Wet Cleaning Approaches to Remove Post-Etching AlOx Residues in Al/AlOx/Al Stacks
Wet cleaning of Al and AlOx surfaces is essential for advanced surface preparation, particularly when ultrathin AlOx layers act as functional barriers in Al/AlOx/Al stacks. Reactive ion etching (RIE) of Al electrodes often leaves amorphous AlOx residues near these barriers and electrodes, where concentrated electric fields can induce defects and compromise device reliability. Effective removal of such residues remains uniquely challenging due to the conflicting requirement of preserving the ultrathin AlOx barrier. This challenge matters broadly because Al/AlOx/Al Josephson junctions are widely adopted in cryogenic electronics and quantum information processing, where barrier integrity directly affects performance and yield.
In this work, we systematically investigate wet cleaning strategies to remove AlOx residues while preserving the structural integrity of Al/AlOx/Al stacks. An Al/AlOx/Al Josephson junction was used as the test vehicle, and several chemistries were evaluated, including a sulfuric acid–hydrogen peroxide mixture (DSP), diluted ammonia (dNH₄OH), diluted phosphoric acid (dH₃PO₄), phosphoric acid–hydrogen peroxide mixtures (PPM), and a commercially available formulated chemistry (Rezi-98). Cleaning performance was assessed through SEM imaging and electrical resistance measurements, focusing on residue removal, Al electrode preservation, and AlOx barrier integrity. An increase in resistance between the top and bottom Al electrodes indicated damage to the AlOx barrier, which is sandwiched between two Al layers.
Among the tested chemistries, diluted H₃PO₄ exhibited the most effective cleaning capability without causing significant structural damage. We hypothesize that an AlPO₄ passivation layer formed during the process suppresses Al corrosion and delays etching of the AlOx barrier. Comparative analysis combined with etching-rate data revealed a strong correlation between residue-removal efficiency and barrier degradation, underscoring a narrow process window between effective cleaning and structural damage. These insights highlight key trade-offs in wet cleans for Al/AlOx/Al junction fabrication and motivate precise process control.
Future work will explore methods to expand the process window. On single-wafer platforms, physical agitation—such as high-velocity spray or megasonic cleaning—may assist in removing post-RIE AlOx residues. Additionally, customized formulations with appropriate inhibitors may help preserve Al/AlOx/Al integrity. By focusing on cleaning mechanisms and controllable parameters, this study highlights the importance of wet cleaning processes in scalable manufacturing of Al/AlOx/Al junction-based technologies and its relevance to advanced superconducting technologies and quantum devices.
BIOGRAPHY
Hsin-Yu Hsu is currently working as an R&D Engineer at imec in Leuven, Belgium. She has more than five years of experience specializing in wet clean process development, having worked at both tsmc and imec. She has process research experience in related fields, including BEOL interconnect cleaning, Si thinning for wafer bonding, and advanced wet solutions for MRAM, 3D DRAM, and superconducting qubits. Her work focuses on developing innovative cleaning strategies that improve yield and reliability for next-generation semiconductor technologies. She enjoys collaborating with multidisciplinary teams to solve complex challenges and translate research into practical solutions. Joining SPCC is an excellent opportunity for her to share her experience, learn from others, and contribute to discussions that shape the future of microelectronics.