Chemical Vapor Deposition (CVD)

What is Chemical Vapor Deposition (CVD)?

Chemical Vapor Deposition (CVD) is a versatile thin-film deposition technique used to deposit high-quality, uniform coatings or layers of materials onto a substrate. It involves the chemical reaction of gaseous precursors on or near a heated substrate surface, forming a solid thin film.

Note: CVD is widely used in semiconductor manufacturing, coatings, and nanotechnology due to its ability to produce precise and conformal films.

How Does CVD Work?

The CVD process typically involves the following steps:

1. Precursor Delivery: Volatile precursor gases (SiH₄, WF₆, TiCl₄) reactants are introduced into a reaction chamber.
2. Chemical Reaction: The precursors react or decompose on the heated (300–1200°C depending on type) substrate surface, forming a solid film.
3. By-Product Removal: Gaseous (HCl, H₂ etc.) by-products are removed from the chamber via an exhaust system.
   Example: SiH₄ → Si (solid) + 2H₂ (gas)
4. Film Growth: The deposited material grows into a thin, uniform film on the substrate.

Types of CVD

Different CVD techniques are used based on the application and material requirements:

• Thermal CVD: Uses heat to initiate chemical reactions (e.g., Low-Pressure CVD, LPCVD) 0.1–10 Torr, 500–900°C.
• Plasma-Enhanced CVD (PECVD): Uses plasma to enhance reaction rates at lower temperatures (200–400°C).
• Metal-Organic CVD (MOCVD): Uses metal-organic precursors for depositing compound semiconductors.
• Atomic Layer Deposition (ALD): A specialized CVD technique for ultra-thin, highly controlled films.
• Hot Filament CVD (HFCVD): For diamond coatings.
• Atmospheric Pressure CVD (APCVD): At normal pressure, high deposition rate.

Applications of CVD

• Semiconductors: Depositing silicon, silicon dioxide, and nitride layers for integrated circuits.
• Coatings: Wear-resistant, anti-corrosive, or decorative coatings (e.g., diamond films).
• Cutting tools: TiN, TiC, Al₂O₃, diamond coatings for wear resistance.
• Optics: Anti-reflective or conductive coatings for lenses and displays.
• Solar cells: Amorphous silicon, CdTe, CIGS thin films.
• Nanotechnology: Fabricating graphene, carbon nanotubes, and other nanomaterials (using special CVD).

Advantages of CVD

• High purity and crystalline quality films.
• Excellent step coverage and conformality (especially LPCVD & ALD).
• Can deposit a wide range of materials: metals, semiconductors, insulators, diamond.
• Precise control over thickness and composition.
• Large-area uniform deposition possible.

Disadvantages of CVD

• High temperature (except PECVD) → Not suitable for low-melting substrates.
• Toxic, corrosive, and flammable precursor gases (safety issues).
• Slow deposition rate in some types (e.g., LPCVD).
• Expensive equipment and maintenance.
• By-product gases need safe disposal.

Key Parameters in CVD

The quality of the deposited film depends on:

• Temperature: Affects reaction rate and film properties.
• Pressure: Influences uniformity and deposition rate.
• Precursor Flow Rate: Determines film growth rate and composition.
• Substrate Material: Impacts adhesion and film quality.

Related Topic: Physical Vapor Deposition (PVD)

CVD MCQs Asked in NEET, IIT-JEE, IIT-JAM GATE, CSIR-NET

JEE Advanced 2022

Q1: In Chemical Vapour Deposition (CVD), the deposition occurs due to

1. Physical adsorption only
2. Chemical reaction on the substrate surface
3. Evaporation of material
4. Sputtering of atoms

Correct Answer: B

Explanation: CVD is a chemical process where gaseous precursors undergo chemical reaction on or near the heated substrate to form a solid thin film. It is not a physical process like PVD.

GATE 2024 (Materials Science)

Q2: Which of the following CVD techniques uses plasma to lower the deposition temperature?

