High Resolution Lens System For Submicron Photolithography

AbstractNew technology in the field of integrated circuits has created a demand forphotolithographic projection lenses capable of producing submicron line structures.Tropel has introduced a series of UV Micro Tropar objectives for this specificapplication.These lenses achieve their superior performance by utilizing the 365nm lineand increasing the numerical aperture to 0.40. Optical design considerations in thedevelopment of these lenses included appropriate materials selection, field vs. aperturetradeoffs, telecentricity constraints, and partial coherence effects due to underfill ofthe lens pupil. Manufacturing problems are accentuated by the extremely tight toleranceson all of the construction parameters of the lens system. Special interferometric testequipment, used to provide precise measurements of each lens element, has proven to beindispensable in the fabrication of these objectives. Mounting of these elements on acommon optical axis is an absolute necessity if the ultimate resolution of the design isto be achieved. Testing of these lenses is also a challenge due to the high spatialfrequencies involved and limitations on visual examination of the image. Methods ofcircumventing these difficulties will be discussed. Typical MTF test results andperformance data will be presented.IntroductionIn recent years, there has been a great deal of interest in the design and manufactureof high resolution projection lenses for optical lithography in the semi -conductor