Electronic Design

Microfabricated Multichannel System Detects, Analyzes Gases Quickly, Accurately

A complete miniature multichannel gas-phase detection and analysis system is under development at Sandia National Laboratories, Livermore, Calif. Using micromachined components, researchers have built a prototype two-board, two-channel system measuring just 8.5 by 5.3 cm. Also, they're not far from assembling the entire system in one package.

Scientists from Sandia's µChemLab program, which is developing the analyzer, say that it provides multichannel gas-phase analysis with fast response, enhanced versatility, and accurate chemical discrimination. The system consists of a sample preconcentrator, a gas-chromatograph separator, and a chemically sensitive surface acoustic-wave (SAW) detector array (Fig. 1).

The preconcentrator stage, which in essence is a microfabricated "hotplate," collects target analytes from the air stream over an extended period of time. It then releases them in a rapid and concentrated pulse, typically 200 ms of full width at half the maximum, in the gas-chromatograph column (Fig. 2). A chemically selective layer, usually a microporous oxide, adsorbs the analytes from the environment. After a sufficient amount of analytes is collected, an embedded platinum heater rapidly raises the preconcentrator's temperature to desorb the analytes. Concentration enhancement factors of over 100 have been achieved after 40 seconds of adsorption.

The heater is fabricated on a silicon-nitride (Si2Ni3) membrane that's suspended over a cavity etched through a silicon substrate. This membrane is 0.5 µm thick, and the cavity is 400 µm deep. Dry reactive-ion etching (DRIE) is used to make the cavity. Si2Ni3 was selected because of its low thermal mass and good thermal isolation, which are important for quickly heating the preconcentrator with a minimum amount of electric power. It takes the preconcentrator about 20 ms to reach a temperature of 200°C. Sustaining this steady-state temperature takes 105 mW.

The tiny gas-chromatograph channel provides temporal separation of the analytes and any interferants that may be collected by the preconcentrator stage. Like the preconcentrator, the gas-chromatograph channel also is made using DRIE. Because of the column's high aspect ratio and the anisotropy of the DRIE process, narrow and closely spaced gas-flow channels can be etched into the silicon substrate to a depth that's many times that of the channel's width. This leads to good performance in a small footprint while maintaining short transit times through the column. A typical column is a spiral that's 1 m long with channels that are 40 to 100 µm wide, separated by walls that are 20 to 40 µm wide and etched to a depth of 300 µm or more. Each column occupies 1 to 1.5 cm2 in area.

After etching the channels, the silicon substrate is thermally oxidized to produce a thin, glass-like layer on their surface. This allows for stationary phase deposition. Closed channels are produced by anodically bonding a Pyrex lid to the substrate's top surface. Microcapillary tubes are inserted through holes machined in the lid to provide gas interconnection

Once the channels are sealed, gas-chromatograph stationary phase materials are deposited on the walls. Conventional polymer solutions are used, as well as a sol-gel coating. The retention of the analytes in the stationary phase produces gaps between the arrivals of the analytes at the acoustic-wave detectors' array. This temporal separation provides an additional means of distinguishing analytes from one another and from interferants.

The system makes use of quartz SAW delay lines. A four-element quartz array is contained within each channel. A center-input interdigitated transducer (IDT) launches a SAW in both directions. Four smaller-output IDTs, two on either side of the input IDT, reconvert the acoustic wave into an electrical signal. Three of the four SAW delay lines are coated with different chemically sorbent materials, each optimized for the analytes of interest. The fourth delay line serves as a reference. Researchers designed and tested delay lines operating between 100 and 700 MHz. The delay line's frequency is determined by the spacing between adjacent IDT fingers and the acoustic velocity of the material.

Sandia hopes to integrate the detectors with drive and signal-conditioning electronics to improve the system's sensing performance. Researchers also aim to further simplify its packaging.

The system was presented at the Sensors Expo Conference, May 9-11, Anaheim, Calif. For more information, contact Jill Hruby at Sandia at (925) 294-2596 or at jmhruby@sandia.gov.

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