# Fabrication of Random Microlens Array for Laser Beam Homogenization with High Efficiency
## 3 Fabrication of Random Microlens Array (rMLA)
### Fabrication of rMLA
#### 3.2.2 Fabrication of Rectangular rMLA  
The fabrication of the rectangular rMLA, which is slightly different from the honeycomb rMLA, requires a mask predesigned and prefabricated. Then, through exposure, development, chromium removal and other processes, the random microporous structure on the mask plate was transferred to the chromium film of the substrate. Finally, chemical etching was performed to generate rMLA.  
The designed mask is a micropores array of random rectangular arrangement, as shown in Figure 7a. The radiuses of micropores are the same as 1.8 \(\upmu\)m, and the distances between adjacent micropores varies randomly from 24 \(\upmu\)m to 45 \(\upmu\)m. The mask was fabricated by high-precision laser direct writing technology with a fabrication accuracy of \(\pm\)250 nm. The pattern of the mask plate observed under the microscope is shown in Figure 7b.  
Subsequently, the chromium film, with a thickness of about 140 nm, was coated on the glass substrate, as shown in Figure 8a. The photoresist of the type AZ MIR-703 Photoresist (14 cp) (AZ Electronic materials, Somerville, MA, USA) was spin-coated on the glass substrate with chromium film at a speed of 4000 r/min with a time of 30 s. The thickness of the photoresist was about 700 nm, as shown in Figure 8b. The photoresist was then exposed by the exposure machine with a center wavelength of 365 nm. The exposure method was contact exposure with an exposure time of 20 s and the exposure power of 3 mW, as shown in Figure 8c. By the development with developer of AZ 300 MIF (AZ Electronic materials, Somerville, MA, USA) for 35 s, the microporous structure on the  
Figure 6: Microscopic image of the honeycomb rMLA.  
Figure 7: Mask part pattern: (**a**) rectangular randomly arranged mask data; and (**b**) microscopy pattern of mask made by laser direct writing technology.  
mask was transferred to the photoresist, as shown in Figure 8d. The substrate was then put into the chromium removal solution for 40 s to transfer the microporous structure on the photoresist to the chromium film to complete the substrate pretreatment, as shown in Figure 8e. The substrate was finally put into the etching solution for etching. The etching solution penetrated from the micropores and contacted with the glass surface to produce a reaction. After the glass substrate was etched for a period of time, micropits were generated on the substrate surface, as shown in Figure 8f. The operation steps in the etching process were consistent with the fabrication of the honeycomb rMLA. During the etching process, the etching solution was continuously vibrated to prevent the falling chromium film from affecting the surface shape of rMLA. The glass substrate was cleaned every 10 min or so. After etching for about 40 min, the rMLA was obtained, as shown in Figure 8g. The experimental result obtained with the microscope is shown in Figure 9a, where it can be clearly seen that the structure is random due to the irregularity of arrangement of microlenses, the surface is smooth with a roughness of 13 nm, the filling ratio is high and the arrangement of rectangular rMLA is basically consistent with the arrangement of Figure 7b. Comparing the profiles data of the microlenses measured with the step profilometer (Stylus Profiler System, Dektak XT, Bruker, Karlsruhe, Germany) with an ideal spherical surface, it can be seen that the profile of the microlenses is almost spherical, and the fitting graph is shown in Figure 9b.  
Figure 8: Fabrication schematic of rectangular rMLA: (**a**) chromium plating; (**b**) coated photoresist; (**c**) exposure; (**d**) development; (**e**) chromium removal; (**f**) under etching; and (**g**) rectangular rMLA obtained by chemical etching.  
Figure 9: Micrographs and simulation profiles: (**a**) microscopic image of rectangular rMLA; and (**b**) comparison of microlenses profile and ideal sphere.