High-Pressure Microdischarges as Microreactors for Materials Applications

Citation

Sankaran, Ramanathan Mohan (2004) High-Pressure Microdischarges as Microreactors for Materials Applications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/DP21-7606. https://resolver.caltech.edu/CaltechETD:etd-06012004-152711

Abstract

This thesis describes a novel "microplasma" source that is suitable for microreactor applications. The high-pressure "microplasma" is a direct current microdischarge, formed between two metal electrodes: a cathode with a pin-hole (diameter~100 µm) and an anode of unspecified shape. Strong radial electric fields are produced in the microhollow cathode geometry, causing electrons to oscillate (Pendel effect). As a result of enhanced ionization processes, it is possible to produce a stable high-intensity discharge at pressures of 1 atmosphere or higher. We have utilized these microdischarges for several applications including pattern transfer, diamond deposition, excimer emission, and nanoparticle synthesis.

Maskless etching of structures for MEMS and microfluidic applications is achieved by forming microplasmas in complex patterns within a stencil mask in contact with the surface to be patterned. In one application, CF₄/Ar microdischarges are formed inside planar copper-polyimide stencil masks that permit direct etching of silicon wafers. To deposit films, microdischarges are formed inside metal capillaries that accommodate flow. A plasma microjet is created and directed towards a heated substrate that enables deposition of polycrystalline diamond films over small spatial scales. Simultaneous operation of multiple micodischarges could form the basis of a combinatorial tool for rapid materials development.

Microdischarges in capillary tubes can serve as sources of intense UV radiation. Excimer emission in argon (Ar₂^*) at 128 nm has been studied by vacuum UV spectroscopy. A method to increase the on-axis excimer radiation is demonstrated by adding discharges in a linear array. Building of emitting volume shows potential for intensifying excimer emission.

The properties of microdischarges are especially conducive to applications as short-residence time reactors. For example, microdischarges are attractive for nanoparticle synthesis since the residence time of particle nucleation can be limited in the reactor to time scales on the order of milliseconds. This methodology has been used to produce silicon particles with mean sizes of 1-5 nm and narrow size distributions. The silicon nanoparticles exhibit blue photoluminescence at room temperature suggesting quantum confinement. The pristine nature of the nanoparticles enables fundamental studies of surface functionalization.

Thesis Files