# Fabrication of Multiscale-Structure Wafer-Level Microlens Array Mold
## 2 Materials and Methods  
### Fabrication Process  
The wafer-level MLA is a thin, 4-8\" diameter wafer, distributed with thousands of microlenses. As shown in Figure 1, the UV-curable resin-based imprint process transfers the microlens shape on the metal mold to a glass wafer substrate, which is free of high pressure, and then the wafer-level microlens array is released from the mold after curing. Several MLA wafers can be stack-assembled and cut into thousands of multisurface lens modules, resulting in very compact microcameras or camera arrays [26]. Currently, the wafer-level MLA process is mainly used to fabricate compact lens modules for low-resolution near-infrared (NIR) eye-ball tracking cameras or disposable endoscope modules. The general requirements of MLA surface characteristics are a PV error <0.5 \(\upmu\)m with a sharp edge, a surface roughness (Sa) <20 nm, and a Fresnel reflectivity <1% to increase sensitivity and avoid ghost images. The wafer-level MLA mold is one of the core components in the whole process. Here, the fabrication of wafer-level MLA molds is followed by an AAO-based self-assembly process to provide periodically arranged nanostructuring. The common ultraprecision machining methods used to generate precision MLA metal molds include fast tool servo (FTS), micromilling, and slow slide servo (SSS) processes [21; 22; 27]. In this study, micromilling and SSS diamond-machining processes were adopted to fabricate the MLA metal molds, using an ultraprecision machine (350 FG, Moore Nanotechnology, Swanzey, NH, USA). For low-density arrays, the SSS-machining process was selected, as its implementation requires only 3 axes of freedom. For high-density MLA molds, the micromilling process was selected, as it allows more flexibility in large-slope microlens design. The key factors that decide the quality of the MLA mold include the lenslet profile accuracy, mold surface roughness, and the lenslet positioning accuracy, which are closely related to the metal mold material, the diamond-machining performance, the machine-tool lifetime, and the machining process parameters. The mold material is the most crucial influencing factor. Metal materials that are used conventionally in diamond machining are usually high-ductility metals, such as aluminum and its alloys, copper alloys, and electroless nickel. Here, considering the further fabrication of nanostructures, various aluminum materials were selected as candidates for mold materials, namely 6061-T6 aluminum alloy, high-purity polycrystalline aluminum, and pure nanocrystalline aluminum. W wafer-level MLA trial molds based on these aluminum materials were diamond-machined and evaluated according to the mold requirements. For the following machining tests, we designed the MLA aspherical shape with the cavity aperture at 1.0 mm and the sag at 85 \(\upmu\)m. 6061-T6 aluminum alloy is the most widely diamond-machined material owing to its relatively high ductility and strength. A 6061-T6 4\" MLA mold with a 20 \(\times\) 20 array was machined via the SSS process. The nose radius of the single-point diamond tool was 0.3 mm, the spindle rotational speed was \(\sim\)100 rpm, and the diamond tool traveled from the periphery to the center of the mold at a speed of 1 mm/min. The depth of the cut for a rough cut was 70 \(\upmu\)m, the semifinished depth of the cut was 8 \(\upmu\)m, and the finished depth of the cut was 2 \(\upmu\)m. Next a high-purity polycrystalline aluminum pin mold was machined via the traditional diamond-turning method. Finally, we adopted the micromilling process for the 2\" pure nanocrystalline aluminum MLA mold to create a dense lenslet array. To generate a uniform layer of nanostructures on the MLA mold, we adopted the two-step AAO process, which is widely used for nanoporous template applications [28; 29]. In the AAO experiments, we investigated characteristics, such as pore size, interpore distance, and nanolayer thickness, by varying the applied voltage, electrolyte concentration, and reaction time.