## 2 Structure Design, Fabrication and Topography Measurement
### Topography Measurement  
First, a VK-X200K laser confocal microscope (Japan Keyence Corporation) is utilized to perform the surface topography measurement. The overall morphology of the CSPA sample is shown in Figure 4. As shown, an initial test area is fabricated for performing equipment debugging before actually processing the CSPA samples. It can be seen that the overall morphology including both the samples with different period such as 25 \(\upmu\)m and 30 \(\upmu\)m and a test area with an unfilled small corner is roughly complete. The CSPA with a period of 30 \(\upmu\)m has attached by a small stain nearing the bottom edge.  
Next, a high-power objective lens is used to observe the details of the fabricated CSPAs. Figures 5 and 6 are morphological diagrams of the CSPAs with a period of 25 \(\upmu\)m and 30 \(\upmu\)m, respectively. As shown in Figure 5, although existing some concave and convex burrs over small region of the CSPA with a period of 30 \(\upmu\)m, the overall structure presents a better surface morphology and a high shaping precision of the phase step. According to the measurements, the structural error between the fabricated CSPA and the designed structure is within 5% to acquire an ideal fabrication result.  
It can be seen from Figure 6 that the structural error of the acquired CSPA is still in a range of not exceeding 5%. However, several small concave and convex burrs on the surface of the structure indicated by areas B and C, which are more clear than that in Figure 5, can be observed. This is mainly because the photoinitiator molecules are excited by incident beams to generate free radicals and then undergo a chain reaction. The monomer molecules are polymerized to form high polymers. When the molecular weight in the polymer network reaches a critical value, the polymer will not be dissolved by subsequent immersion in acetone solution. The appearance of the concave area-B is due to the insufficient molecular weight of the polymer formed by the chain reaction, so as to lead to the subsequent dissolution when soaked in acetone, thereby forming a small pit. The convex area-C is due to the fact that the chain reaction occurs in a relatively large area, so as to result in the surrounding monomer molecules also forming a polymer. As shown, the height curve of the area-A shows an up and down fluctuation, which can be attributed to the machining accuracy limitation. Therefore, Figure 7 is an enlarged view of the overall contour of area-A. The roughness of area a is quantitatively detected with vk analysis software, as shown in Figure 8. The roughness parameters are shown in Table 2. As shown, the maximum roughness is approximately \(\sim\)190 nm which is still less than the 3D printing accuracy of 200 nm. So, the surface of the manufactured sample is not smooth and thus presents a stepped profile, which is a typical diffraction phase outline.  
Figure 4: Measured overall morphology of the sample fabricated. (**a**) Two-dimensional topography, and (**b**) three-dimensional topography.  
Figure 5: Appearance of a CSPA with a period of 25 μm. (**a**) Two-dimensional topography and (**b**) three-dimensional topography and both the surface profiles indicating the height or depth by (**c**) and the featured horizontal size by (**d**).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & **Rp** & **Rv** & **Rz** & **Ra** & **Rq** & **Rsk** & **Rku** & **RAq** & **RSm** \\ \hline Seg.1 & 0.09 μm & 0.10 μm & 0.19 μm & 4.17 μm & 4.17 μm & 1.0002 & 1.0005 & 0.1922 & 0.00 μm \\ \hline \hline \end{tabular}
\end{table}
Table 2: Roughness related parameter table.  
Figure 6: The CSPA with a 30 \(\upmu\)m period. (**a**) Two-dimensional topography and (**b**) three-dimensional topography and both the surface profiles indicating the height or depth by (**c**) and the featured horizontal size by (**d**).  
Figure 7: Enlarged view of the area-A.