2.4.2 Phase-Shifting Masks

Adding a phase-shifting function to the on-off property of binary masks yields a higher resolution at the same or even larger amount of depth of focus. Thus phase-shifting, like off-axis illumination, is a technique to reduce the k₁ parameter in (2.1). The enhancement stems from the fact that both amplitude and phase are used to store the information about the image on the mask. The phase-shifting principle was already introduced in 1982 by Marc Levenson [16], but has long remained a development tool. Recently enormous efforts for industrial application have been made, and pilot production has already started. The operation principle and the most important types of phase-shifting masks are shown in Figure 2.6. A detailed overview on phase-shifting masks including pros and cons of various approaches, the fabrication process as well as repair and inspection issues can be found in [17].

  

Figure 2.6: The most relevant types of phase-shifting masks (PSMs) and their operation principle in comparison to a conventional binary mask. The alternate or Levinson PSM offers considerable enhancement in image contrast and depth of focus but is limited to closely packed patterns to be efficient. The rim PSM approach can be applied to arbitrary mask layouts and is most effective for line and hole openings. Drawbacks are exaggerated proximity effects, the increase in space due to the rim shifter, and the extreme sensitivity on the size of the rim shifter. Similar to rim PSMs, attenuated PSMs are most effective for line and hole openings, apply to arbitrary patterns, can be self-aligned, have proximity effects, and need larger openings than the desired images. However, the attenuated PSMs are better than rim PSMs in all aspects.