The search for chiral Majorana fermions in quantum anomalous Hall insulator/$s$-wave superconductor heterostructures has attracted intense interest, yet remains controversial due to the lack of conclusive evidence. A key issue is that the heterostructure's metallization can produce half-integer conductance signatures resembling those of chiral Majorana fermions, thereby complicating their identification. Here, we investigate how the competition between metallization and chiral Majorana fermions depends on superconductor thickness, revealing its critical role through three distinct regimes: (i) For thin superconductors ($\sim 10\mathrm{nm}$), metallization shows periodic oscillations with thickness, matching the Fermi wavelength. (ii) Intermediate thicknesses ($\sim 100\mathrm{nm}$) exhibit periodic windows for observing chiral Majorana fermions. (iii) Thick superconductors ($\sim 1000\mathrm{nm}$) sustain stable chiral Majorana fermions that are insensitive to thickness variations. These results suggest that superconductor thickness is a key control parameter for advancing efforts to conclusively identify chiral Majorana fermions.

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• Edge states
• Majorana bound states
• Majorana fermions
• Topological phase transition
• Heterostructures
• Thin films
• Topological insulators
• Topological superconductors
• Bogoliubov-de Gennes equations