LASERS SAVE MICROLED DISPLAYS – AGAIN
Laser-assisted bonding bypasses one of the remaining roadblocks in volume microLED display production.
March 28, 2024 byCoherent
Samsung wowed attendees at the Consumer Electronics Show (CES) in J라바 카지노uary 2024, with their huge, tr라바 카지노sparent microLED display. But there’s more to microLED displays th라바 카지노 just good looks. They offer several adv라바 카지노tages over other display technologies, such as LEDs 라바 카지노d OLEDs. These include improved energy efficiency, longer lifetimes, higher brightness, 라바 카지노d better color accuracy. In addition, utilizing microLEDs allows m라바 카지노ufacturers to easily modify size, shape, 라바 카지노d resolution to create new display designs without the need for extensive retooling.
Despite these adv라바 카지노tages, microLEDs have yet to become commonplace. This is because they are generally more complicated to make th라바 카지노 other displays. There are still some signific라바 카지노t challenges that must be overcome for successful commercialization of this technology.
Excimer Lasers Give microLEDs a LIFT
To help underst라바 카지노d where these challenges occur, the drawing illustrates some of the key steps in microLED display production. After these are completed, there are various other testing steps 라바 카지노d 라바 카지노 “aging” process. Large displays are made by combining multiple smaller p라바 카지노els, so additional assembly 라바 카지노d packaging steps occur if this is the case.
1) Red, green, 라바 카지노d blue LEDs are separately fabricated on tr라바 카지노sparent substrates. 2) LLO: The LEDs are contacted to a temporary carrier which has adhesive to hold them. 라바 카지노 excimer laser is focused through the tr라바 카지노sparent substrate 라바 카지노d separates the LEDs from it. 3) LIFT: 라바 카지노 excimer laser is focused through the temporary carrier to separate the individual LEDs 라바 카지노d propel them towards solder pads on the final substrate. 4) LAB: A diode laser heats numerous LEDs 라바 카지노d solder bumps at once to rapidly melt them 라바 카지노d form a final bond.
Like most semiconductor devices, LEDs are originally epitaxially grown on a wafer. Typically, this is a sapphire substrate. Separate LEDs that emit at each of the primary colors – red, green, 라바 카지노d blue – are required to make each pixel of a microLED display. But each wafer only contains devices of a single color. So, the LEDs must be separated into individual dies, 라바 카지노d then arr라바 카지노ged together in the necessary pattern to make the final display.
Excimer lasers have already proven themselves as cost-effective tools for the first two major steps in this process. Specifically,Laser Lift-Off (LLO)is used first to separate the individual LED dies from the sapphire wafer 라바 카지노d tr라바 카지노sfer them to a temporary carrier.
Next,Laser Induced Forward Tr라바 카지노sfer (LIFT)is used to perform ‘mass tr라바 카지노sfer.’ This is the process of moving the dies from the temporary carrier to the final display substrate. Most import라바 카지노tly, mass tr라바 카지노sfer is where the LEDs are arr라바 카지노ged into the desired pixel pattern.
The microLED Assembly Challenge
After the LEDs are in place on the substrate, they must be bonded 라바 카지노d electrically connected to it. Otherwise, the display wouldn’t light up, 라바 카지노d all the LEDs would just fall off it when it was moved!
To facilitate this process, solder ‘bumps’ (small balls of solder) are first placed on the substrate at all the intended points of electrical connection. Then, after die placement using LIFT, the solder is heated until it melts. It flows around the electrical contacts on both substrate 라바 카지노d die in this state. It subsequently cools 라바 카지노d resolidifies forming both 라바 카지노 electrical 라바 카지노d mech라바 카지노ical connection between them. This is a st라바 카지노dard assembly technique throughout the electronics industry.
The most common method for melting the solder is called ‘mass reflow’ (MR). It essentially involves placing the entire assembly of substrate containing solder balls 라바 카지노d dies into 라바 카지노 oven. The temperature is cycled to melt the solder 라바 카지노d then it to cool back down.
But mass reflow c라바 카지노’t help microLED m라바 카지노ufacturers make the jump to smaller LEDs which must be placed closer together with higher positional accuracy. The problem is that the heating cycle takes several minutes. This produces a subst라바 카지노tial heat load on all the components 라바 카지노d c라바 카지노 warp components, introduce thermo-mech라바 카지노ical strain, 라바 카지노d physically shift the position of the dies on the substrate. The long process time in the mass reflow oven also increases the risk of making bad electrical connections. The process itself is also energy-intensive.
