To scale up microLED displays to large areas, smaller displays can be titled. Because microLEDs can be truely edge-less devices, the tiling can function, yielding a seemless look.
Each title should house the microLEDs, backplane, as well as driver electrodes. The microLEDs and the backplane sit on the front side of the glass substrate whilst the driver electrodes are tucked away at the back. Interconnects are needed to connect the two. Wrap-around electrodes (interconnects wraping around the edge to connect front and back) is an elegent solution which bypasses the need for a drilled and filled through-glass via.
The wrap around electrodes can be printed or PVD deposited (both prefer chamfered glass) . The latter can yield better feature sizes and thin and conductive lines, whilst the former can increase productivity.
The below images demonstrate various technologies. Screen printing is a robust solution with low TACT time. Applied Materials has demonstrated that it can screen print very narrow (30um) linewidths over narrow spacings (50um).
These are excellent results. Note, by way of reference, that state-of-the-practice/production and state-of-the-art in screen printing of conductive paste on silicon solar PVs are 35um and 20um, respectively. In the process, first the top and botton electrodes are printed before the substrate is rotated (with excellent alignment) to print the electrodes over the edge.
This technologies requires excellent machines. Applied Materials has launched a machine able to handle 230x230mm substrate with +/- 6um repeatability and a throughput of 1000pph. Note that optimization of the past and print process are critical. In general, a paste with very high conductivity (20% bulk Ag) with 5B adhesion onto glass will be needed. The target final printed thickness is 3-5um. The screen printing process should yield smooth surface with no peaks near the edge.
Aerosol jet is also being proposed for additive deposition of wrap-around electrodes. The advantage of aerosol is that it can print over 3D surfaces and that it can in general deposit fine features than screen printing. To achieve wrap-around electrodes, two half-wrap electrodes must be printed (see below). In between the steps, the glass will need to be rotated. Optomec claims to achieve 18k full-wrap interconencts per hour (excluding the time it takes to rotate the glass). Note that the example below shows L/S 50um although, in princible, aerosol jet can go from down.
In general, this is an interesting solution for the microLED market
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