MicroLED displays have the triple-challenge of transferring millions of components that are vanishingly small with sub-micron precision. One way to achieve this is by using chemistry and the tools of microlithography, leveraging the processes used for nano-scale semiconductor fabrication.
This apporach is developed by Terecircuits who will be presenting on 30 NOV - 1 DEC 2022 at TechBlick's MicroLED conference taking place live online. Here is the agenda www.TechBlick.com/microLEDs
Slide 1 shows one optimization: A deep UV light source is used to expose an entire carrier holding an epi-wafer of MicroLEDs, released and singulated onto the carrier via Laser Lift Off (LLO) or a chemical process. At the same time, a mask is positioned to selectively release only the MicroLEDs at the pixel pitch. In this way tens of thousands of components can be transferred in a single operation, preserving the accuracy of the die-to-die relationships from the fabrication pitch. Masks can also be used to depopulate know bad die and perform defect repair on existing assemblies.
The design of the material which holds the MicroLEDs on the donor carrier is critical. This material needs to both hold the MicroLEDs securely without drift prior to release, then when activated by the Laser-Induced Forward Transfer (LIFT) process, cleanly release and propel the dice towards the substrate without damage and with minimal or no residue.
Slide 2 shows that Optimizing a Transfer Material to work with a mask (M-LIFT) means achieving an activation energy that is below the ablation threshold of the mask material. Most conventional transfer materials such as polyimides and thermosetting polymers are ablative materials which require large activation doses, leave residue, and are difficult to control. They typically require 500-800mJ/cm2 of energy (fluence) to activate, exceeding the mask ablation threshold by 10x. An ideal material decomposes cleanly while imparting a downward placement force to the component (enabling non-contact placement). It should also have minimal “edge effects” which can bleed into adjacent components, negatively affecting the accuracy of subsequent transfers.
Besides facilitating the use of a mask which can provide true patterning without sophisticated optics, achieving lower fluence brings additional Cost of Ownership benefits to the entire transfer process. Lower energy will transfer significantly less damaging heat, and can potentially activate with low cost, highly reliable non-laser UV light sources. Low energy requirement also means a 10x larger surface area can be exposed for parallel release, or alternatively, a 10x smaller laser can be used to save costs. This reduction in tool complexity translates into lower maintenance costs and less downtime.
This apporach is developed by Terecircuits who will be presenting on 30 NOV - 1 DEC 2022 at TechBlick's MicroLED conference taking place live online. Here is the agenda www.TechBlick.com/microLEDs
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