Research On Terahertz Ellipsometry Thickness Measurement Technology Based On Aspherical Lens

Terahertz wave is a kind of electromagnetic radiation between microwave and infrared band,which is widely used in the field of detection due to its low quantum energy and good transmittance to many materials.Terahertz ellipsometry is a new type of detection technology under research and development,it can measure the material parameters of thin films with high precision in a non-contact form,the positioning of the beam and the corresponding system structure require in-depth research.In order to improve the positioning accuracy of the terahertz ellipsometry system and reduce the volume of the system,this paper studied and designed aspherical lenses and applied them to the ellipsometry system,which proved that aspherical lenses can effectively improve the positioning accuracy of the terahertz beam and greatly reduce the system size compared to spherical lenses.The content of this paper is as follows:(1)Based on the gradient descent algorithm,a general method for the design of terahertz aspherical lenses was proposed.Aspherical lenses with working frequencies of0.1 THz,0.3 THz and 0.33 THz were successfully designed by using the finite difference time domain method as the numerical calculation method,and lenses were made by using the corresponding processing methods of different materials.The collimating-focusing experimental system was built at three frequencies to test the performance of aspherical lenses and terahertz spherical lenses.In the experiment,the focusing ability of aspherical lenses was significantly stronger than that of spherical lenses,and it had the highest focusing efficiency in the 0.3 THz system,up to 85%.In addition,it had an excellent "shaping" ability for the low-quality collimated beam and can eliminate the chromatic aberration due to small-scale wavelength changes,the feasibility and versatility of the design method was demonstrated.(2)By studying the simulation of terahertz ellipsometry thickness measurement,it was concluded that the film thickness measurement of the terahertz ellipsometry experimental system was suitable for the condition of incident angle of 45° and incident wave polarization state of circular polarization or elliptical polarization.The data acquisition procedure of the terahertz ellipsometry system was introduced.The 4-F ellipsometry system with a working frequency of 0.3 THz was built using self-designed aspherical lenses as focusing lenses,and the advantages of the system in positioning accuracy and volume were analyzed.The thicknesses of two thin film samples with the different thicknesses were measured at an incident angle of 45°.The results were close to those measured by the micrometer,with deviations of 0.6% and 3.8%,respectively.Compared with the conventional 4-F terahertz ellipsometry system using spherical lenses as focusing lenses,this system had higher measurement positioning accuracy,and can reduce the length of the two arms by about 200 mm,and can accurately measure the thickness of thin film samples.