Dynamics And Extreme Ultraviolet Radiation Of Plasma Produced By Laser Ablation Assisted Discharge

Extreme ultraviolet radiation?output wavelength between 5-50nm?plays a significant role in the application of many fields.For example,the microscope with ultraviolet radiation is an effective solution to high-resolution imaging of nanometer scale structure;the working wavelength for extreme ultraviolet lithography?EUVL?at 13.5nm is the most powerful tool for the fabrication of semiconductor devices with 7nm node or finer characteristic line width in the field of chip manufacturing;extreme ultraviolet light just locates at the resonance of most atom levels,which makes the light a suitable for spectral analysis of materials.The research on extreme ultraviolet radiation is booming and needs further exploration.Laser ablation assisted discharge plasma light source has the advantage of compact size for convenient utilization,which opens up a wide prospect for applications,and becomes a hot spot for research.Laser ablation assisted discharge plasma technique combines laser ablation and discharge produced plasma with the features of high output power,conversion efficiency,space discharge stability,discharge time accuracy and spectral purity.Besides,solid target for the material supplement of discharge plasma is adopted in this method for acquiring an easy tuning of extreme ultraviolet radiation wavelength by the selection of appropriate target materials.Hence,The source is more flexible than the traditional gas discharge extreme ultraviolet light source.The present research has shown that the discharge plasma dynamic process has a dominating influence on the conversion efficiency of extreme ultraviolet radiation.Therefore,a detailed exploration into the influence of laser and discharge parameters on the discharge plasma dynamics is of vital importance in the research on laser ablation assisted discharge plasma generating extreme ultraviolet light source.In this thesis,we carried out research on laser ablation assisted discharge plasma dynamics and extreme ultraviolet radiation from the plasma.A complete set of experimental apparatus for laser ablation assisted discharge plasma was both designed and built,and the selection of the polarity?cathode or anode?and the geometric structure?plane or sphere?of the target were studied.The experimental results have proved that it is beneficial to the generation of Z-pinch plasma when the target material serves as discharge cathode.In the meantime,the intensity for the extreme ultraviolet radiation is 4-fold higher than that when the target material is used as anode.Under the same experimental conditions,a plane target as discharge electrode generates stronger extreme ultraviolet radiation than that from a sphere target.Based on the above results,the polarity and the geometry of the target are determined.Furthermore,we carried out research on experimental conditions for the dynamics of Z-pinch plasma generation and extreme ultraviolet generation from the plasma,the main contents of this thesis include the discharge plasma dynamics?primary stage and pinching stage?and extreme ultraviolet radiation from the plasma.The main achievements are summarized as follows:Firstly,the characteristic of the initial stage of the laser ablation assisted discharge plasma dynamics is investigated.The experimental results showed that two X-ray signals were observed before the discharge current started to increase,the first signal generated near the discharge cathode,the photon energy was in the range of“sub keV”;the second signal generated near the discharge anode,it resulted from the collision of the moving laser plasma with the discharge anode surface.Moreover,the delay time of the second X-ray radiation signal on the dependence of the distance between two discharge electrodes provided that the velocity of the discharge plasma was about 10⁵m/s,and the value depended on the thermal pressure of the plasma and the intensity of the electric field.The results further revealed that the material needed for pinch plasma generation was supplied by laser ablated target material,rather than the small portion of material supplied by laser plasma and discharge current heated target.Th finding is vital importance to the stability of pinch plasma generation.Secondly,the pinching stage of discharge plasma dynamics and extreme ultraviolet radiation from the plasma were studied.The experiments are concentrated on two aspects:the discharge parameters?peak discharge current and the distance between discharge electrodes?and laser parameters?focused diameter of laser beam and laser energy?.In the aspect of discharge parameters,the higher initial power supply voltage was applied,the larger discharge current was obtained,leading to a stronger extreme ultraviolet radiation generated by the plasma.When the distance was increased gradually from 4mm to 10mm,the intensity of ultraviolet radiation showed a tendency of increasing first and followed by decreasing with the optimal distance of 5mm between discharge electrodes.The results showed that when the distance between discharge electrodes was 9mm,zipper effect was observed at the surface of discharge anode and growing toward the surface,restraining the conversion efficiency of extreme ultraviolet radiation.When the distance between discharge electrodes was small,zipper effect was not observed and the method for generating zipper effect as well as that for suppressing it was proposed.In the aspect of laser parameters,the strongest extreme ultraviolet radiation was observed when the beam diameter was about 1mm.When the laser energy was increased gradually from E=40mJ to 90mJ,the ablated material increased slowly as well,leading to a gradually increased extreme ultraviolet radiation.When the laser power was E=90mJ,the diameter of the generated pinch plasma was the smallest,corresponding to the strongest extreme ultraviolet radiation.When the laser energy increased to higher than90mJ,more material was provided by laser ablation,but inner pressure was so high that the electrical energy could not be coupled to plasma effectively,resulting in the weakening of the extreme ultraviolet radiation.Finally,the ultraviolet radiation spectra of laser ablation assisted discharge plasma are studied.The emission ranging from 10nm to 20nm of aluminum and tin targets were obtained with the extreme ultraviolet spectrometer.The results have shown that for aluminum target,the radiation mainly locates around the peaks at 11nm,13nm and 16nm,and those spectral lines were mainly from Al³⁺-Al⁵⁺ions.For tin target,strong radiation signal was observed at13.5nm?2%band width?.In addition,transition probability for ion Sn⁸⁺-Sn¹³⁺and emission wavelengths were calculated by Cowan code,the results show that the spectral peak around13.5nm results from the contribution of Sn¹⁰⁺-Sn¹²⁺ions.And the estimated electron temperature of ions Sn¹⁰⁺-Sn¹²⁺ranges from 20eV to 50eV based on CR model calculation.