CVD+(Chemical+Vapor+Deposition)

= __CVD (Chemical Vapor Deposition)__ =

 Chemical vapor 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 vapor 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.

 //1. 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)



 **2.** //**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.



**//3. Coating Characteristics //**
 CVD coatings are typically:
 * <span style="font-family: Arial,Helvetica,sans-serif;">Fine grained
 * <span style="font-family: Arial,Helvetica,sans-serif;">Impervious
 * <span style="font-family: Arial,Helvetica,sans-serif;">High purity
 * <span style="font-family: Arial,Helvetica,sans-serif;">Harder than similar materials produced using conventional ceramic
 * <span style="font-family: Arial,Helvetica,sans-serif;">fabrication processes

<span style="font-family: Arial,Helvetica,sans-serif;"> 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.

<span style="font-family: Arial,Helvetica,sans-serif;"> //**4. CVD Apparatus**//
<span style="font-family: Arial,Helvetica,sans-serif;"> A CVD apparatus will consist of several basic components:
 * <span style="font-family: Arial,Helvetica,sans-serif;"><span style="font-family: Arial,Helvetica,sans-serif;">__Gas delivery system__ – For the supply of precursors to the reactor chamber
 * <span style="font-family: Arial,Helvetica,sans-serif;"><span style="font-family: Arial,Helvetica,sans-serif;">__Reactor chamber__ – Chamber within which deposition takes place
 * <span style="font-family: Arial,Helvetica,sans-serif;">__Substrate loading mechanism__ – A system for introducing and removing substrates, mandrels etc
 * <span style="font-family: Arial,Helvetica,sans-serif;">__Energy source__ – Provide the energy/heat that is required to get the precursors to react/decompose.
 * <span style="font-family: Arial,Helvetica,sans-serif;">__Vacuum system__ – A system for removal of all other gaseous species other than those required for the reaction/deposition.
 * <span style="font-family: Arial,Helvetica,sans-serif;">__Exhaust system__ – System for removal of volatile by-products from the reaction chamber.
 * <span style="font-family: Arial,Helvetica,sans-serif;">__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.
 * <span style="font-family: Arial,Helvetica,sans-serif;">__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.



//<span style="font-family: Arial,Helvetica,sans-serif;"> 5. Energy Sources //
<span style="font-family: Arial,Helvetica,sans-serif;"> There are several suitable sources of heat for CVD processes. These include: <span style="font-family: Arial,Helvetica,sans-serif;"> Other energy sources may include UV-visible light or lasers as a source of photo energy.
 * <span style="font-family: Arial,Helvetica,sans-serif;">Resistive Heating e.g. tube furnaces
 * <span style="font-family: Arial,Helvetica,sans-serif;">Radiant Heating e.g. halogen lamps
 * <span style="font-family: Arial,Helvetica,sans-serif;">Radio Frequency Heating e.g. induction heating
 * <span style="font-family: Arial,Helvetica,sans-serif;">Lasers

//<span style="font-family: Arial,Helvetica,sans-serif;"> 6. Precursors //
<span style="font-family: Arial,Helvetica,sans-serif;"> 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. <span style="font-family: Arial,Helvetica,sans-serif;"> 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.

<span style="font-family: Arial,Helvetica,sans-serif;"> -__Typical Precursor Materials:__ <span style="font-family: Arial,Helvetica,sans-serif;"> CVD precursor materials fall into a number of categories such as:
 * <span style="font-family: Arial,Helvetica,sans-serif;">Halides. TiCl4, TaCl5, WF6, etc
 * <span style="font-family: Arial,Helvetica,sans-serif;">Hydrides . SiH4, GeH4, AlH3(NMe3)2, NH3, etc
 * <span style="font-family: Arial,Helvetica,sans-serif;">Metal Organic Compounds
 * <span style="font-family: Arial,Helvetica,sans-serif;">Metal Alkyls . AlMe3, Ti(CH2tBu)4, etc
 * <span style="font-family: Arial,Helvetica,sans-serif;">Metal Alkoxides . Ti(OiPr)4, etc
 * <span style="font-family: Arial,Helvetica,sans-serif;">Metal Dialylamides. Ti(NMe2)4, etc
 * <span style="font-family: Arial,Helvetica,sans-serif;">Metal Diketonates. Cu(acac)2, etcMetal Carbonyls. Ni(CO)4, etc
 * <span style="font-family: Arial,Helvetica,sans-serif;">Others. Include a range of other metal organic compounds, complexes and ligands.

//<span style="font-family: Arial,Helvetica,sans-serif;"> 7. Materials That Can be Produced by CVD Processes //
<span style="font-family: Arial,Helvetica,sans-serif;"> CVD is an extremely versatile process that can be used to process almost any metallic or ceramic compound. Some of these include:
 * <span style="font-family: Arial,Helvetica,sans-serif;">Elements
 * <span style="font-family: Arial,Helvetica,sans-serif;">Metals and alloys
 * <span style="font-family: Arial,Helvetica,sans-serif;">Carbides
 * <span style="font-family: Arial,Helvetica,sans-serif;">Nitrides
 * <span style="font-family: Arial,Helvetica,sans-serif;">Borides
 * <span style="font-family: Arial,Helvetica,sans-serif;">Oxides
 * <span style="font-family: Arial,Helvetica,sans-serif;">Intermetallic compounds

//<span style="font-family: Arial,Helvetica,sans-serif;"> 8. CVD Gas Products //
<span style="font-family: Arial,Helvetica,sans-serif;"> 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. <span style="font-family: Arial,Helvetica,sans-serif;"> 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.

<span style="font-family: Arial,Helvetica,sans-serif;"> //9. Applications//
<span style="font-family: Arial,Helvetica,sans-serif;"> CVD has applications across a wide range of industries such as: media type="youtube" key="KjjQf12FgUQ" width="339" height="280" align="center"
 * <span style="font-family: Arial,Helvetica,sans-serif;">__Coatings__. Coatings for a variety of applications such as wear resistance, corrosion resistance, high temperature protection, erosion protection and combinations thereof. In the next video, Dr. Charlie Dunnill shows how we use simple CVD technology to prepare surfaces with functional properties:


 * <span style="font-family: Arial,Helvetica,sans-serif;">__Semiconductors and related devices__. Integrated circuits, sensors and optoelectronic devices.
 * <span style="font-family: Arial,Helvetica,sans-serif;">__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.
 * <span style="font-family: Arial,Helvetica,sans-serif;">__Optical Fibres__. For telecommunications.
 * <span style="font-family: Arial,Helvetica,sans-serif;">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 vapor infiltration or CVI.
 * <span style="font-family: Arial,Helvetica,sans-serif;">__Powder production__. Production of novel powders and fibres.
 * __<span style="font-family: Arial,Helvetica,sans-serif;">Catalysts __
 * __<span style="font-family: Arial,Helvetica,sans-serif;">Nanomachines __

__<span style="font-family: Arial,Helvetica,sans-serif;">References __
<span style="font-family: Arial,Helvetica,sans-serif;"> 1. http://www.azom.com/article.aspx?ArticleID=1552 2. http://en.wikipedia.org/wiki/Chemical_vapor_deposition 3. http://www.youtube.com/watch?v=KjjQf12FgUQ

__Further information__
http://www.asminternational.org/content/ASM/StoreFiles/ACFAA6E.pdf