Chemical vapour deposition or CVD is a generic name for a group of processes that involve depositing a solid material from a gaseous phase and is similar in some respects to physical vapour deposition (PVD).
PVD differs in that the precursors are solid, with the material to be deposited being vaporised from a solid target and deposited onto the substrate.
Types of CVD Processes
CVD covers processes such as:
• Atmospheric Pressure Chemical Vapour Deposition (APCVD)
• Low Pressure Chemical Vapour Deposition (LPCVD)
• Metal-Organic Chemical Vapour Deposition (MOCVD)
• Plasma Assisted Chemical Vapour Deposition (PACVD) or Plasma Enhanced Chemical Vapour Deposition (PECVD)
• Laser Chemical Vapour Deposition (LCVD)
• Photochemical Vapour Deposition (PCVD)
• Chemical Vapour Infiltration (CVI)
• Chemical Beam Epitaxy (CBE)
How Does CVD Work?
Precursor gases (often diluted in carrier gases) are delivered into the reaction chamber at approximately ambient temperatures. As they pass over or come into contact with a heated substrate, they react or decompose forming a solid phase which and are deposited onto the substrate. The substrate temperature is critical and can influence what reactions will take place.
CVD coatings are typically:
• Fine grained
• High purity
• Harder than similar materials produced using conventional ceramic fabrication processes
CVD coatings are usually only a few microns thick and are generally deposited at fairly slow rates, usually of the order of a few hundred microns per hour.
A CVD apparatus will consist of several basic components:
• Gas delivery system – For the supply of precursors to the reactor chamber
• Reactor chamber – Chamber within which deposition takes place
• Substrate loading mechanism – A system for introducing and removing substrates, mandrels etc
• Energy source – Provide the energy/heat that is required to get the precursors to react/decompose.
• Vacuum system – A system for removal of all other gaseous species other than those required for the reaction/deposition.
• Exhaust system – System for removal of volatile by-products from the reaction chamber.
• Exhaust treatment systems – In some instances, exhaust gases may not be suitable for release into the atmosphere and may require treatment or conversion to safe/harmless compounds.
• Process control equipment – Gauges, controls etc to monitor process parameters such as pressure, temperature and time. Alarms and safety devices would also be included in this category.
There are several suitable sources of heat for CVD processes. These include:
• Resistive Heating e.g. tube furnaces
• Radiant Heating e.g. halogen lamps
• Radio Frequency Heating e.g. induction heating
Other energy sources may include UV-visible light or lasers as a source of photo energy.
Materials are deposited from the gaseous state during CVD. Thus precursors for CVD processes must be volatile, but at the same time stable enough to be able to be delivered to the reactor.
Generally precursor compounds will only provide a single element to the deposited material, with others being volatilised during the CVD process. However sometimes precursors may provide more than one. Such materials simplify the delivery system, as they reduce the number of reactants required to produce a given compound.
Typical Precursor Materials
CVD precursor materials fall into a number of categories such as:
• Halides - TiCl4, TaCl5, WF6, etc
• Hydrides - SiH4, GeH4, AlH3(NMe3)2, NH3, etc
• Metal Organic Compounds –
• Metal Alkyls - AlMe3, Ti(CH2tBu)4, etc
• Metal Alkoxides - Ti(OiPr)4, etc
• Metal Dialylamides - Ti(NMe2)4, etc
• Metal Diketonates - Cu(acac)2, etc
• Metal Carbonyls - Ni(CO)4, etc
• Others – include a range of other metal organic compounds, complexes and ligands.
Materials That Can be Produced by CVD Processes
CVD is an extremely versatile process that can be used to process almost any metallic or ceramic compound. Some of these include:
• Metals and alloys
• Intermetallic compounds
CVD Gas Products
An often neglected by-product of the CVD process are volatile gases. However, these gases may be toxic, flammable or corrosive so must be treated appropriately.
Analysis of the off-gases can also lead to a better understanding of the CVD reaction mechanisms and the information used to refine the process.
CVD has applications across a wide range of industries such as:
• Coatings – Coatings for a variety of applications such as wear resistance, corrosion resistance, high temperature protection, erosion protection and combinations thereof.
• Semiconductors and related devices – Integrated circuits, sensors and optoelectronic devices
• Dense structural parts – CVD can be used to produce components that are difficult or uneconomical to produce using conventional fabrication techniques. Dense parts produced via CVD are generally thin walled and maybe deposited onto a mandrel or former.
• Optical Fibres – For telecommunications.
• Composites – Preforms can be infiltrated using CVD techniques to produce ceramic matrix composites such as carbon-carbon, carbon-silicon carbide and silicon carbide-silicon carbide composites. This process is sometimes called chemical vapour infiltration or CVI.
• Powder production – Production of novel powders and fibres