TEGAM Inc., a leading supplier of innovative RF power measurements, was recently awarded Patent No. 8,558,556, “Planar-axial thermistor for bolometry.” This patent describes a novel way to construct very small planar thermistors on a substrate with very low heat transfer. The thermistors are more thermally isolated from the environment than are commercially-produced thermistors. Both the small size and thermal isolation make them useful for measuring RF power at microwave frequencies up to 70 GHz.
TEGAM was able to achieve this high level of thermal isolation by first using a substrate with a low dielectric constant and low heat transfer, unlike typical industrial thermistor substrates. Onto that substrate, they apply thin, planar electrodes using any appropriate metallization process, such that a gap is created between the electrodes. Across the gap, resistive thermistor material having a high temperature coefficient is attached using a thick film process.
The electrodes are much wider than they are thick, and the width and cross-section can be tailored to match a surface waveguide, such as a stripline or the center conductor of a coplanar waveguide. By extending the length of the electrodes a significant distance from the actual resistive thermistor material, the thin metal serves to further thermally isolate the thermistor from metal traces to which the thermistor is attached.
According to Andy Brush, TEGAM CEO, “These special thermistors are allowing TEGAM to develop a next generation of microwave power standards using manufacturing techniques that we couldn’t use before. We’re pretty excited about what we can do when we’re not limited by the bead-on-wire technology.”