Surface Roughness Optimization of 3D Structures Based on Peripheral Photoinhibition and Chemical Quenching Method(Invited)
Objective Surface roughness is a core parameter that affects the quality of micro-and nano-optical devices prepared using laser direct writing technology.For the performance of curved surface light fields,it is crucial to regulate the performance of micro-nano optical devices.Currently,the surface roughness of a device is primarily reduced by optimizing its writing strategy.However,this method has significant limitations in the structure of micro-optical devices and is easily affected by the shrinkage rate of the photoresist,which is not conducive to the preparation of complex structures.When processing micro-nano optical devices with curved profiles,conventional methods can produce unwanted step traces on the surface,which can affect their optical properties.The axial resolution is considered a key factor affecting the step traces produced by uneven structural surfaces.Therefore,it is necessary to propose a method based on axial resolution compression to reduce the surface roughness of optical devices.Based on peripheral photoinhibition and chemical quenching technologies,this study investigates the effect of the compression of axially written feature sizes on the surface roughness of curved surface structures.Methods In this study,5 mg of 7-diethylamino-3-thenoylcoumarin(DETC),4.8 mg of bis(2,2,6,6-tetramethyl-4-piperidyl-1-oxyl)sebacate(BTPOS),and 1 g of polyhedral oligomeric silsesquioxane(POSS)were used to prepare the photoresist.A self-built writing system,in which the excitation beam was a femtosecond laser with a wavelength of 780 nm and the inhibition beam was a continuous laser with a wavelength of 532 nm,was used in the laboratory.The samples were printed using the conventional oil substrate photoresist method.A glass slide was used as the substrate and fixed onto a piezostage using a specially designed adaptor holder.The surface morphologies of the samples were characterized using a Zeiss Sigma300 scanning electron microscope.The surface roughness of the samples was characterized using a Bruker Dimension ICON atomic force microscope.First,the surface roughness of a flat disk was characterized to verify the effect of an inhibition beam modulated by a 0-π phase mask on improving the surface roughness of the planar structure.Second,the surface roughness of the flat disk obtained with and without a quencher was compared to verify the effect of a radical quencher on improving the surface roughness of the planar structure.Finally,by combining peripheral photoinhibition with the chemical quenching effect of the radical quencher,the effects of both methods on improving the surface roughness of the microlens were demonstrated.Results and Discussions First,the flat disk is written on the DETC photoresist with or without an inhibition beam modulated by a 0-π phase mask.It can be seen that the root mean square roughness of the flat disk written using the single Gaussian excitation light mode is 3.52 nm.The roughness of the flat disk written by the single Gaussian excitation light with a 0-π phase inhibition light is 2.48 nm.In comparison,the roughness is reduced by 29.5％.Second,flat disks with and without a quencher are written.By comparing the SEM images,it can clearly be seen that the planar structure changes from a morphology with horizontal lines to a relatively smooth morphology under the effect of the quencher,and the root mean square roughness is measured to be 2.41 nm.Compared with the disk shown in Fig.2(e),the roughness is reduced by 31.5％.Finally,based on the above characterization results,the two effects are combined to explore their impact on the surface roughness.Figure 4(a)shows the SEM morphology of the microlens written in the DETC photoresist using the single Gaussian excitation beam mode.The surface root mean square roughness is measured to be 11.4 nm.Figure 4(b)shows the SEM morphology of a micro-lens written in DETC+BTPOS photoresist with a single Gaussian excitation light and 0-π phase inhibition light mode.The root mean square surface roughness of the lens is 6.84 nm,which is 40％lower than that of the original lens.Conclusions Based on two-photon lithography,this study uses photoresist with polyhedral oligomeric silsesquioxane as the monomer and DETC as the photoinitiator as research object and proposes a new method based on compressed axial resolution to reduce the surface roughness of optical devices.First,peripheral photoinhibition modulated by a 0-π phase mask is introduced to improve the lateral and axial resolution,and the root mean square roughness of the flat disk structure is reduced by 29.5％.Second,adding BTPOS as a radical quencher to the photoresist further improved the spatial resolution of the writing structure,and the root mean square roughness of the corresponding flat disk structure is reduced by 31.5％.Finally,the peripheral photoinhibition and chemical quenching effects are combined to reduce the root mean square roughness of the microlens surface to 60％of the original value,further avoiding step-like traces on the surface of the curved contour device caused by low axial resolution.
laser direct writingquencherperipheral photoinhibitionaxial resolutionsurface roughness
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