Improvements for resolution and focul depth by using tilted illumination and active pupil control in reducing projection optical system

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Recently the reduction of the focal depth becomes important problems as increase the resolved power for the reducing projection exposure systems (stepper). The present study has aimed to establish active reducing projection optical system keeping both high resolved power and deep focal depth, by the use of an annular pupil filter with transparency distribution.At the first stage (1992), 3D image characteristics evaluation experiments have been achieved under the use of annular pupil filter and coherent hollow cone illumination (coherent tilted illumination and its rotation around the optical axis). These experiments have shown that these optical systems basically keep the high resolution and the deep focal depth, although company with the increase of the background level and the reduction of the middle range optical transfer function. An active projection system using a rotatable annular pupil filter with transparency distribution under the tilted illumination synchronized to the rotation of the annular filter has been proposed to restore such degradation.The development of the variable width annular pupil filter with center metal mesh absorber, which is equivalent to the annular pupil filter with transparency distribution, has been done at the next stage (1993). From the evaluation experiments of above active projection system, improvements to the resolution and the projection contrast have been clearly confirmed.Although present approach is not directly connected to the practical projection exposure systems because of the insufficient projection intensity and contrast, it is expected that this technique can be applied to the future steppers.