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

最近，随着缩小投影曝光系统（步进曝光机）的分辨率的增加，焦深的减小成为重要的问题。本研究旨在通过使用具有透明度分布的环形光瞳滤光片，建立同时保持高分辨力和深焦深的主动缩小投影光学系统。 第一阶段（1992年），已经完成了使用环形光瞳滤光片和相干空心锥照明（相干倾斜照明及其绕光轴旋转）的3D图像特征评估实验。这些实验表明，这些光学系统虽然伴随着背景水平的增加和中程光学传递函数的降低，但基本上保持了高分辨率和深焦深。为了恢复这种退化，有人提出了一种使用可旋转环形光瞳滤光片的主动投影系统，该系统在与环形滤光片旋转同步的倾斜照明下具有透明度分布。下一阶段（1993年）已经完成了具有中心金属网吸收器的可变宽度环形光瞳滤光片的开发，该滤光片相当于具有透明度分布的环形光瞳滤光片。从上述主动投影系统的评估实验来看，分辨率和投影对比度的改进已得到明确证实。尽管由于投影强度和对比度不足，目前的方法无法直接连接到实际的投影曝光系统，但预计该技术可以应用于未来的步进机。

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.