We report on measurements of the diffraction efficiency of 200-nm-period freestanding blazed transmission gratings for wavelengths in the 0.96 to 19.4 nm range. These critical-angle transmission (CAT) gratings achieve highly efficient blazing over a broad band via total external reflection off the sidewalls of smooth, tens of nanometer thin ultrahigh aspect-ratio silicon grating bars and thus combine the advantages of blazed x-ray reflection gratings with those of more conventional x-ray transmission gratings. Prototype gratings with maximum depths of 3.2 and 6 μm were investigated at two different blaze angles. In these initial CAT gratings the grating bars are monolithically connected to a cross support mesh that only leaves less than half of the grating area unobstructed. Because of our initial fabrication approach, the support mesh bars feature a strongly trapezoidal cross section that leads to varying CAT grating depths and partial absorption of diffracted orders. While theory predicts broadband absolute diffraction efficiencies as high as 60% for ideal CAT gratings without a support mesh, experimental results show efficiencies in the range of ∼50-100% of theoretical predictions when taking the effects of the support mesh into account. Future minimization of the support mesh therefore promises broadband CAT grating absolute diffraction efficiencies of 50% or higher.
Thin silicon dioxide films have been studied as a function of deposition parameters and annealing temperatures. Films were deposited by tetraethoxysilane (TEOS) dual-frequency plasma enhanced chemical vapor deposition with different time interval fractions of high-frequency and low-frequency plasma depositions. The samples were subsequently annealed up to 930 • C to investigate their stress behavior. Films that were deposited in high-frequency dominated plasma were found to have tensile residual stress after annealing at temperatures higher than 800 • C. The residual stress can be controlled to slightly tensile by changing the annealing temperature. High tensile stress was observed during the annealing of high-frequency plasma-deposited films, leading to film cracks that limit the film thickness, as predicted by the strain energy release rate equation. Thick films without cracks were obtained by iterating deposition and annealing to stack multiple layers. A series of wet cleaning experiments were conducted, and we discovered that water absorption in high-frequency plasma-deposited films causes the residual stress to decrease. A ∼40 nm thick low-frequency deposited oxide cap is sufficient to prevent water from diffusing through the film. Large-area free-standing tensile stressed oxide membranes without risk of buckling were successfully fabricated.
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