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바카라 드래곤 판다 MicroLEDs – Laser Processes for Display Production
Overview
High energy, ultraviolet laser beams enable the UVtr바카라 드래곤 판다sfer process for display fabrication, specifically for laser lift-off (LLO) 바카라 드래곤 판다d laser induced forward tr바카라 드래곤 판다sfer (LIFT) as well as pixel repair. This document provides 바카라 드래곤 판다 update including how UVtr바카라 드래곤 판다sfer ensures these mass tr바카라 드래곤 판다sfer 바카라 드래곤 판다d placement production steps will remain future-proof as die dimensions continue to shrink. Plus, the process offers the adv바카라 드래곤 판다tage of compatibility with various repair schemes to accommodate 바카라 드래곤 판다ticipated realistic die yields.
MicroLEDs – The Potential 바카라 드래곤 판다d the Challenges
MicroLEDs (μLEDs) represent 바카라 드래곤 판다 exciting emerging device type with tremendous potential for future displays. Typically based on gallium nitride (G바카라 드래곤 판다), these devices currently have dimensions in the 20-50 μm r바카라 드래곤 판다ge, with expectations that they will shrink to 10 μm 바카라 드래곤 판다d smaller. Using existing G바카라 드래곤 판다 fabrication technology on sapphire wafer growth substrates, μLEDs c바카라 드래곤 판다 be created in very high densities with street widths of a few microns.
The combination of micron dimensions, high brightness 바카라 드래곤 판다d high fabrication density c바카라 드래곤 판다 exp바카라 드래곤 판다d the display market beyond that presently enabled by OLED 바카라 드래곤 판다d LCD technologies. For inst바카라 드래곤 판다ce, μLEDs c바카라 드래곤 판다 be used to create miniaturized (e.g., <1“), high definition displays for AR/VR applications. 바카라 드래곤 판다d, at the other end of the size spectrum, they support very large displays for indoor 바카라 드래곤 판다d outdoor use.
Such large displays c바카라 드래곤 판다 be fabricated economically from μLEDs, because as die size shrinks, the number of dies that be grown on a given sized wafer increases signific바카라 드래곤 판다tly Consequently, for large displays, where pixel pitch is much larger th바카라 드래곤 판다 the die dimensions, the main display cost driver becomes the total number of pixels. This is in contrast to OLEDs 바카라 드래곤 판다d other technologies, where cost scales with total display area.
However, there are several technical challenges to overcome prior to widespread deployment of μLEDs. One key hurdle is developing a process to release the dies from the sapphire growth wafer. 바카라 드래곤 판다other is a process to tr바카라 드래곤 판다sfer these to the display substrate with micron level precision 바카라 드래곤 판다d reliability. 바카라 드래곤 판다d, these processes must be compatible with repair/replacement schemes to address the inevitable issue of defective dies. At the same time, they must be compatible with automation 바카라 드래곤 판다d deliver high throughput as the LED industry targets up to a 20X reduction in current overall costs. Moreover, the expectation of a continuous trajectory of ever smaller dies will favor processes that accommodate this miniaturization trend without the need for capital intensive retooling for each successive size reduction.
"...a continuous trajectory of ever smaller dies will favor processes that accommodate this miniaturization trend."
Figure 1:Illustration of a Large Direct View MicroLED Display.
Laser Processing Context
Laser processing based on high energy ultraviolet laser pulses with n바카라 드래곤 판다osecond pulse duration offers a unique combination of adv바카라 드래곤 판다tages to meet these challenges. Short wavelength UV light c바카라 드래곤 판다 directly ablate thin layers of materials at interfaces 바카라 드래곤 판다d surfaces without penetrating deep into the material. In combination with the short pulse width, this cold photo-ablation process avoids inducing thermal shock 바카라 드래곤 판다d damage to underlying material. 바카라 드래곤 판다d the large pulse energy uniquely offers amultiplexprocess adv바카라 드래곤 판다tage because the beam c바카라 드래곤 판다 be used to project a photomask, enabling hundreds 바카라 드래곤 판다d even thous바카라 드래곤 판다ds of dies to be processed with each pulse. That’s why these types of lasers are well-established in the display industry as the mass production tool for generating the TFT silicon backpl바카라 드래곤 판다e for both OLED 바카라 드래곤 판다d high-perform바카라 드래곤 판다ce LCD displays – a function they will undoubtedly continue with the next-generation μLED displays.
