Daesha More

Date of Award

8-2007

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

M. Clayton Wheeler

Second Committee Member

William J. DeSisto

Third Committee Member

Robert J. Lad

Abstract

The microhotplate array is a powerful sensor tool. Microhotplates have low thermal' mass and high operating temperatures, resulting in a very fast kinetic response for gas detection. Also, the ability to fabricate arrays of microhotplates allows for combinatorial materials deposition studies and sensor detection techniques. The small size of the hotplates (approx. 100 urn per side) requires only a few milliwatts of power to heat the sensor, which is an advantage to commercialization as a chemical detector. To date, the highest reported operating temperature for a microhotplate is 650°C; we are using fabrication materials that may facilitate operation at temperatures as high as 900°C. Temperature uniformity is a crucial component in the analysis of analyte kinetics; a new heater design has been modeled which decreases the difference in temperature across the surface of the microhotplate to 5%. Along with temperature variation reduction, an increase in surface area per microhotplate has been achieved. Sensor array geometry has been improved so that 10 devices may be placed on one 6 mm x 6 mm chip resulting in a surface area increase of 25 times over the initial instrument. Resistance changes in the metal layers are dependent upon the temperatures of the microhotplate; calibration of the heater layer and RTD's on the surface of the devices facilitates creation of supplied power versus element temperature plots. These data may then be used to precisely control the temperatures at which gas detection occurs, increasing the selectivity of the catalyst layer to target gases. Gas detection occurs from the electrical response of a semiconducting thin-film upon adsorption of a gas analyte. The array allows application of different sensing films on one chip, so multiple films with different responses to a single gas can be used to improve sensor selectivity. The materials deposition will be accomplished by using chemical vapor deposition (CVD) on the surfaces of the individual microhotplates. This thesis focuses on fabrication of microhotplate components: heater geometry, thermal isolation geometry and insulation types and thicknesses. Many challenges to the fabrication of complete devices have been addressed including metal adhesion, photoresist stability, front-side release etching and poor silicon etch rates. Improvements to array layout have increased the number of microhotplates per chip by 2.5 times while maintaining original chip size and number of bond pads. The proportion of total power dissipated in the wiring traces for the square heater was 80% with 20% to the heater only; the newly designed rectangular device wire traces dissipate only 50% of applied power.

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