ADVANCED MATERIALS & PROCESSES | JANUARY/FEBRUARY 2024 22 VAPOR DEPOSITION: CVD/CVI/CMC CVD is a vacuum-based deposition method for producing high-quality and high-performance solid materials. By exposing a substrate to one or more precursor materials in gas form within a reaction chamber, the desired materials with their unique properties are decomposed and deposited on the surface of a substrate while in a vapor phase. The process must be carefully controlled to achieve the desired properties including the thickness of the film and to manage the volatile by-products that are produced. These must be carefully removed from the chamber through gas flow. The breadth of CVD applications has grown well beyond its popularity in the semiconductor industry where thin films are used on silicon wafers. Today, smart phones require thin films made from graphene for flexible displays such as QLED displays and for integrated sensing devices such as micro- electro-mechanical systems (MEMS) accelerometers. The aerospace industry uses hard coatings like titanium nitride and chromium carbide to reduce friction and wear on components, much as the diamond cutting tool sector relies on exotic materials for increasing hardness and wear resistance of drills and end mills. In the energy sectors, CVD is used to produce gas diffusion layers for fuel cells, while copper-indium-gallium- selenide (CIGS) and cadmium telluride (CdTe) are used to create an absorber layer in thin-film solar cells. CVD coatings are also commonly applied in the medical sector to enhance bio- compatibility and reduce wear in orthopedic and dental implants. Chemical vapor infiltration (CVI) and ceramic matrix composite (CMC) are related processes that require many of the same system components. CVI is used to create carbon-carbon composites consisting of carbon fibers embedded in a carbon matrix, whereas CMC is used to embed ceramic fibers in a ceramic matrix. Both are prized for being lightweight while providing high tensile strength, resistance to thermal stress and shock, and ability to withstand extremely high temperatures. The applications for CVI and CMC include gas turbine shrouds and nozzle guide vanes; spacecraft heat shields, thermal tiles, aperture shields, and insulation; missile nose cones and control surfaces; aircraft brake discs and propeller blades; automotive exhaust components; industrial heat exchangers; ceramic cutting tools; and high-temperature furnace components. ORCHESTRATING CUSTOMIZED CVD SOLUTIONS A CVD system is a highly orchestrated process that integrates complex component subsystems, all of which need to be tailored to the unique properties of each material selected for vapor deposition. Principally, these include a reactor, vacuum pumps, treatments for the off-gases that are generated, and oversight through automation controls. In-depth knowledge of how to extract and apply the desired properties from a wide range of exotic materials is first required. It is advisable to use specially designed gas engineering systems that can convert the exotic precursor materials to produce the gases needed for vapor deposition whether they are in solid, gas, or liquid form. Hafnium and zirconium, for example, are among the exotic materials that are used in solid form. For these materials, a physical and chemical engineering provider of CVD coating systems designed a metal chlorination process to create hafnium chloride and zirconium chloride tetrachloride. These gases can then be combined with a hydrocarbon such as propane to produce hafnium carbide and zirconium carbide. Even a seemingly straightforward component like a vacuum pump can be a challenge as the chemical vapor deposition process is very severe for pump components. A device between the reactor and the pumping system cleans the byproducts of the vacuum. Dust and effluent made of corrosive materials make a CVD vacuum very dirty, making the CVD process very hard on pump components. Most of the byproducts are made of hydrogen chloride or hydrogen fluoride and need to be cleaned. Experienced designers provide custom systems that resolve challenges based on unique substrates with varying sizes, uniformity, throughput, and increasingly exotic materials. A recommended starting point is to create a highly integrated CVD solution designed and built to meet the specific requirements of the exotic material, whether the precursor is in gas, liquid, or solid form.
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