Microfabricated self-resonant structure as a passive wireless dielectric constant and conductivity sensor

This paper describes a low cost, passive wireless dielectric constant and conductivity sensor using a microfabricated inductor with interdigitated capacitors (IDC). A self-resonant-structure (SRS) is designed by incorporating IDC electrodes in the inter-winding space of the inductor. The distributed capacitance and conductance of the sensor is affected by dielectric constant (ε) and conductivity (σ) of its environment or material under test (MUT). The ε and σ can be used to provide information about the surrounding environment. This serves as an impedance transducer changing the resonant frequency and phase dip of the SRS. The SRS is interrogated using a non-contact inductively coupled reader coil. The change in resonance frequency and phase dip of the SRS is used to detect material properties of the environment/MUT. The relationship between sensor layout and coupling factor between sensor and reader is investigated. Optimizations of the coupling factor based on this relationship are discussed. IDC design trade-offs between the sensor’s sensitivity and coupling factor are also investigated. The sensor’s response to variety of liquid MUTs with a wide range of dielectric constant and conductivity is presented.