Deutscher Zukunftspreis for “digital light”
The Deutscher Zukunftspreis 2024 was recently awarded to three researchers from ams OSRAM and Fraunhofer IZM for the devel-opment of a light source based on 25,600 individually controllable µLEDs. Their “digital light” makes car headlights smarter and more efficient, and could soon be used in augmented reality glasses, all-optical computer processors, and data transmission.
The path was full of technological hurdles. But none of these could dissuade Dr. Norwin von Malm, Stefan Grötsch and Dr. Hermann Oppermann from the idea of their smart, energy-efficient car headlight: They eliminate the need for manual switching between low beam and high beam by dynamically illuminating the area in front of the vehicle based on its digital control system without blinding oncoming road users. The headlight is now a successful series product—and earned the trio of researchers from ams OSRAM International in Regensburg and the Fraunhofer Institute for Reliability and Microintegration IZM in Berlin the German Future Prize 2024, which was presented to them by Germany’s President Frank-Walter Steinmeier at the end of November.
At the heart of the innovative, digitally controlled headlight is a multi-pixel light source with 320 x 80 pixels on 41 square millimeters of segmented LED surface; each of these 25,600 segments measures 40 x 40 µm and can be controlled individually. That allows those pixels that potentially blind oncoming traffic to be turned off dynamically. It is also possible to project pictograms onto the road, for example, to warn about black ice or wrong-way drivers. The headlight can also reduce the light cone to the width of its own vehicle, making it noticeably easier to drive through highway construction sites and other bottlenecks.
Uncharted territory in connection technology and miniaturization
To implement the “digital light”, the teams led by von Malm and Grötsch at ams OSRAM and Oppermann at Fraunhofer IZM had to repeatedly navigate uncharted technological territory: starting with the µm segmentation of the LED chip into more than 25,000 electrically and optically isolated segments, for which the researchers were able to reduce the surface area from 1 mm² to 40 µm² during the development process. The aim of miniaturization is to maximize the illuminable area with minimal use of expensive, precisely positioned optics. The catch: The tiny format of the segments ruled out the usual lateral contacting of the LED. Instead, the team developed an LED architecture in which the contacts are relocated to the underside. There, each segment is mechanically, thermally and electrically coupled to a structured connection layer, via which it can be individually controlled with a likewise new electronic circuit. This individual control enables energy-efficient operation of the headlights. Alternative: Shading parts of a light source that is permanently illuminated at full power or redirecting parts of the light using a micromirror array. In both cases, a lot of light is wasted in the form of heat which has to be dissipated from the miniaturized system. With the new solution, only the pixels that are currently needed light up—all others remain turned off.
The driver and LED chips are built on CMOS wafers in a layered structure. This is the key to cost-efficient production of the multi-pixel light source and the reliability required with a view to series production. Fraunhofer IZM has created the prerequisite for that by directly connecting the two semiconductor layers with a gold-tin solder. This is essential because the LED and driver layers expand to different degrees under the influence of heat. Each LED segment therefore needs its own electrical circuit and its own heat dissipation. With conventional cable connections, it would be inconceivable to produce more than 1,000 of these combined driver and LED chips on one wafer, each of which consists of 25,600 µm segments.
Photonics paves the way
In addition to the high development requirements for the electronic circuitry below the light-emitting semiconductor layer, the team also faced a highly complex challenge above it: In order for the headlight to generate white light, the LEDs used, which emit in the blue wavelength range, must be coated with a fluorescent material that absorbs parts of the blue light and converts it into yellow light. The mixture of yellow and blue light creates the impression of white light for the human eye. That alone would be nothing new in LED production. However, in the case of the miniaturized headlight light source, a new approach was needed for two reasons: Firstly, the LED layers are built up on a growth substrate and then connected to the electronic driver circuit. This growth substrate is transparent—and would not be a problem with a single LED. However, if the substrate remains on the segmented LEDs before being coated with the yellow fluorescent material, it has a scattering effect: Instead of the individual segment, the entire area lights up. The team found a photonic solution to the problem: A specially developed gentle laser process removes the growth substrate prior to the coating without placing undue strain on the connection to the driver circuit.
But that alone was not enough. The layer thicknesses commonly used in the LED sector also proved to be impractical for coating the µm segments, because they also ensured that entire areas lit up instead of individual pixels. The team solved this problem with the help of a new fluorescent material and a modified coating process. The fluorescent material is based on much finer color particles and can be deposited very thinly on the segmented LED surface using the new process. It really is then only the individual pixels that light up.
Broad consortium and BMBF funding
On the path from the idea to the series headlight, the ams OSRAM core team led by von Malm and Grötsch consulted not only Fraunhofer IZM but also the silicon and electronics know-how of Infineon, the headlight expertise of Hella and the Daimler automotive group as system integrator. In the meantime, this consortium drove development forward in the BMBF-funded project “µ-AFS”, which resulted in a fully functional prototype with a segmented LED chip on a silicon driver in 2016. That laid the basis. Since then, the teams led by the trio of researchers who have now been awarded the German Future Prize have further developed the original light source with 1,024 pixels on 4x4 mm² to its current state (25,600 pixels on 41 mm² LED surface) and at the same time minimized the need for installation space, cooling and projection optics. Thanks to the digital control system, it is also possible to adapt the headlights via software to regional regulations, whether for left-hand or right-hand traffic or to the various regulations in the EU and the U.S.
However, the digitally controllable light is interesting for more than just car headlights. The flexible, miniaturized and highly energy-efficient LED light sources are also of great interest for the projectors in augmented reality (AR) glasses. According to the three award winners, the same applies to data communication between servers or computer chips. Their technology could be used here to integrate the segmented LED chips into arrays. In future, these could feed huge amounts of data into optical cables in the form of two-dimensional QR codes. In purely optical computer processors, this approach could be used to create neural networks in the form of light in the interplay of LED pixels and photodiodes. Not only could higher bandwidths and better computer performance be achieved in these applications, but the rapidly increasing energy requirements of modern supercomputer centers could also be minimized. The benefit can already be quantified for car headlights: Instead of 60 to 70 W for permanently lit LED headlights, the new technology consumes only 19 W on average. At its peak, it can also be 40 W, which then promises maximum light output thanks to the active control.