Fabrication of 50 nm period gratings with multilevel interference lithography.

We have developed a multilevel interference lithography process to fabricate 50 nm period gratings using light with a 351.1 nm wavelength. In this process multiple grating levels patterned by interference lithography are overlaid and spatial-phase aligned to a common reference grating using interferometry. Each grating level is patterned with offset phase shifts and etched into a single layer to achieve spatial-frequency multiplication. The effect of the multilayer periodic structure on interference lithography is examined to optimize the fabrication process. This process presents a general scheme for overlaying periodic structures and can be used to fabricate more complex periodic structures.

Design of a double-pass shear mode acousto-optic modulator.

The design of a compact double-pass shear mode acousto-optic modulator for high power operation in the UV is presented. Using a spherical mirror retroreflector in the second pass to correct for beam deflection, changes in beam position and angle during frequency tuning can be eliminated. The use of the shear mode acousto-optic interaction offers several key advantages, but the strain-induced birefringence creates significant levels of spectral leakage in the system. These effects and the polarization of the beams are analyzed to minimize leakage. Using a heterodyne interferometry scheme, two double-pass acousto-optic modulators with offset acoustic frequency inputs are set up to measure the leakage intensity. The designed double-pass shear mode acousto-optic modulator was tested with lambda=351.1 nm and demonstrated a peak double-pass efficiency of 42.6% and a bandwidth of 28 MHz. Over this bandwidth the spectral leakage was reduced to approximately 0.01%-0.04% relative intensity.

Efficiency of a grazing-incidence off-plane grating in the soft-x-ray region.

Efficiency measurements of a grazing-incidence diffraction grating in the off-plane mount were performed using polarized synchrotron radiation. The grating had 5000 grooves/mm, an effective blaze angle of 14 degrees, and was gold coated. The efficiencies in the two polarization orientations (TM and TE) were measured in the 1.5-5.0 nm wavelength range and were compared with the efficiencies calculated using the PCGrate-SX code. The TM and TE efficiencies differ, offering the possibility of performing unique science studies of astrophysical, solar, and laboratory sources by exploiting the polarization sensitivity of the off-plane grating.

Effects of Gap Width on Vacuum-Ultraviolet Transmission Through Submicrometer-Period, FreeStanding Transmission Gratings.

The effects of gap width on the transmission coefficient of vacuum-ultraviolet light through submicrometer-period, freestanding transmission gratings are reported. Results from computations and an analytical waveguide model are shown to be consistent with experimental measurements. These results show that thin gratings with narrow gaps and thick gratings with wide gaps are equally effective at eliminating 121.6-nm radiation. The thin gratings with the narrow gaps have the advantage of better attenuation of shorter-wavelength radiation than the thick gratings with the larger gaps.

Extreme-ultraviolet polarization and filtering with gold transmission gratings.

The polarization and transmission characteristics of freestanding gold transmission gratings, with 200-nm periods, for extreme-ultraviolet (EUV) radiation (1 < 200 nm) have been measured. We find that EUV transmission through the gratings is dominated by the waveguide characteristics of the gratings and that polarization efficiencies of 90% for wavelengths of 121.6 nm are achievable. Both the EUV polarization and transmission properties are in good agreement with a complete vector, numerical solution of Maxwell's equations. The fraction of open area to total area of the grating has been measured using a 10-keV proton beam and was found to be in good agreement with the microscopic slit and wire dimensions that were obtained by scanning electron microscopy. The use of these gratings for particle measurements in the presence of intense EUV radiation is briefly discussed.

Achromatic holographic configuration for 100-nm-period lithography.

For the fabrication of large-area, spatially coherent gratings with periods of 100 nm or less, a grating interferometer is preferred over a conventional holographic configuration because of the limited coherence of available sources. Using a configuration that employs two matched fused silica phase gratings and an ArF excimer laser, we obtain high-quality 100-nm gratings in polymethyl methacrylate. We analyze the conditions for achieving high-contrast fringes with such an achromatic holographic configuration and show that the depth of focus depends only on the spatial coherence of the source. We also describe a highly accurate method for calculating the diffraction efficiency of the phase gratings as a function of polarization, incidence angle, and grating structure.

Transmission x-ray diffraction grating alignment using a photoelastic modulator.

We have developed a high-resolution alignment technique which utilizes the partial polarization property of fine period transmission gratings. It is especially useful when the grating period is sufficiently small so that there are no visible diffracted orders. This technique uses a photoelastic modulator (PEM) to produce an intensity signal that is proportional to the sine of twice the angle between the grating lines and the PEM crystal axis. The experimentally demonstrated resolution of this technique on 200-nm period gold transmission gratings is better than 1 sec of arc. This technique was developed to align x-ray transmission gratings for spectroscopy and interferometry applications.