The purpose of the end station on Beamline 8 at the Synchrotron Ultraviolet Radiation Facility (SURF III) is to study the degradation of materials irradiated by extreme-ultraviolet (EUV) light in the presence of common outgassing species such as water and hydrocarbons. Beamline 8 was originally designed to address this problem for the EUV lithography (EUVL) industry which operates at 13.5 nm; however, it can also be easily modified to study optics degradation at longer wavelengths relevant to solar-observing satellite instruments.
The sample chamber has a well-controlled vacuum environment with base pressure in the mid 10-8 Pa range and oil-free pumps. To minimize exposure to atmospheric gases, samples are introduced to the main chamber through a load lock where they are first subjected to atomic-hydrogen cleaning to remove any atmospheric carbon contamination.
The output of the synchrotron is collected approximately 4 m from the storage ring tangent point by a near-normal-incidence multi-layer optic designed to reflect 13.5 nm light. A thin foil filter of Zr blocks long-wavelength light while transmitting 50 % to 60 % of the EUV radiation. This filter also serves as a physical barrier separating the exposure chamber environment from the rest of the beamline and synchrotron.
Up to two contamination precursor gases (benzene, toluene, water, etc.) can be admitted to the vacuum chamber through individual leak valves. Absolute partial pressures of admitted gases are determined using a quadrupole mass spectrometer which is calibrated against a spinning rotor gauge. This provides absolute partial pressure measurements with accuracy better than 5 %.
The contamination rate is determined by using x-ray photoelectron spectroscopy (XPS) with 250 mm spatial resolution to measure the distribution of carbon deposited during the exposures. By comparing this map of carbon thickness to the known distribution of EUV intensity on the sample, a single exposure can provide data to determine the contamination rate as a function of intensity.
Base vacuum composition after 150 °C bake:
Water: ~5×10-8 Pa
Hydrogen: ~1×10-7 Pa
Species with mass-to-charge ratio > 44 u/e: <10-11 Pa
Samples are cleaned with atomic-hydrogen in load lock before installation.
EUV intensity on sample:
~1 mW of power in narrow band at 13.5 nm ± 2 %
Spatial distribution is asymmetric Gaussian with full-width at half maxima ~(1.0 mm × 0.6 mm)
Center intensity 4 to 8 mW/mm2
Translation of sample allows multiple exposures to be performed on each sample
Two leak valves enable exposure to a single species or a mixture of species
Partial pressures between (10-9 and 10-3) Pa
Partial pressures measured with absolute accuracy better than 5 %
Automation of beamline and synchrotron allows continuous exposures over many days.
Sample degradation is monitored in situ with single-wavelength, null-field imaging ellipsometer.