Stephen P. Vernon

Multilayer mirror technology for soft-x-ray projection lithography

Recent advances in multilayer mirror technology meet many of the stringent demands of soft-x-ray projection lithography (SXPL). The maximum normal-incidence reflectivity achieved to date is 66% for Mo/Si multilayers at a soft-x-ray wavelength of 13.4 am, which is sufficient to satisfy the x-ray throughput requirements of SXPL. These high-performance coatings can be deposited on figured optics with layer thickness control of ˜ 0.5%. Uniform multilayer coatings are required for SXPL imaging optics, for which maintaining the surface figure is critical to achieving diffraction-limited performance. In contrast the coatings on the condenser optics will be graded to accommodate a large range of angles of incidence. Graded multilayer coatings can also be used to modify the figure of optical substrates without increasing the surface roughness. This offers a potential method for precise fabrication of aspheric imaging optics.

Silicide layer growth rates in Mo/Si multilayers

The thermal stability of sputter-deposited Mo/Si multilayers was investigated by annealing studies at relatively low temperatures (˜ 250-350 °C) for various times (0.5-3000 h). Two distinct stages of thermally activated Mo/Si interlayer growth were found: a primary surge, followed by a (slower) secondary steady-state growth in which the interdiffusion coefficient is constant. The interdiffusion coefficients for the interlayer formed by deposition of Mo-on-Si are higher than those of the interlayer formed by deposition of Si-on-Mo. Assuming that the activation energy is constant, an extrapolation of our results to ambient temperature finds that interlayer growth is negligible, suggesting long-term thermal stability in soft-x-ray projection lithography applications.

The first measurements of soft x-ray flux from ignition scale Hohlraums at the National Ignition Facility using DANTE (invited).

The first 96 and 192 beam vacuum Hohlraum target experiments have been fielded at the National Ignition Facility demonstrating radiation temperatures up to 340 eV and fluxes of 20 TW/sr as viewed by DANTE representing an ∼20 times flux increase over NOVA/Omega scale Hohlraums. The vacuum Hohlraums were irradiated with 2 ns square laser pulses with energies between 150 and 635 kJ. They produced nearly Planckian spectra with about 30±10% more flux than predicted by the preshot radiation hydrodynamic simulations. To validate these results, careful verification of all component calibrations, cable deconvolution, and software analysis routines has been conducted. In addition, a half Hohlraum experiment was conducted using a single 2 ns long axial quad with an irradiance of ∼2×10(15) W/cm(2) for comparison with NIF Early Light experiments completed in 2004. We have also completed a conversion efficiency test using a 128-beam nearly uniformly illuminated gold sphere with intensities kept low (at 1×10(14) W/cm(2) over 5 ns) to avoid sensitivity to modeling uncertainties for nonlocal heat conduction and nonlinear absorption mechanisms, to compare with similar intensity, 3 ns OMEGA sphere results. The 2004 and 2009 NIF half-Hohlraums agreed to 10% in flux, but more importantly, the 2006 OMEGA Au Sphere, the 2009 NIF Au sphere, and the calculated Au conversion efficiency agree to ±5% in flux, which is estimated to be the absolute calibration accuracy of the DANTEs. Hence we conclude that the 30±10% higher than expected radiation fluxes from the 96 and 192 beam vacuum Hohlraums are attributable to differences in physics of the larger Hohlraums.

Ion-assisted sputter deposition of molybdenum-silicon multilayers.

X-ray multilayer (ML) structures that are fabricated by the use of magnetron-sputter deposition exhibit a degradation in structural quality as the deposition pressure is increased. The observed change in morphology is attributed to a reduced mobility of surface adsorbed atoms, which inhibits the formation of smooth, continuous layers. The application of a negative substrate bias produces ion bombardment of the growing film surface by sputtering gas ions extracted from the plasma and permits direct control of the energy density supplied to the film surface during thin-film growth. The technique supplements the energy lost to thermalization in high-pressure deposition and permits the fabrication of high-quality ML structures at elevated processing pressures. A threefold improvement in the soft-x-ray normal-incidence reflectance at 130 A results for substrate bias voltages of the order of ˜ - 150 V for Mo-Si MLs deposited at 10-mTorr Ar.

