(200b) Plasma Coating of Submicron Particles and the Fabrication of Core-Shell Nanostructures for Low-K Elecgtronics

The standard dielectric material for microelectronic devices is SiO₂ with a dielectric constant of 4. The continuous decrease in the size of integrated circuits is fueling the need for materials with lower dielectric constant to improve propagation delays. While the electric properties (dielectric constant, dissipation factor, leakage current) are essential properties, a suitable dielectric must satisfy additional requirements posed by performance and process constraints, including good mechanical and thermal properties (strength, low stresses, stability up to 400 C) good adhesion and compatibility with other materials, low moisture absorption, etchability and integration with current processing practice. A class of materials that has shown promise is based on amorphous hydrogenated carbon produced by plasma assisted deposition. These solids, known as amorphous hydrogenated carbon (a-C:H), diamond-like carbon (DLC) or plasma polymers, consist of a cross-linked network of carbon with variable amounts of hydrogen bonded via a mixture of sp³ (diamond), sp² (graphite), and some sp¹, type C bonding. These materials are characterized by enhanced mechanical and physical properties resulting in excellent resistance to chemical attack, high hardness, low dielectric constant, low friction and variable hydrophobic/oleophobic character.

In this study we are utilizing a low-pressure plasma reactor to produce core-shell nanostructures with a shell consisting of a-C:H and a core that is hollow. The build-in porosity of these materials makes them attractive candidates for low-k applications avoiding the need for a thermal treatment step, as required for organic/inorganic porogens. Particles produced by this method are spherical, submicron in diameter, and contain a hollow center. Sixteen organic molecules were tested for the ability to produce hollow particles. Of these, 12 were found to reproducibly result in the formation of hollow particles while the other 4 either produced non-hollow particles or no particles at all under the conditions investigated here. Cyclohexane derivatives and the presence of an aromatic ring correlate strongly with a tendency to form hollow centers. The particle sizes vary from 70 nm to nearly 1 micron and the core diameter is on the average 18% of the particle diameter. The corresponding shell thickness ranges from 24 nm for styrene to about 360 nm for cyclohexane. We discuss the formation of the core via a liquid-to solid transition and present physical and chemical characterizations of these materials.