We describe the design, fabrication, and evaluation of a new on-wafer measurement platform for the rapid, quantitative determination of the complex permittivity of nL fluid volumes over the frequency range from 10 MHz - 40 GHz. Our measurement platform integrates micron-scale, polydimethylsoloxine-based microfluidic channels with high-frequency coplanar waveguide transmission lines in order to accurately place small fluid volumes at well-defined locations within planar measurement structures. We applied new on-wafer calibration techniques to accurately determine the S-parameters of our integrated devices, and we have developed a novel transmission-line model in order to extract the distributed circuit parameters of the fluid-loaded transmission line segment from the response of the overall test structure. We determine all of the necessary model parameters experimentally directly from a single set of measurements, without requiring a reference fluid of known permittivity. We extract the complex permittivity of the fluid-under-test from the distributed capacitance and conductance per unit length of the fluid-loaded transmission line segment using finite-element analysis of the transmission line cross-section. Our measurements show excellent agreement with bulk fluid permittivity determinations for methanol at room temperature, and yield consistent results for the extracted fluid permittivity for the same microfluidic channel embedded in multiple coplanar waveguide transmission lines of different dimensions.
Citation: IEEE Transactions on Microwave Theory and Techniques
Pub Type: Journals
coplanar waveguide, microfluidics, permittivity measurements