Optical proximity correction

Optical proximity correction (OPC) is a photolithography enhancement technique commonly used to compensate for image errors due to diffraction or process effects. The two most common applications for OPC are linewidth differences between features in regions of different density (e.g., center vs. edge of an array, or nested vs. isolated lines), and line end shortening (e.g., gate overlap on field oxide). For the former case, scattering bars (sub-resolution lines placed adjacent to resolvable lines) or simple linewidth adjustments are applied to the design. For the latter case, "dog-ear" (serif or hammerhead) features are attached to the line end in the design. OPC has a cost impact on photomask fabrication, as the addition of OPC features means more spots for defects to manifest themselves. In addition, the data size of the photomask layout goes up exponentially.

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Impact of resolution: the k₁ factor

The conventional diffraction-limited resolution is given by the Rayleigh criterion as 0.61λ / NA, where NA is the numerical aperture and λ is the wavelength of the illumination source. It is often common to compare the critical feature width to this value, by defining a parameter, k₁, such that feature width equals k₁λ / NA. Nested features with k₁ < 1 benefit less from OPC than isolated features of the same size. The reason is the spatial frequency spectrum of nested features contains fewer components than isolated features. As the feature pitch decreases, more components are truncated by the numerical aperture, resulting in greater difficulty to affect the pattern in the desired fashion.

Impact of spatial coherence

The degree of coherence of the illumination source is determined by the ratio of its angular extent to the numerical aperture. This ratio is often called the partial coherence factor, or σ.^[1] It also affects the pattern quality and hence the application of OPC. The coherence distance in the image plane is given roughly by 0.5λ / (σNA).^[2] Two image points separated by more than this distance will effectively be uncorrelated, allowing a simpler OPC application. This distance is in fact close to the Rayleigh criterion for values of σ close to 1.

Impact of multiple exposure

As the k₁ factor has been steadily shrinking over the past technology generations, the anticipated requirement of moving to multiple exposure to generate circuit patterns becomes more real. This approach will affect the application of OPC, as one will need to take into account the sum of the image intensities from each exposure. This is the case for the complementary photomask technique^[3], where the images of an alternating-aperture phase-shifting mask and a conventional binary mask are added together.

Impact of multiple-etch patterning

In contrast to multiple exposure of the same photoresist film, multiple layer patterning entails repeated photoresist coating, deposition, and etching to pattern the same device layer. This gives an opportunity to use looser design rules to pattern the same layer. Depending on the lithography tool used to image at these looser design rules, the OPC will be different. Multiple-etch patterning may become a popular technique for future technology generations. A specific form of multiple-etch patterning, using sidewall sacrificial features, is currently the only demonstrated way of systematically patterning features less than 10 nm^[4]. The minimum half-pitch corresponds to the deposited thickness of the sacrificial feature.

OPC application today

Today, OPC is rarely practiced without the use of commercial packages from EDA vendors.

The use of OPC is not restricted to the low k₁ features which are commonly encountered today, but can be applied to any desired image correction scheme which can be modeled accurately. For example, proximity effect correction in electron beam lithography is included as an automated capability on commercial electron-beam lithography tools. Since many non-lithographic processes exhibit their own proximity effects, e.g., chemical-mechanical polishing or plasma etching, these effects can be mixed in with the original OPC.

See also

• Phase-shift mask

References

1. ^ K. Ronse et al., J. Vac. Sci. and Tech. B, vol. 12, pp. 589-600 (1994).
2. ^ B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, pp. 364-5 (Wiley, 1991).
3. ^ M. E. Kling et al., Proc. SPIE vol. 3679, pp.10-17 (1999)
4. ^ Y-K Choi et al., J. Phys. Chem. B, vol. 107, pp. 3340-3343 (2003).

External links

• Overview of OPC, with diagrams, by Frank Gennari