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

X-ray lithography is commonly used to build high aspect ratio microstructures (HARMS) in a 1:1 proximity printing process. HARMS fabrication requires high energy X-rays to pattern thick resist layers; therefore the absorber thickness of the working X-ray mask needs to be 1.0-50 mum in order to provide high contrast. To realize high resolution working X-ray masks, it is necessary to use intermediate X-ray masks which have been fabricated using e-beam or laser lithographic techniques. The intermediate masks are characterized by submicron resolution critical dimensions (CD) but comparatively lower structural heights (similar to2 mum). This paper mainly focuses on the fabrication of high resolution X-ray intermediate masks. A three-step approach is used to build the high resolution X-ray masks. First, a so called initial mask with sub-micron absorber thickness is fabricated on a 1 mum thick silicon nitride membrane using a 50KeV e-beam writer and gold electroplating. The initial X-ray mask has a gold thickness of 0.56 mum and a maximum aspect ratio of 4:1. Soft X-ray lithography and gold electroplating processes are used to copy the initial mask to form an intermediate mask with 1 mum of gold. The intermediate mask can be used to fabricate a working X-ray mask by following a similar set of procedures outlined above.