Chemical Vapor Deposition (CVD)
Diamonds are the hardest naturally occurring substance on Earth. In addition to their beauty and brilliance, diamonds are also thermal conductors and chemically inert, making them a prime candidate for use in technology.
Diamonds are rare and expensive to mine, though, plus the quality of the structure is impossible to guarantee. As a result, scientists have spent the last several decades working on methods to create man made diamonds in the laboratory, controlling purity and size. This creation process comes in two forms: high pressure, high temperature (HPHT) and chemical vapor deposition (CVD).
Chemical vapor deposition is a generic name for a group of chemical processes that involve depositing a solid material from a gaseous phase onto a substrate (seed element or mineral). There are several types of vapor deposition processes: atmospheric pressure (APCVD), low pressure (LPCVD), metal-organic (MOCVD), plasma assisted/enhanced (PACVD/PECVD), laser (LCVD), photochemical (PCVD), chemical vapor infiltration (CVI) and chemical beam epitaxy (CBE)
The chemical vapor deposition process functions by delivering precursor gases into a reaction chamber at ambient (room) temperature. In the chamber is a heated substrate (seed material such as a sapphire or small diamond chip). As the gas passes over the substrate, the two either react or decompose forming a solid phase, which is then deposited on the substrate. Temperature is a critical parameter and influences which type of reactions will take place.
Coatings from chemical vapor deposition are typically very finely grained, of high purity, of greater hardness than similar materials, and practically impervious. The CVD coatings are only a few microns thick, and are deposited slowly: i.e., a few hundred microns per hour.
Chemical vapor deposition apparatus consists of several parts:
- Gas Delivery System: to supply the precursors to the reactor chamber.
- Reactor Chamber: chamber where deposition occurs.
- Substrate Loading Mechanism: to introduce and withdraw substrates.
- Energy Source: provides energy required for the reaction.
- Vacuum System: to remove all gases not required for the reaction.
- Exhaust System: to remove any volatile by-products from the chamber.
- Exhaust Treatment Systems: to treat/convert harmful gases.
- Process Control Equipment: gauges, controls, alarms and safety devices.
Chemical vapor deposition can consist of several different energy sources, precursor gases, and substrates, depending on the desired product. Pressure and temperature play an important role in production as well, creating CVD results ranging from small polycrystalline structures to large single crystal structures (diamonds occur naturally as single crystals).
Most modern chemical vapor deposition processes can be further classified according to pressure used, characteristic of the vapors, plasma methods, types of deposition, and heat source used. Man made diamonds, particularly those of gemstone quality, are produced via LPCVD, low pressure chemical vapor deposition.
“Growing” a CVD diamond occurs under pressures of 1 to 27 kPa (kilo pascals), with varying amount of gas in the chamber, which are energized to provide diamond growth on the substrate. Sources of the gas always includes carbon, hydrogen, too, though amounts vary according to the type of diamond being grown. Energy is provided via a hot filament, microwave power, and arc discharges, which generates a plasma. Within this plasma, the gases break down and combine to form more complex chemical compounds. The vapor then deposits onto the substrate, an atom at a time, to form a crystal.
Recent breakthroughs using methane gas with the hydrogen, heated using a hot filament, with a diamond seed crystal created by high pressure, high temperature method as a substrate, have produced a large diamond worthy of gemstone status. Oxides can be added to this process, too, to increase electrical properties of the man made diamond for use in semiconductors.
Chemical vapor deposition has added an exciting component to both the world of jewelry and the world of technology. Man made diamonds that can be grown to specified sizes and thicknesses for use in computers, lasers and radiation detection devices can enhance the future of those products.
The lower cost, plus the possibility of mass production can influence the future of technology itself. Those are still being researched, and may take a decade or more to come to fruition, but in the interim, the world now has an improved method for creating pure, colorless man made diamonds at a fraction of the cost.
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