# High-Aspect-Ratio Silicon Metasurfaces: Design, Fabrication, and Characterization. _Applied_
## 3 Fabrication Process for High-AR Silicon Metasurfaces
This is due to the difficulty to control the sidewall effect related to the etching process, especially for cases with high AR. In the Bosch process, the etch and passivation steps not only cooperate with each other but also compete against each other. The different combination of etch and passivation will lead to very different sidewall effects, thus finding the balanced state between these two steps is crucial to achieve the high-AR silicon etch [51]. As shown in Figure 5, if the passivation step continuously suppresses the etch one, the reactant cannot sufficiently react with the inhibitor, leaving residue on the bottom of the  
Figure 4: Schematics of the workflow of the tri-layered silicon metasurfaces with high AR in three steps (**a**) and the Bosch etch process (**b**) based on the time-multiplexed deep etching technique, where the etch cycle and the passivation cycle switch between each other during the process. silicon to form micro-masking [48]. The roughness of the bottom surface of the substrate will be enhanced after multiplex working cycles, resulting in the presence of micro-grass on the bottom [47]. Such an effect, also known as the black silicon effect, can be readily illustrated with case #3 shown in Figure 5.  
On the other hand, when the etch step suppresses the passivation step, the reactant will exhaust the inhibitor on the surface of the etched structures, and eventually lead to an isotropic etch in the vertical and lateral directions. In this case, the inhibitor on the sidewall will first be removed, and the sidewall of the structure will be etched, resulting in lateral etching. As the time-multiplexed process proceeds, the undercut of the bottom region of the structure will move further in the lateral direction of the pillar, and finally lead to the sidewall slope, even collapsing the micro-pillars, as shown with case #2 in Figure 5.  
Fortunately, once the balance between the etch and passivation cycles has been found, the perfect high-AR silicon etch can be achieved as shown with case #1 in Figure 5. In particular, our fabricated MHWP is of an AR of 20:1 (as show by the yellow dashed line area in the SEM image; the width of the fabricated micro-pillar is about 20 \(\upmu\)m) and the height of pillar of 400 \(\upmu\)m. After optimization of the etch profile by finding the balanced process, we still need to precisely control the etch depth which directly affects the VTD of our tri-layered meta-device. Notice that the presence of the silicon cubes on the bottom side of the silicon wafer will induce non-uniform heat distribution on the bottom side of the wafer, which might lead to a non-uniform etch rate at the bottom side of the structures or causes a micro-grass effect. In order to avoid such negative influences, we adopt a gradient parameter recipe consisting of different processing cycles and platen power in multistage steps, to maintain the balanced state for high-AR silicon etching, as shown in Figure 6. At the beginning of the etching process, the structure is well protected by the thick PR; thus, we apply the etch recipe with a great number of working cycles and a low platen power to increase the etch efficiency and lower the damage to the PR from the reactant. With the increase in the etching depth, the etch rate drops significantly, since it become more difficult for the reactants to reach the bottom of the structure. We increase the  
Figure 5: The etch profile of the fabricated silicon metasurfaces with different combinations of etch phase and passivation phase. Over-etching leads to an undesired sidewall slope of the micro-pillar, while under-etching leads to the micro-grass effect on the bottom of the substrate. The balanced process will guarantee the HAR silicon etch with a straight sidewall profile. planet power, providing a directional accelerating electric field, to maintain the etch profile balance. The increased platen power can also increase the temperature of the substrate and PR, thus increasing the etch rate of the PR.