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.

Laser excitation and ionization of dense atomic vapors.

It has been shown that a dense (>10(14) -cm(-3)) atomic vapor, irradiated by a saturating pulse of resonance radiation, will ionize on a time scale of <10(-6) sec. The ionization can be 95% complete and has been observed in Li, Na, Ca, Sr, and Ba. A large number of physical processes contribute to the ionization with different processes dominating at different stages in the ionization. These processes are discussed, and two models for calculating the ionization are described. The results of the various experiments and the applications to the spectroscopy of ions and to atomic physics studies in general are reviewed.

Capillary array: a new type of window for the vacuum ultraviolet.

Experiments with optical radiation often require separation of a region of relatively high pressure from a lower-pressure region while allowing transmission of radiation between regions. When work is done with vacuum ultraviolet radiation (VUV), the problem is made more difficult by the small number of transparent materials, there being no bulk materials which transmit at shorter wavelengths than the 1050-A cutoff of LiF. In this paper we report the successful use of glass capillary arrays combined with differential pumping to sustain large pressure differences with excellent transmission of radiation throughout the VUV region.

High-stability coaxial flashlamp-pumped dye laser.

Measurements on the stability of a coaxial flashlamp-pumped dye laser demonstrated that the quality of the output can be dramatically improved by isolating the dye cell thermally from the flashlamp and ensuring uniform axially symmetric flow of the dye throughout the cell. A quadraxial laser tube in which the dye cell is surrounded by an evacuated annulus to provide thermal isolation and is terminated by specially designed end caps to provide uniform injection and removal of the dye solution was tested in a standard cavity with a 1200-line/mm grating in Littrow configuration as the dispersive element. The performance characteristics were 1-mrad divergence, 0.5-A bandwidth of the spectral distribution, and 0.04-A jitter of wavelength at maximum intensity.

Even-parity autoionizing states in neutral sodium (350-400 A).

A recently developed laser-excitation technique has been used in the first photoabsorption studies of the even-parity 2p(5)3s3p and 2p(5)3s4p autoionizing levels of Na I. The observed-level values are compared, where applicable, with those previously obtained by ejected-electron spectroscopy of collisionally excited Na, and the identifications are confirmed by ab initio calculations.

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.

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.

Ultraviolet radiometry with synchrotron radiation and cryogenic radiometry.

The combination of a cryogenic radiometer and synchrotron radiation enables detector scale realization in spectral regions that are otherwise difficult to access. Cryogenic radiometry is the most accurate primary detector-based standard available to date, and synchrotron radiation gives a unique broadband and continuous spectrum that extends from x ray to far IR. We describe a new cryogenic radiometer-based UV radiometry facility at the Synchrotron Ultraviolet Radiation Facility II at the National Institute of Standards and Technology. The facility is designed to perform a variety of detector and optical materials characterizations. The facility combines a high-throughput, normal incidence monochromator with an absolute cryogenic radiometer optimized for UV measurements to provide absolute radiometric measurements in the spectral range from 125 nm to approximately 320 nm. We discuss results on photodetector characterizations, including absolute spectroradiometric calibration, spatial responsivity mapping, spectroreflectance, and internal quantum efficiency. In addition, such characterizations are used to study UV radiation damage in photodetectors that can shed light on the mechanism of the damage process. Examples are also given for UV optical materials characterization.