Dry Etching Technology: Principles, Features and Uses

01

Overview of Dry Etching Technology

Dry etching is a core technology in precision processing fields such as semiconductor manufacturing. Its principle involves exposing the wafer surface to a gaseous plasma, which reacts with the wafer through the photoresist window via physical, chemical, or combined mechanisms to remove the exposed surface material. This technology holds a significant position in modern industrial production due to its unique advantages.

02

Core Principles of Dry Etching

(1) Physical Etching Principle

Physical etching primarily utilizes plasma bombardment of the wafer surface. Etching is achieved through collisions between particles, involving only physical changes and generating no new substances. A key feature is anisotropy, with etching occurring strictly along the plasma velocity direction and almost none in other directions. However, this method lacks selectivity and high-energy ions may damage devices. In practice, inert gas argon (Ar) is commonly used for bombardment because it does not alter the chemical properties of the plasma and ensures stable physical etching effects.

(2) Chemical Etching Principle

Chemical etching relies on chemical reactions between the plasma and wafer surface materials to transform the material into gaseous by-products, which are then evacuated. The by-products must be easily removable. While inherently isotropic, polymer deposition on sidewalls during etching enables anisotropic effects. Adjusting the ratio of chemical etching gases allows control over the selectivity for different films. Carbon-fluorine gases (CXFY) are commonly used; F radicals bond with Si to form gaseous SiF₄, enabling material removal. However, unsaturated compounds formed from CF gas reactions generate polymers that deposit on electrodes and in the chamber, causing defects such as surface particles and pattern failure. The fluorine-to-carbon ratio (F/C) determines the amount of polymer formed: lower F/C yields more polymer, while higher F/C yields less.

(3) Reactive Ion Etching Principle

Reactive ion etching combines the advantages of physical and chemical etching and is the mainstream method in industrial dry etching. By adjusting plasma conditions and gas components, etching profiles can shift between isotropy and anisotropy, achieving precise linewidth control and good selectivity, thereby meeting complex process requirements.

03

Technical Characteristics of Dry Etching

(1) Advantages Over Wet Etching

• Flexible profile control: Capable of both anisotropic and isotropic profiles, enabling precise sidewall shape control.
• Excellent critical dimension (CD) control.
• Fewer photoresist issues: Reduces problems of photoresist peeling or adhesion.
• Good etching uniformity: Ensures excellent within-wafer, wafer-to-wafer, and batch-to-batch uniformity.
• Cost advantage: Reduces the cost of using and handling chemicals.

(2) Limitations

• Insufficient selectivity: Poor selectivity in etching underlying materials.
• Risk of device damage: Plasma may cause device damage.
• High equipment cost: Equipment purchase and maintenance are expensive.

04

Application Requirements and Process Adjustment in Dry Etching

(1) Requirements for Successful Dry Etching

• High selectivity: High selectivity for non-etching materials.
• Suitable etch rate: Ensures acceptable throughput.
• Good sidewall control: Enables good sidewall profile control.
• Uniformity assurance: Maintains excellent within-wafer uniformity.
• Low damage: Minimizes device damage.
• Wide process window: Offers broad process manufacturing flexibility.

(2) Process Parameter Adjustment and Control

For each specific application, key etching parameters must be determined through process optimization. The orientation of the RF electric field relative to the wafer surface affects etching characteristics: perpendicular orientation emphasizes physical action and basic chemical reactions; parallel orientation primarily involves chemical reactions between surface materials and active species. Polymers play a key role in etching; their generation is regulated by adding hydrogen (H₂) and oxygen (O₂). Oxygen removes polymers, while hydrogen increases polymer formation, ensuring a stable and efficient etching process.

Glossary of Etching Terms

1. Etch Rate
   The etch rate refers to the speed at which material is removed from the wafer surface during etching, usually expressed in Å/min.
   Å (Angstrom) = 1/10,000,000,000 meters (10⁻¹⁰ m).
   Etch Rate = ΔT / t (Å/s)
   ΔT = thickness of removed material (Å or μm)
   t = time used for etching (seconds)
   Loading effect: Etch rate is inversely proportional to etched area.
2. Etch Profile
   The etch profile refers to the shape of the sidewall of the etched pattern.
   Isotropy vs. Anisotropy: Isotropic etching has equal rates in all directions; anisotropic etching has varying rates in different directions.
3. Etch Bias
   Etch bias refers to the variation in line width or critical dimension spacing after etching.
   Etch Bias = Wb − Wa
   Wb = Line width of photoresist before etching
   Wa = Line width of etched material after photoresist removal
4. Selectivity
   Selectivity is the ratio of the etch rate of one material to another under the same etching conditions.
   Selectivity SR = Ef / Er
   Ef = Etch rate of the target material
   Er = Etch rate of the masking material (e.g., photoresist)
5. Uniformity
   Etch uniformity measures the consistency of etching capability across a single wafer, a batch, or between batches.
6. Residue
   Etch residue refers to unwanted materials left on the wafer surface after etching. They often accumulate on the chamber wall or at the base of etched patterns.
7. Polymer
   Polymers are formed during etching when carbon from the photoresist reacts with etching gases (e.g., C₂F₄) and etch by-products.
8. Plasma-Induced Damage
   A. One major type of damage is trap charge formation in the gate electrode due to non-uniform plasma, leading to breakdown of thin gate oxide.
   B. Another type of device damage is caused by energetic particle bombardment of the exposed gate oxide layer, particularly at the edges during etching.
9. Particle Contamination and Defects
   Plasma-induced wafer damage may also result from particle contamination near the wafer surface. Particles form at the plasma-sheath interface due to potential differences. When the plasma is off, these particles can fall onto the wafer. Fluorine-based plasma generates fewer particles than chlorine- or bromine-based plasmas because etch by-products from fluorine have higher vapor pressures.

Related:

1. Silicon Etching: Gas Choices for Precision and Performance
2. What Is SPT Technology and Its Role in Process Flow?
3. Essential Equipment Needed to Start a New Fab Plant

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