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© 2011 Evince Technology Limited, Netpark, Sedgefield, County Durham, UK, TS21 3FD
Single Crystal Diamond
For high voltage, high power applications the only form of diamond that will deliver the needed performance is synthetic single crystal diamond.
Most synthetic diamond manufactured today is made by replicating the high pressure and high temperatures that create natural diamond. Diamonds produced this way tends to find its way into tools for cutting and abrading other hard materials. However the best method of growth for electronics grade diamond is instead to grow it at a few percent of atmospheric pressure using an ionised soup of hydrocarbons and hydrogen.
The only way, currently, to grow good quality single crystal diamond is to use another diamond as a seed substrate - although there have been some interesting advances using iridum as the seed substrate but this is still early stage. Using implantation methods the seed substrate can be reused again and again. The seed substrate is heated to between 700ºC and 1000ºC in a mixture of a simple carbon based gas such as methane, ethane or ethelyne and hydrogen - all far less toxic than many of the gases used to produce other wide bandgap materials. When the gas mixture is ionised using a powerful microwave source diamond can be grown on top of the seed substrate. The more powerful the microwave source, the faster that diamond grows. Growth rates up 200µm per hour have been reported.
In the past 5 years there have been major advances in the production of very high quality low defect diamond substrates with defect densities of less than 102cm-2. Applications such as quantum computing are helping to drive this down even further. Unlike many of the other wide bandgap electronic materials, diamond has only one crystal type (i.e. it can only form one way!) and crystal defects do not tend to propogate through the material. Hence while ultimate performance may not be yet available, the quality of the electronics grade diamond that exists today is already sufficient for immediate device needs.
One of the received roadblocks for diamond has been limited availability of high quality substrate materials to date. The route to larger area substrates has been well known for over a decade. Only the absence of a major commercial application has deterred materials manufacturers from investing in the work needed to scale-up production of large area substrates.
How To Make Large Area Single Crystal Substrates
The route to larger area substrates is through a process known as "tiling". Today 100mm growth reactors are commercially available, so scaling to this area for single wafers can be accomplished relatively painlessly. For practical applications achieving high yield production does not require a 100% defect free substrate, so long as the wafer has a regular grid-iron pattern of high quality areas of low defect single crystal diamond.
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| A mosaic of carefully cut and polished single crystal diamonds are assembled so that they form a tightly packed array with a flat surface. The seed diamonds are usually sourced from commercially available high pressure high temperature (HTHP) or chemical vapour deposition (CVD) grown substrates. |
The mosaic is heated to between 700ºC and 1000ºC in a mixture methane or ethylene gas and hydrogen at around 1/10 of an atmosphere. A microwave energy source is use to ionise the gas into a plasma that liberates carbon from the gas that bind with the seed substrate to form new diamond. As the new diamond grows the film knits together at the tile boundaries to create a homogenous single crystal diamond substrate with few boundary defects. The thicker the growth layer the fewer the number of tiling defects. Layers as thick as 10mm and more have been successfully grown by several producers. |
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| The grown laminations of diamond film are separated from the tiled seed substrates by implanting a graphitic damage layer prior to starting the new growth. By cycling the implantation and growth stages, multiple free standing substrates can be produced in the same growth run. These substrates can then be either used for device production or the ones with the fewest edge defects can be used as starter tiles to create even larger substrates. |
This is not theory, it's reality with several growers now able to produce large area substrates. The picture below shows 25mm substrates being grown using the tiling method in a CVD reactor. Stepping up to larger areas is just a matter of using bigger tiles.
Single crystal diamond is not the only form of diamond that is available. Already much larger substrates are commercially available based on polycrystalline diamond. For more information read on ...