Nanofabrication and PVD Processes

MEMS, NEMS, and Nanofabrication

• Top-Down Approaches: Start with a large material, create smaller fragments into nanoparticles. Surface imperfections and crystallographic damage from lithography can occur.
• Bottom-Up Approaches: Build nanostructures atom-by-atom or molecule-by-molecule. Chemical and physical processes offer more homogeneous compositions and fewer defects.

Physical Vapor Deposition (PVD)

• Thermal Evaporation: The simplest PVD method, using resistive heating to evaporate material into a vacuum. High deposition rate and low cost, but not suitable for multi-component thin films.
• E-Beam Evaporation: Uses high-energy electrons to heat and vaporize a material. Useful for multi-component films, but more expensive.
• Ion Plating: Energetic species continuously or periodically bombard the substrate/growing film. Influences deposition process and resultant film properties (utilizes plasma or vacuum).
• Sputtering: Ejects particles (atoms, ions, clusters) from a surface bombarded by energetic ions. Wide range of targets available and large-scale uniformity.

Synthesis Approaches

• Different techniques (top-down and bottom-up) exist that can be used in different applications, depending on needs.
• Techniques include ball milling, PVD, CVD, hydrothermal, laser ablation, co-precipitation, sol-gel, etc.

Sputtering

• Advantages: Wide range of targets, uniform coating on large samples, low temperatures, and high-energy inert gas ions.
• Disadvantages: Low sputtering rate and low temperatures.

Magnetron Sputtering

• Mechanism: Uses magnetic fields to increase sputtered material. Applied high DC voltage to the target to initiate discharge. A lower voltage is used for continuous operation.
• Applications: Used for increasing the number of sputtered materials. Initiates discharge, gas regulation system is used. Plasma formation occurs, and electrical and magnetic fields affect electron movement, creating a plasma with separated ions and electrons.
• Types: Balanced magnetron sputtering, where magnets are all equal strength; unbalanced magnetron sputtering with a weaker central magnet; reactive sputtering that involves reactive gas.

Pulsed Laser Deposition (PLD)

• Mechanism: Involves using a laser to ablate material from a target, creating a plasma plume that deposits onto a substrate. Electron relaxation, spontaneous/stimulated emission are part of this process.
• Key Processes: Laser ablation, plasma formation, and expansion.
• Types of Interactions: Non-thermal interactions in the first stage, where incident photons provide enough energy to induce band-to-band transitions and excited states.
  -Thermal interactions may also cause unanticipated side effects.
• Plasma Formation and Expansion: Photoionization, Inverse Bremsstrahlung, and electron and light particle movements.

Other Deposition Techniques

• Molecular Beam Epitaxy (MBE): Method for growing crystals on a specific surface by controlling the angle (orientation) of the incoming atoms.
• Pulsed Laser Deposition (PLD): Targets are ablated using pulsed lasers to create a plasma plume that deposits onto a substrate.
• Vacuum Deposition/Chamber: Techniques that utilize a vacuum environment in the deposition.
• Chemical Vapor Deposition (CVD): Method involving the vaporization of precursor gases, followed by gas-phase reactions/decompositions to create a thin film via different surface reactions.

Properties of Thin Films

• Surface Energy: Reduced lattice constants due to significant bond shortening, leading to higher surface area.

Small Particles

• Energy Reduction: Surface relaxation for inward shifts of surface atoms, surface restructuring (forming strained bonds), composition segregation (impurity enrichment), combining nanostructures (merging), and agglomeration (clustering).

Chemical Vapor Deposition (CVD)

• Mechanism: Precursor gases are converted to a thin film on a substrate.
• Requirements: Volatility, high temperature stability, and controlled decomposition.
• Types: PVD, CVD, MOCVD, and initiated CVD.

Reactor Conditions

• Horizontal: Common orientation, substrates at various angles.
• Vertical: Uses showerhead mixer, improves uniformity and growth rates
• Low Pressure/Atmospheric Pressure: Vacuum pumps required for low-pressure, while atmosphere-pressure doesn't require them.
• Hot Wall/Cold Wall: The entire chamber/only selected areas on the chamber are heated.
• Plasma/Photo-assisted/Laser-assisted: These methods apply energy to promote deposition.
• Metal-organic CVD (MOCVD): Employs volatile metal-organic precursors.
• Hot Filament/Wire CVD: Uses heated filaments to decompose precursors like tungsten, tantalum, and molybdenum.
• Initiated CVD: Uses initiators and monomers.
• Oxidative CVD: Uses oxidants.

Crystal Structures

• Crystalline: Ordered atomic or molecular arrangement, in a 3-D structure.
• Polycrystalline: Aggregate of small single crystals.
• Amorphous: Random arrangement.

Wafer Preparation

• Material Sources: Metallurgical Grade Silicon (MGS), Electronics Grade Silicon (EGS).
• Methods: Czochralski (CZ) and float-zone techniques: differences include oxygen impurities.
• Cutting & Polishing: Various steps to prepare wafers for applications, including sawing, grinding, and polishing.
• Flat: Orientation & identification of flats provide crystal type and orientation.
• Defects/Yield: Quality issues and yield from differing wafer fabrication and quality control, including defect density and dimensions/size of dies.

Surface Micromachining

• Structural Layers: Forms mechanical device.
• Sacrificial Layers: Supports MEMS during fabrication, then etched away.
• Methods: Includes lithography for patterning and etching for removing unwanted material.
• Substrates/Materials: Utilizes SiO2 and polysilicon frequently.
• Anisotropic/Isotropic Etching: Differences impact properties and configurations, isotropic etches all directions, whereas anisotropic etches only certain axes/directions.

Film Deposition

• Techniques include PVD methods (evaporation, sputtering, ion plating) and CVD methods. Each method has various configurations, which determine their efficiency in different applications.

X-ray Diffraction (XRD)

• Applications: Identifying crystal structure and analyzing materials properties.
• Methods: Laue, powder, and rotating crystal methods are used.

Photoresists

• Positive/Negative: Solvents dissolve exposed/unexposed resist areas.
• Use: Precisely defining patterns on wafers.

Explore the intricate processes of MEMS, NEMS, and nanofabrication. This quiz covers top-down and bottom-up approaches, as well as various physical vapor deposition techniques like thermal evaporation and sputtering. Test your understanding of these critical methods in the field of nanotechnology.