Permittivity Measurements of High Dielectric Constant Films at Microwave Frequencies

We have developed a time-domain reflectometry (TDR) technique to measure the dielectric permittivity of high dielectric constant films. The test specimen consists of a planar capacitor terminating coaxial waveguide. The complex permittivity of the dielectric in the frequency domain is obtained from appropriate analysis of the incident and the reflected voltage waves. In order to improve accuracy at higher frequencies, a working equation is proposed to account for propagation in the specimen section. The applicability of the method has been verified at frequencies from 100 MHz to 10 GHz on several polymer composite films, 40 μm to 100 μm thick, having a dielectric constant ranging from 4 to 40. The results obtained in the broad frequency range by using the TDR technique were compared with those obtained from the gap-coupled microstrip resonators at discrete frequencies.