Advanced integration of polymer microlens arrays with GaN microLEDs

Advanced material processing and device fabrication technologies have offered the feasibility not only to create micron-scale light-emitting diodes (microLEDs) with a wide range of accessible wavelengths and low power consumption, but also to integrate them into microsystems as illumination sources for various applications beyond general solid-state lighting. Current ongoing development is focusing on the enhancement of internal quantum efficiencies and methods for improving light extraction efficiency either by modifying the device architecture or integrating other optical elements. However, integrating optical elements is a particular challenge as the dimensions of microLEDs scale down.

In this work, a reproducible processing method to fabricate and directly connect polymer-based microlens arrays on top of microLEDs has been developed and will be described here. Prior to device integration, the InGaN/GaN LED structures were prepared by metalorganic vapour phase epitaxy (MOVPE). The employed technique to realize the microlenses is based on thermal reflow, in which microstructured photoresist structures are created and subsequently transferred onto the desired surfaces (e.g., large-area LED and microLED arrays). Although the thermal softening and roundening effects of such polymers are not desired in some microtechnology processing steps (e.g., metallization), these have been employed for fabricating spin-on-glass-based microlens arrays [1]. However, the structures were only able to be produced on planar large area LEDs due to technology limitation. Therefore, when it comes to the micro-/nanostructured LED surfaces, a more sophisticated transfer methodology is required to overcome that issue. In this case, soft polydimethylsiloxane (PDMS) stamps can be utilized for molding and further embossing the liquid polymer onto the structured microLED surfaces. Moreover, precise positioning of microlens arrays on GaN microLED arrays was demonstrated for an element size down to 10 μm. An optical characterization method based on 3D light beam profile scanning was developed allowing acquisition and analysis of the emission patterns of the integrated devices, in which the obtained results will be presented.

References

[1] Chi-Min Liu, Guo-Dung J Su, “Enhanced light extraction from UV LEDs using spin-on glass microlenses,” J. Micromech. Microeng. 26 (2016).