James Birrell

Low temperature growth of ultrananocrystalline diamond

Ultrananocrystalline diamond (UNCD) films were prepared by microwave plasma chemical vapor deposition using argon-rich Ar∕CH4 plasmas at substrate temperatures from ∼400 to 800°C. Different seeding processes were employed to enhance the initial nucleation density for UNCD growth to about 1011sites∕cm2. High-resolution transmission electron microscopy, near-edge x-ray absorption fine structure, visible and ultraviolet Raman spectroscopy, and scanning electron microscopy were used to study the bonding structure as a function of growth temperature. The results showed that the growth of UNCD films is much less dependent on substrate temperature than for hydrogen-based CH4∕H2 plasmas. UNCD with nearly the same nanoscale structure as those characteristic of high-temperature deposition can be grown at temperatures as low as 400°C with growth rates of about 0.2μm∕hr. The average grain size increased to about 8nm from 3 to 5nm that is characteristic of high-temperature growth, but the relative amounts of sp3 and s...

Materials science and fabrication processes for a new MEMS technology based on ultrananocrystalline diamond thin films

Most MEMS devices are currently based on silicon because of the available surface machining technology. However, Si has poor mechanical and tribological properties which makes it difficult to produce high performance Si based MEMS devices that could work reliably, particularly in harsh environments; diamond, as a superhard material with high mechanical strength, exceptional chemical inertness, outstanding thermal stability and superior tribological performance, could be an ideal material for MEMS. A key challenge for diamond MEMS is the integration of diamond films with other materials. Conventional CVD thin film deposition methods produce diamond films with large grains, high internal stress, poor intergranular adhesion and very rough surfaces, and are consequently ill-suited for MEMS applications. Diamond-like films offer an alternative, but are deposited using physical vapour deposition methods unsuitable for conformal deposition on high aspect ratio features, and generally they do not exhibit the outstanding mechanical properties of diamond. We describe a new ultrananocrystalline diamond (UNCD) film technology based on a microwave plasma technique using argon plasma chemistries that produce UNCD films with morphological and mechanical properties that are ideally suited for producing reliable MEMS devices. We have developed lithographic techniques for the fabrication of UNCD MEMS components, including cantilevers and multilevel devices, acting as precursors to micro-bearings and gears, making UNCD a promising material for the development of high performance MEMS devices. We also review the mechanical, tribological, electronic transport, chemical and biocompatibility properties of UNCD, which make this an ideal material for reliable, long endurance MEMS device use.

Atomic layer deposition for the conformal coating of nanoporous materials

Atomic layer deposition (ALD) is ideal for applying precise and conformal coatings over nanoporous materials. We have recently used ALD to coat two nanoporous solids: anodic aluminum oxide (AAO) and silica aerogels. AAO possesses hexagonally ordered pores with diameters d ∼ 40nm and pore length L ∼ 70 microns. The AAO membranes were coated by ALD to fabricate catalytic membranes that demonstrate remarkable selectivity in the oxidative dehydrogenation of cyclohexane. Additional AAO membranes coated with ALD Pd films show promise as hydrogen sensors. Silica aerogels have the lowest density and highest surface area of any solid material. Consequently, these materials serve as an excellent substrate to fabricate novel catalytic materials and gas sensors by ALD.

Electrical contacts to ultrananocrystalline diamond

The contact behavior of various metals on n-type nitrogen-doped ultrananocrystalline diamond (UNCD) thin films has been investigated. The influences of the following parameters on the current-voltage characteristics of the contacts are presented: (1) electronegativity and work function of various metals, (2) an oxidizing acid surface cleaning step, and (3) oxide formation at the film/contact interface. Near-ideal ohmic contacts are formed in every case, while Schottky barrier contacts prove more elusive. These results counter most work discussed to date on thin diamond films, and are discussed in the context of the unique grain-boundary conductivity mechanism of the nitrogen-doped UNCD.

Tribochemistry and material transfer for the ultrananocrystalline diamond-silicon nitride interface revealed by x-ray photoelectron emission spectromicroscopy

The authors report tribochemical changes due to sliding of a silicon nitride (Si3N4) ball against an ultrananocrystalline diamond (UNCD) thin film. Unidirectional sliding wear measurements were conducted for 2000cycles using a ball-on-disk apparatus with a 3∕16in. diameter Si3N4 ball at a sliding speed of 3.3mm∕s and a normal load of 98.0mN (nominal Hertzian stress of 0.6GPa) in a nitrogen environment at 50% relative humidity at room temperature. The wear track produced on the UNCD film was analyzed by X-ray photoelectron emission spectromicroscopy (X-PEEM) combined with X-ray absorption near-edge structure (XANES) spectroscopy to identify and spatially resolve chemical changes inside the wear track, particularly rehybridization of carbon. XANES spectra show that SiOx complexes are deposited within the wear track. Very little rehybridization of the UNCD from its primarily sp3 bonding configuration to sp2 bonding is observed, and there is no observable oxidation of the UNCD, pointing to the impressive stab...

Investigating the role of hydrogen in ultra-nanocrystalline diamond thin film growth

Hydrogen has long been known to be critical for the growth of high-quality microcrystalline diamond thin films as well as homoepitaxial single-crystal diamond. A hydrogen-poor growth process that results in ultra-nanocrystalline diamond thin films has also been developed, and it has been theorized that diamond growth with this gas chemistry can occur in the absence of hydrogen. This study investigates the role of hydrogen in the growth of ultra-nanocrystalline diamond thin films in two different regimes. First, we add hydrogen to the gas phase during growth, and observe that there seems to be a competitive growth process occurring between microcrystalline diamond and ultra-nanocrystalline diamond, rather than a simple increase in the grain size of ultra-nanocrystalline diamond. Second, we remove hydrogen from the plasma by changing the hydrocarbon precursor from methane to acetylene and observe that there does seem to be some sort of lower limit to the amount of hydrogen that can sustain ultra-nanocrystalline diamond growth. We speculate that this is due to the amount of hydrogen needed to stabilize the surface of the growing diamond nanocrystals.

Mechanical Properties of Ultrananocrystalline Diamond Thin Films for MEMS Applications

Microcantilever deflection and the membrane deflection experiment (MDE) were used to examine the elastic and fracture properties of ultrananocrystalline diamond (UNCD) thin films in relation to their application to microelectromechanical systems (MEMS). Freestanding microcantilevers and membranes were fabricated using standard MEMS fabrication techniques adapted to our UNCD film technology. Elastic moduli measured by both methods described above are in agreement, with the values being in the range 930 and 970 GPa with both techniques showing good reproducibility. The MDE test showed fracture strength to vary from 3.95 to 5.03 GPa when seeding was performed with ultrasonic agitation of nanosized particles.

Characterization of low-temperature Ultrananocrystalline/spl trade/ Diamond RF MEMS resonators

For the first time working MEMS resonators have been produced using low-temperature deposited (550/spl deg/ C) Ultrananocrystalline/spl trade/ Diamond (UNCD/spl trade/) films. Using a lumped-element model to fit experimental data, UNCD materials properties such as a Youngs modulus of 710 GPa and an acoustic velocity of 14,243 m/s have been deduced. This is the highest acoustic velocity measured to date for a diamond MEMS structural layer deposited at low temperatures. A 10 MHz resonator shows a DC-tunability of the resonance frequency of 15% between 15 and 25 V and the breakdown voltage behavior shows electrostatic breakdown rather than electro-mechanical pull-down for higher frequency devices. Good resonant frequency reproducibility is observed when cycling the resonators over bias voltages from 15 to 25 V and over RF power levels of -10 to 10 dBm.