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Meet the demand for smaller LED screens

The search for a revolutionary display technology that meets the requirements of next-generation products has been completed.

A growing number of emerging applications, such as head-up displays, AR / VR headsets and laptops, are exploring new display technologies to enable the development of next-generation products to meet growing global demand. According to Yole Development, the market could reach 330 million units by 2025.

While augmented reality and virtual reality are likely mainstream technologies, augmented reality and virtual reality are increasingly being used in industrial and manufacturing applications, providing qualified and semi-skilled workers access to information that can help them in a variety of ways. Set of tasks. Examples may include showing an employee the correct order for securing and tightening bolts in a motor or riveting in a larger structure such as a hull. As tools are connected, the process becomes more integrated and provides quality assurance because each fix is ​​saved or those that have not yet been backed up are marked.

In addition to increasing reality, head-mounted displays can increase productivity by allowing employees to move freely without being disturbed by large screens or portable tablets.

Display technology is evolving to meet the demand for smaller, lighter helmets that can be worn during a full shift without becoming a burden or a potential health risk. The search for more efficient displays drives innovative manufacturers to MicroLEDs; a technology that promises lighter, smaller and more efficient displays.

New applications

Headsets are an important and emerging application for smaller screens, but also HUDs that are now integrated into road vehicles. Existing applications include handheld devices such as smart watches and of course screens have always been part of the man-machine interface. This will continue as smaller screens will integrate them into a wider range of devices.

Advertisements generally fall into one of two categories: reflective or emissive. Reflective displays typically require a separate light source and mirror assembly while emitting displays, such as LEDs, generate their own light.

Today’s headsets are often based on reflective display technology because of the lack of available emitting displays, which is why they look big and heavy and are usually energy hungry. Moving to an emissive display solution for headsets will reduce overall size, weight and power consumption, resulting in a significant change in market dynamics.

Like other emitting display technologies, MicroLEDs have three LEDs per pixel (red, green, and blue) to deliver the full range of visible light. The name reflects the small size of each LED, often 10μm. m or less, with a pixel pitch of less than 40 .mu.m. m. While suitable for HUD applications, they can theoretically be applied to any display of any size; Scalability is one of the many benefits of technology.

“In simple terms, the same luminance can be achieved for much lower power consumption – a critical parameter for target applications.”

Keith Strickland, Chief Technology Officer of Plessey Semiconductors

For larger screens, such as TVs, tablets or cell phones, the pixel pitch is less demanding. However, applications such as HUDs in vehicles, monitors, and portable devices require pixel densities of a few hundred pixels per inch, which means that the height decreases considerably. In terms of manufacturing, this poses problems for existing technologies such as OLEDs.

The main advantages that microLED technology makes in view of these technical challenges are higher brightness; typically an OLED is made in the 1000 nit region (1 nit corresponds to 1 candela / m 2), while micro LEDs can produce more than 100,000 nit. In simple terms, this means that the same luminance can be achieved for much lower power – a critical parameter for target applications.

Challenges in manufacturing

Conceptually, a screen with three LEDs per pixel consists of the combination of LED chips and an active matrix to control each chip individually. Today, most LEDs are made with gallium arsenide (GaN) on sapphire wafers.

In display applications, each LED must be connected to the active matrix. However, GaN on sapphire is not suitable for integration with CMOS circuits, so that each LED chip must be physically placed on a silicon substrate using thin film transistors. In larger screens with a pixel pitch of 70 μm or more, it is not too difficult. However, as display sizes decrease and pixel steps decrease, manufacturing equipment becomes less efficient.

In fact, the process involves taking the LED chips and placing them on the silicon substrate with extreme precision. While the cost of designing and manufacturing picking equipment is a factor, the ability of companies to provide accurate and efficient pick and place equipment at pixel heights of 20 microns. m or less, a bigger problem.

A major concern is the achievable performance, both in the selection of functional LED chips (and the removal of those that do not work) and their accurate placement. Revising a MicroLED ad would probably not be economically feasible, so the total return is determined by both factors.

The alternative tactic is to use a monolithic approach with multiple transmitters on a single chip. This requires moving LED fabrication onto a substrate that readily accepts silicon, and the most promising technology in this regard is GaN on silicon. Thanks to its surface-emitting properties, it offers the possibility of producing larger LED emitters with better contrast. The technique easily adapts to wafers of 200mm or larger, improving cost and efficiency.

Monolithic approach

Monolithic in this context means that no pick-and-place machines have to be used; LEDs are fabricated on a silicon wafer directly connected to another silicon wafer containing the active matrix. The need to select and place individual LED chips means that the entire manufacturing process becomes more efficient and economically scalable at the same time.

This includes the size of the wafer; Sapphire pads are typically available up to 150mm (6 inches), but 200mm and 300mm silicon wafers are common, as is the manufacturing equipment used to make CMOS devices.

The technology and intellectual property developed by Plessey Semiconductors corresponds to the large-scale production of microLED with GaN on Si.

The company’s track record in developing CMOS sensors is also suitable for micro LEDs, with much of the underlying technology being transferable. While pixels become emitters instead of sensors, much of the addressing technology is comparable.

Another advantage is that GaN on Si LEDs are surface emitting devices, unlike GaN on sapphire, which is a volume emission technology. In simple terms, this means that more of the generated light is emitted in the desired direction. Although a waveguide is always needed, the loss of light is due to the alignment or output vectors of the photon, which further improves the screen’s energy efficiency.

The waveguide’s current interface is a process with many losses, with very little light entering a viewer-usable HUD. Monolithic micro-LED chips could translate a typical VR headset into something more like a pair of glasses.

Below: A demonstrator made in collaboration with Artemis Optical presents the Plessey monolithic display based on a range of MicroLEDs.

graduation

Demand for smaller, energy-efficient displays is based on applications such as HUDs and AR / VR headsets. This will put pressure on existing technologies, which will have difficulty reducing their size, both in practical and economic terms.

The availability of a robust GaN-on-Si process now makes it possible to produce monolithic MicroLED displays. It has been demonstrated that this technology meets the requirements of novel display applications and is now available through technology licensing agreements.

With decreasing screen sizes, GaN appears to Si as the technically and economically feasible approach to meet the demand for high efficiency, high pixel density and low power micro LEDs.

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