Feature Of The Week 08/31/2018: Spectroscopic Characterization of Si/Mo Thin-film Stack at Extreme Ultraviolet Range
Using extreme ultraviolet (EUV) radiation for nanoscale imaging has recently seen much interest. As actinic patterned mask inspection tools are not available, chipmakers have to rely on wafer inspection to identify mask defects. To fulfill the requirements of mask inspection, EUV sources with high brightness, high stability (spatial and temporal), and cost effectiveness are needed. In a laser-produced-plasma (LPP) generated EUV system, a high-intensity laser beam is focused onto a tin target to form a hot plasma, which emits ultraviolet light. In this study, a fiber-laser excitation source was used to generate stable 13.5-nm EUV for spectroscopic optical coherence tomography (OCT) applications. The laser driver for the present tabletop LPP based EUV generation is a four-stage, mirror-free, nanosecond Yb:fiber master oscillator power amplifier system.
In this work, the EUV-OCT system has a common-path configuration. Even though the EUV bandwidth is only a few nm, the 13.5-nm center wavelength warrants its high axial resolution (~65 nm) for device inspection. In the visible and near infrared regions, OCTs with micron-scale axial resolution have been achieved and widely used. As an example at the EUV wavelength range, the characterization of a Si/Mo multilayer beam splitter is demonstrated using the common-path EUV-OCT system. The results strongly suggest the method's applicability as a new nondestructive tomographic tool for nanometer-scale actinic patterned mask inspection. Such an EUV-OCT system is not only good for photolithography, it may also be advantageous for biomedical imaging on organelles in the future.
For more information see recent Article. Courtesy Sheng-Lung Huang from National Taiwan University.