This article is part of TechXchange: Advanced Display Technology.
What you’ll learn:
- Why microLEDs are superior to OLEDs and LCDs.
- How microLEDs can improve electronics displays.
- What makes microLED displays difficult to manufacture.
- New market opportunities for microLED displays.
KLA Corp.’s senior principal scientist John C. Robinson answers burgeoning questions regarding the fast-rising microLED technology.
What is microLED? What is the market opportunity for microLED display technology?
MicroLED is a display technology that’s far superior—in many aspects—to any other that exists today, including organic LEDs (OLEDs) and liquid-crystal displays (LCDs). MicroLED is brighter, uses less power, lasts longer, has better contrast, and withstands moisture, vibration, heat, and more. In all, this presents great market opportunities.
It’s also extremely versatile and can be used in many form factors. Imagine a curved dashboard in your new car. With microLED, the display can be curved like the dashboard. On the other hand, LCDs are difficult to create on an arbitrary form factor like a curved surface, and while OLEDs can be implemented on curved surfaces quite readily, they don’t have the longevity required for a car.
MicroLED displays are versatile in another way: You can assemble more than just LED pixels and include sensors or chips. A sensor might read your fingerprint or detect your body temperature. Or it may identify whether you’re in a dark room or outside, and then adjust the brightness of the screen automatically. Because a microLED is so small, covering only a tiny fraction of a pixel area, it leaves significant display real estate for other purposes.
What are some examples of the way microLED displays can be used in electronics for the benefit of consumers?
It may be quite some time before microLEDs shine in conventional applications such as TVs or phones. However, they may excel in ways not anticipated by consumers.
For example, when you go to a fast-food restaurant, the drive-through menu may be difficult to read because of bright sunlight. MicroLEDs can illuminate brighter than the sunshine, making it possible to easily read the menu.
In the case of large displays, like commercial displays or billboards, you can create large microLEDs out of tiles with no visible borders. This eliminates problems such as moving a particularly wide display in an elevator or through a door. The displays are made of separate high-resolution display tiles assembled on-site.
Near-eye displays are another area where microLEDs can excel. For augmented-reality (AR) glasses, microLED is one of the most viable options if you want to wear them all day. The brightness, versatile form factor, and low power consumption make this technology a top candidate.
What are the barriers to using microLEDs in electronics?
At the moment, microLED displays are too expensive for broad market acceptance. At the same time, other competing technologies—OLED and LCD—are steadily improving, and they’re relatively inexpensive.
Why are microLED displays so difficult to manufacture? What are the challenges?
The manufacturing process for microLEDs is extremely complex and exacting (Fig. 1). The conventional starting point requires expensive, high-quality substrates. The next step is epitaxial growth to create the many layers, including the quantum wells—the structures that create the light. Many tens of layers with many hundreds of parameters must be carefully monitored for temperature, composition, stress, and thickness.
Adding to the complexity, you can't fix anything along the way. You must create the entire structure before you’re able to determine if it's viable.
Once the epitaxial layers are ready, you need to pattern in the microLEDs. This is challenging because you're making extremely small LEDs out of a large planar substrate. Geometries are shrinking, requiring careful control of critical dimensions, profiles, and overlay between layers.
Edge effects become an obstacle; the edge of an LED is lossy because they contain radiative mechanisms that dissipate energy but don't contribute to the light you want. That's fine if your LED is one millimeter across, but when it’s a few microns across, those edges become a dominant problem. As a result, the quality of the etch and deposition strongly influence the LEDs’ efficiency.
Once microLEDs are fabricated, they need to be added to the display. There are two types of microLED displays: high-pixel-density “near-eye” displays, and low-pixel-density “direct view” displays. Near-eye displays, also called monolithic or micro-displays, use an array of microLEDs. A square centimeter of wafer becomes a square centimeter of display. In this case, complex hybrid bonding to a CMOS backplane is required.
The other type of display, the conventional direct view variety, requires mass transfer, which means the transfer of millions of small microLEDs onto a backplane or driver IC. Mass transfer is challenging. In the electronics industry, we're used to moving small electronic parts, but typically in the tens of thousands, and the parts are much bigger. In the microLED process, it’s millions of parts, possibly hundreds of millions, and they're mere tens of microns across. Consequently, the mass transfer and bonding processes are complex and expensive to manage.
The human eye is sensitive to bad pixels, requiring near-perfect displays. Repair is a big issue, and this is one instance where yield becomes important (Fig. 2). If you can achieve five nines—99.999% yield—you may have a relatively manageable “repair bill.” If the yield is four nines or lower, however, it's probably close to unacceptable.
What will it take to overcome these challenges?
Process control is a main pathway to manufacturability. Conventional LED manufacturers, say for >100 µm across, were accustomed to “good enough” and manual control methods. MicroLEDs require a new approach, much like what’s used in the silicon IC fab space.
High-speed, high-sensitivity metrology, and inspection enable advanced process control, which can improve baseline yield and better inform downstream processing such as repair. When combined with advanced process tools, it can further reduce many of the yield and cost issues and make high-volume manufacturing possible.
How will progress with microLED displays change the electronics industry?
MicroLED displays are new and disruptive, and that’s what’s exciting to me. It will inspire new types of displays and ways to interact with displays that we're not even thinking about right now.
MicroLED manufacturing companies and display manufacturers are trying lots of new things, and something will work. Suddenly, there will be a breakthrough and the cost of microLED displays may be cut in half. We don't know which players are going to succeed, but I think there will be market disruption. Once microLED displays are less expensive, electronics designers will create amazing new applications that we can all enjoy.
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