Maskless Grayscale Lithography Using a Positive-Tone Photodefinable Polyimide for MEMS Applications

The novel use of a positive-tone photosensitive polyimide for the rapid production of grayscale features using a maskless lithography system is demonstrated. The removal rate of the polyimide, HD-8820, is characterized as a function of exposure dose. A broad contrast curve is found that is suitable for grayscale lithography. Three-dimensional polyimide structures up to 22 μm thick are demonstrated, and the surface roughness after the final cure is Ra = 4.4 nm, which is suitable for many microelectromechanical systems (MEMS) applications, including many optical applications. Tensile testing of 63 polyimide samples shows excellent mechanical properties for four different polyimide thicknesses produced with grayscale lithography. The modulus of elasticity is found to be 1.92 GPa, the yield strength to be 103 MPa, the fracture strength to be 133 MPa, and the percent elongation to be 51%. The test results show that the mechanical properties are consistent and do not change due to a partial exposure to UV light. The entire fabrication sequence, from computer-aided design file to cured structure, can be performed in less than 4 h, making this a fast low-cost method of producing polymer MEMS devices with excellent mechanical properties.