Laser processing of GaN-based LEDs with ultraviolet picosecond laser pulses

Picosecond (ps) lasers provide a universal tool for material processing. Due to the short pulse length material is removed by a process called "cold ablation", with minimal thermal damage to neighbouring regions. As a result, better defined structures with smother and cleaner side walls can be fabricated than with nanosecond (ns) laser pulses. This offers new possibilities for laser processing in semiconductor technology for both semiconductor materials as well as contact and bond metallizations. The fabrication of optoelectronic devices such as light-emitting diodes (LEDs) typically involves photolithography steps, requiring specific lithography masks be fabricated which, in particular for prototyping, is expensive and time consuming. Therefore it would be attractive for a range of applications to replace these steps by direct writing techniques such as laser processing, which will speed up e.g. the development and prototyping of new devices. We report on fully laser processed planar GaN-based LEDs fabricated without any photolithography steps. On the bare semiconductor wafer, isolation trenches and mesa structures are formed directly by ultraviolet ps laser pulses. For the direct deposition of patterned ohmic contact metallizations, the ps laser fabrication and subsequent use of high resolution shadow masks is presented, which exhibit a significantly reduced sidewall roughness compared to masks produced by ns laser pulses. Due to the higher precision of the laser defined masks it becomes possible to deposit multiple layers, through the use of alignment marks, similar to multiple mask level photolithography. Finally, the ps laser processed LEDs are electrically and optically characterized and their characteristic compared with that of conventionally fabricated mesa LEDs.