news digest ♦ LEDs Mo-Cu wafer substrate for
LED chips A new material which has the same coefficient of thermal expansion as sapphire, minimises cracks in nitride semiconductor layers grown on these wafers which are used to make LEDs
With Mo-Cu R670, Plansee High Performance Materials has developed a new molybdenum-copper composite material for semiconductor wafer substrates.
The high thermal conductivity of Mo-Cu R670 ensures optimised heat dissipation in LED chips. The new material has the same coefficient of thermal expansion as sapphire, and therefore reduces defects in the semiconductor structure which can occur during the hot bonding process. In this way, Mo-Cu R670 helps ensure the reliable production of LED chips and increases the efficiency of the light source.
Plansee can supply Mo-Cu wafer substrates with coatings made from nickel-gold, ruthenium, chromium, silver and other interface materials. These coatings protect the wafer substrate against corrosion and optimise its surface properties for the subsequent bonding process.
As a manufacturer of refractory metals and composite materials, Plansee supplies metallic wafer substrates for heat dissipation and also numerous other components for the production of LED chips and LED packages.
These include furnace components and molybdenum and tungsten crucibles for sapphire single crystal growth, components for MOCVD/MBE reactors, evaporation boats and coils for metallic mirror or barrier layers and base plates for improved thermal management of high-power LEDs.
Cree LEDs illuminate public Mo-Cu R670 substrate
In the most commonly used process for the production of blue spectrum chips- including white LEDs - GaN-based semiconductor layers are grown on sapphire substrate (Al2O3) using epitaxial growth methods like MOCVD.
A metallic wafer, intended for the dissipation of heat, is then bonded to the semiconductor layers at high temperatures using various joining technologies. Stresses caused by different coefficients of thermal expansion can lead to cracks in the semiconductor layers. To prevent these defects, the metal wafer substrate ideally possesses the same coefficient of thermal expansion (CTE) as sapphire.
Offering good thermal conductivity and having a very high heat-resistance, molybdenum is commonly used for such wafer substrates. However, molybdenum has a lower coefficient of thermal expansion than sapphire. That’s why Plansee developed wafer substrates made from the molybdenum- copper composite material R670 especially for sapphire-based and high temperature-bonded LED chips. R670 has a thermal conductivity of 170 W/mK and, with 6.7 ppm/K, the same coefficient of thermal expansion as sapphire.
school in Washington The firm’s LR24 luminaires have been installed at Monroe Elementary School in the Everett district
The school recently installed nearly 450 LED fixtures made by Cree. This makes it the first predominately LED-lit school in the Everett Public Schools district.
Cree LR24 LED troffers now illuminate the newly rebuilt two- story, 68,000-square-foot elementary school, which serves about 600 students in grades K-5.
“We looked at a lot of LED fixture manufacturers but only Cree had fixtures that met all of our requirements for high-quality lighting and cost savings,” says Harold Beumel, director of facilities and planning, Everett Public Schools.
“Since the lights are designed for 50,000 hours of life, the Monroe Elementary School should see ongoing operational savings due to decreased energy consumption and decreased maintenance compared to linear fluorescents. Cree LR24 luminaires provide consistent, even lighting, furthering our goal of enhancing the district’s educational program and providing a better learning environment.”
Everett Public Schools’ leadership first explored broad adoption of LED lighting after realising all of the benefits that came from the installation of LED lights during the conversion of shop classrooms into general classrooms at Heatherwood
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www.compoundsemiconductor.net April/May 2012
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