Bridging the commercialization gap between MicroLED manufacturing and quality

2021-11-25 10:16:24 By : Ms. shirley wu

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Since the appearance of microLED display technology in 2010, analysts, manufacturers, and industry experts have been excited about the potential for its superior performance.

The reason is easy to understand: MicroLED has excellent performance attributes. Compared with organic LED (OLED) displays or LED-backlit liquid crystal displays (LCD), they have lower power consumption, higher pixel density, faster (nanosecond) response time and wider viewing angles.

Crucially, under direct sunlight, they provide an order of magnitude higher brightness than OLED displays or LCDs. This is essential for handheld devices and is also a key driving factor for near-eye displays. 1

The projector based on the MicroLED display is built into Vuzix's latest smart glasses. Video source: Radiant Vision Systems

However, manufacturers are still working to close the gap between promises and market reality, as establishing a cost-effective manufacturing process for microLED displays has proven challenging.

Many steps are required to produce these nanoscale components, and defects may occur in each step. The consumer device market expects today's display devices to have near-perfect performance and visual quality.

Manufacturers must carefully check at every step to meet these high expectations. They must also achieve high volumes to ensure that production costs are still viable for the mass market.

The timetable for the transfer of microLED displays from research laboratories to mass production has been slow and has not yet been completed. Recent advances and developments have helped push the industry closer to the ultimate consumer marketization of microLED devices.

Creating a MicroLED display is a multi-stage process, including some or all of the following steps:

Although LED technology has existed for decades and is widely used in LCD displays, MicroLED requires new processes and methods. Every component needs to be optimized for performance and cost, and almost every step in the manufacturing process must be reinvented. 

For example, micro-micro LEDs have a large relative surface area, which may cause more defects in the manufacturing process. Therefore, solving engineering and manufacturing challenges is essential, including miniaturization of chip size while maintaining chip design, high efficiency, and chip manufacturing technology improvements. 2

Simplified schematic of the microLED display layer (left). Image source: from a source of knowledge sharing. The Plessey 0.26 array is bound to the CMOS backplane for a single-chip microLED display (right). Image source: source.

Their small size (usually <50 μm, some as small as 3 μm) brings new challenges. Most of today's LEDs are manufactured on 100mm and 150mm sapphire wafers, although silicon-based GaN is becoming more and more popular, especially in large silicon-based foundries. 

Historically, LEDs have been assembled in surface mount technology (SMT) packages, wire bonded in place and encapsulated with epoxy or silicone. 

The main difference with microLEDs is that they are used in bare chip form instead of packaged form. In addition to tighter design tolerances, this difference also makes the manufacture of microLEDs a very expensive job. 3 

Developers are working hard to solve challenges such as external quantum efficiency (EQE) reduction, bonding methods, color conversion, low efficiency and low yield in the mass transfer process, and to improve the performance of backplanes and panels. 

Facts have proved that the mass transfer step is particularly challenging, because the traditional LED pick and place and flip chip methods are not suitable for micro-microLEDs. 

Attempts such as laser-induced transfer, elastomer seal, electrostatic transfer, fluid self-assembly (FSA), roll-to-roll (R2R) or roll-to-panel (R2P) transfer have all achieved different successes. 4

One manufacturing method of MicroLED displays uses integrated CMOS drivers to simplify the transfer step. The result is a basic unit consisting of an integrated RGB LED on a CMOS drive circuit. Image source: eenews.

A more sophisticated approach focuses on separating the blue, red, and green LEDs from the wafer and CMOS driver circuit to transmit each LED separately. Image source: eenews.

Despite all these challenges, MicroLED is still the focus of R&D activities, and creative new methods and solutions continue to emerge. The latest sign of progress is the 110-inch microLED TV recently released by Samsung. Experts predict that economies of scale will eventually prevail in this market, even if its price is as high as 155,000 U.S. dollars (USD). 5

Samsung’s 110-inch microLED TV provides a 4K resolution of 8 million pixels. Image source: © Samsung, source.

Some other recent developments include:

A new flexible microLED display that is stretchable, foldable and rollable opens up new possibilities for wearable and embedded displays. Image source: © Royole.

Color or brightness changes, defects, and other irregularities can quickly damage brand reputation, reduce buyer satisfaction, and erode market share. Manufacturers and foundries must meet a near-zero tolerance for defects in finished display devices. 

If quality problems cannot be resolved and corrected at the component level, high production costs and low yield will hinder the viability of microLED display technology for the mass market. 

Because the process of manufacturing microLED displays is complex and multi-stage, quality inspections must also be carried out in multiple stages. 

Manufacturers do not want to miss defects at the chip/wafer stage. These defects may eventually be assembled into assembly equipment and must be discarded along with all invested parts and labor. Therefore, it is very important to check the key points in the whole process.

LED manufacturers are stepping up the use of online metering, automatic optical inspection and testing protocols. Image source: KLA.

No matter which microLED manufacturing technology proves to be the most successful in the end, the requirements for quality assurance throughout the manufacturing process are enduring. 

Radiant has provided a set of effective measurement and inspection solutions for microLEDs from the chip/wafer level to the panel/assembly level. They have nearly 30 years of experience in developing solutions for all stages of electronic product production. 

Radiant’s measurement solutions, including ProMetric® imaging photometers and colorimeters with optional microscope lenses, enable microLED manufacturers to check display performance and uniformity at various manufacturing stages and obtain accurate and repeatable results to Ensure the absolute quality of the microLED display device. 

Since the microLED display is luminous (each microLED pixel is individually powered and emits its own light), the variation of brightness and chromaticity from pixel to pixel can cause uniformity problems. 

Radiant's high-resolution imaging system and TrueTest™ software application patented demura technology enables manufacturers to accurately check and correct each pixel and sub-pixel element to increase yield. 

Radiant's ProMetric system provides the high-resolution imaging required for extremely precise inspection of individual pixels and sub-pixels to capture and measure the discrete output of each pixel and correctly register the sub-pixels, regardless of layout or shape. 

Radiant’s microscope lenses are commonly used for sub-pixel characterization in R&D and laboratory environments, allowing the full resolution of the imaging system to be applied to very small (magnified) parts of the display or wafer, resulting in better measurements in every light Accuracy-light emitting element (each individual microLED). 

After the microLED is transferred to the backplane, Radiant's measurement system can be used to immediately evaluate the brightness and color uniformity of the entire panel. 

The advantage of a 2D imaging colorimeter or photometer is the ability to capture large spatial areas in a single image and compare the measured values ​​to detect and identify any uneven or mura (blemish) areas.

Use Radiant's solution to display MicroLEDs from Jasper Display before and after uniformity correction (demura). Image source: Radiant Vision Systems

Radiant's ultra-high resolution imaging system (up to 61 MP) can also capture a single image of the display for evaluation, and measure the pixel-level brightness and color values ​​of all pixels in the display at the same time

These comprehensive measurements allow extremely efficient pixel uniformity testing and calculation of display calibration (demura) correction factors in production settings. 

Made of materials originally created by Anne Corning of Radiant Vision Systems.

This information is derived from materials provided by Radiant Vision Systems and has been reviewed and adapted.

For more information on this source, please visit Radiant Vision Systems.

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