AVS 49th International Symposium
    Microelectromechanical Systems (MEMS) Thursday Sessions
       Session MM-ThA

Paper MM-ThA10
Micro-Thermal Conductivity Detector for Chemical Sensing

Thursday, November 7, 2002, 5:00 pm, Room C-210

Session: Fabrication, Integration, and Packaging Techniques for MEMS
Presenter: D. Cruz, UCLA and Sandia National Laboratories
Authors: D. Cruz, UCLA and Sandia National Laboratories
J.P. Chang, University of California, Los Angeles
F. Gelbard, Sandia National Laboratories
R.P. Manginell, Sandia National Laboratories
S.K. Showalter, Sandia National Laboratories
L.J. Sanchez, Sandia National Laboratories
S.S. Sokolowski, Sandia National Laboratories
M.G. Blain, Sandia National Laboratories
Correspondent: Click to Email
Microsensors are essential for detecting biological and chemical warfare agents in state-of-the art micro chemical analytical systems. We have designed and fabricated a micro thermal conductivity detector to analyze the effluent from a gas chromatography column (GC). The TCD can be integrated with a micro-GC column to form a complete "lab-on-a-chip" separation-detection scheme. The TCD consists of a two-flow cell Wheatstone bridge circuit where the resistor elements are suspended by a thin SiN@sub x@ membrane in pyramidal and trapezoidal shaped flow cells. A four-flow cell detector can also be constructed for doubling of sensitivity. Rapid computational prototyping by simulating the heat transfer in the TCD with a Boundary Element Method enables a cost-effective way of optimizing the TCD geometries yielding the greatest sensitivity. Two flow patterns, six operating temperatures, five heater sizes, and five channel widths were theoretically investigated, and the optimal geometry along with eight additional promising geometries was fabricated. The change in heat flow versus the change in gas thermal conductivity (dQ/dk) of He was first determined to verify the simulation results. Nitrogen and a carbon-fluoro-carbon were added as effluents to the He gas stream. The voltage response in the Wheatstone bridge changed by approximately 40% and 70% respectively. A four-cell detector was used, where He was flowed at 5 sccm through 2 reference resistors and a mixture of the carrier gas (He) and effluent was flowed through the other two resistors. The measured voltages yielded heat flux values that consistent with the theoretical values. In addition; results verified that convection became a dominant effect over conduction when the carrier gas was flowed at a rate greater than 10 sccm. @FootnoteText@ @footnote 1@Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.