Optimization of the hot-filament chemical vapor deposition setup

Investigating the effect of filament placement and exploring the growth conditions for diamond synthesis

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The hot-filament chemical vapor deposition (HF-CVD) method is widely used for the synthesis of thin films of polycrystalline diamond. These films are used in a broad range of applications, such as coating material on cutting tools, heat spreaders, and electrochemical sensors. HF-CVD offers some unique advantages in terms of simplicity, low equipment cost, and scalability.

Recently, a novel HF-CVD system has been designed and built at dept. PME (TU Delft), which should enable diamond deposition over relatively large substrate areas (i.e., two-inch wafers) by employing an array of multiple straight metallic filaments. However, the use of this setup so far has been primarily restricted due to the premature failure of the tungsten filaments. During the high-temperature chemical vapor deposition process, the filaments undergo large deformations due to carburization, leading to the formation of brittle metal carbides. In the initial configuration, the filaments were placed in a custom-made clamping device, obstructing their ability to expand and contract freely, resulting in residual mechanical stresses that caused premature failure of the filaments after cooling down. To enable diamond growth in this setup, a new approach is needed.

In this research, different placements of the filaments within the existing clamping device were tested to determine the optimized placement that ensures a longer filament lifespan. Additionally, a preliminary experimental parameter study was performed to investigate the effects of different deposition parameters (i.e., filament-substrate distance, stage temperature, and methane concentration) on the diamond growth on Si (100) substrates.
The optimized filament placement was achieved by simply resting the tungsten filament on both electrodes without additional fixations. In this configuration, the filaments could be used for multiple interrupted deposition runs, and even after 162 hours of usage, the filaments remained intact. The average carburization time required to reach a constant filament power consumption and to initiate the thin-film growth of diamond was found to be 8 hours when using 0.5 vol.% methane and 5 hours when using 1.0 vol.%. As the carburization proceeded, the filaments elongated and exhibited a sagging effect. The growth experiments showed that dense polycrystalline diamond films can be synthesized in this setup with growth rates ranging from ~50 nm/h to 300 nm/h depending on the deposition parameters. As diamond growth is a temperature-driven process, the substrate temperature was found to have a particularly strong effect on the film growth rate in the investigated range of 360 to 780 °C. Finally, based on the experimental results, new designs for the clamping device are proposed to maintain straight filaments during the deposition process in future experiments.