February 10, 2012 — Day 3 of the 2012 Strategies in Light conference continued the LED Manufacturing session. Presenters covered lithography for light emitting diode (LED) manufacture, advanced packaging, metrology and testing, high-performance materials, and other topics, such as the value of dimming.
First up, Thomas Uhrmann of EV Group with a review of the lithography and wafer bonding tools that they provide for HV manufacturing of LEDs. Nano-imprint is one of the applicable technologies identified on one chart, but only optical litho was discussed. Several different device structures are being scaled up in production, no longer limited to the original planar device. A variety of bonding schemes were shown, many driven by emerging 3D integration process flows. It remains to be seen whether there will be a process and materials convergence in our future.
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| Figure 1. Chip designs for GaN LEDs. |
The subject switched to metrology and yield management with Mike Plisinski of Rudolph Technologies, focusing on epitaxial process metrology and feedback in LED production. MOCVD Epi process defects and variation are the largest LED cost drivers, impacting both yield and brightness directly.
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| Figure 2. Epi process defects and LED yield/brightness. |
Dan Scharpf of Labsphere talked about optical testing of LEDs. Drive current, junction temperature, stray light and appropriate selection of sphere size are all important parameters for accurate testing that does places all products into the correct bin.
Ilkan Cokgor of Everlight Electronics led us into the LED Packaging session, speaking on packaging trends intended to drive down the cost per lumen. Specifically, he focused on performance and reliability improvements for PLCC packages because they are low cost but have not developed a reputation for reliability before now.
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| Figure 3. Advanced package — reliability improvement. |
Ravi Bhatkal of Cookson took the packaging reliability story up a level to luminaire fabrication, including the introduction of SMT manufacturing methods to LED packaging. Issues in lead-free solder printing and reflowing were discussed. The talk provided some good guidance for engineers just starting to implement such processes.
Geoff Gardner, Lighting Marketing Manager for Dow Corning, talked about innovations in silicone technology as applied to LEDs. As the material most commonly used for the LED primary lens, thermal stability is critical for both transmittance and reflectance over the lifetime of the device. Silicones used in injection molding and screen printing have recently been brought to the market. The chemical physical and mechanical properties make silicones suitable if not preferred for a range of applications within LED modules and luminaries.
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| Figure 4. Material needs in lamps and luminaires. |
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| Figure 5. Silicone technologies for LEDs. |
Marc McClear of Cree, still well-known to those of us from the semiconductor materials business, opened the final session of the LEDs in Lighting track with The Next Big Thing. Among the discarded candidates discussed were LED droop, the green gap, remote phosphors and OLEDs. Rather, the correct answer is 3rd generation direct attach LED chip architecture as the vehicle that will deliver 200 lumens/watt for the industry. The payback time for installed fixtures will drop from 4.5 years to less than 1 year.
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| Figure 6. DOE LED roadmap. |
Nadarajah Narendran, Director of the Lighting Research Center at RPI, gave a program overview of ASSIST, the Alliance for Solid-State Illumination Systems and Technology. In a philosophical observation worthy of “if a tree falls in the forest and no one hears it…” he opined that without a human eye to observe it, light is just another wavelength of radiation. “ASSIST recommends” is a series of publications that the organization has produced over the past decade to disseminate the research to its member companies and throughout the industry. Developing test methods, standards and educational curricula are among the ongoing activities that comprise its mission.
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| Figure 7. ASSIST publications. |
Next on the agenda was Amy Olay (no relation) of the San Jose Department of Transportation talking about the energy savings programs associated with the city’s 62,000 streetlights with an annual operating budget of $6M. Implementation of a wireless control system in which each streetlight was its own node was chosen over hard wired designs for controlling power to and receiving operational data from the lamps. The longitude and latitude of each streetlight is known to itself, and it turns on and off based on its unique sunrise & sunset times, rather than using photocells. Warm white LEDs with a color temperature of 4500°K were found to be preferred by the public. The units could be reduced to 50% power without an adverse effect on safety or public perception. In a nod to scientific collaboration, the city can further dim these lights from midnight to 5am, which are the hours in which the James Lick Observatory above San Jose on Mount Hamilton conducts its astronomical research. The current deployment is for 2,000 streetlights, funded by federal stimulus money. An additional $30M in grants will be needed to complete the conversion to LED streetlights in San Jose.
Sam Klepper of Redwood Systems wrapped up the conference by introducing us to the digital age of lighting. Being dimmable turns out to be a significant feature of LEDs that is not fully exploited. Dimming an LED by 30% reduces power consumption by 50% with no adverse effect. Dimming a CFL by 30% reduces power consumption by only 20%, and reduces the lifetime of the lamp. The digital control system that takes advantage of all of the features of LED lighting brings with it the capability to add a number of other functions of interest to building managers with little additional effort. For example, motion sensors used to turn off lights in unoccupied rooms can also indicate room occupancy for spontaneous roving team meetings searching for an available conference room. The same sensors can provide security warnings for restricted areas or after-hours tresspassers.
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| Figure 8. Key challenges in facilities. |
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| Figure 9. Commercial electricity use by building type. |
Observation from the exhibition floor: I’m accustomed to walking up and down the aisles and wondering how I might use the product being exhibited to build my own product. At this show, the dominant thought was “I wonder how that would look in my living room.” In that sense, it was a bit more like Home Depot than the technology conference that it was. On the other hand, it is so much more interesting to see all of the ready-to-install luminaire designs than it is to see bin after bin of bare and packaged chips and assembly components.
Next year is going to be a tough one for the LED industry, since the 2013 Strategies in Light conference will be held on February 12-14. I think it’s safe to say that not everyone will be able to get home in time for Valentine’s Day to keep peace in the family.
Michael A. Fury, Ph.D., is director & senior technology analyst, Techcet Group in North Plains, OR.