Study Of Optical Design And Compensation On Lithographic Illuminators

As the core fabricating equipment of ultra-large-scale integrated circuits(VLSIC),lithographic exposure tools have long been the fatal flaw in China’s semiconductor industry.Recent years,with the strong support by central government and the unremitting efforts of researchers,some milestones are achieved.However,there is still a huge gap compared with those foreign giants whose lithographic technologies accumulated for decades.Therefore,basal research has still been an insurmountable hurdle in front of China’s lithography development.As an important part of lithography tools,the illuminator impacts significantly on chip manufacturing.Compared with projection objectives,the illumination system has great advantages in terms of design difficulties and manufacturing tolerances,but it strongly demands more complicated modules and higher level of discipline applications.In this paper,basal research on illuminator design and compensation is proposed,and the technical routes for both i-line and ArFi type are given in detail.The design work aims for performance specification and the specific light path,and the compensation targets for performance improvement through understanding and mastering key technologies.A solid establishment of the design target is the premise and guarantee of an excellent design.Since lens design for exposure system is lack of ready-made experience,it is necessary to deepen the understanding of design target and then convert the research results into specific target data.First,based on the relevant principles of physical optics,the key targets of lens design are analyzed and demonstrated at the level of principle deduction,which put the design work to a more theoretical level.To meet the CDU requirements,uniformity of field energy distribution is recommended.To meet the balanced exposing requirements,pupil uniformity and consistency are recommended.To meet the overlay requirements,telecentrisity in a certain matching level is recommended.In addition,the insights of the quantification and decoupling of the above illumination design targets are also presented,too.A qualified designer should fully consider how to achieve balanced optimization at all levels.Accordingly,an excellent design solution has to make various tradeoffs and compromises,avoid redundancy,and maximize the degrees of freedom of optimization variables under a series of constraints.To fully meet the above design target,the design method is proposed,and the key technologies are identified.The root cause of pupil map discreteness is illustrated,and a series of compensation proposed based on the analysis of rod and lenslet integrator,with some verification through an experiment.The pupil energy imbalance has an extremely significant impact on exposure resolution and wafer alignment,which should be considered as a priority in lens design,corresponding to lens aberration decomposition and telecentric matching.Considerations have also been made about aberration control.The level of illumination aberration correction depends entirely on the exposure requirements,rather than “the higher the better”,because of the fact that,the redundant design not only increases design and manufacturing costs,but also reduces system transmittance.Therefore,don’t be too mean to illuminator’s aberration,just make sure of the high level of telecentrisity and consistency across the entire exposure field.Besides,tolerance analysis is an important part of optical design,and a clear tolerance requirement should be listed to provide a solid guarantee for the rest of design work.Some suggestions are also presented on the use of simulation tools and data processing methods.The data process exhibits the rigor of the entire quality of analysis work,which deserves great patience.There should be no ambiguity in the understanding of rules,definitions and concepts.Different from the flexible of design work,the simulation analysis work itself is more like a ruler to measure others,focusing on its own accuracy.Only by calibrating its own "scale" can it precisely measure others,which makes a durable value.At present,i-line lithography technology still has a prosperity and market share.Moreover,with a strong strategic significance in independent research and development,i-line illumination is worthy of deep cultivation in lens design.With the increasing development of aspheric lens manufacturing,i-line optical structure can be greatly simplified and lightweight,thereby improving the transmittance and productivity of the entire exposure system.In this paper,the design target is specifically decomposed,the sub-module design concept and key technology are discussed in detail,and then a lens design solution focusing on telecentric matching is proposed.After the ray tracing simulation,the results show that the matching error at reticle level is better than 1.44 mrad,and the synthetic telecentricity at wafer level is better than 4mrad,which fully meets the requirement,and meets our expectations,too.ArFi lays on the forefront of DUV exposing technology.With the joint efforts of various resolution enhancement technologies(RETs),its application limit can even reach the 7nm process.Accordingly,the requirements and design difficulties have also been greatly improved,mainly on the stricter targets,the more flexible illumination mode transformation and more complicated compensation ability.The research content focuses on the decomposition and solution strategies under the strict requirements,such as telecentric matching,field distribution compensation after lenslet array,etc.The specific design method are expounded with the decomposition of design target on sub modules,then the lens design and its tolerance analysis are completed,with the detailed evaluation through ray tracing simulation.All of which improves the ArFi system design and analysis capabilities.In lithographic illumination,the research of mathematic control and exposure compensation on various sub modules are more complicated than the optical design.The sifting method on pupil shape with its corresponding zoom status for off-axis control is fully studied,and the compensation strategy for pupil uniformity is discussed,too.For off-axis control,by using LightTools macro programming and external execution of Matlab,all possible pupil shapes and their zoom position information are obtained through exhaustive method,and then the database is filtered and reconstructed by erasing useless zoom status,and then a clear zoom-drive connection are established.For pupil compensation,a parametric model of the compensation target is rebuilt,and the overdetermined equation is solved in a way of constrained optimization solution,then a series of the minimum stroke of the compensator are obtained.Besides,the theoretical calculations are in good agreement of simulation results.