Sherry L. Baker

Damage threshold of inorganic solids under free-electron-laser irradiation at 32.5 nm wavelength

We exposed samples of B4C, amorphous C, chemical-vapor-deposition (CVD)-diamond C, Si, and SiC to single 25 fs-long pulses of 32.5 nm free-electron-laser radiation at fluences of up to 2.2 J/cm{sup 2}. The samples were chosen as candidate materials for x-ray free electron laser (XFEL) optics. We found that the threshold for surface-damage is on the order of the fluence required for thermal melting. For larger fluences, the crater depths correspond to temperatures on the order of the critical temperature, suggesting that the craters are formed by two-phase vaporization [1]. XFELs have the promise of producing extremely high-intensity ultrashort pulses of coherent, monochromatic radiation in the 1 to 10 keV regime. The expected high output fluence and short pulse duration pose significant challenges to the optical components, including radiation damage. It has not been possible to obtain direct experimental verification of the expected damage thresholds since appropriate x-ray sources are not yet available. FLASH has allowed us to study the interaction of high-fluence short-duration photon pulses with materials at the shortest wavelength possible to date. With these experiments, we have come closer to the extreme conditions expected in XFEL-matter interaction scenarios than previously possible.

High-performance Mo-Si multilayer coatings for extreme-ultraviolet lithography by ion-beam deposition

An ion-beam deposition system has been used to fabricate Mo-Si multilayer coatings for masks and imaging optics to be used for extreme-ultraviolet lithography. In addition to high reflectivity and excellent profile control, ion-beam deposition has the capability to smooth rough substrates. For example, we achieved reflectivity of 66.8% on a substrate with 0.39-nm roughness. Smoothing can be further enhanced with a second ion source directed at the multilayer coating. The smoothing capabilities relax the requirement on the finish of the mirror and the mask substrates and could dramatically reduce the cost of these components. Thickness profile control is in the +/-0.01% range, and the figure error added to the mirror substrate by errors in the multilayer thickness is less than 0.1 nm. Peak reflectivities obtained on smooth substrates are 67.5-68.6%.

Predicting the coherent X-ray wavefront focal properties at the Linac Coherent Light Source (LCLS) X-ray free electron laser

The first X-ray free electron laser (XFEL) at keV energies will be the Linac Coherent Light Source (LCLS), located at the SLAC National Accelerator Laboratory. Scheduled to begin operation in 2009, this first-of-a-kind X-ray source will produce ultra-short X-ray pulses of unprecedented brightness in the 0.8 to 8 keV first harmonic photon energy regime. Much effort has been invested in predicting and modeling the XFEL photon source properties at the undulator exit; however, as most LCLS experiments are ultimately dependent on the beam focal spot properties it is equally as important to understand the XFEL beam at the endstations where the experiments are performed. Here, we use newly available precision surface metrology data from actual LCLS mirrors combined with a scalar diffraction model to predict the LCLS beam properties in the experiment chambers.

Sub-diffraction-limited multilayer coatings for the 0.3 numerical aperture micro-exposure tool for extreme ultraviolet lithography

Multilayer coating results are discussed for the primary and secondary mirrors of the micro-exposure tool (MET): a 0.30 NA lithographic imaging system with a 200 microm x 600 microm field of view at the wafer plane, operating in the extreme ultraviolet (EUV) region at an illumination wavelength around 13.4 nm. Mo/Si multilayers were deposited by DC-magnetron sputtering on large-area, curved MET camera substrates. A velocity modulation technique was implemented to consistently achieve multilayer thickness profiles with added figure errors below 0.1 nm rms demonstrating sub-diffraction-limited performance, as defined by the classical diffraction limit of Rayleigh (0.25 waves peak to valley) or Marechal (0.07 waves rms). This work is an experimental demonstration of sub-diffraction- limited multilayer coatings for high-NA EUV imaging systems, which resulted in the highest resolution microfield EUV images to date.

