Technical Insight
Nitronex funding targets GaN-on-Si LEDs (LED News)
Nitronex, a developer of GaN devices grown on silicon substrates, has created an optoelectronics division to address applications such as LEDs for solid-state lighting. To fund these initiatives, the company will leverage a recent second round funding injection of $24.5 million from its original investors, who contributed $9.5 million last May. Nitronex intends to use the investment to develop its optoelectronic portfolio, and to expand its existing 10 000 sq. ft facility by a factor of ten. This will primarily support the production of RF devices such as GaN-based HEMTs, which are manufactured on 4 inch Si substrates (see Compound Semiconductor March 2001 p17). Nitronex, based in Raleigh, NC, has developed proprietary MOCVD growth techniques to produce GaN-on-Si layers with relatively low defect densities. This includes its Pendeo epitaxy technique, which reduces the number of dislocations in GaN epilayers by up to four orders of magnitude to around 106/cm2. In addition, the company has patented a growth process called Sigantic, which it describes as the growth of multiple epitaxial layers to produce GaN-on-Si structures with a final defect density of 109/cm2. Such defect levels can be tolerated in optoelectronic devices such as LEDs. Blue LEDs on silicon One of the first initiatives undertaken by the new optoelectronics division was the formation of an alliance with Microsemi (Irvine, CA) to fabricate a line of GaN-based blue and ultraviolet LEDs. The first device, a 470 nm blue LED, will combine Microsemi s chip-scale packaging technology with Nitronex s Sigantic GaN-on-Si structures. These LEDs will sample towards the end of the year. According to Warren Weeks, VP and general manager of Nitronex s new optoelectronics division, the GaN-on-Si approach alleviates the high cost of SiC substrates and the poor thermal conductivity of sapphire. "Our GaN-on-Si technology is the ideal platform for creating cost-effective, large junction-area LEDs," said Weeks. "Additionally, vertically conducting device structures are also permissible using ordinary conductive silicon substrates. These same silicon substrates also offer unique processing routes for improving the overall device brightness." Epoxy-free packaging The primary aim of the packaging procedure developed by Microsemi is to fabricate low-cost, large junction-area LEDs without the use of epoxy encapsulant. Over time, the epoxy can become degraded by UV emission in the device, causing discoloration and failures. Microsemi employs a patented spin-on glass approach that produces a hermetic seal over the whole 4 inch substrate prior to chip dicing (see ). Devices are bump-contacted and diced for subsequent flip-chip attachment, which also eliminates wire bonding. "Most current blue and ultraviolet LEDs are limited in die size as a result of the available wafer size, material non-uniformity and the cost of SiC and sapphire substrates," said Weeks. "Using the Sigantic platform technology, Nitronex and Microsemi can provide larger LEDs that are packaged at the wafer level. This will bypass LED packagers and provide same-size devices that are up to twice the brightness of competing products." Weeks added that Nitronex has a capacity for 100 4-inch GaN-on-Si wafers per month for LEDs, which produces the same number of die as approximately 400 2-inch wafers. While this may suffice for its current LED requirements, Nitronex will consider outsourcing some LED production at a later stage.
