industry LEDs
happening by inserting a carefully selected stack of layers into the epiwafer that address the stresses and strains in this structure. The researchers in Humphreys’ group have mastered this on their single-wafer 6-inch reactor, using the combination of an AlN nucleation layer, a complex buffer structure and layers of AlGaN and GaN. Inserting a SiN layer into this structure cuts the threading dislocation density.
Dennington claims that one of the great features of the Cambridge recipe is the complete neutralisation of the bow and mismatch in the crystalline structure between silicon and GaN. The result is particularly impressive because these wafers, which are sufficiently flat for processing in silicon lines, have a buffer that is just 2.5 µm thick. “We think competition is around 6-8 µm-thick,” remarks Dennington. “So ours is thinner, helping us with wafer bow, and giving us better throughput through the reactor.”
Although the acquisition of CamGaN by Plessey is a perfect fit, it
is not one that many would have foreseen during the latter part of the previous decade. It’s not just that the Plessey 6-inch silicon line was in the hands of X-fab at that time (see box “The evolution of Plessey Semiconductors”); back then Humphreys’ team was involved in a £3 million GaN-on-silicon LED project headed by RFMD’s Newton Aycliffe operation, and involving QinetiQ, Forge Europa and the UK branch of Aixtron. It seemed that if LEDs were ever going to be made in high volume in the UK, production would be out of the RFMD fab.
However, although the engineers at the Plymouth site were not involved in this project, they have been more than just observers in the development of this technology.“[Humphreys] had grown some GaN-on-silicon wafers, and we had a 6-inch wafer facility, so we took some wafers and helped with some very early LED structures,” explains Dennington. Plessey Semiconductors’ CEO, Micheal LeGoff, kept a close eye on the progress of this technology, and then snapped up the company in February 2011.
“We don’t know who else we were bidding against,” remarks Dennington, implying that he doesn’t know if Plessey fought off bids from the likes of RFMD. “Obviously it’s a business deal, but I do believe there was some passion amongst some of the people involved in CamGaN and Cambridge to see that this technology would be planted in a British company.”
Plessey’s acquisition also led to its participation in a European Consortium called “Consumerising Solid-State Lighting”, which had previously included Cambridge University and QinetiQ as partners in the project. “The Consumerising Solid-State Lighting programme was chaired by Philips, and the focus was to create a $9.95 lamp that would replace the 60 W incandescent bulb,” explains Dennington, who adds that Plessey is the only LED manufacturer in that programme.
Alongside the transfer of IP to the Plymouth fab, the acquisition of CamGaN involved the transfer of a handful of employees, who are former post-docs from Cambridge University. “We have plans in the future to open up a Cambridge lab, so that our Cambridge employees can work locally,” says Dennington, who explains that these staff currently split their time between the fab and offices within the university.
After the acquisition of CamGaN closed, the former post-docs had access for one week every month to the Thomas Swan single- wafer reactor, which they used to produce 6-inch GaN-on-silicon material. But since the arrival at the Plessey site of the Aixtron Crius II XL, a reactor capable of accommodating seven 6-inch wafers in a single growth run, they have focused their attention on the higher throughput tool.
Aixtron or Veeco?
Plessey is manufacturing its GaN-on-silicon LEDs on an Aixtron reactor with a capacity of seven 6-inch wafers. The tool is fitted with various in-situ monitoring tools,which measure the curvature of the wafer and its temperature
30
www.compoundsemiconductor.net January / February 2013
The Plessey management found it tough to choose between an Aixtron and a Veeco tool for their 6-inch manufacturing line. “They are both excellent machines,” says Dennington, “and both companies were very, very eager to be associated with Plessey.” The development of the growth technology on an Aixtron showerhead machine tipped the balance in favour of a tool from the Aachen outfit.
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