AVS 50th International Symposium
    Applied Surface Science Wednesday Sessions
       Session AS-WeP

Paper AS-WeP18
Microsensor Technique for Analyte Surface Coverage vs. Sensor Response Correlation

Wednesday, November 5, 2003, 11:00 am, Room Hall A-C

Session: Poster Session
Presenter: A.G. Shirke, The University of Maine
Authors: A.G. Shirke, The University of Maine
R.H. Jackson, The University of Maine
B.G. Frederick, The University of Maine
R.E. Cavicchi, National Institute of Standards and Technology
S. Semancik, National Institute of Standards and Technology
M.C. Wheeler, The University of Maine
Correspondent: Click to Email
This paper presents results based on the combined use of a single microsensor platform (as a Temperature Programmed Desorption [TPD] device) with a sensitive, calibrated mass spectrometer that has a minimum detection limit of 10@super 10@ molecules/s. Desorption kinetics of model systems have been investigated as part of an effort to develop a technique for correlating the relative surface coverages of gas analytes to the electrical responses of chemical sensors. The pulsed-TPD technique takes advantage of the rapid heating characteristics of micromachined sensor platforms (called microhotplates) which are layered structures developed at NIST. Microhotplate sensors include a sensing film, electrodes, and a polysilicon heater all separated by insulator layers. Previous microheater desorption experiments utilized arrays with large numbers of devices to provide enough desorption flux for detection while maintaining the rapid heating rate (over 10@super 6@ K/sec) of the individual devices. Unlike the current experiments that use single microsensors, the large array studies did not explore conductance changes of sensing films. The model system described in this work, condensed benzoic acid on the microsensor surface, is used to evaluate the challenges involved in extension of the technique to actual sensor systems. The experiments were conducted at a base pressure of 5x10@super -10@ Torr, using a constant, directed-dosing arrangement for benzoic acid (vapor pressure: 7x10@super -3@ Torr at 300 K). A voltage pulse of 500 ms duration was used to heat the polysilicon heater to temperatures up to 700 K within approximately 3 ms while the isothermal desorption flux was monitored by the differentially-pumped mass spectrometer. Limitations and modifications in the data acquisition, sample mounting, and minimum detectable desorption flux are discussed along with the presentation of the desorption isotherms.