Greater sharpness, greater contrast, greater brilliance but lower energy consumption: these are the demands that not only new device classes place on the displays of tomorrow. Micro-LEDs can meet these requirements - but their production presents engineers with enormous challenges. AIXTRON has found ways to solve them.
Not so long ago there were only two colour displays in a typical household: the TV and the home computer monitor. The miniaturization of technology then made it possible to incorporate such displays in portable devices: Portable game consoles became possible and experienced an enormous boom. The first mobile phones, still with a rough screen resolution, gave us a first sample of the coming freedom of mobile communication. And finally, smartphones began their triumphant advance just over ten years ago: The small "’always-on computers’ changed our world. We have long since become accustomed to being constantly supplied with information from mobile devices wherever we are.
Even everyday devices - from the desk lamp to the oven, from the bicycle to the car - are becoming increasingly "smarter"
Despite today's omnipresence of portable digital technology, this development is still in its infancy: even everyday devices - from the desk lamp to the oven, from the bicycle to the car - are becoming increasingly "smarter". This means that they collect and process data and network on the Internet of Things. And they have more and more to tell us: Kitchen appliances guide us through the preparation process with recipes, smart watches and fitness trackers inform us about movement data, body values and training progress, offer chat functions, games and much more.
The more we communicate with everyday objects (and they communicate with us), the greater the need for displays that display this variety of information in the smallest of spaces. In addition, the demands we place on display technology are growing.
Liquid crystal displays - LCDs - still dominate the market today. LEDs, i.e. light-emitting diodes, are only used as white versions for backlighting. It is true that colour LEDs can also be produced for a long time, which can be grouped in groups of three (in the RGB colours red, green and blue) into one pixel of a colour display. Due to the size of these LED groups, however, the system is only suitable for large screens, e.g. as advertising boards or displays in stadiums.
The industry has long been keen on LED technology, which can be reduced to such an extent that it can also be used in consumer electronics - especially in mobile devices - not only for lighting but also for the independent display of content in natural colours. And another criterion is immensely important: the lowest possible energy consumption. After all, even the most exciting technological innovation is not much fun for the user if the battery needs to be recharged after just a few hours.
OLEDs (Organic Light Emitting Diode) have been available for some time now - LEDs made of organic semiconducting materials that light up themselves, i.e. do not require backlighting. OLEDs have excellent black values and represent a wide colour spectrum. However, they are currently still quite expensive to manufacture, not very bright, suffer from burn-in effects under certain conditions and do not yet reach the service life of conventional displays.
The hottest contender for the top position on the market is therefore a new competitor - the micro-LED: Compared to the current display technology, it offers a higher pixel density, a longer lifetime, greater brightness, a higher switching speed and a wider colour spectrum - all advantages that make it groundbreaking for a large number of current and future application scenarios. In addition, the micro-LED is particularly characterized by its very low energy consumption - a fact that predestines it especially for future generations of small mobile devices with little space for batteries.
No wonder that virtually all major display manufacturers are now preparing for the future and are looking for production processes that are as reliable as they are economical.
In Herzogenrath near Aachen, Germany, AIXTRON has been driving forward the development of production systems that can also be used for micro-LEDs for many years and is playing a decisive pioneering role in the production of this next generation of displays.
Like conventional LEDs, micro-LEDs are also manufactured using the MOCVD process (metal organic chemical vapor deposition): The semiconductors are applied to the substrate - atomic layer by atomic layer and atomic-layer thin at that. The heart of AIXTRON's production equipment is the planetary reactor: a horizontal-flow reactor with a ring of several wafers orbiting the central gas inlet like satellites. The Gas-Foil-Rotation® process is used, which enables an absolutely uniform growth of structures made of the III-V materials gallium nitride (GaN) and aluminium gallium indium phosphide (AlInGaP) on the wafers.
Compared to conventional LEDs, micro-LEDs place demands on the production process that are many times more demanding. In LED manufacturing, a single particle in the reactor chamber has so far led to a defect on a single chip (a section of the wafer) among thousands. Several hundred defects per wafer are no disadvantage for the production of conventional LEDs. These defects are simply sorted out as part of an automated process. This procedure is no longer practicable in the production of micro-LEDs. Here, thousands of tiny LED chips (arrays) are picked up and transferred using a special pick-and-place process. Each defect leads to a dead pixel, which cannot be replaced and thus makes the whole area unusable. An almost error-free process is therefore necessary, which minimizes defects to such an extent that a yield is achieved that permits the economically sensible production of micro-LEDs.
The AIXTRON engineers' solution is a process in which the inside of the process chamber is prepared for the micro-LED production process by a special in-situ etching process. This cleaning and fully automated wafer handling - a process called cassette-to-cassette handling - can reduce the defect rate to less than 0.1 defects/cm². The AIXTRON system is also the only production system on the market worldwide to offer advanced features such as on-wafer homogeneity control and wafer level temperature control, in which the wafers are automatically measured with high precision using auto-feed-forward and the process chamber is automatically recalibrated.
Thanks to these advanced methods, the yield of usable chip arrays increases dramatically - making the production of micro-LEDs with their high quality requirements possible in the first place.
Today, devices with micro-LED technology are not yet available for the broad consumer market. But market readiness is imminent. One thing is certain: As a provider of consumer electronics, it will be hard to avoid the topic of ‘innovative display technology’ if you want to inspire your customers with exciting new products in the future as well. The advantages of the new display generation over conventional display technologies are so huge that no manufacturer can afford to ignore them.
Initially, the new displays will probably be used in wearables - i.e. in smart watches and fitness trackers. They are the ideal entry point, as their highly compact design means they have to make do with small display sizes, require a high pixel density and place extreme demands on energy efficiency with their small batteries. Another market eagerly awaiting the new technology is that for virtual reality and augmented reality applications. The first to offer a headset that is significantly lighter than comparable models thanks to the use of micro-LEDs and can be operated wirelessly for hours thanks to low power consumption, gains a clear edge over the competition.
And finally, the premium class of consumer electronics is also ready for a further display revolution: if production is only standardized to such an extent that mass production of even very large panels is possible at reasonable costs, manufacturers of screens for the living room at home should also convert to micro LEDs - because the market for ultra-flat, energy-saving devices with brilliant, viewing angle-independent images is huge.