Process Challenges and Optimization of Selective Lateral Etching of Silicon Nitride in 3D Multi-Layer Stack Structures

The performance of dry and wet lateral etching of Si₃N₄ in vertically stacked multilayer semiconductor structures was firstly compared in this study. The result showed the wet lateral etching process significantly outperforms dry etching in terms of within-wafer and inter-layer uniformity (fluctuation < 5nm) and electrical performance. The influence of thermal annealing on the etching behavior of Si₃N₄ films was investigated and indicated furnace annealing at 800°C for 30 minutes produces denser films compared to rapid thermal annealing process (RTP) at 900°C for 30 seconds, resulting in a reduction of the etching rate (7.2%) and an improvement in the verticality of the etched profile. The mechanical stability of different structural designs (hole-like versus staircase) were analyzed and identified critical amounts of lateral etch required for structural collapse in this work. A novel high-selectivity phosphoric acid solution was successfully employed to achieve high selectivity etching between Si₃N₄ and TiN (8.5:1), effectively solved the issues of interfacial electrochemical corrosion and providing a reliable processing solution for the integration of complex 3D structures.