Study On Micro-removal Mechanism Of Single Crystal Calcium Fluoride For Chemical Mechanical Polishing

Due to its high ultra-violet transmittance,laser damage threshold and low refractive index,single crystal calcium fluoride has become one of the indispensable lens materials in extreme violet lithography system.As the 193 nm deep ultraviolet lithography technology approaches its limit,it is imperative to develop 13.5 nm extreme ultraviolet lithography technology to fabricate semiconductor chips with lower feature sizes.Under this background,calcium fluoride crystal has replaced fused quartz as the most important objective material in deep ultraviolet/extreme lithography system.EUV lithography technology puts forward very stringent requirements on the surface accuracy and quality of CaF2 object lens.For instance,subsurface damage in the substrate will significantly reduce the laser damage threshold of the objective material,and deteriorate the processing resolution and the life of the objective system in extreme ultraviolet lithography.Therefore,it is necessary to achieve ultra-precision surface manufacturing of the calcium fluoride optical surface for the development of extreme ultraviolet lithography technology.The study on the theory and technology of ultra-precision surface processing will help to improve the processing quality of calcium fluoride objective lens and enhance the research and development level of domestic lithography.It is significant to promote China's international status in the field of integrated circuit manufacturing.In order to solve the problems in the chemical mechanical polishing of single crystal calcium fluoride,scratch experiments of single crystal calcium fluoride against diamond tip were carried out in different water-containing environments.The role of environmental water in the material removal was detected based on the characterization of the difference in the material removal volume,surface micro-morphology,plastic deformation damage and chemical composition.After that,the tribochemical removal of calcium fluoride was studied and its mechanism was revealed through analyzing the variation of removal volume,surface quality,subsurface lattice structure,friction and adhesion force in various pH value conditions.Finally,the atomic removal limit of calcium fluoride,i.e.,less than single F-Ca-F ionic layer and more than multi F-Ca-F ionic layers,were explored under the tribochemical reaction.Based on the above mentioned results and mechanisms,an optimization processing of chemical mechanical polishing was proposed,an ultra-smooth and low-damage surface was obtained after polishing.Based on these systematical investigations,the main conclusions can be summarized as following:(1)The influence mechanism of water on the mechanical removal single crystal calcium fluoride was detected based on the scratch results in the different water-containing environments.The material removal of calcium fluoride was dominated by stress-induced slipping under dry nitrogen condition,which resulted in the formation of smooth worn surface.Here,the surface damage was mainly attributed to mechanical wear,which could be fitted by the classical Archard plastic wear model.Differently,stress-induced ions dissolution occurred in the water-containing environments,and promoted the crack initiation on the surface and subsurface in the contact area.Then,the spalling or delamination of the calcium fluoride material would finally happen with the sustained dissolution of ions at the crack tip under the reciprocating scratching of the scratch tip.In summary,the degree of plastic deformation and sub-surface damage of the material can be significantly suppressed when the material removal was dominated by ionic dissolution and delamination in water-containing conditions.(2)The mechanism of tribochemical removal of single crystal calcium fluoride was revealed based on the studies in different water-containing environments as well as KOH solutions with various pH value.It was found that the surface of calcium fluoride was neural or little negatively charged in comparison with negatively charged of silicon dioxide tip surface due to hydrolysis and ionization actions,resulting in an electrostatic repulsion between these two surfaces.Then,material removal originating from the formation of chemical bond bridges between sliding interface and the bond broken in substrate can be rule out.Further analysis indicated that the tribochemical removal of calcium fluoride was mainly attributed to the removal of surface softening layer formed under the action of solution corrosion.In addition,the surfacial and subsurfacial crystal structures of the calcium fluoride remained intact after surface softening layer removed.(3)Removal limit of the single crystal calcium fluoride material at atomic level was explored.After etched by KOH solution,less than single F-Ca-F ionic layer(0.315 nm)was removed from the cleavage surface by the tip scratching.The minimum removal depth was around 0.23 nm,indicating that Ca-F~+or F~-partial ion layer removal could occur in F-Ca-F ion layer under certain condition.Meanwhile,multi F-Ca-F ionic layers were removed from the cleavage surface by the tip scratching in KOH solution.This above method may be used to remove the step structure on the polished calcium fluoride surface.(4)Optimizing processing on chemical mechanical polishing of single crystal calcium fluoride was proposed to fabricate a super-smooth and low damage calcium fluoride surface.Based on the previous experimental results and mechanisms,an optimization scheme of CMP process for single crystal calcium fluoride was proposed.A prefect surface with a RMS surface roughness less than 0.4 nm in the scanning region of 10?m×10?m was fabricated,and the sub-surface lattice remained intact.