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

近年来，随着降低投影曝光系统(步进器)分辨率的提高，焦深的减小成为一个重要的问题。在第一阶段(1992)，利用环形光瞳滤光片和相干中空锥体照明(相干倾斜照明及其绕光轴旋转)进行了三维图像特性评价实验。实验表明，这些光学系统基本保持了较高的分辨率和较深的焦深，但随着背景光级的增加和中程光学传递函数的减小，系统的焦深基本保持不变。为了恢复这种退化，提出了一种利用倾斜照明下具有与环形滤光器旋转同步的透明度分布的可旋转环形光瞳滤光片的有源投影系统，并在下一阶段(1993)进行了具有中心金属网吸收的可变宽度环形光瞳滤光片的研制，相当于具有透明度分布的环形光瞳滤光片。通过对上述主动投影系统的评价实验，明确了分辨率和投影对比度的提高。虽然由于投影强度和对比度不足，目前的方法并不能直接与实际的投影曝光系统连接，但该技术有望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.