UNCD®, motivated by the Ultrananocrystalline Diamond material originally developed at Argonne National Laboratory, has been extended by ADT to a family of diamond materials whichfinally enables diamond as a true engineering material.
A key characteristic of all UNCD films is its phase purity. None of the UNCD films consists of a mixture of diamond and graphitic phases, although UNCD Aqua 25 has volume fractions of grain boundaries that amount to 10% of the film by volume and consist of carbon in several different bonding states including sp2 (graphitic). UNCD films are not diamond-like carbons or "NCD" (for nanocrystalline diamond) films reported in the literature. It is this unique structural property of UNCD films that allows attributes such as optical transparency, electrical and thermal conductivity, and film stress to be controlled in ways not possible with other materials.
ADT’s techniques also allow the structure-properties of UNCD to be tuned. This is done by changing the growth process to increase the grain size and to introduce select impurities (dopants like boron, nitrogen, etc.) to engineer UNCD's electrical, thermal and optical properties. Although the typical growth temperatures are 700-800 °C, UNCD films can be deposited at much lower temperatures, down to 350 °C. UNCD can be grown on a wide variety of substrates, including Si, SiO2 (thin oxide films, quartz, Corning glass), refractory metals (Ti, Ta, W, Mo), as well as SiC and SiN. Continuous films can be grown to render the substrate impervious to chemical attack from strong acid solutions such as HF and HNO3. Thanks to recent advances in deposition technology, UNCD films can be routinely deposited onto silicon wafers up to 200 mm in size.
Below are some of the material properties of UNCD that enable its unique characteristics.
- Mechanical Properties. Most UNCD films are essentially equivalent to natural diamond in hardness, Young’s modulus, fracture toughness and strength.
- Tribological Properties. When smooth, UNCD exhibits friction coefficients as low as 0.03 in air. This, combined with the extreme hardness and good adhesion properties, allows for a wide range of applications of UNCD as a tribomechanical coating.
- Transport Properties. By altering the deposition process, the electrical conductivity of UNCD films can be changed over eight orders of magnitude. UNCD Brown films exhibit the highest N-type conductivity reported for a phase-pure diamond film and are more conductive than any doped microcrystalline diamond film or diamond-like carbon film.
- Electron Field Emission Properties. UNCD films consistently exhibit very low threshold fields for field electron emission. In addition, the field electron emission is very stable even when the surface is exposed to 10-4 Torr of oxygen or hydrogen. Emission currents as high as 100 µA from a single UNCD-coated silicon microtip have been observed. Emission currents as high as 1 mA have been achieved from conformally-coated arrays of silicon microtips.