Micromachined Array Studies of Tin Oxide Films: Nucleation, Structure and Gas Sensing Characteristics
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species for various applications including chemical process control, automobile emissions, environmental monitoring and medical processes [1]. A low-cost technology that enables immediate and on-site detection of gaseous species would be beneficial in many of these areas. Conventional technologies like thick film metal oxide sensors have been used, but they lack compatibility with batch fabrication. The problems with the thick film sensors are mainly in the area of reproducibility: compressing and sintering a powder, the deposition of catalysts, the use of binders and other ceramics (e.g. for filtering) are all very difficult to control [2]. Newer technologies, including the use of micromachined structures, offer many advantages including compatibility with batch fabrication and lower power requirements. Sensitive and stable thin films integrated onto microdevice structures could overcome certain problems experienced by conventional thick film sensors. Here we describe a method for controlling the microstructure of sensing films on an array of microdevices. Sensitivity of oxide semiconductor gas sensors can be enhanced by either reducing the grain size, or by dispersing catalytic metal layers on top of the sensing films. Reduction of grain
size increases the surface-to-volume ratio of the sensing film and catalytic additives modify adsorption characteristics [3]. Using microhotplate technology, we have investigated the growth mechanism and chemical sensitivities of tin oxide thin films deposited on seed layers. A seed layer can be defined here as the particle layer that forms at ultra-low coverage when (-1-2 nm) of 213 Mat. Res. Soc. Symp. Proc. Vol. 574 0 1999 Materials Research Society
metals are evaporated on a silicon dioxide substrate. These particles act as varied nucleation sites for tin oxide growth and produce finer grains for enhanced sensitivity and stability. MICROHOTPLATE TECHNOLOGY A micrograph and schematic of a microhotplate gas sensor platform developed at NIST are shown in Figure 1. These micromachined devices have lateral dimensions of -100 Prm and are suspended over a cavity for thermal isolation. Time constants for temperature rise and fall of the microhotplates are typically I ms. The temperature change with applied power to the heater is 8 °C/mW [4]. Multi-element arrays of microhotplates can be fabricated easily and offer opportunities for performing fundamental materials research. For example, the effect of deposition temperature and partial pressure of the reacting gases on the microstructures produced in films by CVD, as well as the gas sensitivities of the same films, can be readily studied on a single array. Advantages of materials processing in these platforms include, simplicity of the process, minimal contamination, and elimination of multiple lithographic steps. The fabrication of microhotplates is described further in reference [4].
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SiO 2Base Layer Figure 1: SEM image of a suspended microhotplate (top view) after etching, and schemat
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