This is a growing collection of concise insights and facts about VLSI, semiconductor fabrication, and electronics. Each point provides a quick understanding of fundamental concepts without lengthy explanations. Keep checking back for new additions!
The process of exposing selective areas of a wafer to light through a photomask is called Printing. There are three basic types of printing systems used. They are listed below:
1] Contact printing: The simplest and most accurate method, where a glass plate (photomask) with the desired pattern comes in direct contact with the wafer. It provides high resolution and low cost but leads to photomask wear and contamination, making it unsuitable for modern VLSI.
2] Proximity printing: The photomask and wafer are placed close to each other without direct contact. While it reduces contamination, increasing the gap lowers resolution. This method is ineffective for feature sizes below 2 µm and is not used in present-day VLSI.
3] Projection printing: A non-contact method where the photomask image is projected onto the wafer using lenses or mirrors. It allows for higher resolution and is the primary technique used in modern VLSI, employing either scanning or step-and-repeat approaches.Photoresist is an organic polymer whose characteristics can be altered when exposed to ultraviolet light. Photoresist is classified into:
1] Positive photoresist: Used to create a mask where patterns exist, meaning the areas exposed to UV light become soluble and are removed during development. This allows for high resolution and is widely used in modern semiconductor fabrication.
2] Negative photoresist: Used to create a mask where patterns do not exist, meaning the areas exposed to UV light harden and remain after development. It offers better adhesion and chemical resistance but lower resolution compared to positive photoresist.There are two basic types of etching techniques: wet etching and dry etching (plasma etching).
Wet etching: Uses chemicals to remove specific materials from the wafer.
- Hydrofluoric acid (HF) → Etches silicon dioxide (SiO₂)
- Phosphoric acid (H₃PO₄) → Removes silicon nitride (Si₃N₄)
- Nitric acid (HNO₃), Acetic acid (CH₃COOH), Hydrofluoric acid (HF) → Used for polysilicon etching
- Potassium hydroxide (KOH) → Etches silicon (Si)
- Phosphoric acid mixture → Used for metal etching
- Wet etching is highly dependent on time and temperature and requires careful handling due to the hazardous nature of the acids used.
Dry etching (Plasma etching): Uses ionized gases that are activated by an RF-generated plasma to etch materials.
- Fluorine-based gases (CF₄, SF₆) → Used for SiO₂ and Si etching
- Chlorine-based gases (Cl₂, BCl₃) → Used for metal etching
- Oxygen plasma (O₂) → Used for photoresist removal (ashing)
- Dry etching provides precise control over feature sizes and is widely used in modern semiconductor manufacturing.
