Research On Microsphere-assisted Lithography And Application Of Refractive Index Sensing

The plasmon resonance sensor based on the interaction between electromagnetic waves and metal-medium composite microstructures is a sensor device with superior sensing performance,small size,and easy integration.It often exhibits sensitive response to environmental humidity or solution refractive index,biomolecule types,concentration,and the inclination angle of incident electromagnetic waves,and shows potential application markets in the chemical,biological,and marine industries.However,there are still many problems in the practical application of the plasmon resonance sensors.For example,the fabrication cost of the plasmon resonance sensors is high,and sensing performance of simple structure sensors is low.In view of the above problems,this thesis firstly studies the low-cost and high-output microsphere-assisted lithography method.Based on the microsphere-assisted lithography method,refractive index sensors with a metal-insulator-metal pattern array structure is prepared,and through the electromagnetic field simulation software CST Microwave Studio to optimize the structural parameters of the sensor device and carry out some experimental verification.The main research contents are summarized as follows:1.In the research on the focusing characteristics of microsphere lenses,the influence of the interaction between adjacent microspheres on the shape of the focus spot is often ignored.Therefore,the study of the effect of the gap between adjacent microspheres on the shape of the focus spot and the results of the microsphere lens assisted lithography was carried out.Numerical studies have found that the microspheres with a diameter and refractive index of 2.06?m and 1.59,respectively,have high Mie near-field efficiency under illumination wavelength of 400nm.When it is among a microspheres array with the same structural parameters as it,and the distance between it and the neighboring microspheres is very large(less than 30nm),there exists strong optical coupling between it and adjacent microspheres,which makes the focus spot by it look like a hexagonal star-shaped.When the thickness of the photoresist(refractive index:1.67)between the microspheres and the Si substrate is 120nm(that is,the optical path is equal to half the wavelength),the photoresist layer plays the role of Fabry-Pérot(FP)cavity to enhance the side lobes appearing at the edge of the focus spot by constructive interference.The shape of the spot is also affected by the polarization direction of the incident light.When the gap between adjacent microspheres gradually increases,the side lobes at the edge of the focus spot gets weaker until a dark ring appears.Experiments have not only verified the above conclusions,but also found that hexagonal holes,hexagonal star-shaped holes,and topological double hole arrays can be obtained when carrying out microsphere assisted lithography with ordered and close-packed microspheres with bigger diameter.These also due to microspheres with larger diameters generally have high Mie near-field efficiency and optical coupling between adjacent microspheres.These findings have certain guiding significance for the fabrication of new patterns using microsphere-assisted lithography.2.In order to solve the problem that the FWHM of the photonic nanojet(PNJ)from a lens in the far field is too large and does not meet the requirements of lithography applications,a immersed double-layer concentric hemispherical lens is proposed.The refractive index of the inner hemisphere/outer spherical shell of the double hemispherical lens is 1.57 and 1.44 respectively,and the inner radius is 2.8?m.Through numerical simulation and ray tracing,it is found that the focal length/FWHM of the hemispherical lens when immersed in air and magnesium fluoride are 5.7?m/422nm and 6.9?m/294nm under the irradiation of parallel light with a wavelength of 365nm,incident along the optical axis.The FWHM of the extended PNJ center is well compressed when the magnesium fluoride is immersion material.This is because the spherical surface of the inner hemisphere at this time totally reflects the incident beam closer to the optical axis,and the incident beam farther from the optical axis participates in the formation of PNJ(that is,similar to immersion lens with high numerical aperture).Based on the formation principle of the long focal length and sub-wavelength focal spot FWHM of this immersed double hemispherical lens,we designed another immersed and engineered single hemispherical lens.The refractive index and diameter of the hemisphere are 1.57 and 9?m,respectively,and the spherical surface of the