Research On Ton Beam Figuring For Lenses Used In Projection Lithographic Objective

As the development of the integrated circuit, the lithographic plants arebecoming more and more important. To develop the projection lithographic objectivesystem with high NA value and large filed range, which is the heart part of thelithographic machine, surface errors of lenses used in the system must be controlledwithin a very low level. The purpose of the paper is to find out a way to fabricatelenses with surface errors less than root mean square value1nanometer, by the useof Ion Beam Figuring technic, to meet the requirement of projection lithographicobjective system. In the meantime, because the level of optical surface test willinfluence the final fabrication accuracy a lot, a serious of researches, includingproblems exist in the vertical interferometric testing and parabolic testing are done tomeet the surface test requirement of lenses used in projection lithographic objective.1. Carrying out researches on optical test, to improve the surface test accuracyand settle the foundation of ion beam figuring with high accuracy. Principles ofFiezau phase-shift interferometry, phase shift algorithms and absolute tests areintroduced, and then the main factors that will influence the final test accuracy areanalyzed, including the phase shift error, source error, vibration, temperature, airpressure and humidity in the interfering cavity. After that, two spheres absolute testis used to calibrate surface error of the transmission sphere when used in Fizeauinterferometer vertically, and compared with results tested when it is usedhorizontally, errors including gravity and mounting forces are analyzed. Finally, parabolic lens is tested by using computer generated holograph and focus ballmethod separately, and the test results are compared with each other, errors ofcomputer generated holograph method are analyzed.2. Principle of ion beam figuring is introduced and the basic characters of ionbeam figuring are researched. By a serious of experiments, the stability of ion beamin four months and ten hours, the uniformity when removing a constant thicknessand also the rapidity of convergence for one single polish are tested. To test thepossibility of using a single lens to correct the wavefront aberration of the wholeprojection lithographic objective system, a surface generated by random Zernikepolynomial is fabricated by ion beam and very good accuracy is achieved.3. Factors that will influence the final fabricate accuracy of ion beam figuringare analyzed. The ion beam removal rate is tested when the dwell time varies from5seconds to150seconds, also the influence of positioning error are calculatedsimulationaly. By control of these factors, surface error with root mean square valueof less than0.45nm is achieved on both sphere and flat. The principle of ion beamfiguring errors in middle and high spatial frequency range is introduced and by theiteration experiments with super smoothing technic, errors in low, middle and highspatial frequency range are all corrected to a perfect level and can meet therequirement of projection lithography objective.4. The way of fabricating aspheric surface is studied and two different methodsare used to polish aspheric surface by ion beam. The first one is to grind the lens toasphere and then to polish the surface error by ion beam, the second one is to grindthe lens to best-fit sphere and then polish to asphere by ion beam directly. Root meansquare value of less than1.5nm is achieved by both of the methods.