Explain the processing steps used in Monolithic IC fabrication.

Monolithic IC Fabrication Processing Steps

1. Substrate Preparation:

• Starting material: Typically a silicon wafer.
• Crystal growth: Silicon ingot is grown using the Czochralski method.
• Wafer slicing: Ingot is sliced into thin wafers using a diamond saw.
• Surface orientation: Wafers are oriented with specific crystallographic directions, typically <100> or <111>.
• Polishing: Wafers are polished to obtain a smooth and defect-free surface.

2. Epitaxial Layer Growth:

• Epitaxial layer deposition: A thin layer of high-quality silicon is deposited on the substrate.
• Chemical vapor deposition (CVD): Silane (SiH4) and other dopant gases are used to deposit the epitaxial layer.
• Thickness control: Epitaxial layer thickness is precisely controlled to achieve desired electrical characteristics.

3. Oxidation:

• Thermal oxidation: Wafer is exposed to high-temperature oxygen to form a silicon dioxide (SiO2) layer.
• Dry oxidation: Pure oxygen is used.
• Wet oxidation: Oxygen and water vapor are used.
• Oxide thickness control: Oxide thickness is controlled to achieve desired electrical properties.

4. Photolithography:

• Photoresist application: A light-sensitive material (photoresist) is applied to the wafer surface.
• Mask alignment and exposure: A mask containing the desired circuit pattern is aligned and exposed to ultraviolet (UV) light.
• Development: Exposed photoresist is removed, leaving the desired pattern in the photoresist.

5. Etching:

• Wet etching: Chemical solutions are used to selectively remove material.
• Dry etching: Reactive gases (e.g., CF4, O2) are used to selectively remove material.
• Anisotropic etching: Etching process that produces vertical sidewalls.
• Isotropic etching: Etching process that produces isotropic (undercut) sidewalls.

6. Ion Implantation:

• Ion source: Ions of the desired dopant species are generated.
• Acceleration: Ions are accelerated to high energy.
• Implantation: Ions are implanted into the wafer, modifying the electrical properties.
• Annealing: Implanted wafers are heated to activate dopants and remove implantation damage.

7. Metallization:

• Sputtering: Metal atoms are deposited onto the wafer surface.
• Evaporation: Metal atoms are evaporated from a heated source onto the wafer surface.
• Chemical vapor deposition (CVD): Metal atoms are deposited using a chemical reaction.
• Patterning: Metal layers are patterned using photolithography and etching.

8. Passivation:

• Nitride deposition: A layer of silicon nitride (Si3N4) is deposited on the wafer to protect the metallization.
• Annealing: Passivation layer is annealed to improve its properties.

9. Backside Processing:

• Backside thinning: Wafer is thinned from the backside.
• Metallization: A metal layer is deposited on the backside for electrical connection.

10. Wafer Dicing:

• Wafer is diced into individual chips (die).
• Die bonding: Die is attached to a package.
• Wire bonding: Electrical connections are made between die and package.

11. Encapsulation:

• Molding: Die is encapsulated in a protective material.
• Lid sealing: Package is sealed with a lid.

12. Testing and Inspection:

• Electrical testing: Chips are tested for functionality.
• Visual inspection: Chips are inspected for defects.

13. Packaging:

• Chips are placed in appropriate packages.
• Packages are sealed and marked.