Improvement of Laser Damage Resistance and Diffraction Efficiency of Multilayer Dielectric Diffraction Gratings by HF-Etchback Linewidth Tailoring Page: 4 of 9
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material Ij
interfaces
11111 I mI Damage is seen along edge of ridge
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Figure 1, A. (Left); Distribution of electric field in the vicinity of a grating with laser radiation incident from the left. Material
interfaces shown in white outline. B (Right); Scanning electron micrograph showing damage along edge of grating ridges. (from
Ref 11).
2. DESIGN OF MLD GRATINGS
The optical design of a high-efficiency MLD grating is subject to a number of constraints related to its
manufacturability12. We choose to design a dichroic multilayer coating that is highly reflective at the use angle and
wavelength, and minimally reflective at the holographic exposure angle and wavelength. This is to minimize
standing-wave effects common to pattern generation in photoresist on reflective structures that impact linewidth
control. The second criterion in particular places demands on the accuracy of the coating deposition. We could
choose to deposit a simpler quarter-wave design and use a sacrificial absorptive coating between the multilayer
stack and the photoresist film, but this increases complexity and risk for other aspects of the grating manufacturing
process, particularly at large apertures. The design must also be insensitive to coating deposition and grating
linewidth variations that can be expected to occur over the meter-size apertures. The grating can be etched into
one or several of the deposited dielectric layers. We choose to have a single thick SiO2 layer comprising the grating
due to the intrinsic high laser damage threshold of SiO2. The high-and low index layers comprising the stack are
made of HfO2 and SiO2. The final design is the result of numerous iterations based on performance and
manufacturing error-tolerance considerations.
Once the MLD stack design is finalized, the grating profiles are optimized for minimal electric field intensities in the
solid grating material, while at the same time maximizing the efficiency. Figure 2 shows a plot of the calculated
diffraction efficiency at 1053 nm, 76.5 incidence angle and TE polarization, of a 1780 line/mm MLD grating as a
function of grating height on the vertical axis and grating duty cycle (linewidth/period) on the horizontal axis, for a
HfO2/SiO2 MLD grating with the grating in the top SiO2 layer. A surface of high efficiency >99% extends from the
lower right to the upper left of this plot, as shown. It is generally true that the electric field strength in the solid
material along a line of constant diffraction efficiency decreases as the grating height increases and the duty cycle
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Nguyen, H T; Larson, C C & Britten, J A. Improvement of Laser Damage Resistance and Diffraction Efficiency of Multilayer Dielectric Diffraction Gratings by HF-Etchback Linewidth Tailoring, article, October 28, 2010; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc845790/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.