EUV-induced carbon growth at contaminant pressures between 10-10 mbar and 10-6 mbar: Experiment and model.

Carbon contamination induced by ultraviolet (UV) radiation affects precision optics in applications as diverse as semiconductor lithography and satellite observations of the Sun. Our previous experiments have shown that low-intensity UV-induced surface contamination depends quasi-logarithmically on the partial pressure of the organic contaminant due to the poly-dispersive nature of the surface-adsorbate system. This complex dependence presents difficulties because, without a physically motivated model, it cannot be extrapolated to low pressures. We present measurements and a model of carbon growth induced by UV exposure in the presence of tetradecane vapor. The model, which includes a coverage-dependent adsorption energy, describes the measurements over four orders of magnitude in pressure, and we expect that it can be extrapolated to the lower pressures of interest to the extreme ultraviolet (EUV) lithography and solar astronomy communities. Our experience with other contaminants leads us to expect that other organic contaminants will behave similar to tetradecane. The results also provide insights into the kinetics governing coverage isotherms at extremely low partial pressures.

Note: Thermally stable thin-film filters for high-power extreme-ultraviolet applications.

We investigated several types of thin-film filters for high intensity work in the extreme-ultraviolet (EUV) spectral range. In our application, with a peak EUV intensity of 2.7 W cm(-2), Ni-mesh-backed Zr filters have a typical lifetime of 20 h, at which point they suffer from pinholes and a 50% loss of transmission. Initial trials with Si filters on Ni meshes resulted in rupture of the filters in less than an hour. A simple thermal calculation showed that the temperature rise in those filters to be about 634 K. A similar calculation indicated that using a finer mesh with thicker wires and made of Cu reduces the temperature increase to about 60 K. We have exposed a Si filter backed by such a mesh for more than 60 h with little loss of transmission and no leaks.

A synchrotron beamline for extreme-ultraviolet photoresist testing.

Before being used in an extreme-ultraviolet (EUV) scanner, photoresists must first be evaluated for sensitivity and tested to ensure that they will not contaminate the scanner optics. The new NIST facility described here provides data on the contamination potential of the outgas products of a candidate resist by simultaneously irradiating a multilayer optic and a nearby resist-coated wafer with EUV radiation. The facility can also be used without changing its configuration to provide accurate resist dose-to-clear measurements. Detailed, real-time information on the rate of contamination growth is given by a unique, in situ imaging ellipsometer. We will describe the optical layout, mechanical design, and capabilities of the beamline, finally presenting experimental examples of its capabilities.

High-efficiency fast scintillators for "optical" soft x-ray arrays for laboratory plasma diagnostics.

Scintillator-based "optical" soft x-ray (OSXR) arrays have been investigated as a replacement for the conventional silicon (Si)-based diode arrays used for imaging, tomographic reconstruction, magnetohydrodynamics, transport, and turbulence studies in magnetically confined fusion plasma research. An experimental survey among several scintillator candidates was performed, measuring the relative and absolute conversion efficiencies of soft x rays to visible light. Further investigations took into account glass and fiber-optic face-plates (FOPs) as substrates, and a thin aluminum foil (150 nm) to reflect the visible light emitted by the scintillator back to the optical detector. Columnar (crystal growth) thallium-doped cesium iodide (CsI:Tl) deposited on an FOP, was found to be the best candidate for the previously mentioned plasma diagnostics. Its luminescence decay time of the order of approximately 1-10 micros is thus suitable for the 10 micros time resolution required for the development of scintillator-based SXR plasma diagnostics. A prototype eight channel OSXR array using CsI:Tl was designed, built, and compared to an absolute extreme ultraviolet diode counterpart: its operation on the National Spherical Torus Experiment showed a lower level of induced noise relative to the Si-based diode arrays, especially during neutral beam injection heated plasma discharges. The OSXR concept can also be implemented in less harsh environments for basic spectroscopic laboratory plasma diagnostics.

Soft-x-ray damage to p-terphenyl coatings for detectors.

The organic phosphor p-terphenyl is used as a wavelength-converter coating in some soft-x-ray detectors. We have measured the absolute photoluminescent efficiency of p-terphenyl as a function of incident photon energy from 36 to 191 eV. We have also measured changes in the efficiency caused by soft-x-ray fluence (total photons absorbed per unit area) at several photon energies in this range. We find that efficiency drops rapidly as a function of fluence, with the rate of decrease increasing with higher soft x-ray energies.

Optical constants of in situ-deposited films of important extreme-ultraviolet multilayer mirror materials.

We have performed angle-dependent reflectance measurements of in situ magnetron sputtered films of B(4)C, C, Mo, Si, and W. The Fresnel relations were used to determine the complex index of refraction from the reflectance data in the region of approximately 35-150 eV. In the cases of Si, C, and B(4)C we found excellent agreement with published data. However, for Mo and W we found that the optical properties from 35 to 60 eV differed significantly from those in the literature.

Influence of electrical isolation on the structure and reflectivity of multilayer coatings deposited on dielectric substrates.

Multilayers prepared with electrically isolated or grounded surfaces during deposition are shown to have dramatically different hard-x-ray, soft-x-ray, and neutron reflectivity characteristics. The effect has been observed for (100) silicon wafers, fused silica, and borate glass substrates of different sizes and with different surface roughness and flatness for multilayer structures prepared by rf and dc magnetron sputtering.

Si/B(4)C narrow-bandpass mirrors for the extreme ultraviolet.

We report the results of extreme-ultraviolet reflectance measurements and structural characterization of multilayer mirrors made by sequential sputter deposition of Si and B(4)C. Compared with Si/Mo multilayers, Si/B(4)C have a much narrower bandpass (deltalambda) and better off-peak rejection but lower peak reflectance (R(0)). Mirrors with three different designs gave the following results: R(0) = 0.275 and deltalambda = 0.31 nm at 13.1 nm and normal incidence; R(0) = 0.34 and deltalambda = 1.1 nm at 18.2 nm and 45 degrees ; and R(0) = 0.30 and deltalambda = 2.0 nm at 23.6 nm and 45 degrees . These multilayers exhibited excellent stability on annealing at temperatures up to 600 degrees C.

The New NIST/ARPA National Soft X-Ray Reflectometry Facility.

We have recently begun a series of upgrades to the NIST/ARPA National Reflectometry Facility at the Synchrotron Ultraviolet Radiation Facility. The facility currently consists of a new monochromator and the original sample manipulator which allows us to measure optical components less than 10 cm in diameter. The monochromator offers high throughput and modest resolution over the wavelength range 3.5-40 nm. In the next year we will be installing a sample manipulator that will be able to accommodate the much larger optics that will be used in future x-ray projection lithography and astronomy instruments. We offer preliminary measurements of the throughput and resolution of the new monochromator.