A new method of dielectric characterization using a genetic algorithm and a coplanar waveguide on bilayer films

International audienceFine characterization of the dielectric properties (r and tanδ) of materials in the microwave, and above, domain is a mandatory step to optimize the performance of components working in this frequency range. When reaching the millimeter frequency range, the conventional parallel plate condenser technique shows its limits by the increasing part of parasitic in the measurement data that makes their de-embedding much more complex. Characterization techniques based on resonant cavity or microring resonator can then be used but they rely on complex fabrication and analytical de-embedding processes. Moreover their validity domain rarely exceeds a few tens of GHz. Using techniques based on propagation lines allows then to overcome this limitation since the material complex permittivity plays a determinant role in their propagation characteristics; those being easily measured using microwave probe stations. Microstrip or (grounded) coplanar waveguide, (G)CPW, lines, for example, can then be designed and measured up to 100 GHz of bandwidth. A novel method to characterize polymer materials has then been built on such a concept [4], it was used with a bandwidth of approximately 40 GHz.We are currently working on a further development of this technique that is now applied to bi-layer dielectric films. Indeed adding a top layer of known material on the unknown material film to be characterized can help to protect the latter from different technological processes that can be aggressive (for example, lithography processes are, for most of polymers, incompatible because of the photoresist solvent). GCPW lines are characterized by their S- parameters in the 450 MHz to 110 GHz bandwidth measured using a vector network analyzer (VNA). Those data are used as reference for a genetic algorithm (GA) which generates r and tanδ. The algorithm offers a good compromise between de-embedding accuracy and time of calculation.This paper will present the details of this determination process. First of all, we will discuss how this solution works and show that the only limitation is the actual bandwidth of the probes used for different VNAs. In a second time, we will show the different advantages of this method, they have been slightly presented earlier but we will advance them more clearly. Finally, we will show the results of characterization on a well-known material, the Benzocyclobutene (BCB), in our case, the GA returns us the couple r and tanδ respectively equal to 2,77 and 7x10−3 which is in good agreement with literature