“When the cavemen lit a lamp and soot was deposited on the wall of a cave,” that was a rudimentary form of CVD. MIT chemical engineering professor Karen Gleason found traces of the technology of chemical vapor deposition in the prehistory.
Today, CVD is a basic tool of manufacturing — used in everything from sunglasses to potato-chip bags — and is fundamental to the production of much of today’s electronics. It is also a technique subject to constant refining and expansion, pushing materials research in new directions — such as the production of large-scale sheets of graphene, or the development of solar cells that could be “printed” onto a sheet of paper or plastic.
This vapor deposition of polymers has opened the door to a variety of materials that would be difficult, and in some cases impossible, to produce in any other way. For example, many useful polymers, such as water-shedding materials to protect industrial components or biological implants, are made from precursors that are not soluble, and thus could not be produced using conventional solution-based methods. In addition, says Gleason, the Alexander and I. Michael Kasser Professor at MIT, the CVD process itself induces chemical reactions between coatings and substrates that can strongly bond the material to the surface.
The process can require a lot of fine-tuning, but is fundamentally a simple set of steps: the material to be coated is placed inside a vacuum chamber — which dictates the maximum size of objects that can be coated. Then, the coating material is heated, or the pressure around it is reduced until the material vaporizes, either inside the vacuum chamber or in an adjacent area from which the vapor can be introduced. There, the suspended material begins to settle onto the substrate material and form a uniform coating. Adjusting the temperature and duration of the process makes it possible to control the thickness of the coating.
With metals or metal compounds, such as those used in the semiconductor industry, or the silvery coatings inside snack bags, the heated metal vapor deposits on a cooler substrate. In the polymer process, it’s a bit more complex: Two or more different precursor compounds, called monomers, are introduced into the chamber, where they react to form polymers as they deposit on the surface.
Even high-temperature CVD processing has evolved, with great potential for commercial applications. For example, the research group of John Hart, an associate professor of mechanical engineering, has built a roll-to-roll processing system using CVD to make sheets of graphene, a material with potential applications ranging from large-screen displays to water-filtration systems. Hart’s group and others have used CVD to produce large arrays of carbon nanotubes, materials with potential as new electrodes for batteries or fuel cells.
One great advantage of CVD processing is that it can create coatings of uniform thickness even over complex shapes. For example, CVD can be used to uniformly coat carbon nanotubes — tiny cylinders of pure carbon that are far more slender than a hair — such as to modify their mechanical properties and make them react chemically to certain substances.