1. LPCVD
2. APCVD
3. PECVD
4. MOCVD

Correct Answer: C

Explanation: Plasma Enhanced CVD (PECVD) uses RF plasma to generate reactive species, enabling deposition at low temperatures (200–400°C), ideal for temperature-sensitive substrates.

CSIR-NET 2023

Q3: In the CVD growth of polysilicon from silane (SiH₄), the overall reaction is:

1. SiH₄ → Si + 2H₂
2. SiH₄ + O₂ → SiO₂ + 2H₂
3. SiCl₄ + 2H₂ → Si + 4HCl
4. SiH₄ → SiH₂ + H₂

Correct Answer: A

Explanation: Thermal decomposition of silane at 600–650°C:
SiH₄(g) → Si(s) + 2H₂(g).
This is widely used in LPCVD for polysilicon and epitaxial silicon.

JEE Main 2024

Q4: Which CVD technique gives the best step coverage and conformality?

1. APCVD
2. PECVD
3. LPCVD
4. Thermal CVD at atmospheric pressure

Correct Answer: C

Explanation: Low-Pressure CVD (LPCVD) operates at low pressure (0.1–10 Torr), increasing mean free path and surface diffusion → excellent conformality and step coverage.

GATE 2023 (ECE)

Q5: Atomic Layer Deposition (ALD) is a special type of CVD because:

1. It is performed at atmospheric pressure
2. It uses continuous flow of precursors
3. It involves self-limiting sequential surface reactions
4. It requires very high temperature (>1000°C)

Correct Answer: C

Explanation: ALD is cyclic CVD with alternate pulsing of precursors and purge steps → each cycle deposits exactly one monolayer → atomic-level thickness control and perfect conformality.

IIT-JAM 2022 (Physics)

Q6: Diamond coatings are generally deposited using:

1. Hot Filament CVD
2. Sputtering
3. Evaporation
4. Electroless plating

Correct Answer: A

Explanation: Hot Filament CVD (HFCVD) uses a hot tungsten filament (~2200°C) to dissociate H₂ and CH₄ → atomic hydrogen and carbon radicals → enables diamond (sp³) growth at 700–900°C.

CSIR-NET 2021

Q7: Metal-Organic Chemical Vapour Deposition (MOCVD) is primarily used for growing:

1. SiO₂ and Si₃N₄
2. III–V compound semiconductors (GaAs, InP, GaN)
3. Polysilicon
4. Tungsten plugs

Correct Answer: B

Explanation: MOCVD uses metal-organic precursors like TMGa, TMIn, AsH₃, PH₃ → high-quality epitaxial layers of GaAs, InP, GaN used in LEDs, lasers, and high-electron-mobility transistors.

JEE Advanced 2021

Q8: Which of the following is a major disadvantage of conventional CVD?

1. Low deposition rate
2. High operating temperature
3. Poor adhesion
4. Non-uniform thickness

Correct Answer: B

Explanation: Most CVD processes require high substrate temperature (500–1200°C), which can damage temperature-sensitive devices. PECVD and ALD were developed to overcome this.

GATE 2025 (Expected Pattern)

Q9: In CVD of tungsten (W) using WF₆ and H₂, the reaction is:

1. WF₆ + 3H₂ → W + 6HF
2. 2WF₆ + 3O₂ → 2WO₃ + 6F₂
3. WF₆ → W + 3F₂
4. WF₆ + H₂ → WHv + 6F

Correct Answer: A

Explanation: Blanket tungsten deposition:
WF₆ + 3H₂ → W(s) + 6HF(g).
This is widely used for via plugs and interconnects in VLSI.

NEET 2024 (Materials in Medicine Context)

Q10: Biocompatible diamond-like carbon (DLC) coatings on medical implants are deposited by:

1. PECVD
2. Thermal evaporation
3. Sputtering only
4. Electroplating

Correct Answer: A

Explanation: DLC is deposited using RF-PECVD from hydrocarbon precursors (CH₄, C₂H₂). It offers extreme hardness, low friction, and excellent biocompatibility.