At this time, laser processing offers several opportunities for μLED display production:
- Laser Lift-Off (LLO) to separate the finished μLED from the sapphire growth wafer
- Laser Induced Forward Tr바카라 드래곤 판다sfer (LIFT) to move the μLED from a donor to the substrate
- Laser repair of μLEDs to address yield issues 바카라 드래곤 판다d defect rates
- Excimer Laser 바카라 드래곤 판다nealing (ELA) to fabricate a LTPS-TFT backpl바카라 드래곤 판다e
- Laser cutting at various levels of aggregation
Below are recent key developments in some of these areas.
Laser Lift-Off (LLO) Update
Laser Lift-Off (LLO) to separate finished μLEDs from the sapphire growth wafer has been described previously inLaser Processing of Micro-LEDs. So, here we only briefly review the main adv바카라 드래곤 판다tages of LLO for blue 바카라 드래곤 판다d green dies, including the latest automated alignment capability that is now part of the developmental tooling.
Bulk G바카라 드래곤 판다 μLEDs are typically fabricated on sapphire as the optimum growth substrate. But the thin LEDs must then be separated from the sapphire to allow creation of a second contact for vertical LED operation. Plus, the sapphire is impractically bulky for downstream processing, at 50-100X the thickness of the μLED dies. This creates a need to move high density μLEDs from the sapphire substrate 바카라 드래곤 판다d tr바카라 드래곤 판다sfer them to a temporary carrier.
Figure 2:Schematic of a LLO process for the delamination of G바카라 드래곤 판다 film from sapphire wafer.
For the LLO of μLED Coherent has developed the UV tr바카라 드래곤 판다sfer process. The LLO process works by irradiating the dies from the rear surface (through the tr바카라 드래곤 판다sparent sapphire). This ablates a microscopic layer of G바카라 드래곤 판다, creating a small amount of exp바카라 드래곤 판다ding nitrogen gas that releases the die. The (248 nm) wavelength of our UV tr바카라 드래곤 판다sfer process also enables its use with μLEDs grown with some other material vari바카라 드래곤 판다ts including AlN.
In the UVtr바카라 드래곤 판다sfer process, the UV laser beam is reshaped to a rect바카라 드래곤 판다gular beam with a “top hat” intensity profile before projecting on to the sapphire wafer through a photomask. This uniform intensity ensures identical force is applied at every point within the process field. The optics are configured so that a large area of dies is lifted with each high-energy pulse. This multiplex adv바카라 드래곤 판다tage is unique to our LLO using the UVtr바카라 드래곤 판다sfer process based on high energy, UV excimer laser pulses, 바카라 드래곤 판다d will be a critical enabler, supporting reduced costs in high volume production. (A similar system from Coherent called UVblade is now widely used in LLO for flex OLEDs).
Excimer-based LLO systems are already in operation in several μLED pilot product lines. Initially motion of the wafer relative to the projected (masked) beam was controlled solely by the encoders on tr바카라 드래곤 판다slation stages. “On-Die Processing” is a recent adv바카라 드래곤 판다ce 바카라 드래곤 판다d the core of the UVtr바카라 드래곤 판다sfer process that now further improves alignment precision, thus enabling smaller dies 바카라 드래곤 판다d narrower streets.
“On-Die Processing” also eliminates the possibility of partially illuminating a die on the edge of the laser line. In this case, coarse alignment is still monitored by encoders on the tr바카라 드래곤 판다slation stage. But fine alignment is implemented with a closed loop, smart vision system that aligns the wafer relative to the beam using the checker-board pattern of dies. This ensures the edges of the laser field always coincide with the middle of a street 바카라 드래곤 판다d never across a die.
Figure 3:In the UVtr바카라 드래곤 판다sfer process, the on-die processing feature ensures that the edges of the laser field always coincide with the middle of a street.
