# Integrated Double-Sided Random Microlens Array Used for Laser Beam Homogenization
## 3 Fabrication of D-rMLA
The crosshair on the mask M\({}_{1}\) for the first exposure is a wider crosshair with a line width of 12 \(\upmu\)m. Furthermore, the crosshair on the mask M\({}_{2}\) for the second exposure is a thinner crosshair with a line width of 4 \(\upmu\)m.  
Firstly, the single-sided rMLA is prepared on one side of the substrate. The process flow is shown in Figure 4, including exposure (using M\({}_{1}\)), development, dechroming, and etching. At this time, the crosshair with a line width of 12 \(\upmu\)m is etched on the edge region which is near the structure of rMLA. Figure 3: Part patterns of mask: (**a**) the first mask M\({}_{1}\); (**b**) the second mask M\({}_{2}\). Secondly, another rMLA structure is fabricated on the other side of the substrate. The preparation method is described in detail in the paragraph below. The first step is the pretreatment of the substrate, including the protection of the prepared single-sided rMLA and the preparation of the mask layer namely the chromium film using chemical etching, as shown in Figure 5a. The fabricated rMLA is filled with photoresist and pasted with acid-fast as protective layer to protect the fabricated structure from corrosion by HF. At the same time, a chromium film with a thickness of 100 nm is plated on the polished surface of the substrate, which is used as a masking layer for chemical etching. The second step is photoresist coating and exposure. The photoresist of the type is AZ MIR--703 Photoresist (14 cp) (AZ Electronic materials, Somerville, MA, USA) which was spin-coated on the glass substrate with chromium film at the speed of 4000 r/min with time of 30 s. The thickness of the photoresist was about 700 nm. In order to ensure that there is no rotation error between both sides of the substrate rMLAs after etching, alignment marks were used to align during the exposure process. The crosshair structure M\({}_{1}\) which had been etched on the substrate was aligned with the crosshair on the mask M\({}_{2}\). In the process of exposure, the alignment marks on the mask M\({}_{2}\) were captured with the microscope. Then the prepared rMLA was placed on the wafer supporting with the rMLA structure face down and the photoresist face up, as shown in Figure 5b. By translating or rotating the wafer supporting, the crosshair with 4 um line width on the mask was nested in the crosshair with 12 um line width which had been prepared on the back of the substrate, as shown in Figure 6a, so as to achieve strict alignment of the rMLA on both sides. Then lock the wafer supporting for exposure. The exposure power density was set as 3 mW/cm\({}^{2}\) with the center wavelength of 365 nm, and the exposure time was 20 s. In the process of alignment exposure, the alignment deviation of the crosshair mark is with \(\pm\)1 um. The third step is development. By the development with developer of AZ 300 MIF DEVELOPER (AZ Electronic Materials, Somerville, MA, USA) for 35 s, the microporous structure on the mask was transferred to the photoresist, as shown in Figure 5c. The fourth step is chromium removing. The developed substrate is put into the chromium-removing solution for about 40 s to transfer the microporous structure on the photoresist to the chromium film, as shown in Figure 5d. The last step is etching. The protected substrate is etched in etching solution (H\({}_{2}\)O:HF: HNO\({}_{3}\) = 5:2:2). In the etching process, the glass substrate is taken out every 3 min and put into the ultrasonic oscillator to clean the chemical reaction residue. The rMLA on  
Figure 4: Fabrication process of single-sided rMLA: (**a**) exposure; (**b**) development; (**c**) chromium removal; (**d**) etching. both sides of the substrate has the same distribution of diameters and sag heights after the same etching time of 30 min, as shown in Figure 5e. The microscope pattern of the rMLA can be obtained after etching. The structure of the front and back sides was mirror symmetrical, whose filling factors are 100%, as shown in Figure 6b,c.