CIP (cleaning-in-place) stability of AlGaN/GaN pH sensors

► Low defect density AlGaN/GaN heterostructures are stable up to 40 CIP cycles. ► Edges at metallization or mesa steps are the weakest points of most passivations. ► A passivation scheme based on SiNx is compatible to conventional device processing. ► A SiNx based, CIP resistant passivation scheme exhibits low long-term drift. ► The integration of nanocrystalline diamond leads to most stable passivation. The CIP stability of pH sensitive ion-sensitive field-effect transistors based on AlGaN/GaN heterostructures was investigated. For epitaxial AlGaN/GaN films with high structural quality, CIP tests did not degrade the sensor surface and pH sensitivities of 55–58mV/pH were achieved. Several different passivation schemes based on SiOx, SiNx, AlN, and nanocrystalline diamond were compared with special attention given to compatibility to standard microelectronic device technologies as well as biocompatibility of the passivation films. The CIP stability was evaluated with a main focus on the morphological stability. All stacks containing a SiO2 or an AlN layer were etched by the NaOH solution in the CIP process. Reliable passivations withstanding the NaOH solution were provided by stacks of ICP-CVD grown and sputtered SiNx as well as diamond reinforced passivations. Drift levels about 0.001pH/h and stable sensitivity over several CIP cycles were achieved for optimized sensor structures.