Photolithography of SU-8 microtowers for a 100-turn, 3-D toroidal microinductor

We present a photolithography scheme for ultra-tall, high-aspect-ratio microstructures. While increased height of microstructures can expand the design capability of various microdevices, it has been challenging to achieve the ultra-tall microstructure, 1 mm or higher, using a well-known negative photoresist, SU-8. One of the reasons is the high absorption rate of 365-nm ultra-violet light during the exposure process, although it used to be recommended for the SU-8 process. We report on optical characteristics of microlithography, in particular the 365- and 405-nm wavelengths, and present the lithography method for ultra-tall micropillars with a height of 1 mm or higher, called microtowers. While the 365-nm wavelength is experimentally validated with its high attenuation inside the SU-8, higher transparency of the 405-nm wavelength with a thicker SU-8 is reported to be suitable for ultra-tall micropillar structures. Assuming exposure time causes the color change of the SU-8, transparency of the SU-8 as a function of exposure time is measured with a thick SU-8. SU-8 microtowers with various heights are reported, including an array of 2000-µm-tall microtowers and a state-of-the-art 7000-µm microtower. To demonstrate usefulness of the proposed fabrication method, an array of 1000-µm-tall microtowers are successfully fabricated to form a 100-turn, 3-D toroid inductor. The fabricated inductor shows average inductance of 950 nH in the frequency range of 0.1 to 10 MHz, a low-frequency resistance of 5.4 Ω at 0.1 MHz, and a quality factor of 22 at 60 MHz.