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A microwave resonator based detector made in LTCC technology
Keywords: LTCC, microwave resonator, microfluidics
Nowadays, there is a noticeable increase of interest in integrated microfluidic-microwave systems. Among different applications of such the systems, two of them are most commonly used. The first one is related to microwave heating of liquid substances and the second one to characterization of electrical parameters of such substances flowing through a microchannel. Both the applications can be used to influence, to control and to detect different chemical reactions. One of methods for detection of physical and chemical changes within a microfluidic-microwave system is based on microwave resonance. In this case the change that occurs in a liquid substance is observed as a change in the resonant frequency of a given microwave circuit. The goal of the research described in this paper was to design and manufacture a ceramic microfluidic-microwave detector to determine changes in parameters of a fluid in the microchannel. The LTCC DuPont 951 was used as a substrate material, which was dictated by the necessity of manufacturing of precise structures, such as microchannels. This LTCC system is characterized by the dielectric constant of 7.8, which was verified by means of the SPDR testing method. We used a microstrip ring resonator as a microwave resonant circuit in our microfluidic-microwave module. The geometry of this resonator was design by means of the ANSYS HFSS Software. Its resonant frequency was tuned to 2.46 GHz under condition of loading the microchannel with deionized water. The position of the microchannel within the device was selected so that the coupling of the resonator to input/output feed lines depends on the permittivity of the liquid under test. Due to the fact that the module was made with the use the LTCC technology, it was necessary to tailor parameters of individual stages of this technology in such a way to manufacture the designed microwave system as accurately as possible, taking into account position of the microchannel. Consequently, after screen printing, the lamination step was divided into two sub-steps. In the first sub-step, separate lamination of the bottom part (with microwave ring resonator) and the upper part was performed. In the second sub-step the lamination of the bottom and upper parts to form the whole module was performed. Then the module was cofired. In the last step, the SMA connectors were soldered and the microchannel inlets were glued. As a preliminary study, the frequency response of the system for different concentrations of ethyl alcohol was measured. All measurements were performed by means of a two-port vector network analyzer (VNA). For the module filled with demineralized water the measured resonance frequency was 2.51 GHz. When it comes to frequency response measurements we observed that the resonant frequency of the microwave resonator was proportional to the concentration of the ethyl alcohol (change of concentration from 10% to 96% was used). A slight difference between the designed and received resonant frequency was also observed. This difference can be compensated by changing dimensions of the resonator. The results of experiment constitute a sound basis for further research and the applications of LTCC substrates in integrated microfluidic-microwave devices.
Laura Jasinska,
Wroclaw University of Science and Technology
Wroclaw, Lower Silesia

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