Green Chemistry: Environmental applications of plasma chemistry

Detection Limits and Decomposition Mechanisms for Organic Contaminants in Water Using Optical Emission Spectroscopy

Well water may contain a variety of potential contaminants. Those associated with fuel oxygenate additives such as methyl tert-butyl ether (MTBE) are of significant concern as they partition into the aqueous phase. Because MTBE is now ubiquitous in water sources worldwide, development of methods for both detection and decontamination have been the objective of many scientific studies. In the Fisher group, we are interested in the application of inductively coupled rf plasmas in the removal of the organic contaminants CH3OH and MTBE from artificially contaminated water. Optical emission spectroscopy (OES) is central to this research as it has the potential for quantitative determination of organic contaminant concentration. As contaminated water vapor is fed into the plasma, fragmented species become excited and emit light as they relax back down to the ground state. The wavelengths of the emitted light are characteristic of the molecule or molecular fragments they came from, as indicated in the MTBE/H2O plasma emission spectrum shown at left. In some cases, the intensity of the emitted light can be correlated to the concentration of organic species in the original water sample through actinometry.

We can currently detect organic species in wastewater by the emission peaks of CO* (marked with blue stars in the OES spectrum) at concentrations as low as 0.01 ppm for both CH3OH and MTBE. The figure at right shows that at different contaminant concentrations, the intensity of the CO* peak varies predictably with applied rf power, suggesting the potential for quantifying contaminant conncetration in these systems. In addition to providing information about the concentration of gas phase species, the presence CO and CH lines in the OES spectra suggests that the parent organic species is being broken down in the plasma. This has implications in the abatement of organic contaminants in water. Mass spectrometric (MS) data were also collected in both the H2O/CH3OH and H2O/MTBE plasma systems, and the results are consistent with decomposition mechanisms implicated in OES spectra.

Chemical Process of Etching Microelectronic Materials with CH3OH

In the 1960’s, plasma etching replaced solution-based etching in the production of microchips, integrated circuits, and solid state devices. Some of the advantages of plasmas over solution-based etching include a significant decrease in waste production, better cost effectiveness, ease of byproduct treatment, and accessible to a production line. Today the primary gases used to etch substrates are fluorocarbons and sulfur fluorides, both of which can be harmful to humans and the environment. For example, fluorine gases produce SiF4 and F2 as byproducts when used in the plasma processing of silicon. All things considered, a safer alternative for the plasma etching of silicon substrates would be extremely valuable from an environmental perspective.

Presently, we are exploring the use of CH3OH, which is both clean and relatively inexpensive, as an alternative plasma etchant for silicon substrates. The carbon and oxygen content of CH3OH show potential to not only etch a silicon wafer but also to form a protective polymer layer on the etched walls of the substrate. This passivating layer prevents undercutting, which is one of the largest problems with etching processes. Based on these properties CH3OH shows promise as a potential plasma etchant.