Towards High Accuracy Reflectometry for Extreme-Ultraviolet Lithography

Abstract

Currently the most demanding application of extreme ultraviolet optics is connected with the development of extreme ultraviolet lithography. Not only does each of the Mo/Si multilayer extreme-ultraviolet stepper mirrors require the highest attainable reflectivity at 13 nm (nearly 70 %), but the central wavelength of the reflectivity of these mirrors must be measured with a wavelength repeatability of 0.001 nm and the peak reflectivity of the reflective masks with a repeatability of 0.12 %. We report on two upgrades of our NIST/DARPA Reflectometry Facility that have given us the ability to achieve 0.1 % repeatability and 0.3 % absolute uncertainty in our reflectivity measurements. A third upgrade, a monochromator with thermal and mechanical stability for improved wavelength repeatability, is currently in the design phase.

Keywords: extreme ultraviolet; lithography; metrology; reflectometry; synchrotron radiation.

Fig. 1

Calculated emitted power from the SURF III storage ring per horizontal milliradian of intercepted orbit, per milliampere of beam current in a 0.1 % bandwidth as a function of wavelength λ for various energies between 180 MeV and 380 MeV and assuming collection of the full vertical extent of the emission. The vertical lines indicate the transmission edges of various filter materials.

Fig. 2

Normalized incident intensity, I₀, as a function of time. Data points were measured every second toward the beginning and every ten seconds after that. The drift is 0.01 % / h and the rms noise is 0.03 %.

Fig. 3

Product U·D·F, representing the radiant flux entering the reflectometer, as a function of wavelength λ for different combinations of electron beam energies, filters, and detectors.