Breaking down the laptop’s screen, at its heart is a plate patterned with red, green, and blue LED pixels arranged edge-to-edge like a meticulous Lite Brite display. I have. When powered, the LEDs together can produce all the shades of the rainbow to produce a full-color display. Over the years, the size of individual pixels has shrunk, allowing more pixels to be packed into devices to produce sharper, higher-resolution digital displays.
But like transistors in computers, LEDs have reached the limit of how small they can be while still functioning effectively. This limitation is especially noticeable on near-field displays such as augmented reality and virtual reality devices. These displays have a limited pixel density, resulting in a “screen door effect” where users perceive streaks in the space between pixels.
Now, MIT engineers have developed a new method to create sharper, defect-free displays. Instead of replacing red, green, and blue light-emitting diodes side by side in a horizontal patchwork, the team invented a way to stack the diodes to create vertical multicolored pixels.
Each stacked pixel can produce the full range of commercial colors and is about 4 microns wide. Tiny pixels, or “micro-LEDs,” can be packed to densities of 5,000 pixels per inch.
“This is the smallest micro-LED pixel and the highest pixel density reported in a journal,” said Jeehwan Kim, associate professor of mechanical engineering at MIT. “It shows that vertical pixelation is a way to achieve high-resolution displays in a smaller footprint.”
Ji-ho Shin, a postdoc in Kim’s research group, said, “For virtual reality, there is a limit to what can look real at the moment.” , it will no longer be possible to distinguish between the virtual and the real.”
The team’s results are Published today in the journal Nature. Kim and Shin’s co-authors include members of Kim’s lab, researchers around MIT, Georgia Tech Europe, Sejong University, and collaborators from multiple universities in the US, France, and South Korea.
Alignment of pixels
Today’s digital displays are lit by Organic Light Emitting Diodes (OLEDs), plastic diodes that emit light in response to electrical current. OLED is the dominant digital display technology, but the diodes degrade over time and can cause permanent burn-in to the screen. This technology also hits the limit of how small the diode can be, limiting sharpness and resolution.
As the next generation display technology, researchers are exploring inorganic micro-LEDs. This is a diode that is 100 times smaller than a conventional LED and is made of inorganic single crystal semiconductor material. Micro LEDs can perform better than OLEDs, require less energy, and last longer.
However, manufacturing micro LEDs requires extremely high precision. To properly reflect and produce different colors, tiny pixels of red, green, and blue are first grown individually on the wafer, then precisely aligned with each other and precisely placed on the plate. need to do it. and tint. Achieving such microscopic accuracy is a daunting task, and if a pixel is found to be misaligned, the entire device must be scrapped.
“With this pick-and-place manufacturing, the potential for pixel misalignment on a very small scale is very high,” says Kim. “If there is misalignment, that material must be discarded or the display may be ruined.”
color stack
An MIT team has come up with a potentially low-waste way to manufacture micro-LEDs that doesn’t require pixel-perfect alignment. This technology is a completely different vertical LED approach as opposed to the traditional horizontal pixel placement.
Kim’s group specializes in developing techniques to produce pure, ultra-thin, high-performance membranes for smaller, thinner, more flexible and functional electronics. The team has previously developed a method for growing and peeling perfect two-dimensional monocrystalline material from silicon and other surface wafers. This is an approach called 2D material-based layer transfer (2DLT).
In the current study, researchers have taken this same approach to grow ultra-thin films of red, green, and blue LEDs. The entire LED membrane was then peeled off from the base wafer and stacked to create a layer cake of red, green and blue membranes. I was able to separate it.
“In conventional displays, the R, G, and B pixels are arranged horizontally, which limits how small each pixel can be. , which could theoretically reduce the pixel area by a factor of three.”
As a demonstration, the team fabricated vertical LED pixels and showed that a single pixel can produce different colors by changing the voltages applied to each of the pixel’s red, green, and blue membranes.
“If the current to red is high and the current to blue is weak, the pixel will appear pink or something like that,” Singh says. “We can create all mixed colors and our displays can cover colors close to the available commercial color spaces.”
The team plans to improve the handling of vertical pixels. So far they have shown that individual structures can be stimulated to produce a full spectrum of colors.
“We need a system to individually control 25 million LEDs,” says Shin. “Here we only partially show it. Active matrix manipulation is something that needs to be further developed.”
“So far we have shown the community that ultra-thin LEDs can be grown, peeled and stacked,” says Kim. “This is the perfect solution for small displays such as smart watches and virtual reality devices where high pixel densities are needed to create vivid, vivid images.”
This work was supported in part by the U.S. National Science Foundation, the U.S. Defense Advanced Research Projects Agency (DARPA), the U.S. Air Force Research Laboratory, the U.S. Department of Energy, LG Electronics, Rohm Semiconductor, and the French National Research Institute. , and the National Research Foundation of Korea.
