High-resolution x-ray masks for the application of high-aspect-ratio microelectromechanical systems (HARMS)

Challenging requirements in optical and BioMEMS application with high aspect ratios of microstructures in access of 20 and smallest structure details of less than I mum have motivated this work to improve x-ray mask fabrication. Several approaches to pattern an intermediate X-ray mask with the gold absorber thickness of 1.6similar to2.2 mum using a I mum thick silicon nitride membrane have been explored. E-beam lithography is employed for primary patterning and experimental results show that high energy (100keV) e-beam lithography is a very promising approach. So-called working X-ray mask can be fabricated from intermediate X-ray mask through x-ray lithography. More than 10 mum thick PMMA x-ray resist has been coated on the silicon nitride membrane by multi-coating process without crack. First exposure results indicate that adhesion and stability of sub-micrometer structures with these heights is critical. In order to overcome these problems a novel approach has been proposed by coating resist on both sides of the silicon nitride membrane and simultaneous patterning of both sides using x-rays. First successful experimental results have been achieved for proving the feasibility.