Chemical Vapor Deposition, commonly referred to as CVD, refers to a broad range of thin film deposition techniques that are widely used for producing high quality, high-performance solid coatings or polymers. While there are a wide variety of specific CVD processes, what they all have in common is a chemical reaction of a gaseous chemical precursor driven by either heat or plasma to produce dense thin films on a substrate.
CVD is a highly versatile and fast method of growing dense, pure coatings with uniform thickness. For thermal CVD, the substrate is heated and a precursor reactant gas is introduced into the deposition chamber which may either be directly absorbed onto the surface of the substrate to be coated, or may form an intermediate reactant in the gas phase which is then deposited onto the substrate. The processes are run at elevated temperatures and at atmospheric or lower pressures.
What is the Difference Between CVD and PVD?
Both CVD and Physical Vapor Deposition (PVD) processes are similar in that they are used to create thin films on an atomic or molecular level of very high purity and density. In some instances, the same equipment can be used to do both.
However, what defines CVD is the chemical reaction that occurs on the surface of the substrate. It is this chemical reaction that distinguishes it from PVD sputtering or thermal evaporation thin film deposition processes that usually don’t involve chemical reactions.
Another key difference is that the deposition of CVD coating is in a flowing gaseous state which is a diffuse multidirectional type of deposition. PVD on the other hand involves vaporizing solid physical particles into a plasma which is a line-of-site deposition. This carries a lot of implications regarding thickness and uniformity of deposition on uneven surfaces.
CVD deposition equipment tends to be more diversified and highly specialized to specific industry applications, and must deal with the toxic chemical byproducts of the process. Whereas PVD utilizing plasma has very little environmental impact.
Where is Chemical Vapor Deposition Used?
CVD is a widely used manufacturing process that is central to the electronics industry, solar panel and optical industries such as sunglasses, optical storage or architectural glass.
Polymerization by CVD — which are small molecules combining chemically to produce a very large chainlike molecule or polymer — is one of its most versatile uses and economical ways of growing films used for packaging such as potato chip bags.
Because CVD coating can be very hard, fine grained and impervious, it is widely used on surfaces to prevent corrosion from weather.
It is used for high performance automotive or aerospace parts where tribology, which is the science of moving parts involving lubricity and hardness, is critically important.
CVD has enabled the production of large-scale sheets of graphene, which are atomically thin sheets of hybridized carbon atoms arranged in a honeycomb structure used for a very wide range of applications from large screen TV displays to water filtration systems.
What are the Advantages of Chemical Vapor Deposition?
One advantage of the CVD process is that because the chemical reactants are gases it is able to take advantage of the physical characteristics of how gases flow over the substrate surface to build uniform, highly conformal films on irregularly shaped surfaces. For many applications, it can result in a more controllable surface morphology.
CVD offers a wide variety of coating materials based on metals, alloys and ceramics. The chemical reactions that characterize CVD can also be used to form alloys.
It can provide an easily scalable and controllable process for many types of batch production runs that provide major cost savings with economies of scale.
CVD deposits very pure films, over 99.995% purity. They are typically fine grained and can be used to enable a very high degree of hardness such as Diamond-like Carbon (DLC) coatings.
CVD usually doesn’t require as high a vacuum as PVD processes, or any vacuum at all. Except for the chemical byproducts that result from the reactions at the outflow, CVD equipment for the most part is self-cleaning.
What are the Disadvantages of Chemical Vapor Deposition?
Many CVD byproducts are hazardous including being highly toxic, explosive or corrosive. Full consideration must be given to safely handling them in a way that is not harmful to humans or the environment. Depending upon the specific precursors, this can be expensive.
Thermal CVD processes are heat driven which can affect the type of substrate that can be coated and not be damaged. Stresses and failures can occur between films with different heat expansion coefficients as a result of the heat involved in the process.
The expense of some precursor gases — particularly those that are metal-organic compounds widely used in chip manufacturing — can be high.
What is Chemical Vapor Deposition Coating Equipment?
There is a very wide variety of specific CVD coating techniques and equipment that are unique to individual industries, but they all share a basic configuration.
A substrate to be coated is placed into the deposition chamber and either heated or exposed to a plasma. All CVD reactions are driven with specialized electrical power sources tailored to the application.
A reactant precursor gas or gases are introduced into the deposition chamber in a controlled manner. Here they may either begin chemically reacting with the substrate surface immediately, or mix forming intermediary gases in the deposition chamber which then chemically react with the surface of the substrate to grow the thin film coatings.
