LEDs industry
Black; Bill Quinn, chief technologist for MOCVD at Veeco Instruments; and Chris Moore, CEO of Semilab USA. SEMI invites participation from all interested parties. Parallel activities are expected to follow with committees in Europe, Taiwan and Japan.
“I see standards as somewhat inevitable,” adds Black, noting that it won’t be realistic to keep using all bespoke materials. “So it’s important to be involved in the process to have some input. It’s time to get the conversation started around what might be possible.”
Finding common ground One key area of common ground is the characterization of incoming materials. This could begin with consistent measurement of purity for chemicals and benchmarks for LED grade materials like indium, gallium, metal hydrides, and packaging materials, which all differ a bit from supplier to supplier. This might not be too difficult because there are a limited number of suppliers.
Wafer standards are more challenging, with multiple different diameters and substrate materials in production employing radically different processes, so some argue that even agreeing on a common thickness for 150 mm sapphire wafers is unlikely. But others counter that one thinner ~1 mm standard for those who thin the wafer down and prefer a thinner substrate, and another thicker ~ 1.3 mm standard for those who remove the epi layer and prefer a thicker substrate, could probably handle most production needs while doing away with much of the individual customization.
Even a few more standard products would allow efficient supply chain management, letting both wafer users and suppliers stock products and buy and sell off the shelf, and to deal with multiple sources in times of shortage. It would also allow tool suppliers to improve process measurement and control. The timing could be good for the fast transition about to come in substrate diameter.
Tool hardware specifications may be another area of common ground, as susceptors, graphite and SiC from different vendors aren’t the same, and measurements of conditions in the tool aren’t measured the same way across suppliers, so results can’t be compared.
Better binning
Metrology and test is another key area for potential gain from consensus on best practice. As the HB LED sector matures and moves from the realm of the development engineer to the manufacturing engineer it will begin to move away from its current focus on volume. The focus will shift to control of the established manufacturing process, and to greater attention to the operational benefits of yield, identifying and tracking yield issues as
“Standards need to focus on the leading edge, 6-inch wafers, 6-inch cassettes,and some sort of wafer or carrier-level identification for traceability as the basics to enable
automation”, Clint Harris, Brooks Automation
early in the process as possible. But the process is currently hindered by everyone measuring different things in different ways.
To measure something as basic as wavelength uniformity on the wafer, for example, some LED makers use electroluminescence tests, others use photoluminescence tests, some measure peak wavelength, and others measure dominant wavelength — and all of those give different results.
“If everyone could agree on one of those measures as best practice, it would get everyone talking about the same issues,” argues Veeco’s Quinn. “If everyone measured things the same way, it would be easier for the epi guys and the rest of the fab suppliers to improve the process for everyone. The potential to improve yields with standard testing and feedback is huge. It’s definitely not too early to start talking about standards.”
“Every company’s fixturing is different, and will give different results,” concurs Dan Morrow, president of Op- Test, citing the difference between probe tests and integrating spheres, and multiple different set ups for each. Morrow suggests that common testing protocols — like standards for thermal junction temperature management during production test — would give makers more useful feedback for process control, and make spec sheet data more useful and meaningful for both LED makers and their customers.
Morrow suggests the traditional transformed CIE XY photometric measurements of what the eye sees, from the traditional lighting industry, may not give precise enough data of what energies are at what wavelengths to drive yield improvements and guarantee that things that look the same really are the same. Spectral power distribution may be the more useful data to improve the production process. “Few are using process control feedback yet, but big players are coming into the market and I expect that will bring big changes,” he adds. Wafer level test systems are still largely a custom business, points out Mark Cejer, Keithley Instruments director of marketing. He notes that the different degrees of precision in color uniformity needed even for edge-lit versus back-array television backlights applications, to say
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