Wafer scale RBG MicroLEDs simplify transfer process: Page 2 of 2

May 15, 2019 //By Julien Happich
At Display Week, researchers from CEA-Leti presented several papers relating to the manufacture of RBG MicroLED pixels and their transfer to large area displays.

Indeed, one of the biggest challenges to support high resolution microLED displays is to improve the performance of the driving electronics, which require more power and faster electronics to power millions of pixels in a fixed-frame time. Today’s TFTs used in active matrix substrates cannot provide the necessary current and speed supported by microLEDs.

With this approach, the receiving backplane could even be fabricated with non-lithographic techniques such as ink-jet or stencil printing, opening the field to display sizes in excess of 100-inches on almost any kind of material such as glass, plastic, metal; rigid or flexible.

The second paper titled “MicroLED Displays based on Transfer with Microtubes Interconnections” gives the solution for a solder-free interconnect mechanism that would be cost effective to implement on the receiving substrate, regardless of the pixel pitch envisaged during transfer.

As an example, the researchers envisage that the small pitch microLEDs initially fabricated at wafer level could be transferred on a substrate at a much larger pixel pitch, ranging from 50 to 500μm. This means the emitting area would leave plenty of free space within each pixel, which OEMs could use to offer more display transparency or to integrate other display functionalities such as optical sensing.

SEM view of a passive matrix receiving hybridized microLEDs
on lines and columns. The close up shows a microLED with
two empty slots of microtubes on both side.

Here the authors propose to secure both electrical and mechanical connections in one press-fitting operation at room temperature, using microtube interconnections they demonstrated in a previous paper. The microtubes are grown directly on the receiving substrate, consisting of micro-cylinders built over a rigid Tungsten Silicide supporting structure. Here, the MicroLEDs are fabricated upside down, light being emitted towards the epitaxial substrate. And for the purpose of the press-fit connection, both N and P contact pads are created on the top side (the N contact being routed through the active to the top side) so the relatively soft and deformable pads end up being pressed onto the hard microtubes, forming a solid electrical and mechanical connection.

Related articles:

Stacking microLEDs onto a full-CMOS active matrix, with micro tubes

BluGlass’s RPCVD yields p-GaN microLED display prototype

Optovate shares details about its microLED transfer process

JB Display aims for 10,000 ppi microLED microdisplays

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