An ion-assisted Mo-Si deposition process for planarizing reticle substrates for extreme ultraviolet lithography

Substrate particles are a serious concern in the fabrication of reticles for extreme ultraviolet lithography (EUVL) because they nucleate defects in the reflective multilayer films that can print in the lithographic image. We have developed a strategy for planarizing reticle substrates with smoothing-layers and, in this letter, we investigate the smoothing properties of an ion-assisted Mo-Si deposition process. We have observed that ion-assistance can significantly improve the particle-smoothing properties of Mo-Si multilayer films and can do so without a significant increase in the high-spatial frequency roughness of the multilayer film. An ion-assisted Mo-Si smoothing-layer approach to reticle substrate planarization, therefore, shows significant promise for defect mitigation in EUVL reticles.

Development and testing of EUV multilayer coatings for the atmospheric imaging assembly instrument aboard the Solar Dynamics Observatory

We present experimental results on the development and testing of the extreme ultraviolet (EUV) reflective multilayer coatings that will be used in the Atmospheric Imaging Assembly (AIA) instrument. The AIA, comprising four normal incidence telescopes, is one of three instruments aboard the Solar Dynamics Observatory mission, part of NASAs Living with a Star program, currently scheduled for launch in 2008. Seven different multilayer coatings will be used, covering the wavelength region from 93.9 to 335.4 Å.

Oxidation resistance and microstructure of ruthenium-capped extreme ultraviolet lithography multilayers

The oxidation resistance of protective capping layers for extreme ultraviolet lithography (EUVL) multilayers depends on their microstructure. Differently prepared Ru-capping layers, deposited on Mo/Si EUVL multilayers, were investigated to establish their baseline structural, optical, and surface properties in as-deposited state. The same capping layer structures were then tested for their thermal stability and oxidation resistance. The best performing Ru-capping layer structure was analyzed in detail with transmission electron microscopy (TEM). As compared to other Ru capping layers preparations studied here it is the only one that shows grains with preferential orientation. This information is essential for modeling and performance optimization of EUVL multilayers.

EUV scattering and flare of 10X projection cameras

Two new Schwarzschild cameras have been fabricated for the EUV 10x microstepper. The surface topography of the mirrors was characterized over the full range of spatial frequencies both before and after multilayer coating. EUV scattering from the individual mirrors was measured and compared with the surface profilometry. A knife-edge test was used to directly measure the flare of the assembled cameras. The flare measured in this way is in excellent agreement with the contrast of isolated printed lines and with the point spread function of the camera as determined by EUV interferometry. The measured flare of the camera is also in good agreement with the flare calculated from the combined surface profile measurements of the individual mirrors. Consistent with the improvements made in the surface finish of the mirror substrates, a significant reduction in the flare is observed as compared with previously existing cameras.

Interaction of short x-ray pulses with low-Z x-ray optics materials at the LCLS free-electron laser

Materials used for hard x-ray-free-electron laser (XFEL) optics must withstand high-intensity x-ray pulses. The advent of the Linac Coherent Light Source has enabled us to expose candidate optical materials, such as bulk B4C and SiC films, to 0.83 keV XFEL pulses with pulse energies between 1 μJ and 2 mJ to determine short-pulse hard x-ray damage thresholds. The fluence required for the onset of damage for single pulses is around the melt fluence and slightly lower for multiple pulses. We observed strong mechanical cracking in the materials, which may be due to the larger penetration depths of the hard x-rays.

Diamond Ablators for Inertial Confinement Fusion

Abstract Diamond has a unique combination of physical properties for the inertial confinement fusion ablator application, such as appropriate optical properties, high atomic density, high yield strength, and high thermal conductivity. Here, we present a feasible concept for fabrication of diamond ablator shells. The fabrication of diamond capsules is a multi-step process which involves diamond chemical vapor deposition on silicon mandrels followed by polishing, microfabrication of holes, and removing of the silicon mandrel by an etch process. We also discuss the pros and cons of coarse-grained optical quality and nanocrystalline chemical vapor deposition diamond films for the ablator application.