Electron roles in extreme ultraviolet lithography mirror contamination and flashover
The purpose of this study was to characterize and study the contamination behavior of residual hydrocarbons and water vapor in a vacuum environment similar to that suited to the performance of EUV Lithography devices. A 50nm ruthenium film deposited on a silicon surface which served to simulate the Ru coated mirrors in a typical lithography setup was bombarded with 13.5 nm wavelength light at a grazing angle. We then studied in real-time the contamination effect by using extreme ultraviolet photoelectron spectroscopy (EUPS). This allowed us to examine the behavior of the contaminants as they formed dipoles. By monitoring X-Ray Photoelectron Spectroscopy (XPS) as a function of angle, we determined that carbon species may be more prevalent in the normal directions, while water and related species tend to be at grazing angles. While characterizing the contamination is certainly important for finding ways of mitigating it, we also attempted to prove that the electrons from a typical EUV source are just as important in terms of causing contamination if not more so than the light itself. By monitoring the contamination with both AES and XPS together, we were able to prove that in the absence of electrons the carbon buildup was far reduced. Similar results were found while studying the grazing angle reflectivity of the ruthenium sample. Electrons also proved to be important in surface flashover, a phenomenon in which the polarity on the surface can suddenly switch from negative to positive over a course of seconds. We saw this effect on gold and examined it in real-time with the help of secondary electron spectroscopy (SES). The flashover effect seemed to be defect driven. Interestingly, there was a dependence on energy change: when changing electrons from a lower to higher energy a flashover was observed, but not while changing from a high to a low energy. While we observed this on gold, silicon, and carbon it could certainly be extended to occurring on the photoresist of a typical EUV lithography setup.
Hassanein, Purdue University.
Computer Engineering|Nuclear engineering
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