The SU8 process is illustrated in Fig. 3. The first layer of SU8-50 was spun on a 100-mm silicon wafer where the thickness was controlled by weighing the material. In this case, 5.2 g SU8-50 was needed to achieve the required thickness of 432 μm and the amount was derived out of previous calibration. Due to the excessive thickness, the edge bead needs to be removed to improve the uniformity. The wafer was then placed onto a carefully leveled stage for a few hours for self-planarization of the SU8. SU8-50 was used as it has relatively lower viscosity and flows more easily on the wafer compared with other formulations, such as SU8-100 or 250. After that the wafer was baked at 65°C for 30 min followed by baking at 95°C for 270 min. It should be noted that, in order to minimize the residue stress in SU8, a slow temperature ramp (i.e., at ) is always necessary during the heat-up process while the wafer is cooled down naturally to room temperature after bake. Once the wafer was cooled down after soft bake, it was exposed in a Cannon PLA-510 mask aligner. Typically, the light coming out of the UV lamp consists of three main lines at 365, 405, and 436 nm. It is known that the long wavelength UV light has the ability to penetrate deeper into the SU8, but is less effective in inducing the photo acid for cross-linking. First, the wafer was exposed with a L39 filter so that only the lines above 400 nm could pass through to achieve deep penetration and a 2-min interval between each of the 40 s was adopted to stabilize the resist. After the first set of exposures, a PL360 filter was used, which allow the i-line 365 nm to pass through, to expose the wafer for another with 2-min interval as well. Then, a postexposure bake (PEB) was conducted at 70°C for 15 min to form a weak cross-link. It is noted that the development was not needed at this stage.