6.3 Chemical vapor deposition and atomic layer deposition

Chemical vapor deposition (CVD) and atomic layer deposition (ALD) are crucial thin film deposition techniques in nanofabrication. These methods enable precise control over film thickness, composition, and structure, allowing the creation of high-quality materials for various applications.

CVD uses chemical reactions of gaseous precursors to deposit films, while ALD achieves atomic-level control through sequential, self-limiting surface reactions. Both techniques offer unique advantages in creating uniform, conformal coatings on complex nanostructures.

Chemical Vapor Deposition (CVD) Techniques

Fundamentals of CVD Process

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• Chemical Vapor Deposition involves chemical reactions of gaseous reactants on or near heated substrate surfaces
• Produces high-quality, pure solid materials with precise control over composition and structure
• Utilizes volatile precursors that react or decompose on the substrate surface to form desired deposit
• Requires careful control of temperature, pressure, and gas flow rates to optimize film growth
• Commonly used to deposit materials like silicon, silicon dioxide, and various metal films

Types of CVD Processes

• Low-Pressure CVD operates at reduced pressures (0.1 to 1 Torr) enhancing film uniformity and step coverage
• LPCVD produces high-quality films with excellent thickness uniformity across large wafer batches
• Plasma-Enhanced CVD uses plasma to enhance chemical reaction rates allowing lower deposition temperatures
• PECVD enables deposition of films on temperature-sensitive substrates (polymers, organics)
• Reaction chamber design varies based on CVD type, substrate size, and desired film properties
• Horizontal tube reactors suit LPCVD while parallel-plate reactors are common for PECVD

Growth Kinetics and Process Control

• Growth rate in CVD depends on precursor concentration, temperature, and pressure
• Surface reaction-limited regime occurs at lower temperatures, growth rate highly temperature-dependent
• Mass transport-limited regime at higher temperatures, growth rate less sensitive to temperature changes
• Precursor depletion along gas flow direction can lead to thickness non-uniformity
• Rotation of substrates or showerhead gas injection improves thickness uniformity
• In-situ monitoring techniques (ellipsometry, interferometry) enable real-time growth rate control

Atomic Layer Deposition (ALD)

ALD Process Principles

• Atomic Layer Deposition achieves precise thickness control through sequential, self-limiting surface reactions
• Consists of alternating pulses of precursor gases separated by purge steps
• Each reaction cycle deposits a single atomic layer of material, enabling angstrom-level thickness control
• Self-limiting nature ensures uniform coverage even on complex 3D structures
• Produces highly conformal coatings on high aspect ratio features (trenches, pores)

ALD Reaction Mechanisms

• Typical ALD cycle consists of four steps: precursor exposure, purge, reactant exposure, purge
• First precursor (A) adsorbs on substrate surface, forming a monolayer
• Excess precursor and byproducts removed during first purge step
• Second precursor (B) reacts with adsorbed layer, forming desired material
• Second purge removes excess reactants and reaction products
• Process repeats until desired film thickness achieved

ALD Applications and Advantages

• Enables deposition of ultra-thin films with precise thickness control (0.1 - 0.3 nm per cycle)
• Produces highly conformal coatings on complex 3D structures (nanotubes, deep trenches)
• Allows deposition of a wide range of materials (oxides, nitrides, metals)
• Used in semiconductor manufacturing for high-k dielectrics, diffusion barriers, and gate oxides
• Enables novel nanostructured materials for catalysis, energy storage, and sensors
• Lower deposition temperatures compared to CVD, suitable for temperature-sensitive substrates

Film Properties and Characteristics

Composition and Structure Control

• Film composition in CVD and ALD determined by precursor chemistry and process conditions
• CVD allows in-situ doping by introducing dopant precursors during deposition
• ALD enables precise control of film stoichiometry through alternating precursor pulses
• Crystallinity of deposited films influenced by substrate temperature and post-deposition annealing
• Amorphous films often deposited at lower temperatures, crystalline films at higher temperatures
• Grain size and orientation in polycrystalline films affect electrical and mechanical properties

Conformality and Step Coverage

• Conformal coating refers to uniform thickness over complex topographies
• ALD provides superior conformality compared to CVD due to self-limiting reactions
• Step coverage in CVD improves with lower pressure and higher temperature
• LPCVD generally offers better step coverage than PECVD
• Conformality crucial for coating high aspect ratio structures (deep trenches, through-silicon vias)
• Sticking coefficient of precursors affects film conformality and growth rate

Growth Kinetics and Precursor Selection

• Growth rate in CVD typically ranges from 10-1000 nm/min, depends on process conditions
• ALD growth rates much slower (0.1-0.3 nm/cycle) but offer precise thickness control
• Precursor selection critical for both CVD and ALD processes
• Ideal precursors have high vapor pressure, thermal stability, and reactivity
• Common CVD precursors include silane (SiH4) for silicon, TEOS for silicon dioxide
• ALD precursors often include organometallic compounds (trimethylaluminum for Al2O3)
• Precursor chemistry influences film purity, deposition temperature, and growth rate