Design And Development Of Near-field Microwave Scanning Imaging Testing System

Near-field scanning microwave microscopy is a combination of near-field measurement technology and microwave measurement technology.It effectively combines the advantages of near-field measurement and microwave measurement.To put it plainly,it can measure the electromagnetic characteristics of materials in the subwavelength scale.Because of its advantages of high spatial resolution,high sensitivity and non-destructive measurement,it has been widely used in the characterization and testing of micron and nano-structured materials,as well as in the microscopic research fields such as Bio-cell scanning in recent years.In this paper,based on the principle of near-field microwave measurement,we have built a near-field scanning microwave microscopy system.By introducing the perturbation theory and imaging theory of coaxial resonator,and deducing these two theories based on Maxwell equation,the feasibility of the scheme of coaxial resonator is proved.The coaxial resonator,the core component of the test system,is modeled and simulated by HFSS software.In addition,according to the functional requirements of the system and the coaxial resonator,we design and assemble the relevant hardware structure to form a complete near-field microwave scanning imaging testing system.Based on the graphical programming software LabVIEW,the related control software is developed.It can satisfy the functions of height scanning,line scanning and plane scanning,which realize the automation of testing.Secondly,based on this system,three different types of samples with single material but uneven surface,flat surface but different materials distribution,diverse materials and uneven surface are tested,and the imaging mechanism of the system is analyzed accordingly.Then,two methods are proposed to optimize the test: locating the scanning area accurately and correcting the influence of the inclination of the sample carrier.For the problem of locating the scanning area,the method of "roughing first and then sweeping carefully" is proposed,which improves the testing efficiency;For the inclination of the platform,a set of "four-point test method" is designed,which can be completed only by one automatic measurement,and the effect is very good.We measured the profiles of NiFe thin films with width of 260 and 470 microns under different tip-to-sample distances.The results show that,with the increase of tip-sample distance,the spatial resolution of the system decreases.Through the analysis of the profile curve,it is found that the line width of the film increases linearly with the increase of the distance between the tip and the sample.Based on the basic theory of nearfield microwave,a measurement method for the linewidth of thin films was proposed,which lays a foundation for further study of near-field scanning microwave microscopy.