The chemical by-products and unreacted atoms or molecules removed from the chamber with the exhaust tend to be toxic, flammable or damaging to the pumps, and therefore need to be treated to make them harmless to people and the environment. These are usually captured with cold traps, wet scrubbers and vents. They are then treated by passing them through a chemical or water trap to protect the pumps. Flammable gases require special attention.
What are Common Precursors for CVD?
- Halides: HSiCl3 SiCl2 TiCl4 WF6
- Hydrides: AlH(NMe3)3 SiH4 GeH4 NH3
- Metal Alkoxides: TEOS, Tetrakis Dimethylamino Titanium (TDMAT)
- Metal Dialkylamides: Ti(NMe2)
- Metal Diketonates: Cu(acac)
- Metal Carbonyls: Ni(CO)
- Metal Alkoxides: Ti(OiPr)4
- Organometallics: AlMe3 Ti(CH2tBu)
- Oxygen
Once in the deposition chamber, the precursors are transported to the substrate by gas diffusion or liquid flow — or some combination of both. Here the atom or molecule has to stay on the surface long enough to create a chemical bond known as the resident time. Atoms that don’t bond are then driven away to make room for more. The desired coating is achieved through control of the thermodynamics and kinetics of temperature, pressure and concentration yields.
What are the Types of CVD Processes?
There is a wide variety of specific CVD processes meeting the demands of a range of industries and applications.
Horizontal and vertical CVD Describes the reactor configuration and direction of gas flow towards the substrate with horizontal tube reactors being the most common.
Low pressure and atmospheric pressure CVD (LPCVD) With LPCVD, a vacuum pump draws the gas through the deposition chamber. Low pressure can result in a more uniform deposition rate and reduce gas-phase reactions. Atmospheric pressure often does not require pumps but can result in a slower deposition rate.
Hot-wall CVD and cold-wall CVD These refer to the manner in which heat is applied. With hot-wall CVD, the entire chamber is heated for a more uniform temperature. With cold-wall CVD, only the substrate is heated which allows for more rapid cooling where overheating of the substrate can be a problem.
Atomic Layer Deposition (ALD) Enables the deposition of successive layers of different substances with a very high degree of uniformity and precise thickness control with easily replicable results. Precursors are introduced sequentially into the chamber with self-limiting absorption and surface reactions, that are driven off by the gas. The next coating is then introduced into the chamber to produce very high-quality thin films that are easy to control.
Metal–organic CVD (MOCVD) Metal-organic precursors used are often-vaporized liquids. This is one of the most popular ways of fabricating III-V semiconductor materials on a chip by being able to grow a number of complex layers on top of each other with varying compositions. MOCVD has also become widely used for the production of optical equipment such as light-emitting diodes and laser diodes, along with solar cells and photodetectors.
Photo-assisted or laser assisted CVD (LACVD) High-intensity lights or laser can be used to activate the reactions with photons tuned to the electrical or vibrational level of the gas.
Hot filament/wire CVD (HFCVD) Utilizes resistive filaments to chemically decompose the source gases. This enables the gas and the substrate to be heated separately with cooler substrate temperatures allowing for better absorption rates.
Initiated CVD (ICVD) Is one variation of hot filament/wire CVD for creating polymers. It uses an initiator and monomers gas reaction which is absorbed on the cooled substrate surface that forms polymers chains. The advantage of the initiator gas is that it enables the use of cooler filament temperatures.
Oxidative CVD (OCVD) Used for creating conducting and semiconducting polymer films, it uses oxidant and monomer vapors that react on the surface of the substrate. Along with Initiative CVD, it enables the creation of polymers with the advantages of the CVD process.
Hybrid physical-chemical vapor deposition (HPCVD) This involves both a CVD chemical decomposition of a precursor gas as well as the physical vaporization or PVD of a solid target material used for coating.
Plasma enhanced CVD (PECVD) A hybrid process that involves the introduction of plasma energy to speed and enhance the chemical reactions that occur with standard CVD techniques. Plasma enhanced CVD — also known as plasma assisted CVD or PACVD — requires relatively low substrate temperatures because the thermal energy is supplied by the plasma rather than heating the substrate and so can be applied to surfaces that could not withstand normal CVD temperatures.
For more information on Semicore’s versatile SC8600 Series that performs plasma enhanced CVD as well as advanced PVD coatings such as HIPMIS please visit our Diamond-like Carbon Tribology Coatings page.
Matt Hughes is President of Semicore Equipment Inc, a leading worldwide Thin Film Deposition Equipment Manufacturer for the electronics, solar energy, optical, medical, military, automotive, and related high tech industries. Let our helpful support staff answer any questions you have on how to implement the best techniques and equipment for your specific needs by contacting us at sales@semicore.com or by calling 925-373-8201.
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