Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flipchip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AIN buffer layer. We optimized the inductively coupled plasma SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was similar to 46 mu m hr(-1) at a high RF bias (400 W), and similar to 7 mu m hr(-1) at a low RF bias (49 W) with very high etch selectivity between SiC and AIN. The high SF6 etch selectivity between SiC and AIN was essential for removing the SiC substrate and exposing a pristine, smooth MN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AIN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by similar to 3 x after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency and power conversion efficiency UV-C LEDs.