低损耗193nm反射膜的设计

采用1/4规整膜系,从电场强度、吸收损耗及散射损耗的分布几个方面,对影响193 nm反射膜性能的因素进行了分析。以分析结果为基础,对低损耗193 nm反射膜的设计进行了探讨。结果表明:在空气侧的外膜层中电场强度较大,随着层数向内过渡,电场强度迅速减小;高折射率材料膜层的吸收损耗明显高于低折射率材料膜层的吸收损耗,而且靠近空气侧最外层的高折射率膜层的吸收损耗最大;按由外层向内层过渡的方向,吸收损耗迅速减小,减小的速度与高低折射率材料折射率的比值相关;表面散射损耗与两种材料的折射率比值成正比,但折射率比值减小后只能通过增加膜层数来获得一定的反射率,而这样又会使表面粗糙度增加,并且引入其它的损耗。因此,选择折射率差值适当大一些的材料对降低散射损耗是有利的。设计了27层膜堆的193 nm反射膜,设计反射率在98%以上。

A quarter-wave conventional film system was adopted to analyze the main effect of the distribution of electric field intensity, absorption loss and the scattering loss on the properties of 193 nm HR mirror. On the basis of the analytic results, the design of a low-loss 193 nm HR mirror was discussed, the results indicate that the electricity field intensity decreases quickly from the outer layer to the center layer, so that the outer layer next to the air side shows the biggest electricity field strength. The absorption losses of the high refractive index layers are higher than that of the low refractive index layers. The absorption losses decrease sharply with the changing rate related to the ratio of the refractive index of the high-low refractive index materials from the outer layer to the center layer. The surface scattering losses increase proportionally to the ratio of the refractive index of the high-low refractive index materials. But the lower ratio of the refractive index will require more layers to gain certain reflectance obviously, it will result in higher surface roughness and other loss, so it is benefit to select relatively high ratio of the refractive index for reduction of the scattering loss. Finally, a 193 nm HR mirror was designed and more than 98% designing reflectance is given.