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

Abstract

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 <i>working</i> X-ray mask needs to be 10-50 &mu;m in order to provide high contrast. To realize high resolution <i>working</i> X ray masks, it is necessary to use <i>intermediate</i> X-ray masks which have been fabricated using e beam or laser lithographic techniques. The <i>intermediate</i> masks are characterized by submicron resolution critical dimensions (CD) but comparatively lower structural heights (~2 &mu;m). This paper mainly focuses on the fabrication of high resolution X-ray <i>intermediate</i> masks. A three-step approach is used to build the high resolution X-ray masks. First, a so called <i>initial</i> mask with sub-micron absorber thickness is fabricated on a 1 &mu;m thick silicon nitride membrane using a 50KeV e beam writer and gold electroplating. The <i>initial</i> X-ray mask has a gold thickness of 0.56 &mu;m and a maximum aspect ratio of 4:1. Soft X-ray lithography and gold electroplating processes are used to copy the <i>initial</i> mask to form an <i>intermediate</i> mask with 1 &mu;m of gold. The <i>intermediate</i> mask can be used to fabricate a <i>working</i> X-ray mask by following a similar set of procedures outlined above.