Study On Direct-writing Photolithography With Optical Vortex Beams

Direct writing is one of the main technologies for micro/nano structure manufacturing.It is being more and more important in material processing.Laser direct writing is a branch of direct writing technology.It has the advantages of low cost,simple structures,high speed and large degree of processing freedom.However,due to the diffraction limit,laser direct writing is facing a difficulty of decreasing linewidths to a value smaller than a wavelength.Introducing vortex beams to the laser direct writing system may help breaking the diffraction limit,because of the dark field distribution in their focused light field which is beyond the diffraction limit.There are two cases of vortex beam direct-writing photolithography.One is direct wiritinng with a single vortex beam.This is a direct usage of vortex beams and they are used as writing beams.The existing theory shows that there is a dark hollow in the focused light field of a vortex beam,and the dark hollow size is beyond the diffraction limit,which provides precondition for super-resolution direct writing.By deriving the exposure dose distribution of vortex beam direct writing,its ability of processing super-resolution structures is further proved.A 146.9 nm super-resolution line was fabricated from a positive photoresist film,which is consistent with the theoretical deduction of direct-write photolithography with a single vortex beam.However,a rather small processing tolerance,which is less than 20 percent,is revealed.To deal with such a defect,the usage of multilevel fractional-order vortex beams is proposed,and the 2-level,1.5-order vortex beam with a processing tolerance close to 100 percent is found.The sub-wavelength lines obtained in this study are not in isolation,but they still have potential applications in the fields of nano waveguides,etch masks,micro/nano fluidics,and so on.The other is direct writing with double beams.This is an indirect usage of vortex beams and they are used as confining beams.The non-indipendence of the subwavelength lines can be overcome by this direct-writing approach.It is shown that the focused light field of a vortex beam can constrain the diffraction-limited wiriting beam to a subwavelength scale.It is essential to introduce a photochromic film to the lithographic substrate to achieve the above constraint by means of absorbance modulation.A simplest composite film consists of a PVA film mixed with DTE and a photoresist film is constructed based on the photochromic performance,thermal stability,water solubility,etc.,of the photochromic material.Existing absorbance-modulated direct-writing photolithography is usually limited to static operations.Through theoretical modeling and simulation,it is proposed that the contrast of the vortex beam is the main factor affecting the static absorbance modulation result.And further more,the theory of exposure of absorbance-modulated direct writing is extended to a stage of dynamic operations,and a smoothing effect,which is the main factor affecting the dynamic absorbance modulation result,is proposed.With guidance of these conclusions,the confining ability of vortex beams with different level and order is analyzed and predicted.Finally,an independent direct-writing line with a linewidth of?400 nm which is beyond the diffraction limit was fabricated in the experiment,though there is a limitation of the performance of the photochromic film.