Reserch On Femtosecond Laser Ablating Metal For Laser Assisted Discharge EUV Source Application

Extreme Ultraviolet lithography(EUVL)technology is the core technology to make chips with feature size less than 22 nm,and extreme ultraviolet light source is an important part of EUVL system.Thus,the research on extreme ultraviolet light source is of great significance for EUVL development.An important method to generate EUV is achieved by laser assistant discharge plasma technology.By laser ablating target material,enough metallic particles will be provided between two high voltage poles as discharge target and form pinched plasma from high-pressure discharge caused by metallic particles,and then produce strong EUV emission.The existing light source employs nanosecond laser for material ablation.During the ablation process,there are metal debris of several microns generated,and this causes unstable discharge as well as results in severe pollution of discharge electrodes and collecting system.To solve this problem,we propose to employ femtosecond laser for material ablation,which can produce smaller metallic particles,so as to provide ideal metallic particle target for plasma discharge electric light source.Due to the low pulse energy of high repetition rate of femtosecond laser,providing enough ablation materials through optimizing experimental parameters to guarantee subsequent ideal discharge is one of the crucial issues in this field currently.In this thesis,we study characteristics of femtosecond laser ablated metallic aluminum and tin target under the condition of low femtosecond laser pulse energy.In vacuum environment,we carry out the study on high frequency femtosecond laser ablated metallic aluminum and metallic tin.By changing the relative position of the focusing lens and the target,the sizes of the ablated spots at different positions are obtained.To further study the relation between laser fluence and the ablation depth of target material,we change laser pulse count for in-depth studies,and obtain the curve of laser fluence vs.ablation depth.Then we derive the required ablated target volume for ideal EUV emission and corresponding laser fluence by calculation.Afterwards,we study the required target ablation efficiency by EUV source.The result shows that the highest ablation efficiency of metallic aluminum reaches the required ablation volume 2×103?m3 when the distance between focusing lens and target is 10 mm.For metallic tin,the highest ablation efficiency reaches the required ablation volume when distance between focusing lens and target is 5mm.It is found that the optimal parameters for ablation efficiency are varied for different targets.In order to accumulate enough metallic particles between two high voltage electrodes with low femtosecond laser pulse energy so as to improve discharge performance and alleviate the serious pollution problem due to particle splashing with femtosecond laser ablation in the vacuum,we carry out the studies on femtosecond laser ablated metal in ambient atmosphere.In this case,we study the relation between the size of ablation spot and laser energy,and obtain the sizes of ablation spots at different positions in front of focus.By studying the relation between pulse count and ablation depth,we get the relation between the laser fluence and ablation depth.Then by utilizing light scattering method,we observe the mass migration trace of femtosecond laser ablated metallic materials in the ambient atmosphere.“Metal cloud” constituted by metallic particles forms at the position 1mm-3mm away from the target surface due to the blocking of environment gases was observed.The experiment result indicates that “Metal Cloud” stays nearby the target for about 25 ms,meaning that the environment gases have confining effect on femtosecond laser ablated materials.This kind of “Metal Cloud” formed by the blocking of environment gases is beneficial to the accumulation of solid materials required between EUV discharge electrodes,namely,it can provide sufficient metallic materials between two electrodes under the condition of high repetition rate and low pulse energy femtosecond laser ablation.This research provides technological support for the development of high repetition rate and stable EUVL industrial light source.