hemisphere is partly cut off perpendicular to the optical axis.This immersed and engineered hemispherical lens is also similar to a immersed high numerical aperture lens,which can form a long focal length,narrow FWHM focal spot under working light illumination,and has good compatibility with light wavelengths.In addition,sub-half-wavelength lithography resolution was obtained during lithography simulation.3.Hydrophilic treatment of substrates with solutions such as hydrogen peroxide has problems that the treatment effect is not good and the hydrophilicity of the substrate surface is kept for a short time.In addition,large-size microspheres are prone to agglomeration during electrophoretic self-assembly.The substrate is cleaned by UV light,the positively charged polymer film is prepared by the spin coating method,and the microspheres in the electrophoretic self-assembly process are heated in a water bath,which greatly optimize the drop coating self-assembly and electrophoretic self-assembly craft and better self-assembly result is obtained.The obtained microsphere array-assisted photolithography was used to prepare hole arrays with different hole sizes,morphologies,and periods.The hexagonal star-shaped holes and the staggered double hole patterns were discovered for the first time.4.Aiming at the problem of low refractive index sensitivity(RIS)and oversized FWHM of the absorption peak of the FP-like cavity resonator sensor of a metal microdisk-insulator film-metal reflective layer structure,a insulator microdisk is proposed to replace the insulator film.By reducing the diameter of the insulator microdisk,environmental exposure of the local electric field at the edge of the metal microdisk will increase,thereby improves the RIS.Meanwhile,the influence of the metal material and the microdisk morphology on the FWHM and RIS of the absorption peak of the FP-like cavity resonator is studied.The results show that the RIS(800nm/RIU)of the FP-like cavity resonator sensor of the Au-SiO₂-Au disk array,in which the middle layer are disks,is about twice of that of the Au disk-Mg F₂film-Au reflective layer structure sensor in the literature at a similar peak position(near 1.6?m).The RIS of the former will double again when reducing the diameter of the SiO₂disk.This phenomenon was also proved in the experimental test of Au-SiO₂-Au disk array structure prepared by microsphere assisted lithography method.Compared with the Au-SiO₂-Au disk array with similar absorption peak position of FP-like cavity resonator(FWHM of absorption peak is about 230nm),the FWHM of the absorption peak of Au-Si₃N₄-Au and Ag-Si₃N₄-Ag square disks arrays are about 160nm,100nm,respectively.The absorption peak FWHM of the square disk array decreases rapidly with the decrease of the side length of the square disk.With the decrease of the side length of the insulator disk,the RIS can increase to more than 1000 nm/RIU,which manifests that the FWHM of the absorption peak of this kind of FP-like cavity resonator structure is related to composing material of the resonator structure,the morphology of the disk,and the size of the disk.By further optimizing the FP-like cavity resonator,it is expected to obtain an absorption peak with a very small FWHM and a large RIS.When reducing the diameter of the middle layer of the Au-Au-Au disk array structure,an absorption peak that is not related to the structure period and sensitive to the environmental refractive index appears.By observing the electric field,magnetic field and energy flow distribution at this absorption peak,it is determined that this is also absorption of the FP-like cavity resonator,and the discovery enriches the structure of the FP-like cavity resonator.5.Using the microsphere array as a deposition mask,triangular disks arrays can be prepared.Different from the previous report,we established the Au-SiO₂-Au structure array model of the triangular disk based on the closely arranged cylinders with a diameter of 900 nm and the obtained triangular disks have elongated tips.Numerical studies have found that the Au triangle tip has a strong optical rod effect,and an enhanced magnetic field appears in the middle insulator layer.These two local modes respond to incident light together,and achieve narrow-band absorption in the near-infrared band(near 900nm,absorption peak FWHM about 5nm).Furthermore,because the Au triangle tip is very fine and long,the strong electric field at the tip make the resonance peak of the unit structure very sensitive to the refractive index of the environment.Finally,RIS of the resonance peak of the Au-SiO₂-Au triangular disk array to the environment is 660 nm/RIU,and the quality factor is as high as 132.