S. Lee, Kim, D., Jin, Y., Han, Y., Desta, Y., Bryant, M. D., and Goettert, J., “A Micro Corona Motor Fabricated by a SU-8 Built-on X-ray Mask,” Microsystems Technologies Journal, vol. 10, pp. 522–526, 2004.
A micro corona motor was fabricated using a membraneless built-on X-ray mask. Sharp stator electrodes of this motor ionize air molecules and ionized charges transfer onto the rotor surface, resulting in rotating rotor motions by Coulomb forces. For good performance, the stator’s electrodes should be wide (axial) and have sharp tips. Therefore, X-ray lithography was adopted for precise, high aspect ratio characteristics. To avoid the fabrication difficulty of a membrane X-ray mask, a built-on X-ray mask (conformal mask) technique was employed with negative toned SU-8 photoresist. SU-8 features X-ray fabrication compatibility, X-ray transparency and a large range of thickness. This technique may be suitable for fast fabrication of prototypes or very tall structures, which can be largely affected by printing gaps. For the X-ray built-on mask, 20 μm SU-8 was patterned and 8 μm gold absorber was electroplated on top of the 300 μm PMMA resist. Tests showed good quality pattern transfer from the SU-8 pattern and smooth sidewalls.