Nanocrystalline Diamond as a Dielectric for SOD Applications

Abstract

Nanocrystalline diamond (NCD) has been grown on oxide coated silicon wafers by microwave plasma assisted chemical vapour deposition using a novel seeding technique followed by optimised growth conditions, and leads to a highly-dense form of this material with grain sizes around 100nm for films approximately 1.5 microns thick. The electrical properties of these films have been investigated using Impedance Spectroscopy, which enables the contributions from sources characterised by differing capacitances, such as grain boundaries and grain interiors, to be isolated. After an initial acid clean the electrical properties of the film are not stable, and both grain boundaries and grains themselves contribute to the frequency dependant impedance values recorded. However, following mild oxidation grain boundary conduction is completely removed and the films become highly resistive (>1013 ohm/sq). This is most unusual, as conduction through NCD material is more normally dominated by grain boundary effects. Interestingly, the AC properties of these films are also excellent with a dielectric loss value (tan δ) as low as 0.002 for frequencies up to 10MHz. The dielectric properties of these NCD films are therefore as good as high quality free-standing (large grain) polycrystalline diamond films, and not too dissimilar to single crystal diamond, and are therefore ideally suited to future ‘silicon-on-diamond’ applications.

The CVD of Nanodiamond Materials

“…[50] A strategy employed to achieve high quality NCD is to seed a substrate with at least a monolayer of the smallest diamonds available (currently the explosively formed nanodiamond powder, ultradisperse diamond (UDD), [5,94,95] then grow homoepitaxially on these seeds under highquality diamond CVD conditions (very low carbon content in the reactants). [50] This high-quality NCD (as determined by optical, [57,96] electrical, [97,98] and mechanical properties [50] ) is grown in 600-1200 W microwave plasmas with 0.1-0.3% CH 4 in H 2 under 5-30 Torr pressure, and a substrate temperature between 400 and 900 8C. Membranes and structures as thin as 30 nm can be fabricated from these films by etching away the substrate or supporting materials.…”

“…[98] Two conduction mechanisms are observed and are attributed to conduction through the grains and along grain boundaries. [98] Both mechanisms display the characteristics of band conduction due to boron acceptor states.…”

“…[98] Two conduction mechanisms are observed and are attributed to conduction through the grains and along grain boundaries. [98] Both mechanisms display the characteristics of band conduction due to boron acceptor states. In the heavily doped films, hole concentrations of $10 20 cm À3 and mobilities of 0.2-0.5 cm 2 V À1 s À1 have been measured, along with a positive magnoresistance at low temperatures.…”

“…[97,130] Boron doping of NCD is achieved by the addition of gaseous boron compounds, such as diborane or trimethylborane, to the growth chemistry. The boron levels can range from <10 15 cm À3 to over 10 21 cm À3 [98,131] with conductivities up to 0.1-1 S m…”