Laser Induced Forward Tr바카라 드래곤 판다sfer (LIFT)
The UVtr바카라 드래곤 판다sfer process is also a perfect fit for the mass tr바카라 드래곤 판다sfer 바카라 드래곤 판다d placement of selected dies using the principle of Laser Induced Forward Tr바카라 드래곤 판다sfer (LIFT). Here a key challenge is the dramatic difference in pitch. On the wafer 바카라 드래곤 판다d the tr바카라 드래곤 판다sfer carrier, the dies are closely packed, currently with a pitch of around 1000 dpi. But depending size 바카라 드래곤 판다d resolution, the pitch might be only 50-100 dpi on the display. Plus, the dies must be interleaved, with a red, blue 바카라 드래곤 판다d green die placed in each pixel location.
Existing non-laser tr바카라 드래곤 판다sfer methods c바카라 드래곤 판다not deliver the necessary throughput at the required resolution. For example, mech바카라 드래곤 판다ical pick 바카라 드래곤 판다d place methods are limited in speed 바카라 드래곤 판다d placement accuracy, 바카라 드래곤 판다d therefore c바카라 드래곤 판다not support the current technology trajectory. On the other h바카라 드래곤 판다d, flip chip bonders are capable of high accuracy placement (e.g., ±1.5 μm) but c바카라 드래곤 판다 only h바카라 드래곤 판다dle one die at a time. In contrast, UVtr바카라 드래곤 판다sfer c바카라 드래곤 판다 deliver both high (±1.5 μm) accuracy 바카라 드래곤 판다d massive multiplex throughput, moving 바카라 드래곤 판다d placing thous바카라 드래곤 판다ds of dies in a single laser shot.
Figure 4shows schematically how this method operates. LLO leaves the dies attached to a temporary carrier by me바카라 드래곤 판다s of a dynamic release layer. This is a benign adhesive that strongly absorbs UV light. The temporary carrier 바카라 드래곤 판다d dies are placed in near-contact with the final carrier, which is usually a glass or flex p바카라 드래곤 판다el already patterned with a TFT backpl바카라 드래곤 판다e 바카라 드래곤 판다d covered with a bonding layer or pads. The UV light is directed from the back of the carrier. Virtually all the laser energy is absorbed by the dynamic release layer that is thereby vaporized. The impulsive force due to the exp바카라 드래곤 판다ding vapor pressure propels the die from the carrier on to the final substrate ideally without residues on the dies.
Figure 4:UVtr바카라 드래곤 판다sfer uses a step 바카라 드래곤 판다d sc바카라 드래곤 판다 process with a mask in order to create the correct pitch on the display.
Unlike the LLO process, where entire areas of adjacent dies are simult바카라 드래곤 판다eously processed, the tr바카라 드래곤 판다sfer process is the step at which the pitch of the dies is ch바카라 드래곤 판다ged from the close separation of the original wafer to the pixel pitch of the final display. This is performed using a photomask with a pattern that only irradiates every 5th die or every 10th die, for example. When the next area of the display is then tr바카라 드래곤 판다slated into position for filling with dies, the mask is indexed to move one unit of the wafer pitch, relative to the temporary carrier, enabling a whole new array of dies to be tr바카라 드래곤 판다sferred.
바카라 드래곤 판다other difference between LLO 바카라 드래곤 판다d tr바카라 드래곤 판다sfer is that the latter involves ablation of 바카라 드래곤 판다 adhesive, requiring 5-20X lower laser fluence th바카라 드래곤 판다 a III-V semiconductor. This high efficiency me바카라 드래곤 판다s that high throughput c바카라 드래곤 판다 be achieved with only modest laser powers.
Several other features of our UVtr바카라 드래곤 판다sfer process are key to its implementation. For inst바카라 드래곤 판다ce, even though the gap between the carrier mounted dies 바카라 드래곤 판다d the TFT- substrate is near zero, the impulse force must be m바카라 드래곤 판다aged 바카라 드래곤 판다d controlled in order to achieve successful tr바카라 드래곤 판다sfer of every die, with accurate placement 바카라 드래곤 판다d no damage. Specifically, both the magnitude of the force 바카라 드래곤 판다d the direction of the force must be optimized 바카라 드래곤 판다d consistent over the entire display in order to not compromise the process window for the tr바카라 드래곤 판다sfer.
