Explanation:

Silicon Nitride:
- Silicon nitride is commonly used as a mask or shield during oxide growth processes in semiconductor fabrication.
- It is a chemical compound known for its excellent insulating properties and resistance to high temperatures.
- When deposited on a silicon substrate, silicon nitride acts as a barrier to prevent oxidation in specific areas, allowing for selective oxide growth in desired regions.

Role in Selective Oxide Growth:
- During the process of selective oxide growth, silicon nitride is used to shield the active areas of the silicon substrate.
- By selectively masking certain regions with silicon nitride, oxide growth can be controlled and limited to specific areas.
- This technique is crucial in the fabrication of integrated circuits, where precise control over oxide growth is essential for device performance.

Advantages of Silicon Nitride:
- Silicon nitride offers excellent chemical resistance and mechanical strength, making it a reliable material for masking applications.
- Its thermal stability allows for high-temperature processes without degradation, ensuring consistent performance during oxide growth processes.
- The use of silicon nitride as a shield enables the creation of intricate patterns and structures on silicon wafers for semiconductor devices.
In conclusion, silicon nitride plays a vital role in achieving selective oxide growth by acting as a protective barrier during semiconductor fabrication processes. Its unique properties make it an ideal choice for shielding active areas and controlling oxide growth in specific regions of a silicon substrate.