They are robust, energy-efficient, and provide up to 50,000 hours of light: light-emitting diodes (LEDs) are considered the light source of the future. High-performance LEDs, in particular, are opening up to new industrial applications that were long unimaginable. However, the high temperatures involved do present a challenge, as LEDs prefer it to be cool. One day soon, microsystem technology from Fraunhofer IZM could make optimum cooling possible.

Light-emitting diodes are making an entrance into more and more new areas, including the optics sector, mechanical engineering, and materials processing. To produce the luminosity required for applications such as these, hundreds of LEDs have to be positioned together in as small a space as possible. The problem, however, is that the power density of these high-output modules produces a lot of heat, and if cooling is inadequate, high temperatures develop. Unfortunately, light-emitting diodes react sensitively to the ambient temperature. At temperatures over 100 °C, the luminosity of most LEDs drops significantly. In order to be really able to use the potential of high-power LEDs, an optimum cooling arrangement is required. The higher the packing density, the greater the challenges faced by the heat management system.

Effective cooling structures
Scientists at the Fraunhofer Institute for Reliability and Microintegration IZM have now – together with two industrial partners (Excelitas Technologies Corp. and Ceram Tec GmbH) and as part of the CooLED project sponsored by the Federal Ministry of Education and Research – developed a new type of high-power LED module with integrated water cooling. These modules have an output of up to 600 watts. By comparison, the high-power LEDs with four chips on the market today have an output of around six watts. The LEDs are packed into the smallest possible space: A luminous area of only 250 x 1 mm contains up to 160 LEDs. They are mounted on a new type of water coolant device – manufactured by Ceram Tec GmbH – which is made of aluminum nitride. The experts at Fraunhofer IZM in Oberpfaffenhofen have designed the microfluidic current in the coolant device with great precision, thus ensuring that the coolant structures conduct heat away as effectively as possible, keeping the temperature inside the module homogeneous. This is important to prevent deviations in output between individual LEDs, as this would shift the peak wavelength – an essential process parameter.

Measuring the temperature inside the chip
A team working at the Berlin division of the institute is responsible for thermal characterization. Up to now, it has only been possible to measure the temperature outside the chip, but even the transition to neighboring material results in incorrect readings. "Now, for the first time, we have been able to measure the temperature electrically at the p-n junction directly. This is where the highest temperatures are usually to be found," explains Dr. Rafael Jordan, division manager of Photonics. This new method, which is known as junction temperature measurement and which is based on the forward voltage, enables temperatures to be determined to within half a degree. The project partners are also going down a new road regarding the die mount technologies: Alternative LED strips are sintered directly to the aluminum nitride piece without liquid solder. This prevents slippage during assembly, which is very important due to the requirements for accurate positioning on the tiny devices involved. Furthermore, the metalized layers of the coolant device, which can only tolerate so much soldering, are not placed under unnecessary strain.

The project partners will present a prototype of the new module at this year's SMT trade fair in Nuremberg. By about the middle of 2011, they hope to have developed a square module (400 LEDs on 16 cm²) with an even higher power density of ~ 1200 watts.