# Fabrication of Multiscale-Structure Wafer-Level Microlens Array Mold
## 3 Results and Discussion
### Pure Nanocrystalline Aluminum MLA Mold with Dense Array  
After numerous trials to identify the right material, such as single crystal/polycrystalline pure aluminum and different aluminum alloys, an aluminum alloy (6061-T6) substrate electroplated with \(\sim\)0.3 mm-thick pure aluminum consisting of nanosized grains (AlumiPlate, Coon Rapids, MN, US) was finally selected as the mold material, by considering the requirements of both the ultraprecision machining process and the subsequent AAO process. The strength of the 6061-T6 alloy substrate assures the shape stability of the mold, while the pure nanocrystalline-plated layer simultaneously supports the generation of a uniform nanopore layer without impurity defects. A 2\" wafer mold was fabricated to validate the process. First, a flat optical surface was created on the aluminum substrate by a diamond-turning process, as shown in Figure 6a. Next, a concave MLA with a dense array was machined by micromilling using a high-speed air-bearing spindle. The measured results demonstrated that the boundary of the lenslets had a sharp appearance, as shown in Figure 6b. The surface roughness of the lenslet was 3.362 nm, as shown in Figure 6c. We then adopted the AAO process to obtain the pure nanocrystalline aluminum-dense MLA mold with nanostructures, as shown in Figure 6d.  
The SEM images of the mold nanostructure, in Figure 7, show the uniformity of the nanoporous layer on the MLA mold cavity, both at the apex flat region and the lens boundary region. Cross-sectional SEM images of the thick metal mold were difficult to record, so Figure 7d shows a SEM image of a nanostructured sheet plate sample that was fabricated using the same process parameters as those of the metal mold. The nanostructures exhibited a V-shaped cross section with an average pore  
Figure 6: Different stages in the fabrication of the hybrid-structure microlens array (MLA) metal wafer mold: photographs of (**a**) the flat mirror surface obtained by diamond-turning and (**b**) the micromilled MLA mold cavities on the substrate and edge profile of the processed sample, (**c**) the MLA mold 3D surface finish of the lenslet apex region without nanostructures (Sa = 3.362 nm), and (**d**) the final MLA mold with antireflection nanostructures, which was fabricated using the anodic aluminum oxidation (AAO) process.  
diameter of ~100 nm and a spatial period of ~120 nm, as shown in Figure 7d. Figure 8a shows the lens profile error of the lenslet mold with AR nanostructures--the PV error is ~0.098 \(\upmu\)m, which indicates that the AAO process has little negative impact on the mold profile. The surface roughness of the nanostructured mold was measured to be 15.683 nm, as shown in Figure 8b, and the mold surface displays almost the same visual mirror quality as before the AAO treatment. The results of the surface profile and the surface roughness measurements showed that the MLA mold of pure nanocrystalline aluminum meets the optical requirements even after the nanostructuring treatment, and it paves the way for a further study to replicate a multiscale wafer-level MLA with an inherent AR capability.  
Figure 8: (**a**) Stylus profile measurement results of the lenslet mold with nanostructures (peak-to-valley (PV) error = 0.0985 \(\upmu\)m). (**b**) Surface roughness of the MLA mold with nanostructures (Sa = 15.683 nm).  
Figure 7: (**a**) SEM images of the nanostructured MLA mold. (**b**) High-magnification SEM image of the lenslet apex region. (**c**) High-magnification SEM image of the lenslet boundary region, with the boundary being indicated by the dashed line. (**d**) Cross-sectional SEM image of the nanopores.