The process of n-well fabrication in integrated circuit manufacturing is a critical part of the CMOS technology. It requires a precise sequence of steps to ensure the proper formation of the well and the subsequent components. Here's a detailed breakdown of the correct sequence:

1. n-well implant (B): At this stage, the n-well is created by implanting donor atoms like phosphorus or arsenic into the silicon substrate. This forms the n-type region necessary for the CMOS transistor.
2. Source/drain diffusion (D): After the n-well is formed, the source and drain regions of the transistors are defined and dopants are diffused into these regions to form the source-drain junctions of the transistor.
3. Metallization (C): This step involves depositing metal contacts to connect the various parts of the circuit. The most commonly used metal is aluminum, which is deposited using techniques like sputtering or chemical vapor deposition.
4. Passivation (A): The final step involves depositing a passivation layer, typically silicon dioxide or silicon nitride, to protect the wafer's surface and the integrated circuit from external contaminants and physical damage.

Therefore, the correct sequence for the n-well fabrication process is B, D, C, A. This ensures that each component is correctly aligned and functional for the integrated circuit's overall performance.

Thus, the correct answer is: B, D, C, A