Cryogenic cleaning of tin-drop contamination on surfaces relevant for extreme ultraviolet light collection

N. Böwering, C. Meier
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引用次数: 2

Abstract

Improvement of tool reliability and uptime is a current focus in development of extreme ultraviolet lithography. The lifetime of collection mirrors for extreme ultraviolet light in tin-based plasma light sources is limited considerably by contamination with thick tin deposits that cannot be removed sufficiently fast by plasma etching. For tin droplet splats sticking to large substrates, we have developed and compared several efficient cleaning techniques based on cryogenic cooling. A silicon carbide substrate and different silicon wafer samples with up to 6 inch diameter with the surface uncoated, multilayer-coated, unstructured and grating-structured were tested. After tin dripping onto heated samples, embrittlement of droplet contamination is induced in-situ by stresses during phase transformation, following the initiation of tin pest with seed crystals of gray tin. Conversion of initially adhesive deposits to loose gray tin has been reached in less than 24 hours on all tested surfaces by continuous cooling with cold nitrogen vapor to temperatures in the range of -30 to -50 °C. Alternatively, stress-initiated tin-removal by delamination of beta-Sn droplet splats has been attained via contraction strain induced by strong cooling to temperatures of around -120 °C. Profilometry has been used to analyze the bottom side of tin droplet splats removed from a grating-structured wafer. The in-situ tin cleaning techniques give results comparable to fast ex-situ cleaning that has been achieved either by sample immersion in liquid nitrogen or by splat removal after CO2 snowflake aerosol impact using a hand-held jet-nozzle. The implementation of the in-situ phase-conversion concept for the cleaning of collector mirrors in commercial light sources for lithography is discussed.
低温清洗与极紫外光收集有关的表面锡滴污染
提高工具的可靠性和正常运行时间是目前极紫外光刻技术发展的重点。在锡基等离子体光源中,极紫外光收集镜的寿命很大程度上受到厚锡沉积物污染的限制,而厚锡沉积物不能通过等离子体蚀刻足够快地去除。对于粘附在大型衬底上的锡滴片,我们已经开发并比较了几种基于低温冷却的高效清洁技术。测试了碳化硅衬底和直径不超过6英寸、表面未涂覆、多层涂覆、非结构化和光栅结构的硅片样品。锡滴入加热试样后,在相变过程中应力的原位作用下,锡害虫由灰锡种子晶体引发,从而导致锡污染的脆化。在所有测试表面上,通过用冷氮蒸汽连续冷却至-30至-50°C的温度范围,在不到24小时内将最初的粘合剂沉积转化为松散的灰色锡。另一种方法是,通过强烈冷却至-120℃左右的温度引起的收缩应变,通过β - sn液滴薄片分层来实现应力引发的锡去除。用轮廓法分析了从光栅结构晶圆片中取出的锡滴片的底部。原位锡清洗技术提供的结果可与通过将样品浸入液氮或使用手持喷射喷嘴去除CO2雪花气溶胶撞击后的飞溅物来实现的快速非原位清洗相媲美。讨论了用于光刻商业光源集电极镜清洗的原位相变概念的实现。
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