Image formation by continuous writing with multi-beam in X-ray nanolithography

Difficulties in X-ray lithography are now condensed into mask related matters. Since the exposure mode is basically 1:1, more strict accuracy is requested for mask fabrication than for the final images. Mask fabrication technology has made remarkable progress, and image formation of 70 nm line width was reported recently (Miyatake et al, 2001). Meanwhile, pattern reduction necessitates narrowing gaps between mask and wafer, since the gaps decrease in proportion to the square of the line width. If we want to form 25-35 nm two-dimensional patterns in future, mask-wafer gaps of 2-4 μm are needed; it is impractical. Therefore, around 50-70 nm image forming has been considered as the limits of X-ray lithography. Recently, three types of X-ray mask were proposed which enable 25-35 nm image formation while keeping a practical proximity gap >8 μm. The first is enlarged pattern masks (EPMs), applying a line-narrowing effect by edge diffraction. The second is interference slit masks (ISMs), which form design images by interference effect from the slits of the mask. The third is focusing x-ray masks (FXMs), which form an array of concave lenses using the absorbing materials on the mask membrane. These masks, which reduce mask patterns partially (PRMs: partially reducing masks), relieve the limitation of the proximity gaps. In this article, we discuss the design of the masks, beamlines and procedures of writing for 25 nm image formations.