Question

Sequence for standard n-well fabrication process is : A. Passivation B. n-well implant C. Metallization D. Source/drain diffusion Choose the correct answer from the options given below :

• A. A, B, C, D
• B. B, D, C, A
• C. C, A, D, B
• D. D, C, B, A

Hint:

Standard CMOS fabrication sequence: \[ \text{Well Formation} \rightarrow \text{Source/Drain Formation} \rightarrow \text{Metallization} \rightarrow \text{Passivation} \] Always remember:
• Passivation is the final protective step.
• Metallization occurs after transistor fabrication.

Answer 1

The Correct Option is B

Concept: In semiconductor IC fabrication, MOS devices are fabricated through a carefully ordered sequence of processing steps. For standard n-well CMOS fabrication:
• First, the n-well region is created.
• Then source and drain regions are formed.
• After active device formation, metal interconnections are created.
• Finally, a passivation layer is added for protection. The chronological order of fabrication is extremely important because each processing step depends on the previously fabricated structure.

Step 1: Understand the role of n-well implantation. The n-well implantation process creates the n-type well region inside the p-type substrate. This step is performed near the beginning of fabrication because:
• Device regions must first be defined.
• Subsequent transistor formation occurs inside these wells. Thus: \[ \text{n-well implant} \] is an early fabrication step. Hence: \[ B \] comes first.

Step 2: Analyze source/drain diffusion process. After well formation:
• Gate structures are fabricated.
• Dopants are introduced to create source and drain terminals. This process is called: \[ \text{Source/drain diffusion} \] Therefore: \[ D \] comes after n-well formation.

Step 3: Analyze metallization process. After transistor regions are completed:
• Metal layers are deposited.
• Interconnections between devices are formed. This step is called: \[ \text{Metallization} \] Hence: \[ C \] comes after source/drain formation.

Step 4: Analyze passivation process. Passivation is the final protective layer added over the IC surface. Its functions are:
• Protection from moisture
• Prevention of contamination
• Mechanical protection Since it is the final protective process: \[ A \] comes last.

Step 5: Determine the complete sequence. Thus, the correct fabrication order is: \[ B \rightarrow D \rightarrow C \rightarrow A \] Therefore, the correct option is: \[ \boxed{(B)\ B,D,C,A} \]