Mod-01 Lec-26 Etching and deposition (growth)

Overview of Lithography and Etching Processes

Introduction to Lithography

• Lithography is a process that transfers patterns from a mask onto wafers, essential for integrated circuit (IC) design.

• The lithography process involves two main steps: transferring the pattern onto a photoresist layer and then onto the wafer itself.

Photoresist Layer

• Photoresist is a photosensitive material that can either dissolve when exposed to light (positive photoresist) or become less soluble (negative photoresist).

• After applying the photoresist, the pattern must be transferred to the wafer through etching or growth processes.

Understanding Etching

Definition and Importance of Etching

• Etching is defined as the removal of material from the top layers of a wafer, often combined with lithography.

• There are two primary types of etching: wet etching and dry etching.

Wet Etching Process

• Wet etching uses liquid chemicals to remove material; wafers are immersed in an etchant under controlled conditions.

• For silicon etching, potassium hydroxide (KOH), typically at 30% concentration and 90°C, is commonly used.

Post-Wet Etch Procedures

• After wet etch, wafers are cleaned with distilled water to remove excess chemicals before further processing.

• Wet etch is suitable for removing large areas but may not provide precision for smaller patterns.

Key Factors Affecting Etch Rate

• Factors such as temperature, concentration of chemicals, and external agitation influence the uniformity and rate of etching.

• Initial experiments help determine optimal conditions for consistent etch rates before actual wafer processing.

Common Issues in Etching

Incomplete and Over-Eting

• Incomplete etch occurs when insufficient time or incorrect parameters prevent complete material removal.

• Overetching can lead to undercutting where lateral material removal widens patterns beyond intended dimensions.

Selectivity in Wet Etchants

• Selectivity refers to how well an etchant removes one material while preserving another; KOH shows high selectivity for silicon over silicon dioxide.

Transitioning to Dry Etching

Advantages of Dry Etching

• Dry etching utilizes gases as primary mediums allowing for precise control over small features compared to wet methods.

Types of Dry Etching Techniques

1. Plasma Etching:

• Plasma generation allows chemical reactions that effectively remove materials like SiO2 using fluoride gases such as CF4.

2. Ion Beam Milling:

This physical removal method bombards surfaces with high-energy ions but lacks selectivity due to its purely physical nature.

3. Reactive Ion Etching:

• Combines both plasma chemistry and ion beam techniques for enhanced directional control during material removal.

Growth Processes Overview

Deposition vs. Growth

• Unlike etched layers which involve removing materials, deposition adds new materials without consuming underlying silicon.

Techniques for Deposited Layers

1. Physical Vapor Deposition (PVD):

• Typically used for metallic layers; details will be covered in future discussions on metallization processes.

2. Chemical Vapor Deposition (CVD):

• Used extensively for growing dielectric materials like silicon dioxide or nitride through gas-phase reactions.

3. Atomic Layer Deposition (ALD):

• A precise method growing materials one atomic layer at a time by alternating gas introductions leading to monolayer formation.

4. Molecular Beam Epitaxy (MBE):

• Utilizes molecular beams for highly controlled growth of thin epitaxial layers with excellent composition accuracy.