CVD Diamond Synthesis

 

Chemical vapour deposition, CVD, is an extremely versatile process for creating thin solid films and is a generic term for a group of chemical processes that involve depositing a solid material from a gas or mixture of gases onto a substrate.  As well as its application in diamond synthesis, CVD processes are widely used in the fabrication of semiconductor devices and in the manufacture of coatings for wear parts and machine tool components, for example.

CVD processes for diamond synthesis can be used for the growth of both single crystal and polycrystalline forms of the material.  The actual conditions required for making each type of diamond are very similar; it is the choice of substrate that determines which will grow.  For single crystal CVD diamond, the substrate needs to be single crystal diamond and the new diamond film grows epitaxially, with the deposited film taking on the lattice structure and orientation identical to those of the substrate.  For polycrystalline diamond, a non-diamond substrate is used.  A variety of materials can be used in this context; primarily silicon, silicon carbide and a range of carbide forming metals including molybdenum and tungsten.

 

Growth conditions

In CVD diamond synthesis, the process generally takes place at below atmospheric pressure, typically between 1 and 200 Torr (a Torr is a non-SI unit of pressure defined as 1/760th of an atmosphere).  The starting materials, or precursors, for diamond synthesis are hydrogen and a hydrocarbon such as methane.  Only a tiny amount of hydrocarbon, often between about 1 and 5%, is normally present in the gas mix and provides the source of carbon from which the diamond is formed.

However, it is the hydrogen that plays a key role in the synthesis process.  Hydrogen needs to be present in the form of the "hydrogen radical", essentially a highly reactive hydrogen atom, in order to make the process work at all.  Under the conditions used for CVD diamond synthesis, the graphite form of carbon is the thermodynamically favoured phase in the bulk material (i.e. most energetically stable phase) rather than diamond.  Fortunately, hydrogen radicals are able to etch away any graphite forming on the substrate far faster, indeed ten to one hundred times faster, than they can remove any diamond.  Thus it is the kinetics of the process at the surface which enable diamond to be the only form of carbon left at the end of a synthesis run.  Another vital role of hydrogen is to terminate the dangling carbon bonds on the carbon atoms at the growing diamond surface, stabilising the surface and preventing it from reconstructing into a non-diamond form.  Without the effect of hydrogen, CVD diamond synthesis could not happen.

Making the hydrogen radicals is an energy-intensive process and providing the energy are hot filaments, arc jets and microwaves, even blowtorches, all of which can generate large numbers of hydrogen radicals.  Commercially hot filaments and microwaves are the most popular means of generating the hydrogen radicals for CVD diamond synthesis.

 

Practical synthesis

Sadly possessing a blow torch or microwave oven of itself is no guarantee of making diamonds consistently or cost-effectively.  There are a number of conditions that need to be satisfied in order to ensure successful diamond synthesis.  At Element Six, a microwave plasma-enhanced CVD process is the main diamond synthesis route that has been developed and refined.  In this system, the substrate sits within a reaction chamber through which the reaction gases flow.  Within the cavity, the microwaves excite the hydrogen-hydrocarbon gas mixture to form a plasma, providing the source of the hydrogen radicals, slightly above substrate.  To ensure good quality diamond growth, the substrate is normally maintained at a temperature of between about 700°C and 1,200°C.

 

Nucleation and growth

To initiate quickly the formation of diamond nuclei or nucleation from which the diamond film grows, scientists realised that the presence of diamond on the substrate was advantageous.  For polycrystalline diamond where other materials form the substrate, the surface of the substrate is "seeded" or scratched with small particles of diamond to encourage nucleation to take place.  In this case growth is rather like saplings of different type of trees growing in a forest: in diamond different crystallographic directions have different growth rates (like different types of tree) with those growing fastest winning out, thus the layer will evolve so that the vast majority of grains present are oriented with a particular crystallographic direction more-or-less parallel to the growth. For single crystal diamond growth, the substrate has to be diamond, so the conditions for diamond growth are already met.  In this case, diamond grows by "step-flow growth" where the diamond layers are built up like waves running over the surface of a pond.

For both single crystal and polycrystalline CVD diamond, the as-grown diamond layer normally requires some kind of post-growth processing to make it into a useful object.  This can range from simply cutting out a plate having specific lateral dimensions for use an electrochemical electrode to polishing to a precise thickness and finish to make an infrared optical window. A key property of diamond is its hardness and this means that processing diamond, in some instances is as demanding as growing it.