## 3 Results  
### Grating Quality Assessment with Scanning Electron Microscopy  
The grating quality evaluation was first performed with an optical and scanning electron microscope. The main quality control points were the uniformity of the grating pattern, the sharpness of structures edges, sidewall geometry, and quantitative characterization of the grating height, period, and duty cycle. The duty cycle (DC) in grating fabrication represents the ratio of the structure width to the period of the pattern. For the gold height from 5 \(\upmu\)m to 15 \(\upmu\)m, it was possible to obtain reproducible structures with periods down to 10 \(\upmu\)m; however, the shape of the structures was affected by diffraction in the corners of the square pillars. For gold structures higher than 20 \(\upmu\)m, the electroplating of structures with a period of 10 \(\upmu\)m was not possible due to excessive cross-linking of the photoresist and significant distortion of the initial pattern shape. An example of the grating of 25 \(\upmu\)m period and 28 \(\upmu\)m gold height with stripped photoresist is shown in Figure 3a. One can see that the grating pattern is clear and uniform; no structures are collapsed or detached. Each pillar of the grating is wider in the bottom and smaller on the top \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Photoresist Thickness, \(\upmu\)m**} & **Pattern Period, \(\upmu\)m** & **Pattern Shape** & **Exposure Dose, mJ/cm\({}^{2}\)** & **Electroplated Thickness** \\ \hline \multirow{4}{*}{10} & 50 & & & \\ \cline{2-5}  & 25 & square & 550 & 5 \\ \cline{2-5}  & 10 & & & \\ \cline{2-5}  & 10 & & & \\ \cline{2-5}  & 50 & round & & \\ \hline \multirow{4}{*}{30} & 50 & & & 22 \\ \cline{2-5}  & 25 & square & 630 & (-) 1 \\ \cline{2-5}  & 10 & & & 22 \\ \cline{2-5}  & 50 & round & & 22 \\ \hline \multirow{4}{*}{40} & 50 & & & 22 \\ \cline{2-5}  & 25 & square & 650 & (-) 1 \\ \cline{2-5}  & 10 & & & 28 \\ \cline{2-5}  & 50 & round & & 28 \\ \hline \hline \end{tabular} \({}^{1}\) No structures could be obtained due to dramatic diffraction-induced shape distortion.  
\end{table} 
Table 1: Main characteristics of the fabricated gratings. (Figure 4a), which is due to the interplay of a negative photoresist response, absorption, and diffraction effects occurring in the UV lithography. The duty cycle of the grating at the top and on the bottom is slightly different from the intended value. Analysis of SEM images has shown that, for the top of the grating, the duty cycle is 0.47, and for the bottom, 0.58. Thus, the average duty cycle is 0.52 \(\pm\) 0.05. Note, that the analyzed grating has the highest aspect ratio (height 28 \(\upmu\)m, structure size 12.5 \(\upmu\)m) among fabricated gratings with sufficient quality (Figure 4a); the distortions observed for such structures are much less for the other fabricated gratings. From the application side such a distorted sidewall shape of the structures might slightly decrease the efficiency of the wavefront modulation, but, on the other hand, it provides structures with additional mechanical stability. The granular surface of the structures (see Figure 4) does not affect the quality of the wavefront modulation, as the gratings are to be used as an absorption mask. As it was mentioned before, different shapes of the structures were tested to compare their final quality. The two-dimensional X-ray gratings of the 50 \(\upmu\)m period and 28 \(\upmu\)m gold heights are presented in Figure 3b,c. As one can see, the grating patterns of both shapes are clear and uniform, and the structures are well-defined and exhibit a high degree of periodicity. Single pillars of the square and round shapes are shown in Figure 4b,c. Both have a wider foundation and narrower top, but the deviation is smaller than the one presented in Figure 4a. For the square pillars, the top duty cycle is 0.49 and the bottom DC is 0.53, which gives the average DC = 0.51 \(\pm\) 0.02.