MOS Fabrication: Understanding the Correct Sequence

The fabrication of a Metal-Oxide-Semiconductor (MOS) device involves a series of precise steps performed in a specific order to create integrated circuits on a silicon wafer. Understanding this sequence is crucial for semiconductor manufacturing.

Key Steps in MOS Fabrication

Let's break down the process step-by-step:

• A. Wafer Formation: This is the foundational step. It begins with a pure silicon ingot, which is sliced into thin discs called wafers. These wafers serve as the base substrate for building the integrated circuits.
• C. Oxidation: The silicon wafer is exposed to oxygen at high temperatures (typically in a furnace). This process grows a thin, uniform layer of silicon dioxide (SiO₂) on the wafer's surface. This oxide layer is essential as an insulator and as a mask for subsequent steps.
• B. Photolithography: This is a pattern transfer technique. A light-sensitive material called photoresist is coated onto the wafer (often over the oxide layer). UV light is then shone through a mask (photomask) containing the desired circuit pattern. The exposed or unexposed photoresist (depending on the type) is developed, leaving a patterned resist layer that protects underlying areas during etching or implantation.
• D. Gate and Source/Drain Formation: This stage involves creating the critical transistor components. Using the pattern defined by photolithography, selective etching removes the oxide layer in specific areas. Materials like polysilicon are deposited to form the gate electrode. Then, processes like ion implantation or diffusion are used to introduce dopants into the silicon substrate, creating the source and drain regions. This step often requires multiple photolithography and etching cycles for different parts.
• E. Metallization: Once the transistors are formed, metal layers (commonly aluminum or copper) are deposited onto the wafer. Photolithography and etching are used again to pattern these metal layers into wires, forming the interconnections between different transistors and components, and connecting them to the external pins of the chip.

Correct Sequence Summary

Based on the standard MOS fabrication process, the correct order of the given steps is:

1. A. Wafer formation - Starting material preparation.
2. C. Oxidation - Growing the insulating layer.
3. B. Photolithography - Pattern definition.
4. D. Gate and source/drain formation - Creating active regions and control gate.
5. E. Metallization - Interconnecting the components.

Therefore, the correct sequence is A, C, B, D, E.