Mandy Gebhardt, marketing and public relations team leader, Arved Kampe, product manager, and Markus Müller, product management team leader, 3D-Micromac
MicroLEDs (μLEDs) have tremendous potential for future displays. However, there are several technical challenges to overcome prior to their widespread deployment. One such challenge is to separate the μLED chips from the sapphire growth substrate; another is to precisely transfer the μLED chips from the carrier substrate to the display substrate. There are several laser technologies for producing μLED displays, including laser lift-off (LLO), for separating the μLED chips from the sapphire growth substrate, and laser-induced forward transfer (LIFT), for transferring the μLED chips from the carrier substrate to the display substrate.
In this article, laser-based systems for the aforementioned μLED manufacturing steps are presented. Integrated process control and monitoring are used to ensure reliable and stable operation, and therefore achieve high-throughput and low yield losses.
Introduction
As the name suggests, μLED is a display technology that relies on LEDs with dimensions in the micrometre range. Manufacturers such as PlayNitride and Sony define μLED screens as having LEDs with dimensions of less than 50 µm or a luminous area of less than 0.003 mm².
μLEDs are self-luminous, dimmable and completely switchable, and have enormous potential for future displays. They score particularly well with their high brightness, high contrast and high production density but are currently very cost-intensive to mass manufacture. Potential applications include very large displays for indoor and outdoor use, as well as high-resolution displays for augmented reality (AR) and virtual reality (VR) applications.
The benefits of microLED applications.
Typically, μLED chips are produced epitaxially from gallium nitride (GaN), in a thin layer, on a sapphire growth substrate. They have dimensions in the range of 20 to 50 µm, but future generations are expected to have significantly smaller dimensions down to below 10 µm. μLEDs can be produced in very high numbers and with linewidths of a few micrometres.
However, before μLEDs can be used in the mass market, there are some technical challenges that need to be overcome in the fabrication process. As previously mentioned, these include separating the μLED chips from the sapphire growth substrate and then precisely transferring them from the carrier substrate to the display substrate. The laser lift-off (LLO) and laser-induced forward transfer (LIFT) processes directly address these challenges. There are also laser technologies for the detection and repair or replacement of defective μLEDs during the manufacturing process.
The market potential of microLED applications.
The μLED manufacturing process
Unlike conventional OLED or LCD panels, the manufacture of a μLED panel is a complex task involving several production lines for different process steps. These are:
- fabrication of the backplane,
- fabrication of the carrier substrate
- fabrication of the μLED chips;
- and transfer and application of the μLED chips to the backplane.
MicroLED chips are built up on the glass backplane using processes that are standard in the semiconductor industry. During LLO, the μLED chips are separated from the sapphire growth substrate. Their functionality is then tested by photo- and electro-luminescence. During LIFT, the μLED chips separate from the carrier substrate and gravitate towards and latch onto the glass backplane. They are then inspected using a camera.
Defective μLED chips are removed during the laser trimming/repair process step. The LLO and LIFT processes are then repeated to replace the removed μLED chips until the desired yield is achieved.
The microLED manufacturing process.
LLO
Laser lift-off is a process for selectively detaching one material from another. The process involves a laser beam penetrating a transparent base material that is coupled to a second material. It is commonly used in the fabrication of LEDs and μLEDs, to separate the chips from the sapphire growth substrate. A significant advantage is that there is no damage to the substrate, meaning it can be reused.
Laser lift-off can also be used for the separation of transparent and absorbing flexible substrates from glass substrates, such as in the manufacture of flexible displays, organic LED (OLED) or active-matrix organic LED (AMOLED). In addition, other LLO applications can be found in the semiconductor industry and sensor manufacturing.
The underlying concept of the LLO process is the varied absorption of the laser light in the different layers that need to be separated from each other. Short-wavelength laser light in the UV wavelength range from 193 nm to 355 nm and pulse lengths in the nanosecond range are ideal for LLO. Exceptional laser beam quality and tight process control are essential for achieving high-performance and high-speed LLO.
The main steps of the LLO process are:
- An appropriate adhesive layer is applied to the carrier substrate and the µLED chip is placed face down on top.
- The UV laser beam penetrates the sapphire growth substrate from above and strikes the impermeable GaN layer. This heats the interface between sapphire and GaN up to 900°C.
- The sapphire growth substrate then lifts off, or separates from, the µLED chip over the entire surface.
A schematic representation of the laser lift-off (LLO) process.
The microMIRA LLO system from 3D-Micromac enables very uniform, force-free lift-off of flexible layers on large areas, at high speeds. The system is capable of processing different substrate materials and sizes. It has been used successfully in mass production by electronics manufacturers globally for years.
LIFTLaser-induced forward transfer is a process that involves using a UV laser to selectively separate and then transfer material from the carrier substrate to the display substrate. The laser does not process any material in the classical sense but is used as a tool that triggers the material transfer via a controlled energy input.
The main steps of the LIFT process are:
- The μLED chip is positioned face down towards the display substrate.
- Similar to LLO, the UV laser heats the interface between the carrier substrate and the μLED chip, resulting in the chip separating from the substrate.
- The heat generated by the process and positioning of the μLED chip ensure its gravitational transference directly onto the display substrate.
*The laser is so selective that single or multiple μLED chips can be separated and transferred.
A schematic representation of the laser-induced forward transfer (LIFT) process.
The transfer of μLEDs is subject to special requirements in terms of accuracy, reliability and speed. In contrast to LIFT, conventional transfer methods do not deliver the required throughput. Mechanical placement methods are limited in terms of speed and positioning/placement accuracy. Flip-chip bonders, on the other hand, are capable of high-precision placement, but can only handle one μLED chip at a time.
The microCETI LIFT system from 3D-Micromac affords an exceptionally high transfer rate of approximately 130 million μLEDs per hour and positioning accuracies of less than 2 µm.
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A comparison of transfer rates of different transfer approaches.
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3D-Micromac’s microCETI LIFT system.
Conclusion
μLEDs represent a promising development for future display applications. However, there are still several manufacturing hurdles to overcome before these displays can be manufactured in high volumes using mass production processes. Despite the technological advances discussed above, there are still some critical bottlenecks that need to be overcome for mass production of μLED displays.
An important issue is the pixel yield of the display. A dead pixel can occur at various stages of manufacturing, such as epitaxy, LED chip processing or the transfer process. To produce a full-colour 1920 × 1080, full-HD (high-definition) display with less than five dead pixels, the yield must be 99.9999 percent. This yield is too high for the technical level achievable today with conventional manufacturing methods.
LLO and LIFT systems have enormous potential to drive mass production of μLED displays. Above all, the high transfer rates enable a cost-efficient process and thus a reduction in the overall costs of display production.
3D-Micromac