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Optical constants in the EUV range

Creation of a database of materials for EUV lithography

PTB-News 2.2022
Especially interesting for

semiconductor industry

At PTB, optical thin-film materials for extreme ultraviolet lithography are systematically investigated at a wavelength of 13.5 nm. The optical constants (i.e., the refractive index and the extinction coefficient) have now been determined in the wavelength range between 10 nm and 20 nm with small uncertainties based on reflection measurements performed with synchrotron radiation.

Extinction coefficient and refractive index of various materials for EUV lithography at a wavelength of 13.5 nm

Introducing EUV lithography into commercial semiconductor manufacturing has extended the spectral range of electromagnetic radiation used in industry to wavelengths of around 13.5 nm. To develop optical components such as mirrors, gratings or photomasks for this spectral range, it is essential to have accurate knowledge of the optical properties of the materials used. In the lithography process, an optical imaging system is used to demagnify and transfer the original image of the semiconductor structure from the photomask to the chip ingot.

The selection of materials is critical, in particular as regards the photomask: The achievable contrast of the image is largely determined by the beam attenuation and phase shift effects in the photomask itself. These effects depend very sensitively on the complex refractive index of the materials used. For many chemical elements, existing databases of refractive indexes in the EUV range only contain estimations based on few measured values. The situation is obviously even worse when it comes to compounds.

Within the scope of EU-funded projects and longterm cooperation projects with research institutes and companies from the semiconductor sector, PTB has systematically investigated the optical properties of materials that are technologically relevant to EUV photomasks. These data can be used to search for and optimize new material systems in order to enhance the optical contrast of the structures to be imaged onto the semiconductor chips.

The results for an initial set of 19 materials have now been published and thus made available for the purpose of technological development. For simple and fast retrieval, they have been made freely accessible as the cornerstone of a database to which new results are to be added continuously.


Richard Ciesielski
Department 7.1
Radiometry with Synchrotron Radiation
Phone: +49 30 3481-7149
Opens local program for sending emailrichard.ciesielski(at)ptb.de

Link to the database

Opens external link in new windowhttps://www.ocdb.ptb.de/home

Scientific publication

R. Ciesielski, Q. Saadeh, V. Philipsen, et al.: Determination of optical constants of thin films in the EUV. Appl. Opt. 61 (2022)