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Materials

Figure 1: A smooth 1 micron thick nanocrystalline diamond film deposited via CVD onto a 100 mm silicon wafer

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routinely using plasma RIE tools. Figure 4 shows a process sequence that can be used to achieve a suspended membrane structure of diamond on a silicon wafer.

Metallization and electrodes

Compatibility with common metals that are used as electrical contacts is a must for diamond when used as an electronic material or a structural material for active MEMS applications. Most commonly used metals make ohmic contacts to diamond except for aluminum and gold, which make Schottky contacts with large-grained undoped microcrystalline diamond. Growing diamond onto traditional electrode heterostructures (e.g., Ti/Au/Ti) is challenging but can be accomplished with the use of diffusion barrier coatings such as TiN and TiCN. Tungsten can be used as an electrode provided thicker layers are used due to its higher electrical resistivity compared to gold.

Integration with advanced materials

Diamond can be directly grown onto many

different types of thin films, including SiO2, SiN, W, Ti, Mo, and TiN. Other materials, like Au, Cu, and ferrous alloys (e.g., steel), require the use of a carbide forming interlayer and/or diffusion barrier such as TiN and WC. High quality ferroelectric films can be integrated with diamond surfaces by the use of oxidation barrier coatings such at TiAl . Piezoelectric materials such as ZnO, AlN, and PZT films can be integrated with diamond by first using CMP to make the diamond average surface roughness less than 1 nm. Integration with advanced materials is important because it enables, for instance, surface acoustic wave (SAW) filters and RF MEMS resonators which leverage the extreme stiffness (i.e., high acoustic velocity) of diamond to make higher frequency filter and time references with superior performance for mobile RF applications.

Figure 2. RF MEMS AC Switch using diamond to achieve over 10 billion cycles in dry air

Wafer Bonding

Anodic, thermocompression, and glass-frit bonding have all been demonstrated with diamond, enabling a number of process flows that utilize handle wafers to integrate diamond with complex heterostructures not achievable using direct deposition techniques. Silicon-on- diamond can now be considered as an alternative material to silicon-on-insulator to engineer substrates with vastly superior thermal performance or low RF loss characteristics.

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