1 July 2004 High-resolution x-ray masks for high aspect ratio microelectromechanical systems applications
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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.
Lin Wang, Todd R. Christenson, Yohannes M. Desta, Rainer K. Fettig, Jost Goettert, "High-resolution x-ray masks for high aspect ratio microelectromechanical systems applications," Journal of Micro/Nanolithography, MEMS, and MOEMS 3(3), (1 July 2004). https://doi.org/10.1117/1.1753271


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