8.1.2 Imaging Enhancement Techniques

As discussed in Chapter 2 various imaging enhancement techniques were introduced to overcome the small depth of focus in DUV lithography without substantial decrease of resolution. For practical applications two major strategies are relevant, namely the introduction of advanced illumination apertures (cf. Figure 2.4) and the modification of the layout data by applying phase-shifting elements or additional opaque patterns like serifs as pursued in optical proximity correction (cf. Figure 2.6 and Section 3.3.1, respectively). Each approach has advantages and drawbacks. The main difference stems from the fact whether the method is optimized for dense or sparse patterns. In the following we present an extensive comparison of closely spaced and isolated contact holes. The width of the contact holes is 0.25 $\mu$m, i.e., again a quarter micron technology is used. The stepper is the same DUV projection printing system as in the previous section. It operates at a wavelength of 248 nm and has a numerical aperture of NA = 0.6.

The following approaches are investigated in Figure 8.8 to Figure 8.15. Schematics of the applied resolution enhancement techniques are shown in Figure 8.7.

(1)

Conventional illumination with a partial coherence factor of $\sigma$ = 0.7. This stepper is the reference system with parameters identical to those used for the entire layout simulation.

(2)

Quadrupole illumination with $\sigma$ = 0.1 and X = Y = 0.7. Quadrupole illumination can be highly optimized for a specific pattern pitch [236]. For complex layouts such a optimization is not possible and for isolated features, i.e., a large pitch, the optimization would yield a conventional aperture.

(3)

Annular illumination with $\sigma_{\mathrm{out}}^{}$ = 0.7 and $\sigma_{\mathrm{in}}^{}$ = 2/3$\sigma_{\mathrm{out}}^{}$. Annular illumination is a compromise between the conventional and quadrupole aperture [237]. Increase in depth of focus is smaller than in quadrupole, but it can be applied to general layouts as well as to isolated features.

(4)

Subresolution serifs at the contact corners. Subresolution serifs considerably reduce optical proximity effects [60]. They are best suited for complex layouts. The depth of focus performance is better for isolated than for dense patterns.

(5)

Alternate PSM. Alternate PSMs are restricted to dense, periodic layouts [16,49]. Since their approach was the first proposed phase-shifting technique they are more interesting from a historical perspective than for industrial applications.

(6)

Rim PSM with a shifter width of 25 nm. Rim PSMs overcome the limitation of alternate PSMs [238]. They are applicable also to isolated features at the expense of minor focus increase.

(7)

Outrigger PSM with a shifter distance of C.5 nm and a shifter width of 25 nm. Outrigger PSMs apply the phase-shifting element outside the nominal opening [239]. The illumination intensity is thus not decreased. However, the focus latitude is smaller in comparison to rim PSMs.

(8)

Attenuated PSM with a transmittance of 10%. Attenuated PSMs do not require opaque elements [240]. The chrome-less fabrication process is easy to handle, self-aligned techniques can be applied. They are especially useful for isolated patterns.

  

Figure 8.7: Resolution enhancement techniques: Advanced apertures and mask layouts. The alternate PSM is not depicted. It is realized by applying a phase-shifter to every second pattern.

quadrupole (2)	annular (3)	subresolution serifs (4)
rim PSM (6)	outrigger PSM (7)	attenuated PSM (8)

An investigation of the results presented in Figure 8.8 to Figure 8.15 exhibits the characteristic properties of the various methods.

• Dense patterns--best focus. All resolution techniques are capable to resolve the contact holes (cf. Figure 8.8 and Figure 8.9). The four PSMs perform slightly better, which follows from the broader valley between the two main lobes. The alternate PSM (5) has the highest resolution. It forces zero intensity in the opaque areas.
  
• Dense patterns--defocus of  1 $\mu$m. Three methods work in the defocus case, namely quadrupole illumination (2), annular illumination (3), and alternate PSM (5) (cf. Figure 8.10 and Figure 8.11). Among them quadrupole illumination operates best. The defocus effects are almost completely compensated.
  
• Isolated pattern--best focus. Again all resolution techniques provide a similar resolution (cf. Figure 8.12 and Figure 8.13). The PSMs approaches are superior. The sharper main lobes are achieved at the expense of higher sidelobes.
  
• Isolated pattern--defocus of  1 $\mu$m. The attenuated PSM (8) is the only approach that resolves the pattern in a tolerable quality (cf. Figure 8.14 and Figure 8.15). However, this aerial image would require a very high resist contrast to yield an acceptable contact hole diameter.

The simulations prove the expected results: In the case of best focus dense as well as isolated patterns can be printed with similar quality, whereby the PSMs provide a higher resolution. In the defocus situation dense patterns are easier to print than sparse patterns since defocus can be reduced with the help of proximity effects. Advanced illumination apertures are for that purpose best suited. The attenuated PSM is the only technique that also operates for defocused isolated patterns.

  

Figure 8.8: Resolution enhancement for dense patterns at best focus: (1) Conventional illumination, (2) quadrupole illumination, (3) annular illumination, (4) optical-proximity correction with serifs.

  

Figure 8.9: Resolution enhancement for dense patterns at best focus (cont.): (5) Alternate PSM, (6) rim PSM, (7) outrigger PSM, (8) attenuated PSM.

  

Figure 8.10: Resolution enhancement for dense patterns at a defocus of 1 $\mu$m: (1) Conventional illumination, (2) quadrupole illumination, (3) annular illumination, (4) optical-proximity correction with serifs.

  

Figure 8.11: Resolution enhancement for dense patterns at a defocus of 1 $\mu$m (cont.): (5) Alternate PSM, (6) rim PSM, (7) outrigger PSM, (8) attenuated PSM.

  

Figure 8.12: Resolution enhancement for sparse patterns at best focus: (1) Conventional illumination, (2) quadrupole illumination, (3) annular illumination, (4) optical-proximity correction with serifs.

  

Figure 8.13: Resolution enhancement for sparse patterns at best focus (cont.): (5) Alternate PSM, (6) rim PSM, (7) outrigger PSM, (8) attenuated PSM.

  

Figure 8.14: Resolution enhancement for sparse patterns at a defocus of 1 $\mu$m: (1) Conventional illumination, (2) quadrupole illumination, (3) annular illumination, (4) optical-proximity correction with serifs.

  

Figure 8.15: Resolution enhancement for sparse patterns at a defocus of 1 $\mu$m (cont.): (5) Alternate PSM, (6) rim PSM, (7) outrigger PSM, (8) attenuated PSM.