A Terahertz Scanning Aperture Near-Field Imaging System

The development of imaging technology impact on the development of biology,medicine,military,public security and other fields,and is closely related to human production and life.Characteristics of security,penetrability,and high-resolution of terahertz near-field imaging may enable it to make up for the deficiencies of existing imaging methods in some respects,such as infrared,ultrasound,X-ray,and imaging of other wavebands,or visible light microscope,atomic force microscope,electron microscope,scanning tunneling microscope,and other microscopic techniques.These widely used imaging methods have their own unique advantages and suitable application fields,but they also have their limitations.The development of terahertz near-field imaging will overcome some of the defects of existing imaging methods.Terahertz emission,propagation,detection,imaging and other related technologies are relatively new technologies.In recent years,terahertz near-field imaging has been widely studied internationally,but there are fewer studies on it in the country.In addition,there is currently a lack of nondestructive testing methods for packaged electronic microcircuits.Conventional nondestructive testing methods,such as X-ray and ultrasonic each have certain undesirable attributes.X-ray nondestructive testing may damage ultraviolet scrubbing chips or photoelectric chips near the packaged electronic microcircuits and components.The resolution of ultrasonic nondestructive testing may not be able to resolve details of packaged electronic microcircuits and components.Aluminum etched antennas are widely used in radio frequency identification(RFID)electronic tags,and are also contemplated to be used in the frequency band of the sixth-generation mobile communication technology,and the current quality control method for aluminum etched antennas is mainly manual quality inspection before packaging.In this way,the accurate layout of the packaged antenna cannot be measured,or whether the antenna is broken and shorted due to falling off,shifting,or foreign objects during the transport and packaging process,which may affect the signal transmission performance.The above two traditional methods are not suitable for nondestructive testing of microcircuits such as packaged aluminum etched antennas.Therefore,a suitable nondestructive testing method is needed to test the packaged aluminum etched antennas.Combined with terahertz near-field imaging,this dissertation demonstrated a system that not only can achieve high-resolution surface imaging but also can perform nondestructive testing of packaged aluminum etched antennas.It has the characteristics of high-resolution,highpenetration,and does not damage the ultraviolet scrubbing chips and optoelectronic chips near the antenna.A set of scanning aperture reflective near-field imaging system was built in this dissertation.A series of size optimization,structural improvement and innovation of the near-field probe,which is the most important component of the system has been demonstrated.Data and image processing algorithms were studied,and possible applications were expanded.These works have laid the foundation for the application of terahertz near-field imaging system.The main research contents are as follows:First,the classification,structure and principle of the terahertz near-field imaging system are investigated,and the system is built.According to the research status of various system structures and their respective advantages and disadvantages,the structure of this near-field imaging system is determined.The application direction of the terahertz near-field imaging system is investigated,and it is found that there are requirements for micro-size surface imaging,subsurface imaging of dielectric packaging,and surface defect imaging.This system has the possibility of imaging the abovementioned types of samples,so the above samples are used as imaging samples.A nearfield imaging system including a vector network analyzer,a spread spectrum module,a scanning platform,a near-field probe,a longitudinal distance measuring microscope,and a computer has been established.Then,the mechanism of near-field imaging is studied,theoretical estimation and simulation calculation are carried out on the structure of the aperture probe,and the structure of the probe is improved gradually.Based on the principle of antenna resonance and impedance matching,the large end face size and total length of the aperture probe are determined.Electromagnetic simulation is performed on the pyramid probe by CST electromagnetic simulation software,and the electric field enhancement factors of different sizes are obtained,and then determine the best-performing pyramid probe size as the first probe structure in this dissertation.Improving the structure on the basis of this pyramid probe,a curved aperture probe which is able to additional bunching and enhance the terahertz beam transmitted through the aperture is proposed,so that the strongest electric field intensity out of the aperture accounts for more than 90% of the internal strongest electric field intensity.This is a significant improvement over the 38%of the first pyramid probe,which in turn extends the working distance of the probe to 10?m.Two new structures with better performance are designed subsequently,including a pyramid probe embedded by hemispherical micro-silicon lens in the tip,and a probe with a combination of cylinder and frustum of a cone.We then evaluated performance of the last three probe structures through electromagnetic simulation results,including electric field enhancement factors and beam widths,and obtained the best size of each kind of probe.The calculated electric field enhancement factor of the probe with the best performance is as high as 25,and the beam width is reduced to 12 ?m,which is a great improvement over the performance of the classic pyramid probe.Next,three types of probes are used to conduct near-field imaging experiments to verify resolutions and other performance characteristics,and the probe with the highest resolution is selected for the two-dimensional near-field imaging experiment to verify the application potential.Three types of probes except pyramid probe embedded by hemispherical micro-silicon lens in the tip are used to imaging multiple samples including PCB boards,dielectric defects embedded in the media,aluminum foil strips attached to the PVC board,and small scratch on the PVC board to verify resolution.The three probes achieve surface imaging resolutions of 15 ?m,10 ?m and 6 ?m,respectively.Among them,the last type of probe achieves an imaging resolution of 12 ?m for a small scratch on the PVC board.Using the probe with a combination of cylinder and frustum of a cone with the highest resolution for application experiments,including twodimensional surface imaging of flexible gold electrodes and two-dimensional subsurface imaging of packaged aluminum etched antennas(coating thickness 50 ?m,200?m).The resolution of surface imaging is 6 ?m(?/450),and the resolutions of subsurface imaging are 110 ?m and 500 ?m,respectively.They are currently the highest reported surface and subsurface resolutions of terahertz aperture-type near-field imaging.Finally,the data and image processing algorithms that can effectively suppress the main noise of the system are studied.Average de-extreme filtering is performed on the measurement data of each measurement point.According to the characteristics of the system and experimental results,the main noise sources include three types.First,the central scattering enhancement phenomenon.Second,the phenomenon that target with weak reflection are submerged by the noise of target with strong reflection.Third,there is a slight difference in the near-field distance between the measuring point and the probe tip at different imaging positions.These three kinds of noises manifest as uneven threshold distribution in the imaging processing stage.Derivative filtering extremum threshold segmentation algorithm which is proposed creatively and adaptive threshold segmentation algorithm of Hilbert scan combined with wavelet transform used in this dissertation successfully suppressed these kinds of noises,accurately identified the submerged targets,improved signal-to-noise ratio of the image,enhanced the consistency of the near-field imaging results and the sample,and restored the boundaries,details and actual appearance of the measured object.The minimum size errors of the processed imaging results can be as small as one scan step.Correspondingly,relative error of the flexible gold electrode is 1.67%,relative error of the aluminum etched antenna with a package thickness of 50 ?m is 5%,and relative error of the aluminum etched antenna with a package thickness of 200 ?m is 2.5%.