Fabrication of Recessed Microelectrodes

All the electrodes are fabricated using a similar method as previously reported^{12 (link)}. We first sputtered a release layer of 70 nm Al on a Si wafer (University Wafer). Followed by photolithography to pattern the bottom SU8: spin coated SU8 2005 (MicroChem) at 5000 rpm for 30 s, resulting in a 4 μm thick layer, then prebaked at 65 °C and 95 °C for 1 min and 4 min, respectively. After exposure, the wafer was post-baked again at 65 °C and 95 °C for 1 min and 4 min, respectively, then developed in SU8 developer for 2 min. Next, LOR 3A was spin-coated at 2000 rpm for 1 min, baked at 100 °C and 150 °C for 2 min and 10 min as lift-off sacrificial layer. Then S1818 was spin-coated at 4000 rpm for 1 min, baked at 115 °C for 1 min. After exposure, the wafer was developed in MF321 for 2 min. Prior to sputtering metal traces, the surface was treated with oxygen plasma at 100 W for 1 min to enhance adhesion. A 100 nm thick Pt layer was sputtered and lifted off with acetone. For gold electrodes, 8 μm-thick Ti was sputtered as an adhesion layer. Next, top insulation SU8 was patterned using the same steps as bottom SU8. For recessed electrodes, a dry etch was applied using Reactive Ion Etcher (Oxford 180) to open up the top SU8 vias for small electrodes. Before releasing, the microelectrodes were hardbaked at 180 °C for 1 hour. The microelectrodes were released using MF 321 for 8 hours.