Self-organized Nanoripples On Bilayer Systems Produced By Ion Bombardment

Ion bombardment(IB),may induce self-organized nanostructures,which is a costeffective method for producing large-area nanostructures with high throughput.Quasiperiodic nanoripples produced by using this method,as templates with high line density,are considered especially promising for a wide range of applications,e.g.,synchrotron radiation,materials and biology.How to improve the orderliness of the IB-induced nanostructures is the emphasis and difficulty in this field.Recently,the growth of the self-organized nanoripples can be guided with fabricating periodic prepatterns by using conventional lithographies,e.g.,electron beam lithography and focused ion beam lithography.However,these conventional methods suffer the disadvantages of expensive and low efficiency of writing prepatterns.Thus,the compatibility of these lithographies with IB for prepatterns needs to be improved.Therefore,in this thesis,a method is proposed to improve the orderliness of self-organized nanostructures by the bombardment of a bilayer system.Moreover,the characterization range of the IBinduced self-organized nanoripples is obviously increased by using extreme ultraviolet(EUV)scatterometry.The main research contents and results of the thesis are as follow.1.The basic ion bombardment properties of a single layer of antireflection coating(ARC)and periodic prepatterns are systematically studied,which is the basis for proposing the method of improving the orderliness of nanoripples by IB of a bilayer system.Firstly,the effect of IB parameters on the IB-induced morphologies formed on the ARC surface is systematically studied,from which it is pointed out that even under an optimized ion parameters,the nanoripples on the ARC surface needs to be further improved.Secondly,diffraction grating structures are prepared as periodic prepatterns using holographic lithography.Prepatterns with optimized the material and the structural parameters preliminarily regulate the evolution of nanoripples during subsequent IB.It was deduced that the prepatterns may be valid to adjust the growth of nanoripples,with the condition that the structural parameters of the prepatterns are close to those of the IB-induced nanoripples to be adjusted.2.To improve the orderliness of IB-induced nanoripples,it is proposed to bombard an optimized bilayer system.In addition,the specific process and basic conditions of the bombardment of the bilayer system for the proposed method are clarified.The evolution of surface morphologies during the IB of a photoresist(PR)/ARC bilayer is systematically studied.Correspondingly,the temporal evolutions of the morphological characteristics and power spectrum density of the nanoripples are analyzed,combined with the statistics of the structural parameters(height and width)of the nanoripples.Furthermore,the influence of the upper layer thickness on the morphological evolution is also investigated.All these results illustrate the process of the proposed method.In detail,IB-induced nanoripples on the upper-and lower-layer materials work as mask and prepatterns of the final morphology.The improvement of the periodicity and low frequency suppression of the induced nanoripples are achieved by the coupling between the upper and lower layers of nanoripples.Compared with the result of a single layer ARC,the full width at half maximum of the high frequency peak of the power spectral density curve of a PR/ARC bilayer system is reduced by 77%.And the number of breakpoints of the nanoripples per unit period is decreased by 74%.The orderliness of the nanoripples on the PR/ARC bilayer is better than that on any single layer material in the bilayer system.The basic conditions for the proposed method are summarized,which include that nanoripples can be induced on the surface of both upper-and lowerlayer materials under their respective bombardment conditions,and the characteristic sizes of the nanoripples on the surface of each material are similar.Both conditions are indispensable.3.Regarding the material combination of a bilayer system with different bombardment conditions for nanoripples and nanoripple modes,a bombardment strategy with variable incidence conditions is proposed.In this way,the directions of the nanoripple vectors produced on the upper-and lower-layer materials are parallel under the respective bombardment condition for each layer.The morphological evolution properties of a bilayer system with Au as the upper-layer material are systematically studied during IB,in order to give full play to the nanowires on the upper-layer capacity to guide the development of the nanoripples on the lower-layer.Finally,the long-range orderliness of the nanoripples on the lower-layer materials has been significantly improved.This work extends the ranges of material and bombardment condition of the proposed method to improve the orderliness of nanoripples by the IB of bilayer systems.The representative nanoripples on a Au/ARC bilayer show a maximal autocorrelation length of 513 nm,longer than that of a single ARC(～142 nm)and a PR/ARC bilayer(～279 nm).Moreover,to acquire a better understanding of the proposed method,the IB of Au/PR and Au/fused silica are also carried out.4.The morphological characteristics of the IB-induced self-organized nanoripples are characterized by using extreme ultraviolet(EUV)scatterometry at Hefei Light Source.The characterization area of self-organized nanostructures has been increased from the square micrometer scale up to the square millimeter scale.The results show that the transverse and longitudinal morphological features of the above samples obtained from the in-plane and conical mode of the EUV scatterometry are in agreement with those obtained by using atomic force scanning microscope.These results further demonstrate that it is feasible to characterize the basic morphological characteristics of quasi-periodic nanoripples by using the EUV scatterometry.They also provide a basis for subsequent quantitative analysis.In addition,Morphologies of samples characterized at the metrology line,Hefei Light Source are extended from conventional optical elements,e.g.,diffraction gratings,reflective coatings,photonic crystals,to selforganized nanostructures.