Probe Design And Simulation Study In Microwave Near-field Scanning

The near-field microwave microscopy technology,that is,the microwave signal is transmitted to the sample through the probe,and the probe is interacted with the sample,and the obtained microwave signal is returned to the test instrument through the probe.Throughout the process,the microwave probe is equivalent to a broadband matching transmission line between the instrument and the sample.From the above description,the core of near-field microwave microscopy has two points: the first is the interaction between the probe and the sample;the second is the importance of the microwave probe in the whole measurement system,so designing microwave probe becomes an important job.Starting from the study of quasi-static theory,this paper introduces two specific methods: spherical mirror image method and medium plane mirror image method,which provides a theoretical basis for the analysis of near-field equivalent model.On the basis of quasi-static theory,the electric field distribution of the near-field equivalent model is analyzed by the charge-like method,which is divided into two cases: the tip-sample soft contact and the tip-sample spacing.Finally,the MATLAB simulation can be used to obtain the near-field.The electric field decays very quickly,and the potential in the sample is much smaller than the potential in the air.For the problem of electromagnetic field,through theoretical derivation combined with simulation analysis,it can be concluded that the quasi-static theory is applicable to the near field.Analyze the theoretical basis of near-field coaxial probes,including the design principles of coaxial probes.Study the measurement principle of the entire measurement system,introduce its measurement method,and determine the 50 ohm characteristic impedance of the coaxial line.Modeling the coaxial probe,analyzing the equivalent model of the interaction between the probe and the sample,determining the equivalent capacitance of the interaction between the tip and the sample,and the relationship between the equivalent capacitance and the reflection coefficient,thereby a relationship is established between the electrical parameters of the sample and the reflection coefficient.Change the parameter variables of the coaxial probe,analyze the change of the reflection coefficient by simulation,and the influence on the dielectric constant of the sample,and draw conclusions:(1)the thickness of the underlay and the size of the flange have little effect on the measurement results.(2)When the sample thickness is less than 2.5 mm,the sample has instability.For the effective dielectric constant,comparing the theoretical calculation results with the simulation results,it is concluded that when the sample thickness is greater than 2.5 mm,the theoretical calculation results of the effective dielectric constant are basically consistent with the simulation results,and the maximum error does not exceed 6%.Study the design theory of coplanar waveguide probes,including structure,advantages and disadvantages,propagation modes,and so on.Design a new coplanar waveguide to determine its size.Modeling and simulating this probe,comparing the electromagnetic field line distribution of the new probe with the field line distribution of the traditional coplanar waveguide,verifying the correctness of the new coplanar waveguide probe,and determining the feasibility of the new probe by analyzing the return loss.In near-field microwave microscopy,this new probe is used to measure the microstrip line and analyze the measurement results,including return loss and electromagnetic field distribution in the near field.Measure the effective dielectric of the microstrip line at 3 GHz.The constants,comparing the measured results with the theoretical values,conclude that the measured values of the effective dielectric constant of the microstrip line are basically consistent with the theoretical values at a frequency of 3 GHz,thus verifying the performance of the novel probe.