Phosphoric Acid Reuse for High-Selectivity Silicon Nitride/Silicon Dioxide Wet Etching via Fixed Automated Process Control in Memory Devices

In this study, a Batch Fix APC(Automated Process Control) system was implemented to address etch rate instability issues in the selective Si 3 N 4 / SiO 2 wet etching process of DRAM devices using phosphoric acid (H₃PO₄) baths. The etch rate instability was primarily attributed to the accumulation of silica residues and chemical concentration fluctuations induced by bath aging, which resulted in increased variability in the SiO 2 etch rate. To mitigate this, the lower limit of the SiO 2 etch rate was defined based on XPS analysis, considering the impact of native oxide formation on the Si 3 N 4 surface, and an appropriate bath condition was established. Notably, the Batch Fix APC system played a critical role in maintaining stable Si 3 N 4 /SiO 2 selectivity and etch rates by precisely controlling the etch time at each process step according to a predefined lookup table, using bath concentration, temperature, and life count as input parameters. This approach ensured stable process performance despite bath aging and real-time chemical concentration fluctuations. The system also reduced the frequency of chemical replacements, resulting in a 5.52% increase in productivity, and improved chemical reuse efficiency, achieving a 33% cost reduction per cycle.Furthermore, by integrating Batch Fix APC with real-time in-line thickness monitoring, this approach can be extended to patterned DRAM device processes, contributing significantly to process stability and yield management in high-volume manufacturing environments.