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New EUV reflectometer

Characterization of optical surfaces – free of contamination and with high-accuracy positioning

PTBnews 3.2023
Especially interesting for

the semiconductor industry

EUV lithography

optical metrology

For 25 years now, PTB has been supporting the development of extreme ultraviolet lithography in Europe by characterizing optical coatings. Since 2019, this European technology has been used in the mass production of high-quality semiconductor components. With this application, the requirements placed on the quality of the measurements have once again significantly increased. To meet this challenge, a new and improved reflectometer has now been put into operation.

Inside the new EUV reflectometer during final assembly

Extreme ultraviolet (EUV) lithography is an optical procedure performed at a working wavelength of 13.5 nanometers and used to structure high-end semiconductor components (e.g., processors) in industrial mass production. The exposure machines used here are equipped with precision mirror optics, whose properties – especially reflectance – must be determined with high accuracy at the working wavelength. To achieve this goal, PTB is cooperating with the German and European supplier industry for EUV lithography.

To meet the increased requirements, a new reflectometer developed at PTB and featuring a hydrocarbon-free vacuum system and improved sample positioning was taken into operation, replacing a system that had been built in the early years of the cooperation projects. The former system used vacuum lubricants that caused molecular contamination of the sample surfaces and thus significantly influenced the measurement data. The corrections made necessary by this influence during evaluation resulted in higher measurement uncertainties. In addition, this contamination was no longer acceptable in the industrial application for which the optics were now being used. The system further features a new mechanism that allows the measuring beam to be more precisely aimed at large components (for example, at collector mirrors for the radiation sources of exposure machines). Due to the local gradients in the layer systems, it is necessary to strike the measurement position with an accuracy of only 0.1 mm, even on large mirrors with diameters of up to 700 mm.

This one-of-a-kind mechanism is the essential component of the system. It was designed by PTB’s Scientific Instrumentation Department, whose staff also fabricated the critical parts. In service since the first quarter of 2023, this instrument is being used every day around the clock in largely automated measurement procedures that will perform more than 2500 measurement hours per year.