## 2 Polymer ML Development
### Fabrication of Polymer MLs  
There are a wide varieties of polymer materials available for fabricating MLs. The properties of these materials that should be paid special attention to are refractive index, viscosity, contraction coefficient, and process temperatures. The refractive indices of most polymer-based chemicals are comparable to that of glass, about 1.5. The viscosity depends on the material itself and the interfacial bonding with the substrate. Due to the relatively large contraction coefficients, the volume of commercial polymer materials may shrink by <5% after dehydration. The process temperature for polymer ranges from room temperature to <400 \({}^{\circ}\)C, and one should select a proper temperature not to degrade the Ge PD beneath the polymer ML. With the additional consideration of passing the reliability qualifications for optical fiber communication applications, three commercially available polymers, AZ [27], ORMO [28], and SU8 [29], are chosen for our ML development (the material properties can be found in Refs. [28; 29; 30]. E.g., the viscosity of the SU8 is 4400 cSt.). To determine the most suitable polymer for the ML, we spin coat a thin film on a glass substrate and then heat the substrate to 125 \({}^{\circ}\)C. The transmission spectra derived from UV-VIS (ultraviolet-visible) spectroscopy of ORMO and SU8 after the materials being baked at 125 \({}^{\circ}\)C for different time durations are shown in Figure 2. Note that AZ is not included in Figure 2 because it does not survive the 125 \({}^{\circ}\)C baking process where cracks appear after 120 h. ORMO can sustain nearly 100% transmission for wavelengths longer than 700 nm after 800 h, while SU8 shows a less than 2% drop in transmission for wavelengths longer than 850 nm after 800 h. The transmission drop is due to the chemical changes in the polymer material responding to the thermal budgets. At the visible wavelengths, for both ORMO and SU8, the transmissions drop rapidly with baking duration. However, because the target operation wavelength is 850 nm, the material decay at shorter wavelengths can be overlooked, both ORMO and SU8 are deemed viable for our application. Finally, after further studies, ORMO is excluded from the consideration due to its high viscosity, leaving SU8 as the only logical choice. The extensive use of SU8 in the industry for its durability and reliability is another reason that we apply SU8 in our work. In the following, two types of ML process developments base on photon-acid diffusion with SU8 are discussed. The process flow of the Method 1 formation process is shown in Figure 3: a 20 \(\upmu\)m thick SU8 layer is spin coated on a substrate at 3000 rpm for 40 s, and then baked at 150 \({}^{\circ}\)C for 5 min. Then, the 1 st photolithography is applied to define the pedestal region with a UV dosage of 140 mJ/mm\({}^{2}\). A 30 \(\upmu\)m thick SU8 layer is then spin coated over the 1 st SU8 layer at 2000 rpm speed for 40 s, followed by a soft bake at 120 \({}^{\circ}\)C for 5 min. After a process of edge bead removal (EBR), the 2 nd photolithography is applied to generate photon-acid in the 2 nd SU8 layer at the ML region with a UV dosage of 100 mJ/mm\({}^{2}\). A thin SU8 layer is then spin coated over the 2 nd SU8 layer at 1000 rpm  
Figure 2: Transmission spectra of (**a**) ORMO and (**b**) SU8 derived from UV-VIS spectroscopy, after 200 h, 500 h, and 800 h of thermal treatment at 125 \({}^{\circ}\)C.  
for 40 s, followed by no soft bake, and placed a in clean dry air (CDA) purging chamber for an hour for photon-acid diffusion. Within this hour, the photon-acids diffuse outward uniformly to form a concentration gradient. Post exposure baking (PEB) at 70 \({}^{\circ}\)C for 5 min and follow-up development remove the additional polymer without a crosslink, forming the shape of the lens.