LED Makers Shifting Focus from Efficacy to Manufacturing Efficiency for Mass-Market Leap
By Paula Doe, SEMI Emerging & Adjacent Markets
At $10, long life and low energy use make LED lighting costs compelling for the consumer. Despite major recent progress, however, current prices are closer to $40.
“Improvements in device efficacy have been driving big improvements in lumens per dollar, but that’s getting close to the theoretical maximum— there’s not room to double it again, but costs still need to come down much more than that. So that means it’s now all about manufacturing efficiency— things like designing devices that are easy to manufacture, and improving cost of ownership of equipment,” says Bryan Bolt, Cascade Microtech’s director of technology development, systems.” Better wafer-level testing is one area that can have a big impact on reducing costs by avoiding the high cost of packaging for bad die. Also key will be bringing traceability to test, for tracking defects back to root causes. But for test suppliers to deliver traceability, the sector will need to come to some degree of consistency in operating conditions and performance parameters from the wide range of different probers, spectrometers, integrating spheres and software now put together in different combinations by individual LED makers.
Better measurement of process conditions can also improve yields. Veeco Instruments Chief Technologist Bill Quinn reports that test results using the near UV pyrometer developed with Sandia National Lab to monitor the temperature of the transparent and often warped sapphire wafer directly during epitaxial deposition, instead of just the temperature in the pocket in which it sits, are showing potential to improve yields even in a well-controlled manufacturing environment. With each 2°C temperature variation in deposition resulting in a 4nm drift in device-emitted wave length, the ability to measure temperature and eventually go to model-based temperature control as in the semiconductor industry could significantly improve yields within the desired 2nm wavelength bins.
Standard A19 warm white bulb, 800lm, no maintenance costs (residential use). Source: Yole Développment
Better ways of doing more to prepare the substrate before epi growth also have potential. The U.S. Department of Energy recently awarded Veeco a $4 million research matching funds grant towards developing a PVD process for growing AlN buffer layers— on either silicon or sapphire. Eliminating this first MOCVD step with this engineered substrate approach could potentially cut out an hour of MOCVD growth time, for a 20% or so increase in throughput. Veeco will work with crystalline nitride materials supplier Kyma Technologies.
Another option for improvement is in the handling of the delicate wafers after laser liftoff, in making vertical structures and in transferring the epi layer from the sapphire to a thermally conductive substrate, notes Ram Trichur, director of the LED/energy business unit at Brewer Science. That requires a solution for temporary bonding the device wafer to a carrier substrate so it survives handling and processing without damage. Debonding choices range from chemical release for low-volume applications to semi-automated and automated processes for medium- or high-volume applications that can bond and debond down to 50µm wafers with low mechanical and thermal stress.
Another option for lower-cost ways to make patterned sapphire substrates is to use common proximity printers instead of projection lithography. SUSS Microtec suggests the 1-2µm feature-size hexagonal pattern etch layer can be printed on sapphire using 3-4µm AZ photo resist and improved mask aligner exposure optics.
Speakers from Cascade Microtech, Veeco Instruments, Brewer Science, and SUSS Microtec will join other experts— from Philips Lumileds, Cree, UC Santa Barbara, KLA-Tencor, Dow Electronic Materials, Redwood Systems, Yole Développement, Fraunhofer IZM, Intematix and NNCrystal— to discuss recent progress and next issues in LED manufacturing technology at the session on “More Lumens per Dollar: Issues and Answers to Bring Costs Down to Create a General Lighting Market,” at SEMICON West July 13.
July 5, 2011