Impact of Water Content in NMP on Ohmic Contacts in GaN HEMT Technologies

Wet chemical lift off in N-Methyl-2-pyrrolidone (NMP) is widely used in GaN HEMT Front End manufacturing. In case of a Ti-Al-Ni-Au based metal stack for ohmic contacts, the quality of the lift-off process is a strong function of the water content in the solvent NMP. In this article, it will be shown that the metal stack can be attacked during lift off in NMP when its water content is exceeding 5%. Additionally, environmental impacts on the hygroscopy of NMP are investigated. Both temperature of NMP and its contact area to air impact the evolution of its water content. Based on these investigations, the lift off sequence was slightly adapted in order to keep moisture below a certain level and avoid optical defects on ohmic contacts.