junctions to be ‘stacked’, that is, built on the same substrate and connected in series such that a monolithic LED die can contain many diodes. For example, in our 7070 series we currently offer a 3V product, and a 12V product which is effectively four 3V junctions in series. In this way, we can build very high-voltage products. This is not possible in sapphire, since with this technology, light is emitted in all directions (Figure 2). While external mirrors can be used, some light is inevitably lost in the substrate. With silicon, the substrate acts as a mirror, reflecting all the light out of the device with minimal internal losses. Combined with silicon’s superior uniformity across the wafer, this means bigger dies can be created – anything from micro-LEDs for displays right up to super-sized lighting products.
Silicon acts as a mirror, reflecting light out of the top of the device, whereas in sapphire, some is inevitably lost inside the substrate.
The benefits of building LED dies in this way are many. Firstly, the cost is a lot lower than competing sapphire and SiC products; silicon wafers cost about a fifth as much as sapphire and a tenth as much as SiC. The cost savings don’t stop there, though – since we use standard 6-inch wafers, rather than sapphire’s and SiC’s 2- or 4-inch ones, there are economies of scale to be realised in the back end part of the process. Many of the processes used for GaN-on-silicon wafers are fully automated, and it takes as little as 7–10 days for Plessey to take bare wafers and turn them into LEDs.
There are also optical benefits for GaN-on-silicon LEDs. A traditional approach using four mid-power LED chips in one package still has an important flaw. While the dies are packaged as COB products, minimising the gaps between the four chips, the chips still have edges, resulting in a tiny non-emissive area shaped like a cross. This non-emissive area is in the centre of the device, the area that the lens is focused on, while the main light-emissive areas are towards the periphery of the optical system. While this isn’t a problem in some applications, other applications that require a tight beam cannot cope with the inevitable dimming in the centre of the field. Using one large LED die completely eliminates this optical problem, offering 30% brighter central beam candela power using the same secondary optics (Figure 3).
Using a single monolithic die eliminates the ‘cross-hairs’ often visible with competitor products.
While LEDs are very efficient, there are still losses inside the devices, and the phosphor used in high-power LEDs also emits heat. Thermal conductivity is essential in these assemblies, as running the LEDs hotter than the recommended level can negatively affect their lifetime. Large, bulky heat sinks are required, which can compromise luminaire aesthetics. With Plessey devices, silicon’s superior thermal properties combine with custom-designed aluminium nitride packaging (Figure 4) to keep thermal resistance below 2o C/W, while GaN-on-silicon devices also have a significantly higher maximum junction temperature of 135o C, meaning heat sinks can be smaller, or in some cases are not necessary, saving money. Single-chip solutions also exhibit greater uniformity, since they don’t suffer from multi-chip solutions’ uneven distribution of heat, which can result in colour changes for different viewing angles.
A Plessey multi-junction GaN-on-silicon device in its packaging.
Plessey GaN-on-silicon LEDs are undergoing extensive testing both in-house and at independent test houses to prove that their lifetime and reliability are equivalent to sapphire products. Initial results show that this is absolutely the case, with silicon products performing particularly well under harsh temperature cycling. For example, 40 of our blue and 40 of our white LEDs in older-style plastic packaging are being driven at 60mA/3V (35A/cm2 ) continuously, held at 85o C (Tj around 105o C). At the time of writing, these parts had successfully passed 14,000 hours of testing, and we expect our newer, ceramic-packaged products to perform even better.
Overall, GaN-on-silicon technology has reached the point where it offers a genuine alternative to multi-die GaN-on-sapphire products for industrial, retail and outdoor lighting applications that require high brightness combined with tight beam angles. Plessey’s expertise in the fields of GaN buffer layer chemistry and silicon wafer process technology is unmatched in the industry, enabling single-die high-power LEDs with superior thermal and optical properties to be offered at literally half the cost of multi-die sapphire products.