New metrology techniques improve the production of silicon diffractive optics
New metrology techniques improve the production of silicon diffractive optics
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新计量技术提高了硅衍射光学器件的生产
DOI：
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发表时间：
2014
期刊：
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通讯作者：
D. Jaffe
中科院分区：
文献类型：
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作者：
C. Brooks;M. Gully;M. Grigas;D. Jaffe
硅浸没光栅和光栅在紧凑性和性能方面优于前表面光栅和由低折射率材料制成的光栅。同时，Si（3.4）的高折射率导致对可允许的凹槽位置误差的非常严格的约束，通常在100 mm内rms < 20 nm，并且重复误差幅度<5 nm。对于这两种类型的器件，我们使用光刻、等离子体蚀刻和湿法蚀刻在硅中产生凹槽。到目前为止，生产商已经使用接触光刻法来图案化UV敏感的光致抗蚀剂作为初始处理步骤，然后将该图案转移到氮化硅层上，该氮化硅层又在将凹槽湿法蚀刻到硅中期间用作硬掩模。对于凹槽生产的每一步，我们都使用了新的和敏感的技术来确定该步骤对相位不均匀性的贡献。了解了错误及其来源，我们就可以对每个步骤实施过程控制。改善了氮化硅掩模蚀刻工艺的等离子体均匀性，并测量了等离子体蚀刻步骤的相位贡献。然后，我们使用灰度光刻技术，其中光致抗蚀剂是故意曝光不足，以测量大规模的非均匀性的UV曝光系统的精度为3- 5%，使我们能够进行校正的光学对准。此外，我们使用了一种新的多重曝光技术结合激光干涉测量UV曝光剂量和线边缘偏移之间的关系。从这些数据中，我们预测的贡献的蚀刻和光刻步骤的光栅表面的相位误差。这些测量结果表明，在曝光步骤期间引入的误差占主导地位的所有其他处理步骤的贡献。本文介绍了用于量化单个工艺对相位误差的贡献的技术，以及为提高整体相位均匀性而采取的步骤。
Silicon immersion gratings and grisms offer significant advantages in compactness and performance over frontsurface gratings and over grisms made from lower-index materials. At the same time, the high refractive index of Si (3.4) leads to very stringent constraints on the allowable groove position errors, typically rms < 20 nm over 100 mm and repetitive error of <5 nm amplitude. For both types of devices, we produce grooves in silicon using photolithography, plasma etching, and wet etching. To date, producers have used contact photolithography to pattern UV sensitive photoresist as the initial processing step, then transferred this pattern to a layer of silicon nitride that, in turn, serves as a hard mask during the wet etching of grooves into silicon. For each step of the groove production, we have used new and sensitive techniques to determine the contribution of that step to the phase non-uniformity. Armed with an understanding of the errors and their origins, we could then implement process controls for each step. The plasma uniformity was improved for the silicon nitride mask etch process and the phase contribution of the plasma etch step was measured. We then used grayscale lithography, a technique in which the photoresist is deliberately underexposed, to measure large-scale nonuniformities in the UV exposure system to an accuracy of 3-5%, allowing us to make corrections to the optical alignment. Additionally, we used a new multiple-exposure technique combined with laser interferometry to measure the relationship between UV exposure dose and line edge shift. From these data we predict the contribution of the etching and photolithographic steps to phase error of the grating surface. These measurements indicate that the errors introduced during the exposure step dominate the contributions of all the other processing steps. This paper presents the techniques used to quantify individual process contributions to phase errors and steps that were taken to improve overall phase uniformity.