8.1.1 Entire Layout

The aerial images of the entire layout depicted in Figure 8.3 and Figure 8.5 show that all four layers can be printed in sufficient quality if no focus errors occur, but the images are completely spoiled for a defocus of 1 $\mu$m since the patterns are totally blurred and the whole image information gets lost. In Figure 8.4 and Figure 8.6 the upper right corner of the layout is zoomed, which clearly exhibits the image degradation. The following particularities can be observed from the plots:

• Edge and ringing effects. Although partially coherent illumination is used overshooting occurs at the edges of large area patterns. For coherent illumination this phenomenon would be more sever (cf. Figure 2.5).
• Line width variations. A comparison of the images of the ACTIVE-AREA with POLY1 and METAL1 exhibits line width variations between isolated and dense lines.
• Proximity effects. The optical proximity phenomenon can clearly be observed in the defocus situation. The image intensity at large are patterns and closely spaced contact holes is large because additive interference occurs between adjacent pixels. Features with large area are thus printed in relatively good quality (cf. ACTIVE-AREA, POLY1, and METAL1). The contact holes are, on the contrary, totally spoiled. Instead of a single contact the whole neighboring area is irradiated.

Compensation techniques for these disturbing effects are discussed in the subsequent section.

  

Figure 8.1: Entire layout of a 4:16 multiplexer fabricated in a 0.25 $\mu$m technology. The following four mask layers are drawn one upon the other: ACTIVE-AREA (green), POLY1 (red), CONTACT (white), and METAL1 (blue). Separate plots of each layer are shown in Figure 8.2.

  

Figure 8.2: The four layers of interest: ACTIVE-AREA (upper-left), POLY1 (upper-right), CONTACT (lower-left), and METAL1 (lower-right). The aerial images for best focus are shown in Figure 8.3, and for a defocus of 1 $\mu$m in Figure 8.5.

  

Figure 8.3: Aerial images for best focus: ACTIVE-AREA (upper-left), POLY1 (upper-right), CONTACT (lower-left), and METAL1 (lower-right).

  

Figure 8.4: Aerial images for best focus (zoomed): ACTIVE-AREA (upper-left), POLY1 (upper-right), CONTACT (lower-left), and METAL1 (lower-right).

  

Figure 8.5: Aerial images for a defocus of 1 $\mu$m: ACTIVE-AREA (upper-left), POLY1 (upper-right), CONTACT (lower-left), and METAL1 (lower-right).

  

Figure 8.6: Aerial images for a defocus of 1 $\mu$m (zoomed): ACTIVE-AREA (upper-left), POLY1 (upper-right), CONTACT (lower-left), and METAL1 (lower-right).