Optical Design And Imaging Performance Compensation For The Lithographic Lens

To meet the industrial production of integrated circuit below the 100nm node, this paper focuses on the optical design and simulation for the high numerical aperture (NA), deep ultraviolet (DUV) projection lithographic lens. It means to design a ArF lithographic lens (working wavelength 193nm, NA0.75) with the considerations for the technical requirements and constraints such as optical fabrications, optical test, coatings, optical material, mechanical structure, and various kinds of resolution enhancement technology (RET). While minimizing the manufacturing cost without the violation of the specifications, this paper also focuses on the methods and solutions for the imaging performance compensations.Currently, the most advanced projection lithographic lens achieves the exposure node less than 25nm, while the number of lens components (including non-spherical components and reflection components) is over 20, and wavefront aberration is better than 1nm (RMS) across full field 26mm×8mm. As the most sophisticated and complex optical systems ever since, the 193nm (ArF) projection exposure system involves the very extensive and cutting-edge disciplines, these disciplines includes system design, fabrications, system integrations and so on. These disciplines intertwined and penetrated very closely in the projection exposure system, and it is impossible to design a modern lithographic lens just simply rely on brain power, experience, and experimental exploration. It is necessary to use of modern commercial optical design code and collaborate with all the key technology in the manufacture stage to aid the optical design and integration for 193nm exposure system.In order to complete the optical design for the NA0.75 lithographic lens, this paper carried out the following research:First, the image quality evaluation: It is very important to find an appropriate image quality evaluation for high-quality lithographic lens which has extremely small aberrations. Based on the different impacts for lithography imaging, it is usually divide the wavefront aberration into low-frequency, mid-frequency, and high- frequency by using the concept of spatial frequency and equivalent pupil. This paper summarizes the basis of this spatial frequency-based division. And taking into account the limited resolution of the actual wavefront measurement system, this paper put forward a new classification and definition of wavefront aberration. This new image quality evaluation method is very suitable for optical design and tolerance analysis of the NA0.75 lithographic lens; in particular, this evaluation is particularly conducive to selecting the optimal compensators.Second, the optical design and simulation analysis: There are some critical innovated methods and implementation to achieve high NA lithographic lens, such as the use of aspherical components, liquid immersion and reflection components. By learning and review the history and evolution of lithographic lens, several lens design forms are proposed for the NA0.75, 193nm (ArF) lithographic lens. This paper compares the four different design forms, and taking into account the advantage and disadvantages, one lens form is selected and then optimized by the CODE V and custom macro programs. The final results and simulation analysis are also presented.Third, tolerance analysis and seeking the optimal ways of image performance compensations: In order to achieve near-perfect image quality of high-NA lithographic lens, there are a series ways of image performance compensation to relax the extremly tight lens manufacturing tolerances, such as recomputation, computer aided assembly adjustment, fine surface figuring, real-time adjustment by using the deformable mirror technology and so on. According to the image quality evaluation for the NA0.75 lithographic lens, this paper analyzes the impacts at the different wavefront aberration spatial frequency for different tolerances. Thus gives the preferred ways of compensation for the low-order and high-order wavefront aberration. Especially for fine surface figuring technology which is used to compensate the high-order aberrations, this paper describes the calculation for the surface shape removal functions and also gives two simulation results.Final results and simulation analysis show that the image quality evaluation, lens design form, the imaging performance compensation, and the calculation of surface shape removal functions described in this paper can effectively reduce manufacturing tolerances of the NA0.75 lithographic lens. All of these methods and analysis may provided some useful theoretical support for the manufacture of lithographic lens with the 100nm node ,or even smaller nodes.