Inductively coupled plasma (ICP) etch rates for GaN are reported as a function of plasma pressure, plasma chemistry, rf power, and ICP power. Using a Cl2/H2/Ar plasma chemistry, GaN etch rates as high as 6875 Å/min are reported. The GaN surface morphology remains smooth over a wide range of plasma conditions as quantified using atomic force microscopy. Several etch conditions yield highly anisotropic profiles with smooth sidewalls. These results have direct application to the fabrication of group-III nitride etched laser facets.

Using both wet and plasma etching, we have fabricated micro-channels in silicon substrates suitable for use as gas chromatography (GC) columns. Micro-channel dimensions range from 10 to 80 pm wide, 200 to 400 p m deep, and 10 cm to 100 cm long. Micro-channels 100 cm long take up as little as 1 cm' on the substrate when fabricated with a high aspect ratio silicon etch (HARSE) process. Channels are sealed by anodically bonding Pyrex lids to the Si substrates. We have studied micro-channel flow characteristics to establish model parameters for system optimization. We have also coated these micro-channels with stationary phases and demonstrated GC separations. We believe separation performance can be improved by increasing stationary phase coating uniformity through micro-channel surface treatment prior to stationary phase deposition. To this end, we have developed microfabrication techniques to etch through silicon wafers using the HARSE process. Etching completely through the Si substrate facilitates the treatment and characterization of the micro-channel sidewalls, which dominate the GC physico-chemical interaction. With this approach. we separately treat the Pyrex lid surfaces that form the top and bottom surfaces of the GC flow channel.

The soluble polydiacetylenes' -3) provide a unique system for the study of rod-like polymers. Because of the conjugated chemical structure, the electronic states are strongly coupled to the conformation of the polymer backbone; conformational changes are known to cause striking color changes in these polymer^^.^). The solubility is especially important, because it permits studies at concentrations ranging from the very dilute to the melt. For example, this solubility was crucial for a recent light scattering study" of very dilute polydiacetylene solutions that identified a single chain, rod-to-coil transition coincident with the red to yellow color change; the first example of this kind of transition in a conjugated polymer. The macroscopic properties of polydiacetylene in the melt and in the solid state have also been investigated5s7). These condensed phases often exhibit optical anisotropy which is not unexpected for systems of aligned rod-like polymers8). In addition, the same single chain transition found in dilute solutions has been identified in gels and in the melt 5*7, 9).Recent observations suggest a coupling between the single polymer chain conformation and the macroscopic state of the polydiacetylene. Plachetta et al. ' ) have noted the disappearance of the optical anisotropy coincident with the color change (indicative of the rod-coil transition) in the melt. In our laboratory, a gel-sol transition has been observed9) in a semidilute solution coincident with the rod-coil transition.In this communication, we report observations of a reversible anisotropic-isotropic transition in an aligned film of polydiacetylene 4BCMUa) also coincident with the rod-coil transition. The transition temperature is intermediate between the rod-coil transition temperature for a dilute solution of polydiacetylene 4BCMU and a single crystal of the solid. We relate these results to the concept of induced rigidity as put forward in the mean field model of nematic polymers developed by Pincus et al. lo -12). The anisotropic absorption of the aligned films is used to spectroscopically characterize the fully extended polydiacetylene 4BCMU conformation. Comparison of these results with solution spectra demonstrates that in the red solution phase the polymer is fully extended. a) Systematic IUPAC name of 4BCMU: dibutyl 4,19-dioxo-5,18-dioxa-3,20-diaza-10,12docosadiynedioate.

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Chemiresistorsare fabricatedfrom materialsthat changetheir electricalresistancewhen exposedto certain chemical species.Composites of soluble polymers with metallic particles have shown remarkable sensitivity to many volatile organic chemicals, depending on the ability of the rmalyte molecules to swell the polymer matrix. These sensors can be made extremely small (< 100 square microns), operate at ambient temperatures, and require almost no power to read-out. However, the chemiresistor itself is only a part of a more complex sensor system that delivers chemical information to a user who can act on the information. We present the design, fabrication and performance of a chemiresistor array chip with four different chemiresistor materials, heaters and a temperature sensor. We also show the design and fabrication of an integrated chemiresistor array, where the electronics to read-out the chemiresistors is on the same chip with the electrodes for the chemiresistors. The circuit was designed to perform several fi,mctions to make the sensor data more usefi,d. This low-power, integrated chemiresistor array is small enough to be deployed on a Sandia-developed microrobot platform.