Clean Sky's HiLiCo: Bright ideas take flight

Dimmable windows are a popular cabin design feature for passengers flying high above the clouds when the cabin becomes too bright, but for obvious safety reasons this is not an option in the cockpit. Consequently, the avionics instrument displays on which pilots rely for monitoring every aspect of the aircraft's situation, performance, navigation and other key flight parameters have to be exceptionally bright. However, this means that they also consume a lot of electricity and generate unwanted heat. Clean Sky's HiLiCo project is en route to deliver the technologies for a next generation 'see-through' pilot's headset-based instrumentation display system using low-energy-consuming Gan LED technology.

The cockpit can be a dazzlingly bright working environment where pilots are tasked with balancing an awareness of what's happening visually outside the cockpit windshield while invigilating a high workload of information displayed on bright, energy-consuming avionics screens. Could there be a more ergonomic and more ecological solution? Clean Sky's three-year HiLiCo (High Luminescence In Cockpit) project, which runs until May 2020, set out to explore the possibilities.

"This project focuses on developing technologies that contribute to European competitiveness through deployment of innovative products in the growing micro-display market, particularly for head-up displays, head-mounted displays and smart eyewear – in this case for the pilot to improve their situational awareness and focus better on operational tasks" says Jaime Perez de Diego, Clean Sky Project Officer. "The main challenge is to achieve the desired brightness for use in a micro-projector device with very low power consumption in a compact unit. The problem is that when compactness is acceptable the brightness is usually low and lifetimes short. Usually these optical systems are bulky, so the idea is to make them more flexible and comfortable for the pilot".

The HiLiCo consortium consists of Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA), supported by Swiss semiconductor specialists Novagan Sàrl, and French companies Nexdot, specialists in the synthesis and production of quantum colloidal dots; and MicroOLED Sàrl, manufacturers of headset micro-displays. The proposed technology for the project is LEDs made from Indium Gallium Nitride (GaN), a semiconductor material that provides high efficiency, outstanding reliability, long term life – and temperature ranges within acceptable limits. Heat management is imperative, as the micro-display is intended to be integrated into the pilot's headset to enable avionics information to be projected onto the cockpit windscreen so that it is legible while the pilot is looking out through the windscreen. Heat is of course wasted energy, so there's an ecological issue at stake too.

In terms of deliverables, the project aims at developing a new generation of monochrome and full-color emissive GaN micro-displays with 1920 x 1200 pixel resolution (WUXGA), 8-μm pixel pitch, and very high brightness (over 1MCd/cm2) – all in an ergonomic design. 

To achieve this, five significant technological hurdles need to be overcome: The development of high-quality GaN based LED epilayers (pure, crystalline material which is grown in a vacuum); design and fabrication of a CMOS active matrix able to control each individual pixel; coupling of the LED structure to the CMOS matrix followed by high precision LED pixelisation (8-10μm pixel pitch); transfer on blue emitting devices of dedicated light conversion layers (quantum dots or 2D Multi-Quantum Wells) to manufacture single-, bi- or full-colour display demonstrators; and finally, design and manufacture of the electronics followed by the test and evaluation of the complete micro display device. First demonstrators will be qualified for future commercialisation.

"HiLiCo is focused on the development of the technological building blocks necessary to manufacture the micro-projector – we start from scratch and at the end we should have different demonstrators, small micro-displays which we'll give to Thales, the topic manager" says Dr. Etienne Quesnel, Research Engineer at the CEA. "The issue we have is to be able to have a micro-projector which is bright enough to project an image or text onto the inside of the cockpit windshield in a very bright environment – you can imagine how sunny it can be in the cockpit of the plane, and so if you want to have some image contrast you really need to have a very bright projection of your image. For that you really have to develop micro-displays or micro-projectors with much higher performances than exists today – we have to increase the brightness by a factor of at least 10 or 100 compared to existing systems. The first challenge is to have the display working correctly with a single colour, because if you want to have a display which is very bright you need to have a conversion of electricity to light that is efficient, otherwise you end up building a kind of heater".

"Gallium Nitride is used in the manufacture of LED lighting. What we want to do is miniaturize it – make it really work at the micro level. This is combined with the element Indium to 'tune' the colour of the light emission of the material. When you build an LED from scratch you get blue light. But for a full colour display you also need green and red, so we add a colour conversion layer to convert blue into green light, and another conversion layer to convert blue into red light. The system is a stack, with 2.3 million pixels, so it's really a challenge of micro technology" says Dr. Quesnel.

Now, two thirds of the way into the project timeline, the development of the main technological building blocks is well underway. And while there are still some technological challenges ahead, several key results and advances have already been achieved: Fully optimized 4’’ blue LED epilayers were grown at Novagan, providing single micro-LEDs with external quantum efficiency exceeding 9% at 2.8V. The final architecture and appropriate CMOS technology of the driving circuit of 9.5 μm-pixel pitch LED arrays are now chosen, and the IC design step has been started. A suitable architecture has been found that relies on the combination of an analog current control for homogeneity compensation combined with a digital PWM-type control for the video. 

Successful GaN-epilayer transfer on 8’’ CMOS-like silicon wafer and further processing of
8μm pixel/10μm pitch LED array have been demonstrated, and assessment of two light conversion strategies have been carried out, paving the way to efficient blue to green and blue to red innovative light conversion solutions. Additionally, as an alternative approach to 2D conversion layers, photo-patterned resin layers containing 2D-nanoplatelets (NPL) as light converters is also being considered, and synthesis of more absorbing and more stable core-shell NPLs has been successfully achieved at Nexdot. First photo-patterning of red NPL/resist composite was demonstrated on the CEA technological platform with pixel pitch resolution down to 10μm, though further work is underway to control the localization of the conversion layer.

HiLiCo is now focusing on several specific technology building blocks necessary for the realisation of the various micro-display demonstrators, with the design and fabrication of the active CMOS matrix and manufacturing of a green micro-LED array demonstrating 1 Mcd/cm2 in powering conditions compatible with a see-through visualisation system. The consortium is also addressing the integration of colour conversion layers into blue micro-LED arrays enabling the manufacturing of bi- or full RGB- colour displays capable of demonstrating at least 100,000 cd/m2.
 

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