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Polycrystalline Diamond
Other forms of diamond are available that offer potential for electronics use. Because these forms are made up of thousands, or more, of individual crystals of diamond the performance of polycrystalline diamond is compromised from that of perfect single crystal diamond. However in most cases the properties of polycrystal diamond are still superior to that of most other semiconductor materials.
Polycrystalline diamond is cheaper to produce than single crystal diamond and there are already established methods to grow these types of material on large carrier substrates. It can be grown using either a CVD process like that used for single crystal diamond or using a hot filament method (which involves placing the growth substrate under what is effectively a giant light bulb at low pressure). Through careful control of the growth process, polycrystalline diamond can be grown to exhibit near intrinsic properties or be capable of being electrically conductive.
Today 200mm substrates with a diamond film on them are already available, which makes this form of diamond compatible with modern production level semiconductor fabrication systems. It is being increasingly used as a way of manufacturing a very efficient heat spreader for silicon and gallium arsenide devices and, chances are your modern pda type mobile phone already contains a tiny amount of polycrystal diamond.
Going beyond the thermal use of diamond, polycrystalline diamond is now being widely explored for MEMs, high-frequency, high temperature and radiation hard device applications.
There are 3 classifications of polycrystalline diamond:
growth direction |
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type: | Polycrystal diamond | Nanocrystalline diamond | Ultra-nanocrystalline diamond |
crystal size: | 1-100µm grain size | 10-200nm grain size | 1-10nm grain size |
cross section: |
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seed substrate: | Usually grown on a refractive metal substrate such as tungsten which does not chemically bond to the diamond | Uses a dispersion of nano-diamond powder as the seeding material therefore can be grown on a variety of substrates including other semiconductors and metals | Uses a dispersion of nano-diamond powder as the seeding material therefore can be grown on a variety of substrates including other semiconductors and metals |
growth method: | CVD (high grade), hot filament (thermal grade) | CVD (high grade), hot filament (thermal or mechanical grade) | CVD |
availability: | 100mm free standing near intrinsic substrates (for electronic applications) already commercially available | 200mm diamond on silicon wafers typically 1-5µm thickness commercially available - resistivity selectable from conductive to insulating | 100mm diamond on silicon wafers typically 0.5-2µm thickness commercially available - resistivity selectable from conductive to insulating |
What's The Difference Between Nanocrystalline (NCD) and Ultra-Nanocrystalline (UNCD) Diamond ?
The two forms of nanocrystalline diamond stem from the same starting point, with the application of a single layer dispersion of nanocrystalline diamond power on a substrate. NCD films tend to consist of closely packed grass-like long thin diamond crystals that are aligned with the growth axis. However for UNCD a noble gas (usually argon) is also added to the normal hydrogen/ hydrocarbon growth mix. This has the effect of breaking up the diamond crystals along the growth axis forcing the diamond to instead create a densely packed film of nano-sized diamond crystals.
The two types of materials therefore offer similar but customisable performance depending on the specific application in mind.