Backscatter measurements for NIF ignition targets (invited)

Backscattered light via laser-plasma instabilities has been measured in early NIF hohlraum experiments on two beam quads using a suite of detectors. A full aperture backscatter system and near backscatter imager (NBI) instrument separately measure the stimulated Brillouin and stimulated Raman scattered light. Both instruments work in conjunction to determine the total backscattered power to an accuracy of ∼15%. In order to achieve the power accuracy we have added time-resolution to the NBI for the first time. This capability provides a temporally resolved spatial image of the backscatter which can be viewed as a movie.

Images of the laser entrance hole from the static x-ray imager at NIF

The static x-ray imager at the National Ignition Facility is a pinhole camera using a CCD detector to obtain images of Hohlraum wall x-ray drive illumination patterns seen through the laser entrance hole (LEH). Carefully chosen filters, combined with the CCD response, allow recording images in the x-ray range of 3-5 keV with 60 μm spatial resolution. The routines used to obtain the apparent size of the backlit LEH and the location and intensity of beam spots are discussed and compared to predictions. A new soft x-ray channel centered at 870 eV (near the x-ray peak of a 300 eV temperature ignition Hohlraum) is discussed.

Mask blanks for extreme ultraviolet lithography: Ion beam sputter deposition of low defect density Mo/Si multilayers

We report on the growth of low defect density Mo/Si multilayer (ML) coatings. The coatings were grown in a deposition system specifically designed for extreme ultraviolet lithography mask blank fabrication. Complete, 81 layer, high reflectance Mo/Si ML coatings were deposited on 150 mm diam (100) oriented Si wafer substrates using ion beam sputter deposition. Process added defect densities correspond to 2×10−2/cm−2 larger than 0.13 μm as measured by optical scattering. This represents a reduction in defect density of Mo/Si ML coatings by a factor of 105.

Soft x‐ray projection lithography

Recent advances in x‐ray optics have made possible the practical consideration of soft x‐ray projection imaging for the fabrication of high density integrated circuits.

High-performance multilayer mirrors for soft x-ray projection lithography

We present a comparative study of three metal Si multilayer coatings, Mo-Si, Ru-Si, and Rh-Si, designed for optimum normal incidence reflectivity at x-ray wavelengths near 130 angstroms. The ML structures are characterized using x- ray diffraction and high-resolution electron microscopy, and the results are correlated with the normal incidence reflectivity measured using synchrotron radiation. It is found that interlayers of mixed composition are formed to various degrees in the as-deposited structures for all three material compositions. The thinnest interlayers are observed in Mo-Si and the most intermixing occurs in Rh-Si, where the pure Rh layers are completely consumed. The stoichiometry of the interlayers in all cases is most consistent with the formation of the Si rich silicide. The highest peak normal incidence reflectivity of 61% at approximately 130 angstroms is achieved with Mo-Si, whereas the highest integrated reflectivity of 4.4 angstroms is achieved with Ru-Si. In a separate study, the formation of the interlayers in as-deposited Mo-Si multilayer coatings is investigated by systematically varying the thickness of either the Mo or Si layers. We observe a contraction of the multilayer period which is consistent with formation of MoSi2 at the interfaces. The presence of the MoSi2 interlayers decreases the normal incidence reflectivity, thereby limiting the x-ray throughput in soft x-ray projection lithography applications.

Thermal stability of Mo/Si multilayers

The thermal stability of Mo/Si multilayers for x-ray mirror applications was investigated by annealing studies at relatively low temperatures for various times. The as-deposited and annealed multilayers were examined using conventional small and large angle x-ray diffraction, normal incidence x-ray reflectance measurements using a synchrotron source, selected area electron diffraction, and high-resolution electron microscopy. The as-deposited structure consists of pure layers of crystalline Mo and amorphous Si separated by thin regions of amorphous Mo-Si. At temperatures between 200 - 400 degree(s)C the amorphous Mo-Si interlayers grow and hexagonal MoSi2 forms by a thermally activated process(es) and the bilayer spacing and x-ray reflectivity decrease. A determination of the effective activation energy of the process(es) suggests long-term stability at the mirror operating temperature, although additional low temperature testing is warranted.