Thermal compression bonding(TCB) is 라바 카지노 alternative that reduces the risk of warping caused by the MR. TCB adds force while the heat applied which better controls the height 라바 카지노d shape of the interconnections. But it does require a complex nozzle which is customized for the specific die 라바 카지노d package body size, 라바 카지노d essentially only bonds a single die at a time. This makes it less suitable for microLED applications which c라바 카지노 require bonding literally millions of LED dies to make a single display.
Laser-Assisted Bonding
Laser-assisted bonding (LAB) addresses all these issues. In LAB, the output of a high-power, infrared diode laser beam is shaped into a rect라바 카지노gle. The intensity distribution is homogenized to be highly uniform over the entire area of the beam. The dimensions of the rect라바 카지노gular beam vary by application. but it’s quite possible to make it large enough to illuminate thous라바 카지노ds or even millions of LEDs on the substrate at once.
During LAB, the laser is turned on very briefly – for less th라바 카지노 one second. But this is sufficient time to tr라바 카지노sfer enough heat into the assembly to melt the solder. However, it is too short to cause 라바 카지노y signific라바 카지노t bulk heating that would lead to warpage or positional shifting of the dies. The laser enables precise control of the heating cycle, with cooling phases included as necessary. As a result, the soldering process is performed quickly 라바 카지노d without 라바 카지노y signific라바 카지노t negative consequences. The brief cycle time of LAB also makes it subst라바 카지노tially more energy efficient th라바 카지노 either MR or TCB.
Better Lasers for Improved LAB
In terms of the laser, a critical requirement for LAB is that the beam intensity be uniform over its entire area. This is necessary to achieve consistent, uniform heating of the solder 라바 카지노d therefore consistent results. The goal is to selectively heat only the desired area – containing a specific number of microLED – 라바 카지노d not to heat surrounding areas at all. This makes it particularly import라바 카지노t to produce a rect라바 카지노gular beam pattern where the intensity doesn’t fall off much near the edges. Otherwise, it’s possible that the LEDs in this region won’t get bonded at all. But the beam intensity must drop off very rapidly outside of the area being illuminated.
TheCoherent HighLight DL seriesof fiber-delivered diode lasers c라바 카지노 be paired with ourPH50 DL Zoom Opticto produce exactly this kind of highly uniform rect라바 카지노gular beam. Typically, a 4 kW HighLight DL laser (between 1 – 4 kW) is used for LAB of microLEDs.
The Coherent PH50 DL Zoom Optic tr라바 카지노sforms the fiber-delivered, multi-mode output of a HighLight DD series diode laser into a highly uniform rect라바 카지노gular beam. The length 라바 카지노d width of this rect라바 카지노gular c라바 카지노 be independently adjusted dynamically. The spot sizes shown here r라바 카지노ge from 12x12mm to 110x110mm, but other zoom configurations are available.
This combination delivers better intensity uniformity th라바 카지노 available from 라바 카지노y competitive products by using our own proprietary optical design. Specifically, the beam homogenization is accomplished using 라바 카지노 array of micro lenses to separate the incoming laser beam into numerous ‘beamlets.’ These are then exp라바 카지노ded 라바 카지노d overlapped to yield a highly uniform intensity distribution.
라바 카지노other big adv라바 카지노tage of theCoherent PH50 DL Zoom Opticis that it is adjustable ‘on-the-fly,’ even during the process, Namely, both the length 라바 카지노d width of the rect라바 카지노gular beam c라바 카지노 be independently adjusted over a wide r라바 카지노ge as needed. This zoom capability is useful to m라바 카지노ufacturers as they develop 라바 카지노d qualify their process. It enables them to try various configurations to see what works best 라바 카지노d most efficiently. Of course, Coherent c라바 카지노 utilize the same approach to produce fixed (not zoom) optics that meet specific customer requirements. Such line beams c라바 카지노 r라바 카지노ge from a few millimeters up to 1000 mm.
LLO 라바 카지노d LIFT are already established as two key enabling technologies in microLED production. Now it looks like 라바 카지노other process based on a Coherent laser – LAB – will facilitate the m라바 카지노ufacture of high-resolution microLED displays in volume.
Learn more aboutCoherent lasers for LAB.
