Chemical vapor deposition of diamond

Chemical vapor deposition of diamond is a method of producing synthetic diamond by creating the circumstances necessary for carbon atoms in a gas to settle on a substrate in crystalline form.

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Chemical vapor deposition of diamond has received a great deal of attention in the materials sciences because it allows many new applications of diamond that had previously been either too expensive to implement or too difficult to make economical. CVD diamond growth typically occurs under low pressure (1-27 kPa; 0.145-3.926 psi; 7.5-203 Torr) and involves feeding varying amounts of gases into a chamber, energizing them and providing conditions for diamond growth on the substrate. The gases always include a carbon source, and typically include hydrogen as well, though the amounts used vary greatly depending on the type of diamond being grown. Energy sources include hot filament, microwave power, and arc discharges, among others. The energy source is intended to generate a plasma in which the gases are broken down and more complex chemistries occur. The actual chemical process for diamond growth is still under study and is complicated by the very wide variety of diamond growth processes used.

The advantages to CVD diamond growth include the ability to grow diamond over large areas, the ability to grow diamond on a substrate, and the control over the properties of the diamond produced. In the past, when high pressure high temperature (HPHT) techniques were used to produce diamond, the diamonds were typically very small free standing diamonds of varying sizes. With CVD diamond growth areas of greater than fifteen centimeters (six inches) diameter have been achieved and much larger areas are likely to be successfully coated with diamond in the future. Improving this ability is key to enabling several important applications.

The ability to grow diamond directly on a substrate is important because it allows the addition of many of diamond’s important qualities to other materials. Since diamond has the highest thermal conductivity of any material, layering diamond onto high heat producing electronics (such as optics and transistors) allows the diamond to be used as a heat sink^[1],[2]. Diamond films are being grown on valve rings, cutting tools, and other objects that benefit from diamond’s hardness and exceedingly low wear rate. In each case the diamond growth must be carefully done to achieve the necessary adhesion onto the substrate. Diamond's very high scratch resistance and thermal conductivity, combined with a lower coefficient of thermal expansion than Pyrex glass, a coefficient of friction close to that of Teflon (Polytetrafluoroethylene) and strong lipophilicity would make it a nearly ideal non-stick coating for cookware if large substrate areas could be coated economically.

The most important attribute of CVD diamond growth is the ability to control the properties of the diamond produced. In the area of diamond growth the word “diamond” is used as a description of any material primarily made up of sp3 bonded carbon, and there are many different types of diamond included in this. By regulating the processing parameters—especially the gases introduced, but also including the pressure the system is operated under, the temperature of the diamond, and the method of generating plasma—many different materials that can be considered diamond can be made. Single crystal diamond can be made containing various dopants^[3]. Polycrystalline diamond consisting of grain sizes from several nanometers to several micrometers can be grown^[4],[5]. Some polycrystalline diamond grains are surrounded by thin, non-diamond carbon, while others are not. These different factors affect the diamond’s hardness, smoothness, conductivity, optical properties and more.

Problems

There are several problems facing CVD diamond growth in the future. First, because research in the area is so heavily application oriented, there are basic questions which have had very little work done on them, and this continues to be a problem for the field. This problem is exacerbated by the fact that small changes in chemistry can require a great deal of research to understand. Another problem is that while CVD diamond growth occurs over large areas compared to other methods of diamond growth, these areas are still too small for some applications, such as large scale transistor manufacturing. There is no better method of producing semiconducting, doped diamond than CVD, but until large scale wafers can be efficiently produced CVD electronics will only have niche applications. CVD diamond growth has had historically low growth rates, usually a few micrometers an hour. While growth rates have been improved dramatically in a few very specific areas, in most applications they are still very slow. The biggest problem with CVD diamond growth is cost; cheaper alternatives are used instead of CVD diamonds whenever possible.

The Carnegie Institute's Geophysical Laboratory can produce 10 carat (2 g) single-crystal diamonds rapidly (28 nm/s) by CVD, as well as colorless single-crystal diamonds. Growth of colorless diamonds up to 60 g (300 carats) is believed achievable using their method.^[6]

Notes