## 2 Polymer ML Development
### Experimental Results of Polymer ML  
The optical microscope (OM) images in Figure 5 show two MLs on pedestals fabricated using Method 1 with different dosages at the 2 nd exposure step. Note that the two MLs on pedestals are placed at the opposite side of a spacer, prepared for protecting the devices from damages during the stage of wafer dicing and chip packaging. The pedestal is built from the 1 st SU8 layer, which was formed by the 1 st spin-coating and the 1 st exposure. After the 2 nd SU8 layer is coated, we have two essentially non-identical materials, i.e., the 1 st exposed layer and the 2 nd non-exposed layer, even though they originated from the same polymer material. The interface between them is hereafter referred to as \"inhomogeneous interface\". Since the 2 nd SU8 layer without UV exposure does not dissolve well in the 1 st SU8 with UV exposure, the correspondingly weak adhesion through the inhomogeneous interface as well as the resultant anisotropic photo-acid diffusion lead to a distorted spherical ML with its vertex shifted away from the center. After adjusting the UV dosage to balance the photon-acid generation within the 1 st and 2 nd SU8 layers, the best spherical ML is achieved at the UV dosage of 100 mJ/mm\({}^{2}\), whereas its shape becomes worse with increasing UV dosages. Still, Method 1 cannot produce a satisfactory ML shape when its diameter is roughly larger than 45 \(\upmu\)m.  
On the other hand, an ML on a pedestal fabricated by Method 2 is shown in Figure 6. Since the 2 nd SU8 layer is processed right after the 1 st SU8 layer so that both layers are exposed to UV at the same time, the dimensions of the ML and the pedestal are defined simultaneously through the generated photon-acids. This way, one can avoid the problem of inhomogeneous interface observed in Method 1. In Figure 6a, the scanning electron microscope (SEM) of a nearly perfect spherical ML on a pedestal with a diameter of 54 \(\upmu\)m is shown. The total height between the vertex of the ML and the bottom of the pedestal is about 65 \(\upmu\)m, which is close to the design target shown in Figure 1b. In Figure 6b, the SEM image of arrays of ML separated by arrays of spacers is shown. High uniformity of the fabricated MLs on pedestals using Method 2 can be observed. Note that the height of the spacer is intentionally made higher than that of the ML on pedestal so that the spacers can protect the surface of the MLs on pedestals from scratching when wafer dicing and chip packaging are performed. Figure 6c is the 3D profile of the fabricated ML using Method 2  
Figure 4: (**a**) The 1 st spin-coating and soft bake to derive a uniform SU8 film of 20 \(\upmu\)m thickness. (**b**) The 2 nd spin-coating and soft bake to place a uniform SU8 layer of 30 \(\upmu\)m thickness over the 1 st SU8 layer. (**c**) UV exposure to define the ML and pedestal dimensions and generate photo-acids in the 1 st and the 2 nd SU8 layers. (**d**) The 3 rd spin-coating of an SU8 layer, which is left idled for about 1 hour to control the photo-acid diffusion. (**e**) PEB and (**f**) follow-up development performed to remove the extra polymer without a crosslink, forming the shape of the lens. (**g**) Hard bake to generate higher degrees of crosslink and dehydration. for checking ML sphericity. The result shows clear concentric circular interference patterns, which means a nearly ideal symmetric lens shape is derived. The un-resolved slopes within the dark region are due to the tool limitation, but they can be seen in the SEM result as continuous and smooth. These results demonstrate a promising method for fabricating giant polymer ML arrays on an 8-inch Si substrate with a high yield, low thermal budget, and cost-efficient process. Figure 5: Two MLs on pedestals (leftmost and rightmost objects) formed by using Method 1 with a UV dosage of (**a**) 100 m\(\mathrm{J/mm^{2}}\) (**b**), 140 m\(\mathrm{J/mm^{2}}\), and (**c**) 160 m\(\mathrm{J/mm^{2}}\). Figure 6: (**a**) SEM image of an ML on a pedestal with a 54 \(\upmu\)m diameter fabricated by Method 2.