Design and operation of chemical -sensing films for a microfabricated sensor from platinum -titanium bilayers.

Abstract

Thin Pt/TiO_x films have proven to be chemically sensitive to their gap environment. To better understand the structure and surface composition of these films, a quantitative determination of grain boundary diffusion of titanium through the platinum layer has been carried out. The accumulation of partially oxidized titanium at the top surface of the platinum layer was measured by x-ray photoelectron spectroscopy (XPS) to determine the diffusion pre-exponential factor and the activation energy for gram boundary diffusion coefficient. For Pt layers thinner than 200 A, there was a thickness dependence on the diffusion kinetics, resulting in activation energies as low as 20 +/- 4 kJ/mol. Above 200 A, the activation energy remained constant at an average value near 118 +/- 15 kJ/mol. The XPS results were validated by conducting in-situ atomic force microscopy experiments on a microelectromechanical structure. The structure is capable of operating from ambient room temperature up to 800&deg;C without damage to the microscope. With this device, topographical images of platinum supported titanium films, have been acquired at temperatures between 25--375&deg;C. <italic> In-situ</italic> imaging of the titanium underlayer diffusing through the platinum film has been observed at 375&deg;C. Titanium migration to the surface near this temperature is also shown on blanket 35 A Pt/65 A Ti films with x-ray photoelectron spectroscopy (XPS). The conductometric changes in Pt/TiOx films upon exposure to hydrogen and oxygen can be described by a coupled surface reaction diffusion/bulk diffusion model. In this model, the mechanism assumes the disruption of the surface oxygen equilibrium by catalytic reaction with hydrogen. The reaction on the surface provides a driving force for diffusion of oxygen from the layer of TiOx to the surface. The model depends on two parameters: a single parameter to describe the reaction of hydrogen on the surface and a parameter to describe the diffusion of oxygen through the TiOx, layer. The temperature dependence of both model parameters follows an Arrhenius relationship. The activation energies for the reaction and diffusion parameters are 16 kJ/mol and 39 kJ/mol respectively. This work provides valuable information regarding Pt/Ti surface concentrations in thin film chemical sensors, microelectronic applications, and for better understanding the mechanisms of conductometric sensor response.