Optical Coatings For Projection Objectives Of Immersion Lithography

In semiconductor technology,in order to increase the degree of device integration,it is necessary to advance to a new process node according to Moore's Law,and the core issue is how to improve the lithography resolution.With the mainstream process wavelength fixed at 193 nm,and the process factor has been reduced to the limit,immersion lithography,which adds immersion liquid to increase the NA value between the last element and the silicon wafer,has become the only choice.Through the objective lens immersion method,the NA value of the projection objective can be increased to 1.3¹.4,which meets the requirements of the dozens of nanometer lithography technology nodes.However,the application of the objective immersion liquid technology makes the corresponding optical design and manufacturing more demanding technical challenge.The thin-film optical elements in the objective lens are faced with many problems such as the realization of optical quotas,the adjustment of the immersion environment,and the ensurence of the laser irradiation life.The transmission and polarization aberration of the elements in the objective lens must be strictly met.In order to solve the problems mentioned above,this paper has studied the following contents:1.Design of broad angle polarization preserving film and its preparation.The broad angle anti-reflection thin-films ware designed using the method of combining a ultra-low refractive index with a conventional PVD material.When the refractive index 1.18 was used,we can achieve an anti-reflection coating with a reflection of less than 1.1%,a polarization separation of less than 0.16%with in incident angle of 0to 70 degree.To satisfying the requirements of immersion lens,SiO₂ was used to be outer layer material and the adjustable refractive index film layer were used to design an anti-reflection coating system.We can achieve an anti-reflection coating with a reflection of less than 0.2%,a polarization separation of less than 0.08%with in incident angle of 0 to 70 degree.In addition,an ultra-low refractive index film layer with a refractive index of 1.28 was prepared by sol-gel method.And the homogeneity of the optical properties on the steep concave and convex lens was improved by using a mixed material,and polarization aberration was suppressed.2.Study on lifetime of optical film in immersion environment.First,through the combination of ambient temperature immersion experiments and high temperature accelerated immersion experiments,thin film materials suitable for immersion environments were screened.Through experiments,only the SiO₂ film prepared by the PVD method has long-term stability,and the optical properties of the film system with SiO₂ as the outermost layer have not changed significantly after the experiment.Second,the sol-gel method is used to prepare the dense SiO₂ film,and its surface was hydrophobic modified to achieve the optical film hydrophobic properties,and then the surface contact angle was up to 110 degree.Thirdly,the immersed environment stability of the hydrophobic SiO₂ film layer prepared by sol-gel was tested.After the experiment,the material has a certain immersion environment stability.3.Study on laser irradiation life of coated elements.An accelerated laser irradiation life evaluation scheme for lithographic lens elements was established.Through in-situ and continuous monitoring of the sample transmission,combined with changes in ellipsometric parameters,surface morphology before and after irradiation experiment,the laser irradiation life of the sample was evaluated.The anti-reflection film prepared by the thermal evaporation and ion beam processes remained intact after 1.2MJ/cm² cumulative irradiation of a 4kHz high-frequency laser light source.