22 October 2018

Case Study from Coherent: Laser Manufacturing Micro-LED Displays

Micro-LEDs based on GaN hold great promise for displays that outperform existing technology in terms of resolution, efficiency, brightness, lifetime and operating temperature range. Moving from LEDs (approx. 200 μm die size) to Micro-LEDs (approx. 20 μm die size) requires advanced fabrication methods, creating new opportunities for laser-based processing.

The vast majority of LED production today utilizes sapphire wafer as the growth substrate due to its small lattice mismatch and relatively low cost. However, as a final (permanent) carrier material it severely hampers the performance of GaN-LEDs in terms of optical efficiency and heat dissipation. Hence, an active area for HB-LED development and manufacturing involves the integration of GaN layers with dissimilar host substrates through wafer-bonding, following non-contact sapphire delamination via Laser Lift-Off (LLO). The new types of Micro-LEDs also require that the sapphire be removed to produce thin or even flexible display devices.

In the LLO process, a Micro-LED wafer is exposed to high intensity 248 nm pulses directed through the sapphire substrate. The interfacial GaN layer of some 10 nm absorbs the UV laser photons, and undergoes thermal decomposition into liquid gallium and nitrogen gas. The sapphire wafer is then easily removed with nearly zero force exerted on the Micro-LED dies.

For example, a six-inch wafer can be covered within a single scan using a novel 248 nm excimer laser line beam system collaboratively developed by Coherent and Fraunhofer ILT (Aachen, Germany) (see image).  The system uses a top-hat line beam profile to ensure that the entire wafer area is exposed at the same optimum fluence.

The assembly of a high resolution display from many million Micro-LEDs brings its own challenges. In fact, a 4K display with 3,840 x 2,160 x 3 RGB subpixels requires nearly 24 million Micro-LEDs to be transferred to the display backplane. Individual “pick and place” methods would thus need 6 weeks for each display!

Parallel transfer using a large excimer laser beam now provides a high speed solution to this challenge. To this end, the processed Micro-LED epi-wafer may be bonded to a temporary carrier wafer by means of a UV absorbing polymer adhesive film. Illumination with a 248 nm excimer laser beam can then vaporize the adhesive.  This detaches the Micro-LED chips and simultaneously accelerates them on to the receiving backplane panel. The high energy of excimer beam pulses enables the beam to cover many square millimeters, transferring tens of thousands of Micro-LEDs with a single shot.  This translates into a transfer rate of millions of subpixels per second. This excimer laser induced forward transfer (LIFT) process compresses the total LED transfer time for assembling a 4K display down to less than a minute.

In summary, excimer lasers are well-suited to support emerging applications in the display industry. Their unique characteristics such as short UV wavelength, and short pulse length, combined with scalable field size and throughput are key enablers for laser manufacturing of MicroLEDs.

Image Courtesy: Fraunhofer ILT, Aachen, Germany / A. Steindl.

Handover of the Coherent laser system LineBeam 155 to Fraunhofer ILT Aachen -People are holding a lightweight CFRP part for an automotive application. 

From left to right: C. Hoerdemann (Fraunhofer ILT), T. Geuking (Coherent), R. Paetzel (Coherent), A. Gillner (Fraunhofer ILT) and R. Delmdahl (Coherent).

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