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Nano-diamond building blocks for micro-device applications

Final Report Summary - NANODIA (Nano-diamond building blocks for micro-device applications)

Nano-diamond thin films can solve actual reliability problems in micro-electromechanical systems that are associated with contacting surfaces in relative motion and harsh operation environments that lead frequently to wear and damage of system components. Synthetic diamond is attractive due to the unique combination of material properties such as extremely high hardness and Young’s modulus, high chemical inertness, and very high thermal conductivity. The aim of research project NANODIA is the growth of nano-diamond building blocks with well-controlled surface roughness and film properties for micro-devices. The mechanisms of nucleation and growth occurring during chemical vapor deposition of nano-diamond are investigated experimentally and theoretically, and the tribomechanical behavior of nano-diamond is studied to unravel friction and wear processes at the micro scale.

The first and second objectives of the NANODIA project are to enhance the nucleation of nano-diamond to achieve a uniform and highly dense nucleation and to study the effectiveness of seed layers in promoting the growth of nano-crystalline diamond (NCD) thin films.
The synthesis of smooth NCD thin films with different microstructure and phase purity has been carried by hot-filament chemical vapor deposition by a systematic variation of the seeding density and the CH4/H2 ratio of the reactive gases. The enhancement of the nucleation and subsequent growth of NCD films with a submicron thickness control on mirror-polished silicon (100) substrates has been demonstrated by using an RF sputter deposition of six different metallic (Cr, Mo, Nb, Ti, V, and W) seed nanolayers (thickness about 90 nm) in a Veeco Nexus 800 sputter deposition tool. Seeding by nano-sized (<10 nm) diamond particles was done by ultrasonication in alcoholic dispersions. Deposition of nano-diamond was performed at a substrate temperature of 650 °C, filament temperature of 2,200 °C, system pressure of 15 mbar, and total gas flow of 304.5 SCCM. The methane and hydrogen gas flows were fixed at 7.5 SCCM and 297 SCCM, respectively. It was found that the number density of nanodiamond particles embedded on the nanorough metallic surfaces after an ultrasonic seeding step together with the dynamic surface chemistry during hot-filament chemical vapor deposition of diamond determine the nucleation kinetics, microstructure and surface topography of the NCD films. Overall, the smoothest NCD layers (surface roughness 10 nm) are obtained with the highest seed density of diamond nanoparticles (about 3.5×10E10 cm-2) anchored to the metallic (W) surface. In particular, the rapid carbide-forming metals Mo, Nb and W show the highest number density of diamond crystallites formed during the NCD nucleation stage, which results in dense, uniform and very smooth NCD films. Much rougher NCD films (17-37 nm) are obtained on the Cr, Ti, and V nanolayers that do not form carbides rapidly. Importantly, the carbon phase purity of the grown NCD films remains unaffected by the presence of different metallic seed nanolayers. Furthermore, the metallic nanolayer surface morphology (film roughness in the range 0.5-1.5 nm) does not play a relevant role in the enhancement of the seeding step and thus seeding by diamond nanoparticles is mainly determined by the surface chemistry of the substrate and diamond nanoparticles.

The third project objective is to investigate the growth dynamics of different nano-diamond film deposition processes to get a scientific insight into the evolution of the film surface morphology and roughness with deposition time.
The growth dynamics of nano-diamond and diamond-like carbon films grown by hot-filament and plasma enhanced chemical vapor deposition processes have been investigated by analyzing the film morphology evolution by using atomic force microscopy. The NCD films grown from CH4/H2 gas mixtures of 1.5% and 2.0% at a substrate temperature of 800 degrees C show the same cauliflower-like surface morphology defined by large aggregates of nanogranular structures that are smaller for the higher CH4 content. The roughness correlation that takes place below the micrometer scale proves to be related with the surface fluctuations induced by the nanogranular structure, which results from the gas mixture-dependent renucleation processes. The large (micro-sized) cauliflower-like aggregate structures coarsen with deposition time in a specific manner which is controlled by the very low sticking probability of the main growth species on the NCD growth surface. Both sets of NCD films share the same asymptotical growth regime obeying the Edwards Wilkinson scaling properties, characterized by the absence of roughening. In particular, the cauliflower-like growth pattern is a type of fractal pattern that is ubiquitous and can be spotted in numerous (non-)living systems. By tailoring the chemical vapor deposition growth process, thin films of diamond-like carbon were grown into shapes similar to those seen on a cauliflower, but limited to the submicron scales. In a joint experimental/theoretical research collaboration with EU researchers from Comillas Pontifical and Carlos III Universities (ES), Instituto de Ciencia de Materiales-CSIC (ES), and École Polytechnique (FR), for the first time, the laws that govern how these intricate surface patterns form and develop over time have been described.

The fourth and fifth objectives of the NANODIA project are devoted to the study of the tribomechanical behavior of nano-diamond in sliding micro-contacts and to implement nano-diamond building blocks in industrial practice, and their evaluation.
Related to the development of NCD-based micro-systems with moving mechanical elements, a key role in the success of high resolution Scanning Spreading Resistance Microscopy is played by the full diamond AFM probes recently developed at Imec (Interuniversity Microelectronics Centre), Leuven. Scientific insight into the wear resistance and mechanism as well as hardness of B-doped thin films and nanoscopic tips made thereof is needed for being able to manufacture high-performing B-doped tips for nanoscale probing applications. Together with researchers from Imec and Tallinn University of Technology (Estonia), the friction and wear behavior of boron-doped hot filament chemical vapor deposited diamond films has been studied. Fretting sliding tests at the macro scale have thus been performed on diamond samples with different film thickness and varying boron doping levels. A direct comparison of the tribological performance of boron-doped 1-μm and 200-nm thick NCD films and 1-μm thick microcrystalline diamond (MCD) films sliding against a variety of ceramic and diamond counterfaces was carried out. It was first quantitatively confirmed that an increase in the boron doping level drastically influences the wear rate. The latter increases about four times as the boron doping is increased from background doped to 2.8 at.% for NCD, and 0.6 at.% for MCD. Also, a clear trend in the running-in period of the COF as a function of the boron-doping has been observed and nanoindentation measurements show that higher boron doping levels lead to softer NCD films.