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

近年来，随着投影曝光系统（步进机）分辨率的提高，焦深的减小成为重要的问题。本研究的目的是利用具有透明度分布的环形光瞳滤波器，建立既保持高分辨力又保持深焦深的主动缩小投影光学系统。在环形光瞳滤波器和相干空心锥照明下进行了三维图像特性评价实验（相干倾斜照明及其围绕光轴的旋转）。实验结果表明，虽然伴随着背景能级的增加和中程光学传递函数的减小，但这些光学系统基本上保持了高分辨率和深焦深。为了恢复这种退化，人们提出了一种主动投影系统，该系统使用一个可旋转的环形光瞳滤波器，该滤波器在与环形滤波器的旋转同步的倾斜照明下具有透明分布。下一阶段（1993年）已经完成了具有中心金属网吸收体的可变宽度环形光瞳滤波器的开发，该滤波器相当于具有透明分布的环形光瞳滤波器。通过对上述主动投影系统的评价实验，清晰地证实了其分辨率和投影对比度的提高，尽管由于投影强度和对比度不足，目前的方法不能直接与实际投影曝光系统相联系，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.