Effect of Dielectric Constant Variation of High Purity Alumina Substrate on RF Performance

Surface roughness and stability of permittivity over batch are the important parameters of substrate. Moreover, at higher frequencies, variation in these parameters are detrimental to circuit performance. The selection of alumina substrate is due to higher dielectric constant and maturity in fabrication process which results in reliable reproducible circuit performance. Slight variation in the permittivity value results in change of the measured value of the actual hardware. Hence, proper characterization at the desired frequency is an important consideration. This article presents the permittivity characterization of alumina substrate at microwave frequencies. Band pass filter performance variation is studied and correlated with the permittivity variation. Further concept of substrate purity and micromachining on alumina substrate is also detailed. The effective dielectric constant of the material drastically changes by > 40% due to bulk micromachining. By adopting bulk micromachining, alumina substrate can be used for realization of Antenna elements which requires low permittivity materials. The article also discusses various techniques to measure dielectric constant, circuit realization, frequency shift analysis on alumina substrates.