Reflectance Improvement of Mo/Si Multilayer Mirrors and Masks for Extreme Ultra-Violet Lithography
|Keywords:||極紫外光微影;多層膜;屏障層;覆蓋層;介面工程;Extreme Ultra-Violet Lithography (EUVL);Multilayer;Barrier layer;Capping layer;Interface-Engineered|
|Abstract:||本研究主要探討極紫外光微影之鉬矽多層結構之設計與特性。其四分之一波膜反射率可以藉由調整膜層厚度的最佳化程序達到較好效能，而於商業用的極紫外光微影系統其要求多層膜需具備高反射率效果及較好的穩定性。本研究以介面工程(interface-engineered)完成了在13.5-nm波長具有72.62%反射率及0.828 nm半高全寬介面工程鉬矽(Mo/Si)多層膜結構之開發，此結構則包含被boron carbide (B4C) 與ruthenium (Ru) 材料所分隔的鉬矽交錯層，Ru 和B4C 的屏障層(barrier layer)則被分別插入於鉬和矽(Mo-on-Si)及矽和鉬(Si-on-Mo)的介面中形成54對的Ru/Mo/B4C/Si多層膜。與傳統的四分之一波膜相比，在13.5-nm波長大約有4%的反射率改善，將1.4 nm厚的Ru的0.2 nm厚的 B4C插入鉬矽多層膜的介面可以達到此最佳反射率。另外，現今主要關注的議題之一是由於表面光子輻射所導致的光學式表面汙染將會減低光電元件的使用壽命；而從模擬結果看來最有希望成為覆蓋層(capping layer)的材料是Ru。藉由將Ru覆蓋於介面工程鉬矽多層膜結構最上層可達到對極紫外光的氧化抵抗力，在經由極紫外光曝光環境的過程中這個結構能夠具有高反射率以及抗氧化力。|
In this study, the design and characteristics of Mo/Si multilayers for extreme ultra-violet (EUV) lithography have been investigated. The reflectance of the quarter-wavelength multilayers can be enhanced further by optimized procedures with which the layer thicknesses are varied for best performance. For commercial EUV lithographic systems multilayer mirrors and masks require with higher reflectance and better stability. Interface-engineered Mo/Si multilayers with 72.62% reflectance and a FWHM of 0.828 nm at 13.5-nm wavelength have been developed and simulated. The design was achieved with 54 pairs of Ru/Mo/B4C/Si multilayer. This structure consist of alternating Mo and Si layers separated by thin boron carbide (B4C) and ruthenium (Ru) layers. Ru barrier layer was inserted into a Mo-on-Si interface and B4C barrier layer was inserted into a Si-on-Mo interface to improve the EUV reflective multilayer properties. About 4 % improvement of the reflectance at 13.5-nm wavelength compared to standard quarter-wave stacks can be acquired by the design. The best results according to simulation were obtained with 1.4-nm-thick Ru layers for the Mo-on-Si interfaces and 0.2-nm-thick B4C layers for the Si-on-Mo interfaces. The contaminations of optical surfaces by photon irradiation in the presence shorten optics lifetime become one of the main concerns. From simulation results, the most promising candidate seems to be Ru. The improvement in oxidation resistance of EUV multilayers has been achieved with Ru -capped interface-engineered Mo/Si multilayers. This structure achieves high reflectance and the great oxidation resistance during the EUV exposure in a water-vapor (oxidized) environment. Based on simulation results, we calculated reflectance for the reflective multilayer mirrors in a EUVL procedure and concluded that Ru-capped Ru/Mo/B4C/Si multilayers have a higher performance than quarter-wave Mo/Si multilayers. The simulation results clearly show reflectance improvement of our designed structure compared with standard Mo/Si multilayer structures.
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