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Figure 2: Full process and assembly flow for mass transfer using adhesive bonding.
this technique less attractive as careful fine that releases the devices from the growth the diode-pumped solid state (DPSS)-LLO
tuning of process conditions is required substrate. The same adhesive, deposited on process and the Ni stress buffer layer allows
to attain substantial yields. It also requires a lithographically-patterned glass stamp, us to attain nearly 100% LLO yield for the
the use of specialized equipment, which is used to transfer print selected arrays of InGaN/GaN devices on sapphire.
may not be available in most outsourced programmably-debonded microLEDs from Figure 3 gives experimental details
semiconductor assembly and test suppliers the temporary carrier to the target substrate, of the aforementioned mass transfer and
(OSATS) or packaging facilities. which in our case is an ultra-flexible assembly process at different critical
For a mass transfer process to be PDMS substrate called FlexTrate™ [3]. stages. The devices that were electrically
commercially viable, transfer yields of Because simple adhesive bonding and laser measured while on the stamp – after pickup
>99.9999% (six 9s of yield or <1ppm failure) debonding are used for the mass transfer, for final assembly – showed less than 5%
are to be achieved as even a few dead pixels process complexity is significantly reduced, degradation in forward current at operating
in a commercial display are unacceptable. and the process can be easily performed forward voltage of 4V when compared to
The mass transfer techniques explored in at any packaging facility without the need virgin devices on sapphire indicating that
this summary have demonstrated around for specialized equipment (only a substrate the devices were undamaged by the mass
99.99% yield, which is impressive, however bonder and laser debonding system are transfer process. The devices are finally
falls short of the stringent yield requirement required). The full process and assembly flow printed onto a thermal release tape (TRT)
of > six 9s for commercialization. Even are given in Figure 2. As strong adhesive laminated on a 4” carrier wafer. This tape
though using post-assembly repairs and bonding, instead of weaker electrostatic/ also contains other components like Si
redundant pixels has been suggested to electromagnetic or Van der Waals (VdW) dielets flip-chip bonded on it. A die-first fan-
overcome this yield issue, such solutions are forces is used for mass transfer, potentially out wafer-level packaging (FOWLP) process
not economically viable given the high cost higher yields of transfer (<1ppm defect) and called FlexTrate™ is then carried out, which
and time of repairs and material cost of the finer alignments can be achieved because involves compression molding of PDMS to
microLEDs. devices will not fall off or shift during the reconstitute the packaging substrate with
To overcome the yield issues and other transfer. Before the substrate release process, embedded microLEDs and dielets, followed
drawbacks, we are currently developing the microLEDs are also protected with a by fabrication of metal interconnects using a
a transfer process using thermoplastic 5-10µm electroplated Ni stress buffer to back end of line (BEOL) Cu plating process
adhesive bonding and programmable-laser prevent any damage during the LLO process. and surface passivation using Parylene-C.
debonding to selectively pick up and mass The LLO process itself is done using a novel
transfer InGaN/GaN MQW microLEDs DPSS laser system (DFL7560L) developed Summary
grown on c-plane sapphire substrates [2]. A by DISCO Corporation that uses gentle Commercialization of microLED display
thermoplastic, laser-debondable polyimide (fluence per pulse <<1J/cm ) overlapping technology requires the development of mass
2
based adhesive (HD3007) is used to attach gaussian beams (>>10s of Hz repetition rate) transfer approaches that are cost effective,
the microLEDs to a temporary glass carrier with a small spot size to achieve low-stress, high yield, and easy to implement. Current
before the laser lift-off (LLO) process gap-free device liftoff [4]. A combination of mass transfer approaches in literature and

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