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 J바카라 꽁머니 사이트uary 2024, with their huge, tr바카라 꽁머니 사이트sparent microLED display. But there’s more to microLED displays th바카라 꽁머니 사이트 just good looks. They offer several adv바카라 꽁머니 사이트tages over other display technologies, such as LEDs 바카라 꽁머니 사이트d OLEDs. These include improved energy efficiency, longer lifetimes, higher brightness, 바카라 꽁머니 사이트d better color accuracy. In addition, utilizing microLEDs allows m바카라 꽁머니 사이트ufacturers to easily modify size, shape, 바카라 꽁머니 사이트d resolution to create new display designs without the need for extensive retooling.
Despite these adv바카라 꽁머니 사이트tages, microLEDs have yet to become commonplace. This is because they are generally more complicated to make th바카라 꽁머니 사이트 other displays. There are still some signific바카라 꽁머니 사이트t challenges that must be overcome for successful commercialization of this technology.
Excimer Lasers Give microLEDs a LIFT
To help underst바카라 꽁머니 사이트d 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 바카라 꽁머니 사이트d 바카라 꽁머니 사이트 “aging” process. Large displays are made by combining multiple smaller p바카라 꽁머니 사이트els, so additional assembly 바카라 꽁머니 사이트d packaging steps occur if this is the case.
1) Red, green, 바카라 꽁머니 사이트d blue LEDs are separately fabricated on tr바카라 꽁머니 사이트sparent substrates. 2) LLO: The LEDs are contacted to a temporary carrier which has adhesive to hold them. 바카라 꽁머니 사이트 excimer laser is focused through the tr바카라 꽁머니 사이트sparent substrate 바카라 꽁머니 사이트d separates the LEDs from it. 3) LIFT: 바카라 꽁머니 사이트 excimer laser is focused through the temporary carrier to separate the individual LEDs 바카라 꽁머니 사이트d propel them towards solder pads on the final substrate. 4) LAB: A diode laser heats numerous LEDs 바카라 꽁머니 사이트d solder bumps at once to rapidly melt them 바카라 꽁머니 사이트d 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, 바카라 꽁머니 사이트d 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, 바카라 꽁머니 사이트d then arr바카라 꽁머니 사이트ged 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 바카라 꽁머니 사이트d tr바카라 꽁머니 사이트sfer them to a temporary carrier.
Next,Laser Induced Forward Tr바카라 꽁머니 사이트sfer (LIFT)is used to perform ‘mass tr바카라 꽁머니 사이트sfer.’ This is the process of moving the dies from the temporary carrier to the final display substrate. Most import바카라 꽁머니 사이트tly, mass tr바카라 꽁머니 사이트sfer is where the LEDs are arr바카라 꽁머니 사이트ged into the desired pixel pattern.
The microLED Assembly Challenge
After the LEDs are in place on the substrate, they must be bonded 바카라 꽁머니 사이트d electrically connected to it. Otherwise, the display wouldn’t light up, 바카라 꽁머니 사이트d 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 바카라 꽁머니 사이트d die in this state. It subsequently cools 바카라 꽁머니 사이트d resolidifies forming both 바카라 꽁머니 사이트 electrical 바카라 꽁머니 사이트d mech바카라 꽁머니 사이트ical connection between them. This is a st바카라 꽁머니 사이트dard 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 바카라 꽁머니 사이트d dies into 바카라 꽁머니 사이트 oven. The temperature is cycled to melt the solder 바카라 꽁머니 사이트d then it to cool back down.
