Manufacturability of high power ultraviolet-C light emitting diodes on bulk aluminum nitride substrates (2011)

Summary form only given: Bulk AlN substrates are an ideal solution for the growth of ultraviolet-C light emitting diodes (UVCLED). They are optically transparent at this wavelength and closely lattice and thermally expansion matched to the layers required for the device structure. In addition pseudomorphic growth can be achieved allowing for low dislocation density in the active region of the device. This approach has been used to achieve the best efficiency and output power, as well as lifetime, in the 265 nm wavelength range. However, currently these substrates are only available in limit quantities and sizes preventing large scale manufacturing such as is occurring with visible LEDs. Regardless, we have been able to succeed in setting up pilot production of UVCLEDs to enable low volume manufacturing and allow for rapid increase in volumes as supply and size of the AlN substrates are increased. This pilot production line utilizes the bulk substrate manufacturing and epitaxial growth at Crystal IS and the high volume fabrication facility at Sanan Optoelectronics. The current process uses 10 x 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> AlN substrates. The epitaxial growth process as tracked using both non contact sheet resistance measurements and X-ray diffraction for analyzing the variation in aluminum content of the n type AlGaN layer of each wafer. The sheet resistance and Al content over several lots of wafers is shown. The sheet resistance shows a normal distribution with a mean of 341 ohms/sq. and dispersion of 89 ohms/sq. This value is desired to be as low as possible but in the current range is more than adequate for high performance UVCLEDs. The Al content is targeted at 70% with an upper specification limit (USL) of 75% and a lower specification limit (LSL) of 65%. The distribution is bimodal but does have a mean value of 70.0% and a dispersion of 2.2% indicating the Al% is well controlled with the specification limits. The fabrication process is similar to high volume production of visible LEDs but differs in several key areas. As fabrication facilities are moving beyond 2" substrates and relying on more sophisticated automation to improve yields, the processing of 10x10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> substrates presents challenges in wafer handling and processing. We have been successful in implementing the fabrication of these wafers in pilot production. By carefully tracking key process parameters such as etch depths, metal thickness, and contact resistances devices can be fabricated with optimal parameters. The output power measured on wafer after the fabrication process is shown. This output power is measured through the partially absorbing AlN substrate at 100 mA of input current and typically at an emission wavelength of 265 nm. This results in a mean output power over 1 mW. This value can be increased significantly once the device is packaged and we have currently demonstrated over an order of magnitude improvement by introducing enhanced photon extraction methods to the device. Additional parameters such as forward voltage and wavelength will be discussed.