Phase-shifting masks are widely used in optical lithography, and the question is whether the technology can be extended to much shorter wavelengths. We have extensively modeled the use of clear phase-mask materials as absorbers. for x-ray exposures. Similar to the optical domain. the imaging from x-ray phase masks is highly nonlinear, and thus can be used to produce a feature reduction of 3-5x compared to the mask pattern. This observation suggests that mask fabrication might be easier, but the technique is most suited for less dense pattern requirements. In this article we review some of the salient modeling, examine the requirements for effective fabrication of the masks, and provide some experimental verification of this approach to reach the sub-50 nm region. It is rift essential to have exactly a 180degrees phase shift in the clear material. because mask feature reductions can be achieved with 45, 90, 180, and 270degrees shifts by choosing the appropriate thicknesses of the clear material. A mask with a 235 nm feature and a 45degrees thickness is transformed into a 134 nm aerial feature, but the same dose for a 180degrees thickness results in an 86 nm feature. The best resolution is obtained in the so-called bright peak enhanced x-ray phase mask (BPEXPM) mode with thicknesses appropriate to 180degrees or greater. Previous modeling of the clear phase feature indicated a strong dependence on the wall slope, and fabrication processes are being pursued to verify the modeling and demonstrate the slope dependence on the feature profile. Silicon nitride with a phase shift of 126degrees is currently employed, but other materials could also be used in the mask fabrication. Experimentally, we are pursuing the feature width reduction achieved from correctly fabricated masks to produce gate width structures at the sub-50 nm region. (C) 2004 American Vacuum Society.