Research On Measuring Method Of Dielectric Constant Of Materials Based On Near Field Microwave Scanning

Near-field microwave scanning microscopy is used to detect cancer cells,surface morphological imaging of samples,and measurement of electrical parameters of materials.Its advantage is that it can break the diffraction limit and exceed the resolution of the optical microscope.The near-field microwave scanning microscopy implementation process is: using electromagnetic waves to interact with materials,the intrinsic properties of the measured materials are inversely derived by measuring weak signals in the near field.Each point of information can be obtained by scanning technology.It is easy to find that the core technology of near-field microwave scanning microscopy is the interaction between the probe and the sample.This article mainly studies the relationship between probe and sample interactions in the near field.The first research point is the distribution of the field formed by the interaction of the probe and the sample.The second research point is how to convert the weak near-field signal into an intuitively measurable quantity and measurement principle.From the perspective of the near field formed by the tip of the microwave probe,the quasi-static theory is used to equate the tip of the probe to a metal microsphere with potential.The radius of the microsphere is the radius of curvature of the tip.Using the image-charge method in the theory of electromagnetic fields,the electric field distribution of a spherical surface interacting with a dielectric plane is derived by a spherical mirror image method and a medium plane mirror method.The mathematical model of the interaction between the probe and the sample is derived in theory.The second is to verify the correctness of the theoretical model.The verification is mainly divided into two parts.The first part is using MATLAB to calculate and image the theoretically deduced mathematical model.The second part is: using the finite element method(electromagnetic field numerical solution method)to construct a near-field model,which is calculated by software.Imaging the potential,and compared with the results in MATLAB.Thus,the correctness of the interaction between the probe and the sample in the near field based on the quasi-static theory was proved,and the near-field fast decay was also proved.The interaction between the probe and the sample can be modeled as an impedance model in the measurement.The relationship between the impedance model and the dielectric constant of the sample can be established through theoretical analysis.The mathematical relationship between the S parameters and the impedance model can be known,and the S parameters can be measured by the vector network analyzer.Then we can achieve the purpose of measuring the dielectric constant of the sample.According to the characteristics of near-field,the probe structure is designed in wide frequency band.The probe and sample model are constructed by HFSS and stimulated by it.The simulation results verify the measurement principle of the near-field measurement platform.It is concluded that the near-field measurement principle can be simulated by the mathematical relationship between the S parameters and the impedance model.There is a certain error between the theoretical calculation result and the simulation result,but the correctness of the calibration theory of the calibration part can be explained by the simulation.