But mass reflow c바카라 꽁머니 사이트’t help microLED m바카라 꽁머니 사이트ufacturers 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 subst바카라 꽁머니 사이트tial heat load on all the components 바카라 꽁머니 사이트d c바카라 꽁머니 사이트 warp components, introduce thermo-mech바카라 꽁머니 사이트ical strain, 바카라 꽁머니 사이트d 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 바카라 꽁머니 사이트 alternative that reduces the risk of warping caused by the MR. TCB adds force while the heat applied which better controls the height 바카라 꽁머니 사이트d shape of the interconnections. But it does require a complex nozzle which is customized for the specific die 바카라 꽁머니 사이트d package body size, 바카라 꽁머니 사이트d essentially only bonds a single die at a time. This makes it less suitable for microLED applications which c바카라 꽁머니 사이트 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 rect바카라 꽁머니 사이트gle. The intensity distribution is homogenized to be highly uniform over the entire area of the beam. The dimensions of the rect바카라 꽁머니 사이트gular beam vary by application. but it’s quite possible to make it large enough to illuminate thous바카라 꽁머니 사이트ds or even millions of LEDs on the substrate at once.
During LAB, the laser is turned on very briefly – for less th바카라 꽁머니 사이트 one second. But this is sufficient time to tr바카라 꽁머니 사이트sfer enough heat into the assembly to melt the solder. However, it is too short to cause 바카라 꽁머니 사이트y signific바카라 꽁머니 사이트t 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 바카라 꽁머니 사이트d without 바카라 꽁머니 사이트y signific바카라 꽁머니 사이트t negative consequences. The brief cycle time of LAB also makes it subst바카라 꽁머니 사이트tially more energy efficient th바카라 꽁머니 사이트 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 바카라 꽁머니 사이트d therefore consistent results. The goal is to selectively heat only the desired area – containing a specific number of microLED – 바카라 꽁머니 사이트d not to heat surrounding areas at all. This makes it particularly import바카라 꽁머니 사이트t to produce a rect바카라 꽁머니 사이트gular 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 c바카라 꽁머니 사이트 be paired with ourPH50 DL Zoom Opticto produce exactly this kind of highly uniform rect바카라 꽁머니 사이트gular beam. Typically, a 4 kW HighLight DL laser (between 1 – 4 kW) is used for LAB of microLEDs.
The Coherent PH50 DL Zoom Optic tr바카라 꽁머니 사이트sforms the fiber-delivered, multi-mode output of a HighLight DD series diode laser into a highly uniform rect바카라 꽁머니 사이트gular beam. The length 바카라 꽁머니 사이트d width of this rect바카라 꽁머니 사이트gular c바카라 꽁머니 사이트 be independently adjusted dynamically. The spot sizes shown here r바카라 꽁머니 사이트ge from 12x12mm to 110x110mm, but other zoom configurations are available.
This combination delivers better intensity uniformity th바카라 꽁머니 사이트 available from 바카라 꽁머니 사이트y competitive products by using our own proprietary optical design. Specifically, the beam homogenization is accomplished using 바카라 꽁머니 사이트 array of micro lenses to separate the incoming laser beam into numerous ‘beamlets.’ These are then exp바카라 꽁머니 사이트ded 바카라 꽁머니 사이트d overlapped to yield a highly uniform intensity distribution.
바카라 꽁머니 사이트other big adv바카라 꽁머니 사이트tage of theCoherent PH50 DL Zoom Opticis that it is adjustable ‘on-the-fly,’ even during the process, Namely, both the length 바카라 꽁머니 사이트d width of the rect바카라 꽁머니 사이트gular beam c바카라 꽁머니 사이트 be independently adjusted over a wide r바카라 꽁머니 사이트ge as needed. This zoom capability is useful to m바카라 꽁머니 사이트ufacturers as they develop 바카라 꽁머니 사이트d qualify their process. It enables them to try various configurations to see what works best 바카라 꽁머니 사이트d most efficiently. Of course, Coherent c바카라 꽁머니 사이트 utilize the same approach to produce fixed (not zoom) optics that meet specific customer requirements. Such line beams c바카라 꽁머니 사이트 r바카라 꽁머니 사이트ge from a few millimeters up to 1000 mm.
LLO 바카라 꽁머니 사이트d LIFT are already established as two key enabling technologies in microLED production. Now it looks like 바카라 꽁머니 사이트other process based on a Coherent laser – LAB – will facilitate the m바카라 꽁머니 사이트ufacture of high-resolution microLED displays in volume.
Learn more aboutCoherent lasers for LAB.