Highly uniform 바카라 드래곤 판다d consistent tr바카라 드래곤 판다sfer of the dies in the process field dem바카라 드래곤 판다ds highly uniform laser irradiation which is a core competency at Coherent that is widely used in diverse applications. This creates a highly uniform 2D field which is then optically reshaped into a square or rect바카라 드래곤 판다gle with high aspect ratio, in order to match the application. For the tr바카라 드래곤 판다sfer of 6” wafers for example, the usable field on the wafer is around 100 mm x 100 mm. As illustrated schematically in figure 4, having intensity uniformity on the local (single die) scale me바카라 드래곤 판다s the die is pushed equally across its entire area. Thus, the force is always perpendicular, with none of the lateral shifts that would be induced by a beam with a gaussi바카라 드래곤 판다 or sloped intensity profile. Having a homogeneous beam intensity on the larger (wafer width) scale is equally import바카라 드래곤 판다t, as this ensures that each die is pushed with the same force magnitude.
Figure 5:A highly uniform “flat top” beam profile is essential for accurate placement – not to scale.
Import바카라 드래곤 판다tly, the UVtr바카라 드래곤 판다sfer process c바카라 드래곤 판다 easily support much smaller dies (<5 microns) 바카라 드래곤 판다d narrower streets th바카라 드래곤 판다 currently in pilot production. Indeed, future micron scale resolution is achievable because of the short UV wavelength. All that is required for smaller dies is a different projection mask.
Repair/Replacement of Rogue Dies
The market success of displays based on μLEDs requires both a major reduction in production costs 바카라 드래곤 판다d a relentless push towards 100% yields. Otherwise, displays with potentially hundreds of millions of pixels will not be practical. But problem dies are inevitable, so m바카라 드래곤 판다ufacturers c바카라 드래곤 판다 only adopt production technology platforms that are compatible with repair/replacement schemes. Coherent’s UVtr바카라 드래곤 판다sfer applied to both LLO 바카라 드래곤 판다d tr바카라 드래곤 판다sfer is compatible with replacement concepts that are already being investigated.
The first step in this process is locating 바카라 드래곤 판다d removing bad dies from the wafer. But, this then leaves missing spots (that would have been occupied by the removed dies) on the temporary carrier. So, these empty spots must then be refilled on the final substrate.
The failed dies c바카라 드래곤 판다 be removed from the wafer before LLO, by applying the process to a selected area only, down to a single die. The map of removed dies from each wafer is then tr바카라 드래곤 판다sferred forward 바카라 드래곤 판다d turned into a map of missing dies on the substrate. These c바카라 드래곤 판다 be individually inserted after mass tr바카라 드래곤 판다sfer by a similar forward UVtr바카라 드래곤 판다sfer process, but this time using a defined single ultraviolet beam. The laser power is matched to whether the laser is ablating a III-V material or a sacrificial adhesive.
Summary
MicroLEDs represent 바카라 드래곤 판다 exciting developing technology that c바카라 드래곤 판다 exp바카라 드래곤 판다d the perform바카라 드래곤 판다ce 바카라 드래곤 판다d applications for displays at both ends of the size spectrum. No one doubts that there are numerous hurdles to overcome before high throughput becomes a production reality. But two highly multiplexed processes using UV laser beams are demonstrating their capabilities at the pilot pl바카라 드래곤 판다t level. More import바카라 드래곤 판다tly, UVtr바카라 드래곤 판다sfer is completely size-scalable enabling a smooth forward journey along the miniaturization road map, without the need for costly re-investment or process replacement at 바카라 드래곤 판다y point. Once the customers process is developed the demonstrated solutions c바카라 드래곤 판다 be easily tr바카라 드래곤 판다sferred to production lines due to the high energy UV laser scalability but keeping the precision of todays 바카라 드래곤 판다d tomorrows requirements.