MicroLED display is one of the most important developments in the LED field. In addition to its beauty, it also has many advantages compared to other display technologies (such as LED and OLED), including improved energy efficiency, longer service life, higher brightness and better color accuracy. Additionally, with MicroLED technology, manufacturers can easily modify panel size, shape and resolution to create new display designs without having to specifically purchase new equipment.
MicroLED displays have many advantages, including aesthetics, improved energy efficiency, longer service life, higher brightness and better colors.Color accuracy, etc.
Despite the many advantages mentioned above, microLED is not yet popular. This is because the manufacturing process is typically more complex than other display technologies. There are still significant challenges that must be overcome for the technology to be successfully commercialized.
Excimer lasers power MicroLED development
To help understand where these challenges come from, the following figure shows some of the key steps in MicroLED display manufacturing. After these steps are completed, there are various other testing steps and "burn-in" processes. Large displays are made by combining multiple panels of smaller sizes, in which case additional assembly and packaging steps are required.
1) Red,Green and blue LEDs are respectively produced on transparent substrate growth wafers. 2) LLO: The LED on the growing wafer is contacted and fixed on a temporary carrier with adhesive, and the excimer laser is focused through the transparent substrate and separates the LED from it. 3) LIFT: The excimer laser is focused through the temporary carrier to selectively separate individual LEDs and transfer them to the pad locations on the final substrate. 4) LAB: The semiconductor laser heats multiple LEDs and solder at one time to quickly melt and form the final bond.
Like most semiconductor devices, LEDs are initially epitaxially grown on wafers, usually using sapphire substrates. Each pixel of a MicroLED display requires a separate LED that emits the three primary colors of red, green, and blue, but each grown wafer contains only a single color LED light-emitting device. Therefore, the LED epitaxial layer must be divided into individual bare dies and then arranged together according to the necessary design patterns to form the final display screen.
Excimer laser has been recognized by the industry as an efficient and economical solution for the first two main processes. Among them, laser lift-off technology (LLO) first separates a single LED chipDetach the sapphire wafer and transfer it to a temporary carrier.
Next, laser-induced forward transfer (LIFT) is used as "megatransfer." This process transfers the LED chips from a temporary carrier to the final display substrate. Importantly, mass transfer can match the LED chip arrangement to the desired pixel pattern.
MicroLED Assembly Challenges
After the LED is transferred to the substrate, it must be electrically connected to the substrate through a bonding process. Otherwise, the display will not light up and the LED chips will fall off when moved!
In order to perform the bonding process, solder "bumps" (small solder balls) are first placed on all predetermined electrical connection points on the substrate. The LED chips are then placed into place using a LIFT transfer device and the solder is heated until it melts. In this state, solder flows around the electrical contacts on the substrate and chip, the solder then cools and re-solidifies, forming an electrical and mechanical connection between them. This is a standard assembly technique throughout the electronic materials industry.
The most common method of melting solder is called "batch reflow" (MR), a process in which the entire substrate assembly, including solder balls and chips, is placed in an oven, circulates temperatures to melt the solder, and then cools again.
But batch reflow soldering is not very helpful for MicroLED display manufacturing. The LED chips used are extremely small in size, very close to each other and have extremely high positional accuracy. The key issue with reflow soldering is that the heating cycle takes several minutes, which creates a large thermal load on all components and can cause component deformation, introduce thermomechanical strain, and shift the position of the LED chip on the substrate. Longer processing times in reflow ovens increase the risk of poor electrical connections. The process itself is also energy-intensive.
Thermocompression bonding (TCB) is an alternative method that reduces the risk of warpage caused by reflow soldering. Thermocompression bonding applies pressure at the same time as heat, allowing greater control over the height and height of the interconnects formed.shape. But it requires a complex nozzle that is customized for a specific chip and package size and can only bond one chip at a time. Since MicroLED technology may require the bonding of millions of LED chips to create a display, this makes the thermocompression bonding process less suitable.
Laser-assisted bonding (LAB) can solve problems in the MicroLED assembly process
Laser-assisted bonding
Laser-assisted bonding (LAB) solves all these problems. In the LAB process, the high-power infrared band semiconductor laser is shaped into a rectangular spot. After homogenization, the intensity distribution in the entire spot area is highly consistent. Rectangular spot sizes vary depending on the application and can cover thousands or even millions of LEDs on a substrate at once.
During the LAB process, the laser is on for a very short time - less than a second, but this is enough to transfer enough heat into the component to melt the solder. Due to the extremely short time, the LAB does not produce any overall heating that could cause the substrate to warp or shift the position of the LED chip. The laser process enables precise control of the heating cycle and, as needed, the cooling phase, so the welding process can be performed quickly and without any noticeable negative consequences. LAB’s short cycle times also make it more energy efficient than reflow or thermocompression bonding.
Better lasers for improved LAB
As far as lasers are concerned, a key and necessary requirement for LAB is high consistency of beam intensity across the entire area to achieve a consistent and uniform solder heating process and obtain consistent bonding results. The goal is to selectively heat only the desired area (containing a specific number of LED chips) without heating the surrounding area at all. Therefore, it is particularly important to output a high-quality rectangular light spot, which requires that the beam intensity does not drop too much near the edge of the light spot, otherwise the L of this areaED chips may not bond at all. At the same time, the beam intensity of the rectangular spot must drop rapidly outside the irradiation area.
Coherent HighLight DL series semiconductor lasers, coupled through fiber output, can be used with our PH50 DL Zoom Optic zoom optical assembly to produce this highly uniform rectangular spot. Typically, HighLight DL lasers with a typical power of 4 kW are used for MicroLED laser-assisted bonding processes.
Coherent PH50 DL Zoom Optic zoom optical component uses fiber coupling to connect Hilight DL The multi-mode laser output from the series of semiconductor lasers is shaped into a highly homogenized rectangular spot, the length and width of which can be independently and dynamically adjusted. Spot sizes shown above range from 12x12mm to 110110mm, with additionalConfigurations are available.
By using our own proprietary optical design, this combination delivers better beam intensity consistency than any competing product. Specifically, beam homogenization is achieved by using a microlens array to split the incident laser into many "beamlets," which are then expanded and overlapped to produce a highly consistent intensity distribution.
Another great advantage of the Coherent PH50 DL Zoom Optic zoom optical component is that it can be adjusted "on the fly" during the processing, that is, the length and width of the rectangular spot can be independently adjusted within a wide range as needed. This scaling capability is useful for manufacturers developing and validating processes, allowing them to try various configurations in search of optimal process conditions. Of course, Coherent can also use the same approach to produce fixed (non-zoom) optics to meet customer specific requirements, with line spot lengths ranging from a few millimeters to 1000mm.
LLO and LIFT have become two key technologies that enable MicroLED display manufacturing. Now it seems that based onAnother process from Coherent lasers -- LAB -- will facilitate volume production of high-resolution MicroLED displays.
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