University of California, Lawrence Livermore National Laboratory, Livermore, California, 94551.
J Xray Sci Technol. 1994 Jan 1;4(3):167-81. doi: 10.3233/XST-1993-4301.
Soft x-ray projection lithography (SXPL) is an attractive technique for the fabrication of high-speed, high-density integrated circuits. In an SXPL stepper, the x-ray imaging mirrors consist of multilayer coatings deposited onto high precision substrates. The stepper is intended to fabricate ultra-high spatial-resolution structures with a minimum feature size of <0.1 μm. To achieve this resolution, the imaging mirrors must maintain a very precise surface figure while being exposed to x radiation. Failure to achieve and maintain the mirror surface figure will distort the wavefront propagating through the imaging system and will degrade system resolution. The required surface figure accuracy for each imaging mirror depends upon the required resolution, the wavelength, and the optical design. For conventional SXPL stepper designs, the total (peak-to-valley) surface figure error budget per mirror is approximately ±1 nm. Due to material properties at soft x-ray wavelengths and practical fabrication considerations, x-ray multilayer mirrors have limited reflectivities. A fraction of the incident x radiation is absorbed in the multilayer coating. This absorbed radiation constitutes a thermal load on the mirror, thereby distorting its shape and compromising the accuracy of its surface figure. In this paper, we analyze the thermally induced distortion on the imaging optics and conclude that the maximum allowable thermal distortion limits the maximum allowable x-ray power transported to the wafer and limits the minimum acceptable multilayer mirror reflectivity. The penalty for either insensitive x-ray resists or inefficient mirror reflectivity is a decrease in system throughput which cannot be compensated with increased source power either collected by condenser optics or generated by the source.
软 X 射线投影光刻（SXPL）是一种用于制造高速、高密度集成电路的有吸引力的技术。在 SXPL 光刻机中，X 射线成像反射镜由沉积在高精度基底上的多层涂层组成。光刻机旨在制造具有<0.1μm 最小特征尺寸的超高空间分辨率结构。为了实现这一分辨率，成像反射镜在暴露于 X 射线时必须保持非常精确的表面形状。未能实现和保持反射镜表面形状将扭曲通过成像系统传播的波前，并降低系统分辨率。每个成像反射镜所需的表面形状精度取决于所需的分辨率、波长和光学设计。对于传统的 SXPL 光刻机设计，每个反射镜的总（峰谷）表面形状误差预算约为±1nm。由于软 X 射线波长的材料特性和实际制造考虑因素，X 射线多层反射镜的反射率有限。一部分入射 X 射线被多层涂层吸收。这种吸收的辐射构成了镜的热负荷，从而扭曲其形状并损害其表面形状精度。在本文中，我们分析了成像光学器件的热诱导变形，并得出结论，最大允许热变形限制了可传输到晶圆的最大允许 X 射线功率，并限制了最小可接受的多层反射镜反射率。对不敏感的 X 射线抗蚀剂或低效反射镜反射率的惩罚是系统吞吐量的降低，这不能通过增加收集器光学器件收集或源产生的源功率来补偿。
