Damping of the resonant modes of a rectangular metal package (MMICs)

It is shown that the lowest order TM/sub 110/ resonant mode of a metal package can be suppressed by fixing a substrate coated with a thin resistive film to the upper wall of the package cavity. Design information is presented to allow unwanted modes to be easily and effectively damped with this technique. It was found that it is possible to obtain resonant mode quality factors of 20 or less with film resistivities between 10 and 30 Omega /sq on high-dielectric-constant substrates. Thin films of this resistivity can be formed on alumina, which has a high dielectric constant, by evaporating approximately 100-1000 AA of chrome or titanium on its surface. The backside of the alumina substrate can then be metallized and soldered to the lid of a package containing a large monolithic microwave integrated circuit, using the same proven technology used to affix the circuit to the bottom of the package, effectively damping the lowest order cavity mode supported by the package.>