6.2 Physical vapor deposition methods

Physical vapor deposition is a key nanofabrication technique for creating thin films. It involves vaporizing a source material and condensing it onto a substrate in a vacuum chamber. This process allows precise control over film thickness and composition.

Two main types of physical vapor deposition are evaporation and sputtering. Evaporation uses heat to vaporize materials, while sputtering bombards a target with energetic particles to eject atoms. Both methods enable the creation of high-quality thin films for various nanoelectronic applications.

Evaporation Techniques

Fundamentals of Evaporation Deposition

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• Evaporation involves heating a source material to its vapor pressure point in a vacuum chamber
• Vaporized atoms travel in straight lines and condense on the substrate surface
• Requires high vacuum conditions (~10^-6 Torr) to minimize collisions and impurities
• Produces high-purity thin films with excellent uniformity and adhesion
• Commonly used for depositing metals (aluminum, gold) and some dielectrics (silicon dioxide)

Electron Beam and Thermal Evaporation Methods

• Electron beam evaporation uses a focused electron beam to heat the source material
  • Generates intense localized heating, suitable for high melting point materials (tungsten, molybdenum)
  • Allows precise control of deposition rate and film thickness
  • Minimizes contamination from the crucible due to localized heating
• Thermal evaporation heats the source material in a resistive boat or filament
  • Simple and cost-effective method for low melting point materials (copper, silver)
  • Limited to materials with vapor pressures below ~10^-2 Torr at reasonable temperatures
  • May introduce impurities from the heating element at high temperatures

Molecular Beam Epitaxy (MBE)

• Ultra-high vacuum technique (~10^-10 Torr) for growing epitaxial films with atomic-layer precision
• Uses effusion cells to generate molecular beams of atoms or molecules
• Allows precise control of composition and doping profiles in semiconductor heterostructures
• Incorporates in-situ monitoring tools (RHEED) for real-time growth analysis
• Enables growth of complex layered structures (quantum wells, superlattices) for advanced electronic and optoelectronic devices

Sputtering Techniques

Principles of Sputtering Deposition

• Sputtering ejects atoms from a target material through bombardment with energetic particles
• Operates at higher pressures (~10^-3 Torr) compared to evaporation techniques
• Generates a plasma of ionized gas (typically argon) to accelerate ions towards the target
• Produces films with better step coverage and adhesion compared to evaporation
• Allows deposition of materials with high melting points and complex compositions (alloys, compounds)

Advanced Sputtering Methods

• Magnetron sputtering uses strong magnetic fields to confine electrons near the target surface
  • Increases ionization efficiency and deposition rates
  • Reduces substrate heating and damage
  • Enables lower operating pressures and improved film quality
• Pulsed Laser Deposition (PLD) uses high-power laser pulses to ablate material from a target
  • Generates a highly energetic plume of atoms and ions for deposition
  • Preserves stoichiometry of complex materials (high-temperature superconductors, multiferroics)
  • Allows growth of metastable phases and nanostructured films
  • Requires careful control of laser parameters and substrate temperature

Reactive and Co-Sputtering Techniques

• Reactive sputtering introduces reactive gases (oxygen, nitrogen) to form compound films
  • Enables deposition of oxides, nitrides, and carbides with controlled composition
  • Requires careful control of gas flow rates and partial pressures
• Co-sputtering uses multiple targets to deposit alloys or composite materials
  • Allows precise control of film composition and graded structures
  • Enables fabrication of novel materials with tailored properties

Deposition Parameters

Critical Factors in Film Growth

• Film thickness determines optical, electrical, and mechanical properties of the deposited layer
  • Measured using various techniques (profilometry, ellipsometry, X-ray reflectivity)
  • Affects stress, grain structure, and surface roughness of the film
  • Critical for device performance in applications like thin-film transistors and optical coatings
• Deposition rate influences film structure, composition, and properties
  • Faster rates may lead to increased defects and reduced film density
  • Slower rates allow better control of film morphology but reduce throughput
  • Typically measured in Angstroms or nanometers per second

Process Control and Optimization

• Substrate temperature affects adatom mobility and film microstructure
  • Higher temperatures promote crystallinity and grain growth
  • Lower temperatures may result in amorphous or nanocrystalline films
• Working pressure impacts mean free path of sputtered atoms and film properties
  • Lower pressures increase directionality and film density
  • Higher pressures may improve step coverage but reduce deposition rate
• Target-to-substrate distance affects deposition uniformity and rate
  • Shorter distances increase deposition rate but may reduce uniformity
  • Longer distances improve uniformity but decrease efficiency