Chemical Vapor Deposition (CVD)

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Chemical vapor deposition (CVD)

Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high-performance solid materials.or Chemical vapor deposition (CVD), i.e. the deposition of a solid by a chemical reaction involving one or several gaseous chemical species and usually thermally activated. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer (substrate) is exposed to one or more volatileprecursors, which react and/or decompose on the substrate surface to produce the desired deposit.

Microfabrication processes widely use CVD to deposit materials in various forms, including:, polycrystalline, amorphous. These materials include: carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO2, silicon carbide, silicon nitride, The CVD process is also used to produce synthetic diamonds. and used for many years in different kinds of applications (e.g. oxidation or/and wear resistant coatings for cemented carbides, steels or alloys, etc...). In most cases, the substrates considered here have a rather simple shape and are made of non-porous materials.

Among the new composite materials, fiber-reinforced metal-matrix composites and ceramic-matrix composites have been given special attention for their potential uses in a variety of fields. A successful fabrication process for a fiber-reinforced composite requires that the fiber be protected, usually by a coating, during fabrication and service. The chemical vapor deposition process is a key technology for fiber coating.

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CVD is practiced in a variety of formats. These processes generally differ in the means by which chemical reactions are initiated.

Classified by operating pressure:*

1)Atmospheric pressure CVD (APCVD) – CVD

process at atmospheric pressure.

2)Low-pressure CVD (LPCVD) – CVD

process at sub-atmospheric pressures. Reduced pressures tend to reduce unwanted gas-phase reactions and improve film uniformity across the wafer.

3)Ultrahigh vacuum CVD (UHVCVD) – CVD

process at very low pressure, typically below 10−6Pa

Most modern CVD processes are either LPCVD or UHVCVD.

Classified by physical characteristics of vapor: *

1)Direct liquid injection CVD (DLICVD) – A CVD

process in which the precursors are in liquid form (liquid or solid dissolved in a convenient solvent). Liquid solutions are injected in a vaporization chamber towards injectors. The precursor vapors are then transported to the substrate as in classical CVD process. This technique is suitable for use on liquid or solid precursors. High growth rates can be reached using this technique.

Plasma methods *

plasma to enhance chemical reaction rates of the precursors. PECVD processing allows deposition at lower temperatures, which is often critical in the manufacture of semiconductors. The lower temperatures also allow for the deposition of organic coatings, such as plasma polymers, that have been used for nanoparticle surface.

Atomic layer CVD (ALCVD) *

Deposits successive layers of different substances to produce layered, crystalline films.

Combustion Chemical Vapor Deposition(CCVD) 4) Chemical Vapor Deposition or flame pyrolysis is an open-atmosphere, flame-based technique for depositing high-quality thin films and nanomaterials.

*Hot wire CVD (HWCVD)

this process uses a hot filament to chemically decompose the source gases.

Repid thermal CVD (RTCVD) *

This CVD process uses heating lamps or other methods to rapidly heat the wafer substrate. Heating only the substrate rather than the gas or chamber walls helps reduce unwanted gas-phase reactions that can lead to particle formation.

Lec. No. (28)

Powder metallurgy

Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape or form (compacting), and then heating the compressed material in a controlled atmosphere to bond the material (sintering). The powder metallurgy process generally consists of four basic steps: powder manufacture, powder blending, compacting, and sintering.

Several techniques have been developed which permit large production rates of powdered particles, often with considerable control over the size ranges of the final grain population. Powders may be prepared by grinding, chemical reactions, or electrolytic deposition.

Powder compactionis the process of compacting powder in a die through the application of high pressures. Typically the tools are held in the vertical orientation with the punch tool forming the bottom of the cavity. The powder is then compacted into a shape and then ejected from the die cavity. The density of the compacted powder is directly proportional to the amount of pressure applied. Compacting is generally performed at room temperature, and the elevated-temperature process of sintering is usually conducted at atmospheric pressure.

Hot pressing

In most applications of powder metallurgy the compact is hot-pressed. Hot pressing lowers the pressures required to reduce porosity and speeds welding and grain deformation processes. Also it permits better dimensional control of the product, lessened sensitivity to physical characteristics of starting materials, and allows powder to be driven to higher densities than with cold pressing, resulting in higher strength. Negative aspects of hot pressing include shorter die life, slower throughput because of powder heating, and the frequent necessity for protective atmospheres during forming and cooling stages.

Isostatic powder compacting

Isostatic powder compacting is a mass-conserving shaping process. Fine metal particles are placed into a flexible mould and then high gas or fluid pressure is applied to the mould. The resulting article is then sintered in a furnace. This increases the strength of the part by bonding the metal particles. This manufacturing process produces very little scrap metal and can be used to make many different shapes. This is the most efficient type of powder compacting. This operation is generally applicable on small production quantities, as it is more costly to run due to its slow operating speed and the need for expendable tooling.

Compacting pressures range from (14,000kPa) to (69,000 kPa) for non-metals. The density of isostatic compacted parts is 5% to 10% higher than with other powder metallurgy processes.

Hot isostatic pressing

Hot isostatic pressing (HIP) compresses and sinters the part simultaneously by applying heat ranging from (480°C) to (1230°C). Argon gas is the most common gas used in HIP because it is an inert gas, thus prevents chemical reactions during the operation. and used extensively in the production of high-temperature and high-strength parts such as turbine blades for jet engines.