Publications

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Present methods of air sampling for low concentrations of chemicals like explosives and bioagents involve noisy and power hungry collectors with mechanical parts for moving large volumes of air. However there are biological systems that are capable of detecting very low concentrations of molecules with no mechanical moving parts. An example is the silkworm moth antenna which is a highly branched structure where each of 100 branches contains about 200 sensory 'hairs' which have dimensions of 2 microns wide by 100 microns long. The hairs contain about 3000 pores which is where the gas phase molecules enter the aqueous (lymph) phase for detection. Simulations of diffusion of molecules indicate that this 'forest' of hairs is 'designed' to maximize the extraction of the vapor phase molecules. Since typical molecules lose about 4 decades in diffusion constant upon entering the liquid phase, it is important to allow air diffusion to bring the molecule as close to the 'sensor' as possible. The moth acts on concentrations as low as 1000 molecules per cubic cm. (one part in 1e16). A 3-D collection system of these dimensions could be fabricated by micromachining techniques available at Sandia. This LDRD addresses the issues involved with extracting molecules from air onto micromachined structures and then delivering those molecules to microsensors for detection.

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This LDRD is aimed to place Sandia at the forefront of GaN-based technologies. Two important themes of this LDRD are: (1) The demonstration of novel GaN-based devices which have not yet been much explored and yet are coherent with Sandia's and DOE's mission objectives. UV optoelectronic and piezoelectric devices are just two examples. (2) To demonstrate front-end monolithic integration of GaN with Si-based microelectronics. Key issues pertinent to the successful completion of this LDRD have been identified to be (1) The growth and defect control of AlGaN and GaN, and (2) strain relief during/after the heteroepitaxy of GaN on Si and the separation/transfer of GaN layers to different wafer templates.

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A four-channel surface acoustic wave (SAW) chemical sensor array with associated RF electronics is monolithically integrated onto one GaAs IC. The sensor operates at 690 MHz from an on-chip SAW based oscillator and provides simple DC voltage outputs by using integrated phase detectors. This sensor array represents a significant advance in microsensor technology offering miniaturization, increased chemical selectivity, simplified system assembly, improved sensitivity, and inherent temperature compensation.

Monolithic, integrated acoustic wave chemical microsensors are being developed on gallium arsenide (GaAs) substrates. With this approach, arrays of microsensors and the high frequency electronic components needed to operate them reside on a single substrate, increasing the range of detectable analytes, reducing overall system size, minimizing systematic errors, and simplifying assembly and packaging. GaAs is employed because it is both piezoelectric, a property required to produce the acoustic wave devices, and a semiconductor with a mature microelectronics fabrication technology. Many aspects of integrated GaAs chemical sensors have been investigated, including: surface acoustic wave (SAW) sensors; monolithic SAW delay line oscillators; GaAs application specific integrated circuits (ASIC) for sensor operation; a hybrid sensor array utilizing these ASICS; and the fully monolithic, integrated SAW array. Details of the design, fabrication, and performance of these devices are discussed. In addition, the ability to produce heteroepitaxial layers of GaAs and aluminum gallium arsenide (AlGaAs) makes possible micromachined membrane sensors with improved sensitivity compared to conventional SAW sensors. Micromachining techniques for fabricating flexural plate wave (FPW) and thickness shear mode (TSM) microsensors on thin GaAs membranes are presented and GaAs FPW delay line and TSM resonator performance is described.