High-resolution x-ray masks for high aspect ratio microelectromechanical systems applications

The advanced requirements of bio-MEMS and MOEMS, i.e., low sidewall surface roughness, submicron critical dimension, and high aspect ratio, necessitate the use of an intermediate mask and a soft x-ray lithography process to fabricate working x-ray masks that are suitable for deep x-ray lithography. Intermediate masks consist of 2 to 2.5-µm gold patterns on membranes/substrates that are highly transparent to x-ray radiation, whereas working masks possess greater than 5 µm of gold patterns. In this work, 1-µm silicon nitride membranes are produced by a low pressure chemical vapor deposition (LPCVD) process on both the front and backside of 100 prime grade wafers and anisotropic wet etch through silicon nitride etch masks. E-beam lithography is used to pattern 0.8- to 3-µm-thick resist layers with submicron resolution. In the case of the 3-µm resist layers, the features are electroplated with approximately 2 µm of gold to form an intermediate mask. The 0.8-µm-thick layers are electroplated with gold up to a thickness of 0.6 µm and form initial masks, which are in turn used in a soft x-ray lithographical process to make intermediate masks. The process of building a high-resolution intermediate x-ray mask, directly by e-beam patterning a 3 µm layer of e-beam resist, followed by gold electroplating, is found to be viable but requires the use of a high energy (>100 keV) e-beam writer. The stability of the resist pattern during soft x-ray lithography (SXRL) by use of an initial mask is found to be problematic. Double-side lithography and gold electroplating, can effectively reduce the aspect ratio of the mask pattern, eliminates the problems associated with the use of an initial mask to fabricate intermediate x-ray masks.