New-Generation Optical Stepper With High N.A. g-Line Lens

A new optical stepper, NSR-1505G6E, has been developed and it will be one of the most promising exposure apparatuses for half-micron lithography in the next era. g-line is used as an exposure wavelength, and the 5X-type projection lens has 15 mm by 15 mm field size. Its N.A. (numerical aperture) is 0.54, the maximum value among the 5X-type g-line projection lenses ever announced. First, the capability of high N.A. is demonstrated by a theoretical approach. Conventional optical theory says that resolution power is proportional to λ/N.A., but multi-reflection within the photoresist is not taken into consideration. To investigate resolution power based upon resist patterns, one of the most powerful approaches is "the vector model" developed by M. Yeungl in 1988. A similar simulator has been developed and has been used to confirm that a high N.A. lens has high resolution power even with the multi-reflection effect within photoresist. Secondly, performance of the projection lens is shown on experimental data basis. Advantages and disadvantages of the projection lens are discussed in comparison with an i-line lens with same resolution power. Investigation on N.A. is also made, comparing resist patterns exposed by some lenses with various N.A. values. Finally, the total system, including such new functions as chip-leveling and auto-focus, is described.